Indications, clinical and laboratory stages of manufacturing of slip-casted and combined crowns. Indications, clinical and laboratory stages of production of slip-cast dental bridges
1. Patient examination:
Subjectively objectively
complaints, anamnesis core methods:
– check-up;
– percussion;
– palpation;
additional methods:
-roentgenography;
-electroodontodiagnostics
-diagnostical models.
2. Tooth and dentition preparation for prosthetics:
-treatment of dental pulp disease;
– treatment of periodontium disease,
– decay cavities filling;
– insert manufacture;
– stumppostinsertmanufacture;
– orthoapedicstructuresandorthoapedicapparatusmanufacture
.
3. Anaesthetization:
General Local anaesthetization:
– conductory;
– infiltrative;
– apical;
– intraligamentous
4. Tooth preparation:
With a ledge
– circular; -straight:
– on vestibular surface – with a hollow;
– with a beveled edge;
– with a beveled top
– Without a ledge
5. Gum edge retraction:
– mechanical;
– medicamental; – threats and rings with vasoconstrictive medicine;
– surgical
– cutting gum edge
6. Obtaining impressions
Working impression:
– by ring method;
– combinedimpression
1. Single-stage
2. Double-stage:
– With putting correction mass on basic impression;
– With putting correction mass into dentogingival groove.
Auxiliary impression.
Stages of production of slip-cast dental bridges
1. Examination of a patient with partial loss of teeth:
Subjective examination:
1.Complaints, which were addressed by the patient.
2.Life history.
3.History of the disease (the cause and duration of tooth loss)
І. Clinical methods of examination:
1.Examination: a) Face;
b) Teeth, dentitions , mucous membrane, defect topography;
c) Defining a type of one’s bite;
2.Palpation: a) Chewing muscles;
b) Temporomandibular joint;
3.Percussion: а) Teeth
ІІ. Paraclinical examination methods:
1. Analysis of diagnostic models of jaws in articulator, identification of dentoalveolar deformities, increased abrasion of teeth, reduce of the bite height, pathological interdental contacts, prosthesis construction planning.
2. X-ray examination:
а) Sighting radiographs of teeth;
б) Orthopantomogram;
в) Computed tomography of temporomandibular joint.
Indications for use of bridge prostheses in orthopedic treatment of patients with defects of dentitions. Choice of their construction is determined mainly by the following factors: size and topography of the defect, its localization, the state of hard tissuesand by periodontal abutment teeth and their antagonists, as well as occlusal relationships.
Defects of dentitions are commonly divided into small ones (in case of absence from 1 to 3 teeth on the jaw), medium(in case of absence from 4 to6 teeth)and big ones in case of absence of more than 6 teeth). Indications for use of fixed prosthesis arebounded edentulous spaces, theseare dentitions bounded on both sides with teeth. Depending on extent and topography of the defect (quantity of removed teeth and a functional quality of the rest) it is determine the possibility of using non-removable dental prostheses. Non-removable prostheses are used for treatment in the following cases: 1) loss of one or four incisor; 2) loss of canine; 3) loss of one or more premolars; 4) the loss of two premolars and the first molar; 5) permissible loss of two premolars on one side of the jaw, the first and the second molars with preserved and well-developed third molar.
Bounded edentulous spaces are not always an indication for making non-removable types of prostheses. For example, the absence of the canine, two premolars and one molar on one or two sides of the jaw is also considered a bounded edentulous space. However, in case of such extended defects the use of non-removable prosthesis is contraindicated.
Many years of clinical observation and study of mastication physiology determined that grinding and chewing food is done by 2-3 teeth of the upper and lower jaws. Therefore, for replacement of defect of 2 lateral teeth it is enough to fix a bridge prosthesis on 2 healthy teeth. If the defect is formed from the loss of more than 2 teeth, the prosthesis can also have two abutments (a molar andacanine). This position applies only to the side teeth and a canine, located at the junction of two differently functionally oriented groups of teeth.
Clinical stages of making solid-cast bridge prostheses include:
1. Preparation of the abutment teeth with preliminary retraction.
2. Getting the primary and secondary impressions.
3. Determination or fixation of occlusion.
4. Fitting the frame of bridge prosthesis.
5. Fixingbridge prosthesis.
FIXED BRIDGE PROTHESIS (soldered, cast in block)
INDICATIONS:
1. Absence from 1-4 teeth before the frontal section.
2. Absence of the 2 molars with of distal support.
3. Absence of the 3rd lateral teeth through one with the presence of distal support.
4. Absence of the 3rd next confronting teeth the presence of distal support (to consider the standing of supporting teeth and antagonists)
CONTRA-EVIDENCE:
1. Included defects with in 4-5 teeth.
2. Mobility of the teeth (atrophy of bone hole down 1/2 and more).
3 Deformations of a bite and dental numbers with the partial loss of teeth.
4. Production of cantilever prothesis in the region of molars.
CLINICAL STAGES
First visit:Preparation of supporting teeth under selected construction of supporting parts crowns, half-crown’s, pin teeth and so forth the removal of the anatomical of vises.
Second visit:Adjustment of crowns (or other supporting elements) the determination of central occlusion, the removal of occlusion impression.
Third visit:Adjustment (fitting of bridge-shaped prosthesis. Identification of the color of plastic for the facets.
Fourth visit:Fitting and the fixation of bridge-shaped prosthesis by hanging-fop- cement.
LABORATORY STAGES
1. Obtaining model, making of supporting parts.
2. Casting of models to articulator (or casting occlusion impression), modeling of the intermediate part of the bridge-shaped prosthesis from wax, casting. Soldering immediate part to supporting crowns.
3. Polishing, chromium-plating, the simulation of facets from wax, the replacement of wax down the plastic.
This invention relates to a method of making crowns, such as gold crowns, for teeth, and a method of making a form for use in making such crowns by an investment casting technique.
The conventional technique for producing crowns, such as gold crowns, utilises an impression which is made by the dentist in a hardenable resilient material such as a hardenable rubber composition. This impression will include an impression of at least the teeth surrounding the one to be crowned and will also include an impression of the decayed tooth which has been ground down by the dentist in preparation for crowning.
This impression is then utilised, usually by a dental laboratory, to produce a crown. A plaster cast of the stump remaining after the dentist has ground down the decayed tooth is produced on a brass pin and a model of the teeth in that jaw is also cast in such a way that the stump can be removably located in it by means of the brass pin.
The next stage of the process is to cast a layer of wax on the stump and to carve the wax by hand to give the external shape of the desired crown. The carved shape will generally be such as to be compatible with the other teeth. After carving, a rod of wax, generally referred to as a sprue, is attached at one end to the carved wax and at the other end to a conical wax reservoir.
This assembly can then be used to produce a mould which is achieved by casting in an investment casting ring a heat-resisting investment compound with the wax reservoir at the base of the ring. When the plaster compound is set hard it is heated in a furnace to remove all the wax and subsequently a gold crown is cast by introducing molten gold through the sprue into the mould cavity formed by the carved wax.
The specification for US-A-3,058,216 discloses a method of fabricating a porcