Delimitation of partial dentures

Delimitation of partial dentures. Identification and fixation of central occlusion by I, II, III dentition defect groups. Methods of fixation of partial dentures.

The mucous membrane of the orthopedic bed is compressed during pressure, and when the pressure is stopped it comes from the state of compression and attempts to assume the initial form. This physiological property is called compliance.

Different authors explain this ability in different ways. E. I. Gavrilov /1963/ explains the vertical mobility of the mucous membrane by the arrangement of the area vasculosa, by the ability of vessels to be emptied and filled with the blood rapidly. The regions with the extensive area vasculosa are called buffer zones. The threshold compression is the greatest on the alveolar process, and the smallest along the sagittal raphe. The greatest compression on the lower jaw is observed in the region of transition of the mucous membrane from the alveolar process onto the floor of the oral cavity, smallest – on the alveolar process.

The classification of Lunde

1. The region of the sagittal raphe.

2. The alveolar process

3. The area of the hard palate in the region of the transverse folds.

4. The posterior third of the hard palate.

The clinical classification of Suplie.

1. The prosthetic bed is covered with the slightly compliant mucous membrane.

2. The prosthetic bed is covered with the atrophied mucous membrane – “dry mouth”.

3. The prosthetic bed is covered with the loosened mucosa – “soft mouth”.

4. There are mobile strands on the prosthetic bed.

   The compliance of the mucous membrane was measured by V.I. Kulazhenko with the help of the electron- vacuum apparatus /1964/. It is 0.5 mm on an average on the upper jaw, 0.3-0.6 mm in the region of the hard palate. It is 0.4-0.5 mm on an average on the lower jaw in the region of the alveolar process.

     The degree of the compliance of the mucous membrane of the prosthetic bed influences on the choice of the impression material and procedure of taking of the functional impressions as well as the relief of the removable denture base. It is necessary to make recesses in the base of the denture in the places of slightly compliant mucous membrane (torus, exostoses, etc), i.e. microchambers in their projection for leveling compliance. This is done in the following way. Having examined the prosthetic bed the doctor outlines places with slightly compliant mucous membrane with the pencil on the gypsum model and indicates to the dental technician thickness of the liner, which should be placed in this place. For example, the compliance in the region of middle- palatine raphe is equal to 0.3 mm, and in the adjacent area – 0.5-0.6 mm, the liner will be 0.2-0.3 mm of thickness. In such construction the denture base will have the capability of evenly transferring pressure to the entire prosthetic bed during mastication. But if we do not do this, then it will balance on the torus. It is best to cut out the liner according to the outline made by the doctor from the lead foil and to stick it on the model before packing of plastic in the cuvette. The role of the mucous membrane compliance during taking of the functional impressions will be explained in the following guide.

A CONCEPT OF THE VALVE ZONE.

     The place of the transition of the mucous membrane of the oral cavity from the alveolar process onto the cheeks and lip is called “transitional fold”. The place of the transition of the mucous membrane from the hard palate onto the soft one is called a line “A”. If we draw off cheeks and lips with the aid of two dental mirrors, then this transition will be clearly seen as the arched appearance in the form of folds. The passively mobile mucous membrane is located over this valve line. The border of the full removable prosthesis base must be located within it on the vestibular side on the upper jaw. In this case the borders of the prosthesis coincide with the edges of the functional impressions, designed with the aid of the functional tests.

     The mucous membrane, which lies on the edges of the base is called a “valve zone”. During the displacement of the prosthesis during mastication the mucous membrane mobility increases. It is raised and lowered together with the prosthesis, which allows to preserve the negative pressure, which retains the prosthesis on the jaw under the prosthesis; this phenomenon is called the “functional suction”. From the palatine side the “valve zone” is formed in wide opening of the patient’s mouth and the pronunciation of the loud sound “A”, “I”. The borders of the prosthesis base goes behind the mobile soft palate in the form of the torus from the inside.

     The retention of the prosthesis on the jaw at rest is called fixation. It is provided by adhesion and cohesion.

     The retention during nonmasticatory movements (conversation, mimicry) is called the stabilization of the full removable prosthesis, which is provided by adhesion, cohesion and functional suction. The force of suction depends on the area of the prosthesis base accuracy of the peripheral valve (“valve zone”) and is equal to 1 kg per 1 cm².

     And finally the retention of the prostheses during mastication is called the equilibrium (balance) of the prostheses and is provided by adhesion, cohesion, functional suction and correct position of the teeth.

     The historical description of the methods of the prosthesis fixation on the edentulous jaw was as follows. There were proposed:

1. Repulsive springs of Foshar and the suckers of Rauer.

2. Prostheses with repulsive magnets.

Modern methods of improvement in the fixation include:

1. Adhesives.

2. Implantation.

3. Anatomical retention

The work beginning from the casts moulding (at using the alginate cast materials the model can be to mould by the dentist). At the model cutting is important to save the place which corresponds to a transitive cord.

Next step is defining the margins of basis of the future denture. Will be more rationally to make this manipulation with the dentist.

 The size of the denture basis depend from amount of the saved teeth and its location, degree of the alveolar process atrophy, expressions of the hard palatum crest,  degree of pliability of the mucosa under the denture field, torus presence and methods of the denture fixing.

On the maxilla: if teeth loss, then the basis size is bigger; if teeth amount is small, then basis size is smaller. On the mandible the size of basises from the lingual side are constant, but from the vestibular depend from the loss teeth amount. Defining the margins of the denture making by the chemical pencil, because common pencils can be removed by the water.

Denture margins on the upper jaw.

From the vestibular part:on the transitive cord level, bypassing bridles and cords of the mucosa closely to the oral side.

From the oral side:at the frontal natural teeth having on the level of бугорковof teeth, without blocking them. In the molars and premolars area on the 2/3 level of the crowns height. In the presence of a torus it’s should to block by basis with correspond isolation of a site. Trying not to dilate the torus margins on a model the torus is outlined by circular line. In the future denture in the torus area from the gingival side of the denture will be a small thick which allow a denture not to rest on a torus.

Distal marginof the denture to round in the spaces between last molars. Distal border of the denture will represent a line spent at once behind last molar teeth of a jaw.

Denture margins on the mandible.

 

From the vestibular side:by the transitive cord level, bypassing mucosa bridles and cords closely to the oral side.

From the oral side:to block all remain teeth on the 2/3 from the crowns height. The lingual margin of the denture passes on the transitive cord, having corresponding cut for the lingual bridle in the form of a semilunar cutting.

After the model is outlined, starting the wax template manufacturing with the occlusial platens. They are need for defining the condition of the central occlusion at the patient in the clinic by the doctor and fix it which help the dental-technique to plaster the models in the articulator in the central occlusion position.

Templates and patens are making from the basis wax. The wax is issued in the plates diameter 20×10 sm. Preliminary cutting the plate on the square of the outlined zone on the model. On the lower jaw the plate should to fold double. The next step is warming up the wax plate over the torch or the spirit-lamp flame (for the lower jaw – the wax plate fold double, in warmed up by a part, then again warming up the folded double plate); the back side to the warmed big fingers is press to the gingival surface of the model. Surplus of wax on the borders is cutting by the warmed spatula. The wax basis is fixing by the wire to prevent the deformation in the oral cavity. Cutting 4-6 sm of the wax diameter 0.8 sm, bend horseshoe-like by the oral cavity part form of the alveolar process, try on the wax template, for it. The wire taking by a tweezers and warming over a torch. After, the wire piece smoothly to lover in the wax outside of template at the basis of the alveolar process from the pallatinal side. The wax template is ready.

Further start to manufacturing the occlusial platens. The warmed wax plate rolling down in the platen and stack in a free site from a teeth. The platen should be monolithic, height 1-1,5 sm, width 1 sm; based at the average of the alveolar process, stuck densely together with the wax basis (should to spend the warm spatula on the inner and outer surface of platen for it). Theplatensgiveasmoothsurface. The distal platen’s sites are making it the form of a bias. In the presence of a natural teeth the platens making on the 2-3 mm higher than teeth level.

The finished wax templates with the occlusal platens on the models transfer to the clinic. The dentist is defining the central occlusion.

The next step of the partial lamellar dentures manufacturing is casting models in the articulator.

The articulator represents the simulator of the vertical movements of the lower jaw (opening and closing the mouth). It is consists from the upper and lower frames, axis which fastening this frames, articulator high pin which help to regulate the height of the occlusion. The articulator is helping to statement the teeth on existing antagonists, and also to fix the central occlusion. 

Surpluses of the gypsum cut off from the models so that the pin of height of an articulator rested against a platform and didn’t interfere with the connection and disconnection of the articulator. On socle parts of the models do notches crosswisely for the better gypsum fixation. The lower frame of an articulator moistenswithwater.

Get mixed up gypsum and impose it on a smooth table surface and immerse in it the lower frame of an articulator. After add a small layer of gypsum and putting the models on it which fixing it so the overage line of the models is coincided with the overage line of an articulator in the central occlusion position. Further the layer of gypsum imposing on a lower jaw model and omit the upper frame of an articulator. Gypsum compare so that it has covered with an equal layer a framework of the model and articulator. After the gypsum will fall asleep, models will be plastered in an articulator in the central occlusion position.

After an articulator opening the occlusal platens are deleting, correct wax templates and start to partial lamellar denture manufacturing.

As is it shows any partial lamellar denture consist of a three parts: basis, artificial teeth and clasp system.

Clasp system is fixing denture attachments which hold it in the oral cavity at the chewing and non-chewing movements. Clasps are sectioned by a material: on the plastic and metal.

Plastic		Metal
Located only on a gum	Located on a gum and on a teeth	Wire(bent)		Casted
Pelots	Dentoalveolar clasp	Flat	Round	Keeping	Basic-keeping
Fingers-like processes		Clasp using at the insufficient conditions for the denture fixation. Representstheroundclaspflattenedouton an anvil.	Themostwidespreadclasp. It is made of preparations of the clasps which are let out by factory. Manufacturing from a wire from stainless steel is possible diameter 0.8-1.0-1.2 sm.	Only fixes a denture on a tooth because the clasp settles down below equator of tooth and it’s can’t effort without pass through equator (pic. 4.10).	Part of such clasp is the overlay – small metal site which lean on the chewing surface of the tooth and interfering lowering of a denture below the fixed position. Receive chewing pressure on a tooth.
All plastic clasps are using only in cases when should to hide the clasp in a cosmetic goals, or at appreciable abbration of firm tissues; using only at well expressed alveolar process.		All metal clasps have traumatic effect on firm tissues of tooth. Under the influence of a clasp can develop a sphenoidal defect. Therefore teeth which will influence a clasp, it is better to cover a bugel crowns (look “Non-removable prosthetics”).

 

Teeth on which settle down a clasp are called as basic. If connect basic teeth  a conditional line we will receive a clasp line. If in denture has one basis tooth so that denture fixation called dotted, if two (one clasp line) – linear. If exist two cross clasp lines – fixation is plane (it’s the most optimal fixation type for the partial lamellar dentures). The clasp lines are subdivided on: sagittal (in front back or on the contrary) and transversal (on the right in front – back at the left or on the contrary).

Fully-casted basis-keeping clasps are not using at the partial lamellar prosthetics. Theirdestinyisbugeldentures. 

Methodic of fix metal clasps manufacturing is next. On tooth on which the clasp will be located modeling the fixing clasp from the wax that the main part of a clasp were lay more low than the equator line (the most convex part of tooth). The clasp termination was blunted (to do not traumatize the soft tissues) and deduce a little (no more than 1 mm) above an equatorial line. The clasp accurately take out from the model and give it to the moulding. The moulded clasp fitting, hot spatula paste in wax.

The most widespread for this type dentitions is wired clasps. They are simply in manufacturing and enough functionally. The wired clasps has three parts: the process (fixed in the plastic part of clasp), the body (springing part of clasp) and the shoulder (located on the tooth part of clasp).

The wired round clasp manufacturing is next. In the beginning select a clasp from a set or cut off by scissors for metal a part of wire length 2-2.5 sm.

The clasp manufacturing begins with sharpening of the extremity of a wire by a file. Keeping the wire by a left arm, bend forcepses a clasp shoulder adjusting it to a vestibular surface of a tooth. Further at the straight angle from top to bottom bend a wire creating a body of clasp. Then follows the third flexure – unbend a process on the center of alveolar process in a denture basis depth.

The process flatten out on an anvil (if it’s the flat clasp then flatten full clasp) and make the notches by a file on the process for the better fixation in a plastic.

Plastic clasps are casting from the wax like it’s sown on an image.

After manufacturing the clasps on all teeth on which it need, begin the artificial teeth fixation.

All artificial teeth, which using for the fixation at the partial lamellar denture, subdivide on the plastic and porcelain.

Let’s try to understand the plastic teeth fixation. That’s teeth are issued by factory in sets (or by 28 teeth, or separately sets front and side teeth). The plastic teeth are differing by a form, color and design.

Teeth need to be picked up in the beginning depending on defect size, color and form of remain teeth.

At the tooth fitting in the beginning it is necessary to give width by cutting it on the polishing-motor or by milling cutter on a handpiece by dentitechnical a part of plastic. Further fitting it gingival part, on height, watching that the gingival part settet down in a gum and didn’t press the wax. The next step in teeth fitting is specification of a parity of plastic tooth with antagonists.

The right fitted tooth must:

– the vertical axis of tooth must to coincide with the alveolar process overage;

– each tooth (except lower central incisors and second upper molar) should to have two antagonists and it contact should to be  the maximal (decently) on full chewing surface;

– the neck of an artificial tooth should to be on the natural tooth level;

– the plastic teeth are located decently to each other, fitting is making without trems and diastemas.

 The porcelain teeth in the past were popular enough for the partial lamellar dentures using. They are much more cosmetic then a plastic teeth, more strongly the last. But at the same time a plastic teeth was chemically bridged with the basis of denture (they are manufactured from the one type materials), and the porcelain just by a retention (special attachments presence for fixation in the porcelain teeth).

Fitting of the porcelain teeth is next.

Teeth steals up depending on defect size, color and form of the remain teeth.

Requirements which present to that porcelain tooth same that to the plastic. Fitting features:

– at the porcelain teeth sharpening should to  preserve the crampons from grinding;

– the cylindrical form crampons to bend downward and aside at right angle;

– between the teeth and alveolar crest of the model should to be lumeot less than 1-2 mm.

Have the cases when in cosmetic goals recede from these fitting teeth rules, and put the teeth on “sharpening”. At this method the requirements to present tooth the same. Features of fitting on “sharpening”: at the expressed alveolar process the frontal teeth installing on sharpening, that is ahead of alveolar process, grinding off them so each artificial tooth densely adjoined by neck part to the gingival margin of the alveolar process.

After the clasp fixation fixing the artificial teeth, the model of future denture giving in an articulator to the clinic. Dentist checking correctness of selection of a teeth, correctness of the fixing and defining of the central occlusion, flexure or equipment of the clasps. In case of errors in previous steps in common with doctor making a relocation of teeth, to lower or to raise one or group of teeth, clasp position managing.

After begin modeling of the denture basis. At the necks of front teeth the vestibular surface of an artificial gum modeling with the small round prominence above the roots and the cambers which imitating relief of the alveolar process. Gingival margin at the side teeth from neck side modeling with the prominent crest. It is necessary to consider and release the stagnation places of mucosa to preserve the denture dropping at the mouth opening.

On the lower jaw denture the gingival surface in the front teeth area is modeling to create a relief that so at teeth biting don’t disturbance the occlusion and don’t be the thickness. Configuration of the gingival part of a denture should to repeat the features of patient palatum configuration including the palatinal cords. Margin transition of a denture by “A” line should to be a uniform thickness and it’s shown “on is not present”.

The denture basis margins rounding, making a smooth by a hot spatula; the margins should to repeat the margins of neutral zone.

On the lower jawat the expressed bridle of a lower lip and buccal bridle the margins of an artificial gum modeling with their tension at the mouth opening (it’s necessary that the denture margin lagged behind an attachment place of bridle on the 1-1.5 mm). In the side teeth region from the vestibular side modeling a bends for cheeks which is promotes a fixation of denture and correct participation in chewing act.

The lingual surface of basis in the sublingual process region the front teeth make a slightly bending for a free adhering and movement of end of tongue. In the side teeth area modeling a sublingual processes more thickness, with a bends in the average part, which in will be located the side surfaces of tongue.

The basis margins of the lower jaw denture is bend and also observe their dimensions according to the planned model margins by a doctor. The back margins of denture are locating on the back-molar triangle on the inside surface of the lower jaw branch.

Lead off on the basis margin the hot spatula to bond the basis to gypsum. After that easy blows of a hammer beat off models from an articulator. In the presence of auxiliary model it deletes. Start to replace the denture basis from wax to plastic.

Plastic polymerization is carried out in a ditch under the influence of temperature and pressure.

As is it shown we have a three casting methods of model in the ditch. Direct (when in the lower part of ditch remaining the clasps, artificial teeth and the model), reverse (when the plastic teeth and clasps are moved at the lower part of ditch) and combined (when the clasps remain on the model, but a teeth moving to the upper part of ditch). At the partial lamellar denture manufacturing can be all three paths.

 

Casting models into the ditch by the reverse way.

Plastering teeth are cutting off from the vestibular side with the bend to the model level. Cutting off also and teeth basis teeth clasps release a shoulder of clasp and immerse a model on 10 minutes in water.

Get mixed up gypsum to the sour cream-like consistence, then filling it the upper part of ditch and putting the model basis to artificial gum into it. Watching up for the wax basis with a teeth, clasp shoulders and artificial gum towering over edges ditches didn’t stay a free from a gypsum. Fill in gypsum, comparing it to level of edge ditches.

The ditch with the plastered model place in 10 minutes in cold water after the gypsum induration. After placing the ditch basis, putting off the cover, moulding gypsum to the more liquid consistence and filling the ditch basis by small portions and slightly tapping on a table to excision for bubbles. Filling in the basis to the margins by gypsum densely imposing a cover, thus surpluses of gypsum are squeezed out by a dentitechnical press.

 

Plastering models in the ditch by a direct way.

The model basis with a partial lamellar denture cutting off that so the ditch margin with the putted there model will be a little above the artificial teeth level. Part of the gypsum teeth which have clasps is cutting off for the better clasp shoulder plastering. Wet the model.

Moulding the liquid gypsum, filling in the ditch basis by it and imposing the model in it, but the basis should be turned to a bottom of a ditch.

From the squeezed up gypsum forming a platen above the teeth covering vestibular surface, cutting edges of the front teeth and the chewing surface of the side teeth. Unfixed remain only palatinal surfaces of the upper teeth and lingual surfaces of the lower teeth.

To impose the upper part of a ditch without a cover. Moulding gypsum, filling a control ditch by a liquid gypsum constantly stirring a ditch. Densely close it’s by a cover and squeeze out the rests of a gypsum from the ditch by press.

 

Plastering the models in the ditch by a combined way.

This way is using for the frontal teeth fixing on sharpening.  The prepared model placed in the basis of ditch, preliminary fill it by liquid gypsum. The frontal teeth fixed without the artificial gum is cover by the gypsum platen like at the direct plastering, and the side teeth remain unfixed like at the reverse plastering. The basis of ditch place into the cold water on 10 minutes. Further imposing the part of ditch without a cover on it. Mouldinggypsum. Filling the control ditch by a cover and squeeze out the rests of gypsum from the ditch by press.

 After the plastering into ditch start wax evaporation when the gypsum is stiffened. The ditch with the plastered denture wax composition place into the boiling water bath and also boil throughout 10-15 minutes to a full ramollissement of wax.

Putting off the ditch from a water, unsealing it by a spatula. Removing a ramollisse wax and definitively etch the wax from a ditch by a boil water. After etch the wax grease the prosthetic field on the model and gypsum in the second half of ditch by an isolation varnish type “Isokol” (a thin layer by brush) while a gypsum still warm. When the layer has stiffened again imposing isolation liquid and should to look that the varnish hasn’t got on the artificial teeth and clasps.

In parallel with preparation of model for drawing prepare plastic dough. With this goal using the basis plastic of the warm polymerization type “Ethakryl”, “Phtorax” and other.

Plastic powder take from calculation 1 gr. on 1 artificial tooth or 8-12 gr. on a denture and add the monometer before full saturation of a powder. Moulding a plastic in a porcelain or glass jar. Removing a rest of monometer that the admixture surface remained glossy, and densely close by a cover. That granules of polymer (powder) in regular intervals bulked up, and also homogeneous dough was formed, should periodically quickly to mould the plastic and again densely to close a vessel by a cover. Readiness of plastic for formation in stage dough-like judge on disappearance of lasting threads at rapture.

When the plastic is ready the artificial teeth and clasps are degrease. Take from the vessel portion of dough for denture by clean hands through the cellophane and then filling it the ditch basis at the reverse plastering, control ditch – at the direct, and both half – at the combined way of plastering.

Plastic was cover by a worming plate of cellophane, connecting both half of ditch, placing it into a dentitechnical press and slowly pressing and don’t close both half on 1,0-1,5 mm, remain in a press on 3-5 minutes. This pressing is called a trial that so pressing of plastic isn’t ended (thick 1-1.5 mm).

After the trial pressing the ditch remove from the press, open, delete cellophane. To prevent the monomer evaporating it is quickly removing the rest of plastic cutting off it on a denture margin.

Folding a ditch and definitively press having finished both parts to full closing, taking under press in 10-15 minutes; then take out from press, fixing in the bugel and start to plastic polymerization. With this goal in metal ware or electric sterilizer fill in a cold water and place there a ditch. Lifting of water temperature from room to 80°Сis carried out in a current of 60-70 minutes, after that warmed up strengthen and lead up temperature to boiling and boil the ditch in 20-30 minutes. After this time having stop warmed up. The ditch cooling together with water or take out from a vessel and cooling in air.

When the ditch completely cool down to the room temperature, take off both the covers by a gypsum knife and hammer, accurate squeeze up gypsum from a ditch in a special press. Further separate the rest of gypsum from a plastic denture by a knife. The rests of gypsum wash off water with soap by a brush.

By a scrapers and files take out a rests of plastic and roughness from a denture. Denture margins process on a polish-motor or by a flexible sleeve and a handpiece, milling cutters and carborund stones. Make out necks of an artificial teeth and intervals between them. Furnish reach a uniform and smooth surface of a denture.

Polishing of denture making by an emery paper of different granularity since more rasping and finishing more thin. Polishing only the outside surface and denture margin.

On screw cutting of a handpiece grinding-motor input a cone filz and polishing sites of a denture between the teeth constantly moistening a denture with a meal from water and pumice. Than polishing other surfaces of a denture by a cone filz until external surface doesn’t become absolutely smooth. The final mirror shine give to a denture by a brush and meal from a chalk and denture powder with water. After the polishing washed up by a water with soap.

The polished denture is transferring to a clinic for fixation.

 DETERMINATION OF THE CENTRAL OCCLUSION

     The central occlusion may be at the beginning and at the end of the masticatory act, i.e. during swallowing of the food or saliva. The central occlusion is characterized by maximal joining of the dentitions, predominant technical tension of the muscles that raise the lower jaw as well as by position of the articular heads at the basis of the articular tuber slope. The height of the bite is a distance between the alveolar processes of the upper and lower jaws which is determined in the position of the central occlusion. There are four groups of dentition defects while determining the central occlusion.

     I group is characterized by presence of at least three pairs of teeth-antagonists that are located in the triangular and models may be put together according to tubercle-fissure guiding lines.

     II group is characterized by presence of antagonists on one or two sides of the jaws.

     III group – there are teeth in the mouth but they do not have antagonists.

     IV group – edentulous jaws.

     Complexity of determining the central occlusion increases in every consequent group and in IV group of defects it is studied in the section “Prosthesis of edentulous jaws”

     I group of defects: the models are put together in the position of the central occlusion without using occlusion rims.

     II group of defects: the central occlusion is determined with the aid of the occlusion rims.

     To do this the occlusion rims on the wax base are washed in the cold water and then inserted into the oral cavity of the patient. The occlusion rims are prepared in such a way that the teeth-antagonists don’t join by 2 mm. After that the occlusion rims are transferred onto the models and their surface is soften with a hot spatula at the depth of 3-4 mm. Then they are inserted into the oral cavity again and the patient is asked to close the jaws. The central occlusion is correctly determined in case there is maximum contact between antagonists.

     III group of defects: determination of the central occlusion is composed of two parts: determination of the height of the bite and medio-distal position of the lower jaw.

     The state of tonic balance of the masticatory muscles (ascending and descending) is characterized as the sate of the physiological rest. In this state the distance between the alveolar processes is 2-4-5 mm longer (depending on age) than the distance in the central occlusion.

     To determine the height of the bite it is necessary to determine the points on the chin and at the basis of the nasal septum. Then it is necessary that the patient speak for some time and then swallow the saliva. After that the masticatory muscles became relax and the jaws are established in the position of the physiological rest.

     The distance between these points is measured on the paper or wax plate and it is decreased by 2-3 mm, so we obtain the distance of the lower part of the face at the state of the physiological rest. Then we warm the rim against the mandible teeth at the depth of 2-3- mm and insert it into the mouth and ask the patient to close the jaws. Not to have the protrusion of the lower jaw it is necessary for the patient to throw the head back, raise the tongue to the palate and swallow the saliva. His lower jaw will close in the position of the central occlusion. There may some pressure of the palm on the patient’s chin but it should be made very carefully not to get the protrusion of the lower jaw. Having closed the jaws in the position of the central occlusion the patient bites the rim at the depth of 2-3 mm, i.e. up to the second mark at the wax plate. Then the occlusion rim is cooled in the cold water and wedge-shaped threads are made on it. After that both rims are inserted into the oral cavity and the lower one is adjusted to the first mark on the wax plate.  Then its surface is warmed at the depth of 3-4 mm, it is inserted in the oral cavity and the patient is asked to close the jaws together with a number of above stated manipulations. Then the rim is taken out of the oral cavity, cooled in the cold water and again placed on the jaw. Determination of the central occlusion is checked several times: when joining of the rims coincides in all cases then the central occlusion is determined correctly.

      After that the rims are placed on the model, put them together in the position of the central occlusion and fixed with the aid of matches and molten wax. After determination of the central occlusion the color of the natural teeth is determined and registered in the accompanying note.

 

CONSTRUCTION OF THE PARTIAL PLATE DENTURE. BASE BORDERS.

     The size of the partial plate denture base depends on the size of the dentition defect. The plate denture is known to disturb taste, tactile and temperature sensitivity of the oral cavity. Therefore, it is desirable to decrease the size of the denture base. On the other hand, the smaller the base, the greater pressure falls on the unit of the prosthetic bed area resulting in atrophy of the mucous membrane and alveolar processes. The denture base borders from the vestibular side end in the neutral zone. The denture base on the upper jaw that covers the hard palate may be decreased not more than one third of the hard palate length. Natural lateral teeth are covered with a plate up to the masticatory surface and anterior teeth – by 1-2 mm in the area of the tooth neck. On the lower jaw the base border from the oral side comes to the movable mucous membrane and all natural teeth are covered with the plate up to the cutting edge or masticatory surface.

 

FIXATION OF THE MODEL IN THE OCCLUDATOR OR ARTICULATOR

     After fixation of the models with the occlusion rims in the position of the central occlusion they should be plastered in the occludator or articulator.

     Occludators (wire, cast, etc) allow to make vertical hinged movement –closing and opening of the dentitions.

     Articulators (medium and complex) reproduce vertical and horizontal movements of the lower jaw. Wire occludators are usually used in construction of the partial plate dentures. Gypsum is kneaded up to sore cream – like consistency and placed on the lower frame of the occludator. Then the models are placed in such a way that their middle line coincides with the middle line of the occludator. The upper frame is lowered and it is fixed by gypsum to the upper model. After consolidation of the gypsum its residues are carefully cut off.

   Adhesion is the use of two identical surfaces when there is a thin sublayer of fluid between them.

      The adhesion fixation force is 100g/cm², the denture of the upper jaw with an area of 20 cm² is fixed with force of 2 kg. During the masticatory function the denture withstands pressure of 15-20 kg in force. Therefore, adhesion for fixation of the partial plate denture is insufficient.

      Anatomical retention points

      The so-called clasp-free dentures are fixed with the aid of the anatomical retention points. This kind of dentures is satisfactory as to the cosmetic appearance. Although denture fixation becomes weaker in some period of time as the base loosens the abutment teeth. Clasp-free dentures are justifiable in using elastic plastic.

     Plastic dentoalveolar clasps by Kemeni.

     The Hungarian dentist I.Kemeni proposed to use dentoalveolar clasps made of plastic for fixation of the partial plate dentures. These dentures are called retention ones. The dentoalveolar clasps may be constructed in case when the vestibular surface of the alveolar process has a straight or sloping structure and the teeth are positioned in conformity with the alveolar crest. When the vestibular surface of the alveolar process is thread-like or the teeth are inclined vestibularly, retention dentures cannot be constructed.

     Fixation of the removable plate dentures with the help of telescopic crowns

     This kind of dentures is widely applied in the Western countries. The singly standing or separate groups of the teeth are constructed metal crowns-caps that are fixed on the teeth by phosphate cement. Other crowns are constructed to be placed on these crowns that are, in their turn, fixed in the denture. While placing the denture the external crowns are applied on the internal ones, thus achieving satisfactory fixation of the dentures. But the masticatory load falling on the whole denture is transferred to a small number of the teeth resulting in overload of the parodont of these teeth with different consequences.

     Fixation of the dentures by Rumpel

      The singly standing teeth or separate groups of the teeth are covered with crowns with soldered metal bar that passes in the middle of the alveolar process. A long slot is made in the denture base where the bar fit. The masticatory load is transferred on the bar via the denture, and in this way on the abutment teeth resulting in the fast loosening due to overload.

     Fixation of the partial plate dentures with the help of vestibularly-directed clasps

      To strengthen the single teeth or separate groups of the teeth and to fix plate dentures V.I.Kulazhenko proposed a construction of the continuous vestibular clasp in 1962. The vestibular clasp is a metal bar that connects natural teeth from the vestibular side at the neck. Such construction of dentures contributes to strengthening of the loosen teeth and provides good fixation of the plate dentures.

      Clasp fixation

      A clasp is a hook that fixes the denture. Natural teeth where the clasps are placed are called the abutment teeth.

      To obtain the best fixation of the denture the following should be taken into consideration: mobility of the abutment teeth, their number, choice and solution of the problem of the kind of the clasp strengthening.

      There are three kinds of clasp fixation:

– Point fixation – only one tooth is used for abutment. In such fixation the abutment tooth becomes loosen rapidly because of lever-like force of the clasp and denture base on the abutment tooth.

– Linear fixation – two teeth are used for abutment. The clasp line may be sagittal, diagonal and transversal. When there is a free choice, it is better to construct a diagonal one on the upper jaw and transversal clasp fixation – on the lower jaw. The sagittal fixation is the least fitting for both jaws.

– Plane strengthening is the most rational strengthening as 3-4 teeth are used as abutment teeth.

 Clasps are divided into:

1. By the method of making – wire (labile) and cast (stable).

2. By the shape: circular, semicircular and strip.

3. By the degree of coverage – one-chain, multichain and continuous

4. By the number of arms – one-arm and two- arm.

5. By the fixation – retaining, tooth-supported, molar clasper

      The retainer is composed of the arm, body and process (anchor part). The arm covers the abutment tooth from the vestibular side between the equator and the tooth neck. The body is located on the equator with approximal surface of the tooth; and the process connects the clasp with the denture base.

      Even the clasp of the most rational construction is not safe for the abutment teeth. The more flexible the clasp, the less it loosens the abutment teeth. Clasp flexibility depends on the method of its making and the arm of length. Cast clasps do not have flexibility, which is characteristic of the flexible ones.

Indications and constructive features of crown construction for clasps of the plate dentures

     As it was said, the crown construction in prosthesis with partial denture is not obligatory.

     The crowns are indicated 1) in defects of the hard tissues of the tooth; 2) in devitalization of the teeth; 3) for strengthening of the loosen teeth by soldering several crowns; 4) for leveling of the occlusion plane.

      The abutment teeth, on which the crowns are constructed, are prepared in such a way that their sides are equilateral. In restoration of the anatomical shape of the prepared teeth on the model, the equator is formed only on the vestibular side that has an adjacent tooth. The lateral sides are modeled as equilateral ones.

Aremovable partial denture must have sup­port that is derived from the abutment

teeth through the use of rests and from the residual ridge through well-fitting bases. It must be stabilized against horizontal movement through the use of rigid connectors, indin_d retainers, and other stabilizing components. In addition, the removable partial denture must have sufficient retention to resist reasonable dislodging forces.

Primary retention for the removable partial denture is accomplished mechanically by plac­ing retaining elements on the abutment teeth. Secondary retention is provided by the intimate relationship of minor connector contact with the guiding planes, denture bases, and major con­nectors (maxillary) with the underlying tissues. The latter is similar to the retention of complete dentures and is proportionate to the accuracy of the impression registration, the accuracy of the fit of the denture bases, and the total area of contact involved.

Mechanical retention of removable partial dentures is accomplished by means of direct retainers of one type or another. A direct retainer is any unit of a removable dental prosthesis that engages an abutment tooth in

such a manner as to resist displacement of the prosthesis away from basal seat tissues. This may be accomplished by frictional means, by engaging a depression in the abutment tooth, or by engaging a tooth undercut lying cervically to its height of contour.

There are two basic types of direct retainers. One is the intra coronal retainer, which is cast or attached totally within the restored natural contours of an abutment tooth. This type of retainer is composed of a prefabricated ma­chined key and keyway, with opposing vertical parallel walls that serve to limit movement and resist removal of the partial denture through frictional resistance. The other type of retainer is the extracoronal retainer, which uses mechanical resistance to displacement by com­ponents placed on or attached to the external surfaces of an abutment tooth. There are three principal types of extracoronal retainers. One is the manufactured attachment, such as the Dalbo. Others use a spring-loaded plunger that engages a contoured or restored depression on the external surface of the abutment tooth and thereby resists displacement. A second type is a manufactured attachment that uses flexible clips or rings that engage a rigid component that is cast or attached to the external surface of an abutment crown. A third type is the clasp-type retainer , in which a flexible arm engages an external surface of an abutment tooth in an area cervical to the greatest convexity of the tooth or engages a depression prepared to receive the terminal tip of the arm. The most common extra coronal attachment is the reten­tive clasp arm.

  lntracoronal retainer consists of a key and keyway with extremely small tolerance. A, Keyways are contained within abutment crowns, and B, keys are attached to removable partial denture framework. Frictional resistance to removal and placement and limitation of movement serve to retain and stabilize prosthesis.

 The intracoronal retainer is usually regarded as an intemal attachment, or a precision attach­ment. The principle of the internal attachment was first formulated by Dr. Herman E.S. Chayes in 1906. One such attachment manufactured commercially still carries his name. Although it may be fabricated by the dental technician as a cast dovetail fitting into a counterpart receptacle in the abutment crown, the alloys used in manufactured attachments and the precision with which they are constructed make the ready-made attachment much. preferable to any of this type that can be fabricated in the dental laboratory. Much credit is due the manufactur­ers of metals used in dentistry for the contin­ued improvements in the design of internal attachments.

Numerous well-designed internal attach­ments are available in the dental marke\ that may be used in situations requiring special retention. Descriptive literature and technique manuals are available from the manufacturers.

 

 

 

 Dalbo extracoronal attachment. Components consist of A, L-shaped male portion that is attached to an abutment crown; B, female sleeve that is placed in artificial tooth adjacent to abutment, and coiled spring that fits into female portion. Design permits some vertical movement of denture under force through compression of coiled spring.

  Extracoronal circumferential direct retainer. Assembly consists of A, buccal retentive arm; B, rigid lingual stabilizing (reciprocal) arm; and C, support­ing occlusal rest. Terminal portion of retentive arm is flexible and engages measured undercut. Assem­bly remains passive until activated by placement or removal of restoration or when subjected to masticatory forces that tend to dislodge the den­ture base.

The internal attachment has two major advan­tages over the extracoronal attachment: elimi­nation of visible retentive and support compo­nents, and better vertical support through a rest seat located more favorably in relation to the horizontal axis of the abutment tooth. For these reasons, the internal attachment may be preferable in selected situations. It provides horizontal stabilization similar to that of an internal rest; however, additional extracoronal stabilization is usually desirable. It has been claimed that stimulation to the underlying tissues is greater when internal attachments are used because of intermittent vertical massage. This is probably no more than is possible with extracoronal retainers of similar construction.

Some of the disadvantages of internal attach­ments are that (1) they require prepared abut­ments and castings; (2) they require somewhat complicated clinical and laboratory procedures; (3) they eventually wear, with progressive loss of frictional resistance to denture removal; (4) they are difficult to repair and replace; (5) they are effective in proportion to their length and are therefore least effective on short teeth; (6) they are difficult to place completely within the circumference of an abutment tooth because of the size of the pulp; and (7) they are considered more costly.

Because the principle of the internal attach­ment does not permit horizontal movement, all horizontal, tipping, and rotational movements of the prosthesis are transmitted directly to the abutment tooth. The internal attachment therefore should not be used in conjunction with extensive tissue-supported distal exten­sion denture bases unless some form of stress-breaker i:’i u:’ied between the movable base and the rigid attachment. Although stress-breakers may be used, they do have some disadvantages, which are discussed later, and their use adds to the cost of the partial denture.

EXTRA CORONAL DIRECT RETAINERS

 

Although the extracoronal, or clasp direct, retainer is used more often than the internal attachment, it is commonly misused. It is hoped that a better understanding of the principles of clasp design -will lead to a more intelligent use of this retainer.

Critical areas of an abutment that provide for retention and stabilization (reciprocation) can only be identified with the use of a dental cast surveyor . To enhance the under­ standing of direct retainers, an introduction of the dental cast surveyor is appropriate at this time.

The cast surveyor is a simple instrument essential to planning partial denture treatment. Its main working parts are the vertical arm and the adjustable table that holds the cast in a fixed relation to the vertical arm.

This relationship of the vertical arm to the cast represents the path of placement that the partial denture will ultimately take when inserted or removed from the mouth.

The adjustable table may be tilted in rela­tion to the vertical arm of the surveyor, until a path that best satisfies all the factors involved can be found. A cast in a horizontal relation­ship to the vertical arm represents a vertical path of placement; a cast in a tilted relation­ship represents a path of placement toward the side of the cast that is tilted upward. The vertical arm, when brought in contact with a tooth surface, will indicate the areas avail­able for retention and those available for support, as well as the existence of tooth and other tissue interference to the path of placement.

When the surveyor blade contacts a tooth on the cast at its greatest convexity, a triangle is formed. The apex of the triangle is at the point of contact of the surveyor blade with the tooth, and the base is the area of the cast representing the gingival tissues.

The apical angle is called the angle of cervical convergence. This angle may be measured as described in Chapter 11, or it may be estimated by observing the triangle of light visible between the tooth and the surveyor blade. For this reason a wide sur­veyor blade rather than a small cylindric tool is used so that the triangle of light may be more easily seen.

The following factors determine the amount of retention a clasp is capable of generating:

 Adjustable table

 

 Essential parts of a dental surveyor (Ney Parallelometer), showing vertical spindle in relation to adjustable table.

 1. Size of the angle of cervical convergence (depth of undercut)

2 How far into the angle of cervical convergence the clasp terminal is placed

3. Flexibility of the clasp arm, which is the product of:

  a. Its length, measured from its point of origin to its terminal end

  b. Its relative diameter, regardless of its cross-sectional form

  c. Its cross-sectional form or shape-that is, whether it is round, half-round, or some other form

d. The material of which the clasp is made­that is, whether it is made of a cast gold alloy, cast chrome alloy, wrought gold alloy, wrought chrome alloy, tita­nium, or titanium alloy (each alloy has its own characteristics in both cast and wrought form)

 

To be retentive a tooth must have an angle of col\vergence cervical to the height of contour. When it is surveyed, any single tooth will have a height of contour or an area of greatest convex­ ity; areas of cervical convergence may not exist when the tooth is viewed in relation to a given path of placement. Also, certain areas of cervical convergence may not be usable for the placement of retentive clasps because of their proximity to gingival tissues.

This is best illustrated by mounting a spheri­cal object, such as an egg, on the adjustable table of a dental surveyor.

 The egg now represents the cast of a dental arch or, more correctly, one tooth of a dental arch. The egg is first placed perpendicular to the base of the surveyor and surveyed to determine the height of contour. The vertical arm of the surveyor represents the path of placement that a denture would take and, conversely, its path of removal if it were placed and removed over this object.

With a carbon marker a circumferential line is drawn on the egg at its greatest circumference.

This line, which Kennedy called the height of contour, is its greatest convexity. Cummer spoke of it as the guideline because it is used as a guide in the placement of retentive and nonretentive clasps. To this, DeVan added the terms supra­bulge, denoting the surfaces sloping superiorly, and infrabulge, denoting the surfaces sloping inferiorly.

Any areas cervical to the height of contour may be used for the placement of retentive clasp components, whereas areas occlusal to the height of contour may be used for the placement of nonretentive, stabilizing, or reciprocating components. Obviously, only flexible compo­nents may be placed gingivally to the height of contour because rigid elements would not flex over the height of contour or contact the tooth in the undercut area.

With the original height of contour marked on the egg, the egg is now tilted from the perpendicular to an angular relation with the base of the surveyor . Its relation to the vertical arm of the surveyor has now been changed, just as a change in the position of a dental cast would bring about a different relationship with the surveyor. The vertical arm of the surveyor still represents the path of placement; however, its relation to the egg is totally different.

Again, the carbon marker is used to delineate the height of contour or the greatest convexity. It will be seen that areas that were formerly infrabulge are now suprabulge, and vice versa. A retentive clasp arm placed below the height of contour in the original position may now be either excessively retentive or totally nonreten­tive, whereas a nonretentive stabilizing or recip­rocal arm that is located above the height of contour in the first positioow may be located in an area of undercut

 A, When an egg is placed with its long axis parallel to surveying tool, height of contour is found at its greatest circumference. Similarly, height of contour may be identified on a single tooth when its long axis is placed parallel to surveying tool. Rigid parts of partial denture framework may be located in suprabulge areas above height of contour, whereas only flexible portions of clasp retainers may be placed in infrabulge areas below the height of contour. Those infrabulge surfaces that will be crossed by rigid parts of partial denture framework must be eliminated either during mouth preparations or by blockout. B, If same egg is tilted in relation to vertical spindle of surveyor, areas formerly infrabulge are now found to be suprabulge and will accommodate only nonretentive denture components. At the same time, however, areas formerly suprabulge or only slightly infrabulge are found to be so severely undercut that design and location of clasp retainers must be changed. Unfortunately, no single tooth in a partially edentulous arch may govern relation of cast to surveyor and thus the path of placement of partial denture. A compromise position must be found that, following mouth preparations, will satisfy all four factors: (1) no interference to placement; (2) effective location of retentive components; (3) most esthetic placement of all component parts; and (4) existence of guiding planes that will ensure stabilization of the partial denture and a definite path of placement and removal.

The location and depth of a tooth undercut available for retention are therefore only relative to the path of placement and removal of the partial denture; at the same time, nonretentive areas on which rigid components of the clasp may be placed exist only for a given path of placement.

If conditions are found that are not favorable for the particular path of placement being considered, the conditions produced by a differ­ent path of placement should be studied.

The cast is merely tilted in relation to the vertical arm until the most suitable path is found. The most suitable path of placement is generally considered to be the path of placement that will require the least amount of mouth prepa­ration necessary to place the components of the partial denture in their ideal position on the tooth surfaces and in relation to the soft tissues. Then mouth preparations are planned with a definite path of placement in mind.

 

 Retention is provided primarily by flexible portion of clasp assembly. Retentive terminals are ideally located in measured undercuts in gingival third of abutment crowns. When force acts to dislodge restoration in occlusal direction, retentive arm is forced to deform as it passes from undercut location over height of contour. Amount of retention provided by clasp arm is determined by its length, diameter, taper, cross-sectional form, contour, type of alloy, and location and depth of undercut engaged.

It is of primary importance to remember that tooth surfaces can be recontoured by selective grinding or the placement of restorations (mouth preparations) to achieve a more suitable path of placement or removal. The path of placement also must take into consideration the presence of tissue undercuts that will interfere with the placement of major connectors, the location of vertical minor connectors, the origin of bar clasp arms, and the denture bases.

When the theory of clasp retention is applied to the abutment teeth in a dental arch during the surveying of the dental cast, each tooth may be considered individually and in relation to the other abutment teeth as far as the designs of retentive and stabilizing (reciprocating) compo­nents are concerned. This is necessary because the relationship of each tooth to the rest of the arch and to the design of the rest of the prosthesis has been considered previously in selecting or modifying the teeth to achieve the most suitable path of placement. Once this relationship of the cast to the surveyor has been established, the height of contour on each abutment tooth becomes fixed, and the clasp design for each must be considered separately.

Clasp retention is based on the resistance of metal to deformation. For a clasp to be retentive, it must be placed in an undercut area of the tooth, where it is forced to deform when a vertical dislodging force is applied. It is this resistance to deformation that generates retention. Such resistance is proportionate to the flexibility of the clasp arm.

A positive path of placement and removal is made possible by the contact of rigid parts of the denture framework with parallel tooth surfaces, which act as guiding planes. Because guiding planes control the path of placement and removal, they can also provide additional reten­tion for the partial denture by limiting the possibilities that exist for its dislodgement. The more vertical walls (guiding planes) that are prepared parallelvthe fewer the possibilities that

exist for dislodgement. If some degree of parallelism does not exist during placement and removal, trauma to the teeth and supporting structures, as well as strain on the denture parts, is inevitable. This ultimately results in damage either to the teeth and their periodontal support or to the denture itself or both. Therefore without guiding planes, clasp retention will either be detrimental or practically nonexis­tent. If clasp retention is only frictional because of an active relationship of the clasp to the teeth, then orthodontic movement, damage to peri­odontal tissues, or both will result. Instead, a clasp should bear a passive relationship to the teeth except when a dislodging force is applied.

 

Relative uniformity of retention

The size of the angle of convergence will determine how far into that ;;mgle a given clasp arm should be placed. Disregarding, for the time being, variations in clasp flexibility, relative uniformity of retention will depend on the location of the retentive part of the clasp arm, not in relation to the height of con­tour, but in relation to the angle of cervical convergence.

The retention on all principal abut­ments should be as nearly equal as possible.

 

 

 Retentive cast clasp arm should be tapered uniformly from its point of attachment at clasp body to its tip. Dimensions at tip are about half those at point of attachment. Clasp arm so tapered is approx­imately twice as flexible as one without any taper. Tis clasp thickness.

 

 length of cast retentive clasp arm is measured along center portion of arm until it either joins clasp body (circumferential) or until it becomes part of denture base or is embedded in the base (bar-type clasp).

Although esthetic placement of clasp arms is desirable, it may not be poSsible to place all clasp arms in the same occlusocervical relation­ship because of variations in tooth contours. However, retentive surfaces may be made simi­lar by altering tooth contours or by placing cast restorations with similar contours.

Retentive clasp arms must be located so that they lie in the same approximate degree of undercut on each abutment tooth. Should both clasp arms be placed equidistant below the height of contour, the higher location on tooth B would have too little retention, whereas the lower location on tooth A would be too retentive.

The measurement of the degree of undercut by mechanical means is therefore most impor­tant. Although experience with undercut gauges is important, the student should have a thor­ough comprehension of all the factors influenc­ing clasp retention and be able to apply them intelligently.

 

Flexibility of clasp arms

The following factors influence the flexibility of a clasp arm.

Length of clasp arm

The longer the clasp arm, the more flexible it will be, all other factors being equal. The length of a circumferential clasp arm is mea­sured from the point at which a uniform taper begins. The retentive circumferential clasp arm should be tapered uniformly from its point of origin through the full length of the clasp arm.

The length of a bar clasp arm also is measured from the point at which a uniform taper begins. Generally the taper of a bar clasp ” arm should begin at its point of origin from a metal base or at the point at which it emerges from a resin base. Although a bar clasp arm will usually be longer than a circum­ ferential clasp arm, its flexibility will be less because its half-round form lies in several planes, which prevents its flexibility from being proportionate to its total length.  Based on a proportional limit of 60,000 psi and on the assumption that the clasp arm is properly tapered, the clasp arm should be able to flex repeatedly within the limits stated without hardening or ruptur­ ing because of fatigue. It has been estimated that alternate stress applications of the fatigue type are placed on a retainer arm during mastication and other force-inducing functions about 300,000 times a year.

Diameter of clasp arm

The greater the average diamete-r of a clasp arm, the less flexible it will be, all other factors being equal. If its taper is absolutely uniform, the average diameter will be at a point midway between its origin and its terminal end. If its taper is not uniform, a point of flexure, and therefore a point of weakness, will exist that will then be the determining factor in its flexibility, regardless of the average diameter of its entire length.

Cross-sectional form of the clasp arm

Flexibility may exist in any form, but it is limited to only one direction in the case of the half-round form. The only universally flexible form is the round form, which is practically impossible to obtain by casting and polishing.

Because most cast clasps are essentially half round in form, they may flex away from the tooth, but edgewise flexing (and edgewise adjustment) is limited. For this Jeason, cast retentive clasp arms are more acceptable in tooth­ supported partial dentures in which they are called on to flex only during placement and removal of the prosthesis. A retentive clasp arm on an abutment adjacent to a distal extension base not only must flex during placement and removal but also must be capable of flexing during functional movement of the distal extension base. It must either have universal flexibility to avoid trans­mission of tipping stresses to the abutment tooth or be capable of disengaging the undercut when vertical forces directed against the denture are toward the residual ridge. A round clasp form is the only circumferential clasp form that may be safely used to engage a tooth undercut on the side of an abutment tooth away from the distal extension base. The location of the undercut is perhaps the single most important factor in selecting a clasp for use with distal extension partial dentures.

Material used for the clasp arm

Although all cast alloys used in partial denture construction possess flexibility, their flexibility is proportionate to their bulk. If this were not true, other components of the partial denture could not have the necessary rigidity. A disadvantage of a cast gold partial denture is that its bulk must be increased to obtain the needed rigidity at the expense of added weight and increased cost. It cannot be denied that greater rigidity with less bulk is possible through the use of chromium-cobalt alloys.

Although cast gold alloys may have greater resiliency than do cast chromium-cobalt alloys, the fact remains that the structural nature of the cast clasp does not approach the flexibility and adjustability of the wrought-wire clasp. Having been formed by being drawn into a wire, the wrought-wire clasp arm has toughness exceed­ing that of a cast clasp arm. The tensile strength of a wrought structure is at least 25% greater than that of the cast alloy from which it was made. It may therefore be used in smaller diamete’rs to provide greater flexibility without fatigue and ultimate fracture.

 

 Reciprocal arm of direct retainer assembly should be rigid. Arm tapered both lengthwise and widthwise is more flexible than arm of the same dimensions tapered only lengthwise. T is clasp thickness.

 

Stabilizing-reciprocal cast clasp arm

A stabilizing (reciprocal) clasp arm should be rigid. Therefore it is shaped somewhat differ­ently than is the cast retentive clasp arm, which must be flexible. Its average diameter must be greater than the average diameter of the oppos­ing retentive arm to increase desired rigidity. A cast retentive arm is tapered in two dimensions, whereas a reciprocal arm should be tapered in one dimension only. To achieve such a form for the arm, freehand waxing of patterns is required.

 

 

CRITERIA FOR SELECTING

A GIVEN CLASP DESIGN

 

In selecting a particular clasp design for a given situation, its function and limitations must be carefully evaluated. The dentist should not expect the technician to make the decision as to which clasp design is to be used. The choice of clasp design must be both biologically and mechanically sound, based on the diagnosis and treatment plan previ­ously established. Extracoronal direct retainers should be considered as a combination of components of a removable partial denture framework, designed and located to perform the specific functions of support, stabilization, reciprocation, and retention. It matters not whether the direct retainer assembly compo­nents are physically attached directly to each other or originate from major and minor connectors of the framework. If attention is directed to the separate function of each component of the direct retainer assembly, then designing a direct retainer for a particular situation is simplified.

The advantages of any particular clasp de­sign should lie in an affirmative answer to most (or all) of the following questions:

1. Is it flexible enough to satisfy the purpose for which it is being used? (On an abutment adjacent to a distal extension base, will tipping and torque be avoided?)

2. Will adequate stabilization be provided to resist horizontal and rotational movements?

3. Will rigidity be provided where it is needed?

4. Is the clasp design applicable to malposed or rotated abutment teeth?

5. Can it be used despite the presence of tissue undercuts?

6. Can the clasp terminal be adjusted to increase or decrease retention?

7. Does the clasp arm cover a minimum of tooth surface?

8. Will the clasp arm be as inconspicuous as possible?

9. Will the width of the occlusal table remain the same or be decreased?

10. Is the clasp arm likely to become distorted or broken? If so, can it be replaced?

 

 

 Choice and definitive location of each component of direct retainer assembly must be based on preserving health of periodontal attachment in spite of rotational tendencies of distal extension denture. Knowledge of characteristics of each component of collective assemblies for a particular arch and rationalized rotational tendencies of denture simplifies design of removable restorations

With this background the various types of clasps will be considered. The choice of a clasp is like the choice of a tool to be used in a given situation. Knowing what types are available and being familiar with their various advantages and limitations permit selection of a clasp design that best meets the needs of the individ­ual situation.

Although there are some rather complex designs for clasp arms, they may all be classified into one of two basic categories. One is the circumferential clasp arm, which approaches the retentive undercut from an occlusal direc­tion. The other is the bar clasp arm, which approaches the retentive undercut from a cervi­cal direction.

 

 

 Clasp assembly (with mirror views), including rest, may be combination of circumferential and bar clasp arms in one of several possible combinations. These mirror views are for abutments bounding a modification space. A, Cast circumferential retentive clasp arm with nonretentive bar clasp arm on opposite side for stabilization and reciprocation. B, Tapered wrought-wire circumferential retentive clasp arm with nonretentive bar clasp arm on opposite side for stabilization and reciprocation. C, Retentive bar clasp arm with nonretentive cast circumferential clasp arm on opposite Side for stabilization and reciprocation.

 

A clasp assembly may be a combination of cast circumferential and bar clasp arms and/or wrought-wire retentive arms in one of several possible combinations.

No confusion should exist between the choice of clasp arm and the purpose for which it is used. Either type of cast clasp arm may be made tapered and retentive or nontapered (rigid) and nonretentive, depending on whether it is used for retention, stabilization, or reciprocation. A clasp assembly should consist of (1) one or more minor connectors from which the clasp compo::: nents originate; (2) a principal rest; (3) a retentive arm engaging a tooth undercut only at its terminus; and (4) a nonretentive arm or other component on the opposite side of the tooth for stabilization and reciprocation against horizon­tal movement of the prosthesis. Rigidity of this clasp arm is essential to its purpose. An auxiliary occlusal rest may be used rather than a reciprocal clasp arm if it is located in a way to accomplish the same purpose . The addition of a lingual apron to a cast reciprocal clasp arm alters neither its primary purpose nor the need for proper location to accomplish that purpose.

 

 

BASIC PRINCIPLES OF CLASP DESIGN

 

Any clasp assembly must satisfy the basic principle of clasp design, which is that more than 180 degrees of the greatest circumference of the crown of the tooth must be included, passing from diverging axial surfaces to con­verging axial surfaces. This may be in the form of continuous contact when circumfer­ential clasp arms are used. When bar clasp arms are used, at least three widely separated areas of tooth contact must embrace more than one half of tooth circumference. These are the occlusal rest area, the retentive terminal area, and the reciprocal terminal area. Other prin­ciples to be considered in the design of a clasp are as follows:

1. The occlusal rest must be designed so that movement of the clasp arms cervlcally is prevented.

2. Each retentive terminal should be opposed by a reciprocal component capable of resist­ing any orthodontic pressures exerted by the retentive arm. Stabilizing and reciprocal com­ponents must be rigidly connected bilaterally (cross-arch) if reciprocation to the retentive elements is to be realized.

3. Unless guiding planes will positively con­trol the path of removal and stabilize abut­ments against rotational movements, reten­tive clasps should be bilaterally opposed; that is, buccal retention on one side of the arch should be opposed by buccal retention on the other, or lingual on one side opposed by lingual on the other. In Class II situations the third abutment may have either buccal or lingual retention. In Class III situations, retention may be either bilaterally or diamet­rically opposed.

4. The path of escapement of each retentive clasp terminal must be other than parallel to the path of removal of the prosthesis.

5. The amount of retention should always be the minimum necessary to resist reasonable dis­lodging forces.

6. Clasp retainers on abutment teeth adjacent to distal extension bases should be designed so that they will avoid direct transmission of tipping and rotational forces to the abutment. In effect, they must act as stress­breakers either by their design or by their construction. This is accomplished by proper location of the retentive terminal or by use of a more flexible clasp arm in relation to prospective rotation of the denture under varying directed forces.

7. Ideally, reciprocal elements of the clasp assembly should be located at the junction of the gingival and middle thirds of the crowns of abutment teeth. The terminal end of the retentive arm is optimally placed in the gingival third of the crown. These locations will permit the abutment teeth to better resist horizontal and torquing forces than they could if the reten­tive and reciprocal elements were located nearer the occlusal or incisal surfaces. As a metaphor, remember that a fencepost is more easily loosened by applying horizontal forcesnear the top than by applying the same forces nearer ground level.

 

A, Retentive clasps should be bilater­ally opposed. B, In Class II situations the reten­tion on the third abutment may be on the buccal or the lingual. C, In Class III situations, retention may be either (a) bilateral or (b) diametrically opposed.

 

Simple mechanical laws demonstrate that the nearer stabilizing-reciprocal and retentive ele­ments of direct retainer assemblies are located to horizontal axis of rotation of abutment, the less likely that physiologic tolerance of periodontal liga­ment will be exceeded. Horizontal axis of rota­tion of abutment tooth is located somewhere in its root.

The reciprocal clasp arm has the following three functions:

  1. The reciprocal clasp arm should provide stabilization and reciprocation against the action of the retentive arm. This is particu­larly important if the retentive arm is accidentally distorted toward the tooth, where it would become an active orthodon­tic force. Remember, the retentive clasp arm should be passive until a dislodging force is applied. During placement and removal, reciprocation is needed as the retentive arm flexes over the height of contour. Unless the abutment tooth has been specifically con­toured, the reciprocal clasp arm will not come into contact with the tooth until the denture is fully seated and the retentive clasp arm has again become passive. When this happens, a momentary tipping force is applied to the abutment teeth during each placement and removal. This may not be a damaging force, because it is transient, so long as the force does not exceed the normal elasticity of the periodontal attach­ments. True reciprocation during placement and removal is possible only through the use of crown surfaces made parallel to the path of placement. The use of cast res­torations permits the paralleling of the sur­faces to be contacted by the reciprocal arm in such a manner that true reciproca­tion is made possible.

2. The reciprocal clasp arm should be located so that the denture is stabilized against horizon­tal movement. Stabilization is possible only through the use of rigid clasp arms, rigid minor connectors, and a rigid major connec­tor. Horizontal forces applied on one side of the dental arch are resisted by the stabilizing components on the opposite side providing cross-arch stability. Obviously the greater the number of such components, within reason, the greater will be the distribution of horizon­tal stresses.

3. The reciprocal clasp arm may act to a minor degree as an indirect retainer. This is only true when it rests on a suprabulge surface of an abutment tooth lying anterior to the fulcrum line. Lifting of a distal extension base away from the tissues is thus resisted by a rigid arm, which is not easily displaced cervically. The effectiveness of such an indirect re­tainer is limited by its proximity to the fulcrum line, which gives it a relatively poor leverage advantage, and by the fact that slippage along tooth inclines is always possible. The latter may be prevented by the use of a ledge on a cast restoration; how­ever, enamel surfaces are not ordinarily so prepared.

 

Circumferential clasp

Although a thorough knowledge of the prin­ciples of clasp design should lead to a logical application of those principles, it is better that some of the more common clasp designs be considered individually. The circumferential clasp will be considered first as an all-cast clasp.

The circumferential clasp is usually the most logical clasp to use with all tooth-supported partial dentures because of its retentive and stabilizing ability. Only when the retentive undercut may be approached better with a bar clasp arm or when esthetics will be enhanced should the latter be used. The circumferential clasp arm does have the following disadvantages:

1. More tooth surface is covered than with a bar clasp arm because of its occlusal origin.

2. On some tooth surfaces, particularly the buccal surface of mandibular teeth and the lingual surfaces of maxillary teeth, its occlu­sal approach may increase the width of the occlusal surface of the tooth.

3. In the mandibular arch, more metal may be displayed than with the bar clasp arm.

4. As with all cast clasps, its half-round form prevents adjustment to increase or decrease retention. Adjustments in the retention afforded by a cast clasp arm should be made by moving a clasp terminal cervically into the angle of cervical convergence or occlusally into a lesser area of undercut. Tightening a clasp against the tooth or loosening it away from the tooth increases or decreases frictional resistance and does not affect the retentive potential of the clasp. True adjustment is, therefore, impossible with most cast clasps.

Despite its disadvantages the cast circumfer­ential clasp arm may be used effectively, and many of these disadvantages may be minimized by mouth preparation. Adequate mouth prepa­ration will permit its point of origin to be placed far enough below the occlusal surface to avoid poor estheticsJand increased tooth dimension . Although some of the disadvan­tages listed imply that the bar-type clasp may be preferable, the circumferential clasp is actually superior to a bar clasp arm that is improperly used or poorly designed.

Experience has shown that the possible advantages of the bar clasp arm are too often negated by faulty application and design, whereas the circumferential clasp arm is not as easily misused.

 

Lingual

 

 Occlusal

 

 

 

Buccal

 

 Bar-type clasp on mandibular premolar. A, Support is provided by occlusal rest. B, Stabilization is provided by occlusal rest and mesial and distal minor connectors. C, Retention is provided by buccal I-bar. Reciprocation is obtained through location of minor connectors. Engagement of more than 180 degrees of circumference of the abutment is accomplished by proper location of components contacting axial surfaces. (Minor connector supports occlusal rest, proximal plate minor connector, and buccal I-bar.)

 

 

 

 

 Cast circumferential retentive clasp arms properly designed. They originate on or occlusal to height of contour, which they then cross in their terminal third, and engage retentive undercuts pro­gressively as their taper decreases and their flexibility increases

 

 Example of the two types of cast clasps in use. Molar abutment is engaged by circumferential clasp originating occlusal to height of contour, whereas premolar abutment is engaged by bar clasp originating from base gingival to height of contour. However, only terminal tip of this clasp is placed in measured undercut.

 

Ring clasp

The circumferential type of clasp may be used in several forms. It appears as though many of these forms of the basic circumferential clasp design were developed to accommodate situa­tions in which corrected tooth modifications could not be or were not accomplished by the dentist. One is the ring clasp, which encircles nearly all of a tooth from its point of origin. It is used when a proximal undercut cannot be approached by other means. For example, when a mesiolingual undercut on a lower molar abutment cannot be approached directly because of its proximity to the occlusal rest area and cannot be approached with a bar clasp arm because of lingual inclination of the tooth, the ring clasp encircling the tooth allows the undercut to be approached from the distal aspect of the tooth.

The clasp should never be used as an unsupported ring because if it is free to open and close as a ring, it cannot provide either reciprocation or stabilization. Instead the ring-type clasp should always be used with a supporting strut on the non retentive side, with or without an auxiliary occlusal rest on the opposite marginal ridge. The advantage of an auxiliary rest is that further movement of a mesially inclined tooth is prevented by the presence of a distal rest. In any event the supporting strut should be regarded as being a minor connector from which the flexible retentive arm originates. Reciproca­tion then comes from the rigid portion of the clasp lying between the supporting strut and the principal occlusal rest.

The ring-type clasp should be used on protected abutments whenever possible because it covers such a large area of tooth surface. Esthetics need not be considered on such a posteriorly located tooth.

A ring-type clasp may be used in reverse on an abutment located anterior to a tooth-bounded edentulous space. Although poten­tially an effective clasp, this clasp covers an excessive amount of tooth surface and can be esthetically objectionable. The only justification for its use is when a distobuccal or distolingual undercut cannot be approached directly from the occlusal rest area and/or tissue undercuts prevent its approach from a gingival direction with a bar clasp arm

 

 

 

 

 Some improper applications of circumferential clasp design and their recom­mended corrections. A, Tooth with undesirable height of contour in its occlusal third. B, Unsuitable contour and location of retentive clasp arm on unmodified abutment. C, More favorable height of contour achieved by modification of abutment. D, Retentive clasp arm properly designed and located on modified abutment. E, Unsuitable contour and location of retentive arm in relation to height of contour (straight arm configuration provides poor approach to retentive area and is less resistant to dislodging force). F, Terminal portion of retentive clasp arm located too close to gingival margin. G, Clasp arm that is properly designed and located.

 

Back-action clasp

The back-action clasp is a modification of the ring clasp, with all of the same disadvantages and no apparent advantages. It is difficult to justify its use. The undercut can usually be approached just as well using a conventional circumferential clasp, with less tooth coverage and less display of metal. With the circumferential clasp the proximal tooth surface can be used as a guiding plane, as it should be, and the occlusal rest can have the rigid support it requires. An occlusal rest always should be attached to some rigid minor

 

 

 Ring clasp(s) en!=ircling nearly all of tooth from its point of origin. A, Clasp originates on mesiobuccal surface and encircles tooth to engage mesiolingual undercut. B, Clasp originates on mesio­lingual surface and encircles tooth to engage me­siobuccal undercut. In either example, supporting strut is used oonretentive side (drawn both as direct view of near side of tooth and as mirror view of opposite side).

 

 Improperly designed ring clasp lacking necessary support. Such a clasp lacks any reciprocat­ing or stabilizing action because entire circumference of clasp is free to open and close. Supporting strut should always be added oonretentive side of abutment tooth, which then becomes, in effect, a minor connector from which tapered and flexible retentive clasp arm originates.

 

 

 Buccal strut supporting mesially originat­ing ring clasp. Flexible retentive arm begins at distal occlusal rest and engages mesiolingual under­cut. Despite its resemblance to bar-type clasp, this is a circumferential clasp by reason of its point of origin, the strut being actually an auxiliary minor connector.

 

 Ring clasp engaging mesiobuccal undercut on mesially inclined lower right molar requires supporting bar on lingual surface to limit flexure to only retentive portion of clasp.

Removable partial dentures by design are intended to be removed from and replaced into the mouth. Because of this, they are not rigidly connected to the teeth or tissue, which means they are subject to movement in response to functional loads such as those created by mastication. It is important for clinicians provid­ing removable partial denture service to under­stand the possible movements in response to function and to be able to logically design the component parts of the removable partial den­ture to help control these movements. The following biomechanical considerations provide a background regarding principles of the move­ment potential associated with removable par­tial dentures, and the subsequent chapters covering the various component parts describe how these components are designed and how they are used to control the resultant move­ments of the prostheses.

BIOMECHANICAL CONSIDERATIONS

As Maxfield stated, “Common observation clearly indicates that the ability of living things to tolerate force is largely dependent upon the magnitude or intensity of the force.” The supporting structures for removable partial dentures (abutment teeth and residual ridges) are living things and are subjected to forces. In consideration of maintaining the health of these structures, the dentist must consider direction, duration, and frequency of force application, as well as the magnitude of the force.

In the final analysis it is bone that pro­vides the support for a removable prosthesis, that is, alveolar bone by way of the periodon­tal ligament and bone of the residual ridge through its soft tissue covering. If potentially destructive forces can be minimized, then the physiologic tolerances of the supporting struc­tures are generally capable of withstanding these forces without physiologic or pathologic change. To a great extent, the forces occurring through a removable prosthesis can be widely distributed, directed, and minimized by the selection, the design, and the location of components of the removable partial den­ture and by development of harmonious occlusion.

Unquestionably, the design of removable partial dentures requires mechanical and bio­logic considerations. Most dentists are capable of applying simple mechanical principles to the design of a removable partial denture. For example, the lid of a paint can is more easily pried off with a screwdriver than it is with a half dollar! The longer the handle, the less effort (force) it takes. This is a simple application of the mechanics of leverage. By the same token, a lever system represented by a distal extension removable partial denture can magnify the applied force to the terminal abutments, which is most undesirable.

Tylman correctly stated, “Great caution and reserve are essential whenever an attempt is made to interpret biological phenomena entirely by mathematical computation.” However, an understanding of simple machines should en­hance our rationalization of the design of re­movable partial dentures to accomplish the ob­

jective to preserve oral structures. A removable

partial denture can be, and often is, unknow­ingly designed as a destructive machine.

Machines may be classified into two general categories: simple and complex. Complex ma­chines are combinations of many simple ma­chines. The six simple machines are lever, wedge, screw, wheel and axle, pulley, and inclined plane. Of the simple ma­chines, the lever and the inclined plane should be avoided in designing removable partial dentures.

In its simplest form, a lever is a rigid bar supported somewhere along its length. It may rest on the support or may be supported from above. The support point of the lever is called the fulcrum, and the lever can move around the fulcrum .

 

 

 Lever is simply a rigid bar supported somewhere between its two ends. It can be used to move objects by application of force (weight) much less than weight of object being moved.

 

 

 Distal extension removable partial denture will rotate when force is directed on denture base. Differences in displaceability of periodontal ligament, supporting abutment teeth, and soft tissues covering residual ridge permit this rotation. It would seem that rotation of denture is in combination of directions rather than unidirectional

 

The rotational movement of an exten­sion base type of removable partial denture, when a force is placed on the denture base. It will rotate in relation to the three cranial planes because of differences in the support characteristics of the abutment teeth and the soft tissues cover­ing the residual ridge. Even though the gross movement of the denture may be small, the potential exists for detrimental_ leverlike forces to be imposed on abutment teeth, especially when servicing (that is, relin­ing) the prosthesis is neglected over a long period. There are three types of levers: first, second, and third class. The potential of a lever system to relatively magnify a force.

A cantilever is a beam supported only at one end and can act as a first-class lever. A cantilever design should be avoided. Examples of other leverlike de­signs, as well as suggestions for alterna­tive designs, to avoid or to minimize their destructive potential.

 

 There are three classes of levers. Classification is based on location of fulcrum, F; resistance, R; and direction of effort (force), E. Examples of each class are illustrated.

 

A tooth is apparently better able to toler­ate vertically directed forces than off-vertical, torquing, or near horizontal forces. This char­acteristic is observed clinically and was sub­stantiated many years ago by the work of Box and Synge* of Toronto. It seems rational that more periodontal fibers are activated to resist the application of vertical forces to teeth than are activated to resist the application of off-vertical forces. applied to the artificial teeth attached to the extension base. Because it can be assumed that this rotation must create predominantly off-vertical forces, location of stabilizing and retentive compo­nents in relation to the horizontal axis of rotation of the abutment becomes extremely important. An abutment tooth will better toler­ate off-vertical forces if these forces accrue as near as possible to the horizontal axis of rotation of the abutment. The axial surface contours of abutment teeth must be altered to locate components of direct retainer assem­blies more favorably in relation to the abut­ment’s horizontal axis.

 

 Cantilever can be described as rigid beam supported only at one end. When force is directed against unsupported end of beam, cantilever can act as first-class lever. Mechanical advantage in this illustration is in favor of effoit arm.

 

 Design often seen for distal extension removable partial denture. Cast circumferential direct retainer engages mesiobuccal undercut and is sup­ported by distoclusal rest. This could be considered a cantilever design, and it may impart detrimental first-class lever force to abutment if tissue support under extension base allows excessive vertical movement toward the residual ridge.

 

 

 

 

 

 Potential for first-class lever action exists in this Class II, modification 1, removable partial denture framework. If cast circumferential direct retainer with a mesiobuccal undercut on right first premolar were used, force placed on denture base could impart upward and posteriorly moving force on premolar, resulting in loss of contact between premolar and canine. Tissue support from extension base area is most important to minimize lever action of clasp. Retainer design could help accommodate more of an anteriorly directed force during rotation of the denture base in an attempt to maintain tooth contact. Other alternatives to first premolar design of direct retainer would be tapered wrought-wire retentive arm that uses mesiobuccal undercut or just has buccal stabilizing arm above height of contour

 Illustration A uses bar type of retainer, minor connector contacting guiding plane on distal surface of premolar, and mesio-occlusal rest, to reduce cantilever or first-class lever force when and if denture rotates toward residual ridge. B, Tapered wrought-wire retentive arm, minor connector contacting guiding plane on distal surface of premolar, and mesio-occlusal rest. This design is applicable when distobuccal undercut cannot be found or created or when tissue undercut contraindicates placing bar-type retentive arm. This design would be kinder to periodontal ligament than would cast, half-round retentive arm. Again, tissue support of extension base is key factor in reducing lever action of clasp arm. Note: Depending on amount of contact of minor connector proximal plate with guiding plane, fulcrum point will change.

 

 

 

 

 

 

 More periodontal fibers are activated to resist forces directed vertically on tooth than are activated to resist horizontally (off-vertical) directed force. Horizontal axis of rotation is located somewhere in root of tooth.

 Fencepost is more readily removed by application of force near its top than by applying same force nearer ground level. B, Retentive (buccal sur­face) and reciprocal (lingual surface) components (mirror view) of this direct retainer assembly are located much nearer occlusal surface than they should be. This represents similar effect of force application shown in top figure of illustration A.

 

POSSIBLE MOVEMENTS OF PARTIAL DENTURE

 

Presuming that direct retainers are functioning to minimize vertical displacement, rotational movement will occur about some axis as the distal extension base or bases either move toward, away, or horizontally across the un­derlying tissues. Unfortunately, these possible movements do not occur singularly or inde­pendently but tend to be dynamic and all occur at the same time. The greatest movement possible is found in the tooth-tissue-supported prosthesis because of the reliance on the distal extension supporting tissue to share the func­tional loads with the teeth. Movement of a distal extension base toward the ridge tissues will be proportionate to the quality of those tissues, the accuracy and extent of the den­ture base, and the total functional load applied. A review of prosthesis rotational movement that is possible around various axes in the mouth provides some understanding of how component parts of removable partial dentures should be prescribed to control ‘prosthesis movement.

One movement is rotation about an axis through the most posterior abutments. This axis may be through occlusal rests or any other rigid portion of a direct retainer assembly located occlusally or incisally to the height of contour of the primary abutments. This axis, known as the fulcrum line, is the center of rotation as the distal extension base moves toward the supporting tissues when an occlusal load is applied. The axis of rotation may shift toward more anteriorly placed com­ponents, occlusal or incisal to the height of contour of the abutment, as the base moves away from the supporting tissues when vertical dislodging forces act on the partial denture. These dislodging forces result from the vertical pull of food between opposing tooth surfaces, the effect of moving border tissues, and the forces of gravity against a maxillary partial denture. Presuming that the direct retainers are functional and that the supportive anterior components remain seated, rotation rather than total displacement should occur. Vertical tis­sueward movement of the denture base is resisted by the tissues of the residual ridge in proportion to the supporting quality of those tissues, the accuracy of the fit of the denture base, and the total amount of occlusal load applied. Movement of the base in the oppo­site direction is resisted by the action of the retentive clasp arms on terminal abut­ments and the action of stabilizing minor connectors in conjunction with seated, verti­cal support elements of the framework ante­rior to the terminal abutments acting as indirect retainers. Indirect retainers should be placed as far as possible from the distal exten­sion base, affording the best possible leverage advantage against the liftlng of the distal extension base.

A second movement is rotation about a longitudinal axis as the distal extension base­moves in a rotary direction about the residual ridge. This movement is resisted primarily by the rigidity of the major and minor connectors and their ability to resist torque. If the connectors are not rigid or if a stress-breaker exists between the distal extension base and the major connector, this rotation about a longitudi­nal axis either applies undue stress to the sides of the supporting ridge or causes horizontal shifting of the denture base.

A third movement is rotation about an imaginary vertical axis located near the center of the dental arch. This movement occurs under function as diagonal and horizon­tal occlusal forces are brought to bear on the partial denture. It is resisted by stabilizing components, such as reciprocal clasp arms and minor connectors that are in contact with vertical tooth surfaces. Such stabilizing compo­nents are essential to any partial denture design regardless of the manner of support and the type of direct retention employed. Stabilizing components on one side of the arch act to stabilize the partial denture against horizontal forces applied from the opposite side. It is obvious that rigid connectors must be used to make this effect possible.

 

 

 

 

 

  Three possible movements of distal exten­sion partial denture. A, Rotation around fulcrum line passing through the most posterior abutments when denture base moves vertically toward or away from supporting residual ridges. B, Rotation around longi­tudinal axis formed by crest of residual ridge. C, Ro­tation around vertical axis located near center of arch.

Horizontal forces always will exist to some degree because of lateral stresses occurring during mastication, bruxism, clenching, and other patient habits. These forces are accen­tuated by failure to consider the orientation of the occlusal plane, the influence of malposi­tioned teeth in the arch, and the effect of abnormal jaw relationships. The magnitude of lateral stress may be minimized by fabricating an occlusion that is in harmony with the opposing dentition and that is free of lateral interference during eccentric jaw movements.

The amount of horizontal movement occurring in the partial denture therefore depends on the magnitude of the lateral forces that are applied and on the effectiveness of the stabilizing components.

In a tooth-supported partial denture, move­ment of the base toward the edentulous ridge is prevented primarify by the rests on the abutment teeth and to some degree by any rigid portion of the framework located occlusal to the height of contour. Movement away from the edentulous ridge is prevented by the action of direct retainers on the abutments that are situated at each end of each edentulous space and by the rigid, minor connector stabilizing components. Therefore the first of the three possible movements can be controlled in the tooth-supported denture. The second possible movement, which is about a longitudinal axis, is prevented by the rigid components of the direct retainers on the abutment teeth, as well as by the ability of the major connector to resist torque. This movement is much less in the tooth­supported denture because of the presence of posterior abutments. The third possible move­ ment occurs in all partial dentures; there­fore stabilizing components against horizontal movement must be incorporated into any partial denture design.

For prostheses capable of movement in three planes, occlusal rests should only provide occlu­sal support to resist tissueward movement. All movements of the partial denture other than those in a tissueward direction should be resisted by components other than occlusal rests. For the occlusal rest to enter into a stabilizing function would result in a direct transfer of torque to the abutment tooth. Because move­ments around three different axes are possible in a distal extension partial denture, an occlusal rest for such a partial denture should not have steep vertical walls or locking dovetails, which could possibly cause horizontal and torquing forces to be applied intracoronally to the abut­ment tooth.

In the tooth-supported denture, the only movements of any significance are horizontal, and these may be resisted by the stabilizing effect of components placed on the axial surfaces of the abutments. Therefore in the tooth­supported denture, the use of intracoronal rests is permissible. In these instances, the rests provide not only occlusal support but also significant horizontal stabilization.

In contrast, all Class I and Class II partial dentures, having one or more distal extension bases, are not totally tooth supported; neither are they completely retained by bounding abutments. Any extensive Class III or Class IV partial denture that does not have adequate abutment support falls into the same category. These latter dentures may derive some support from the edentulous ridge and therefore may have a composite support from both teeth and ridge tissues.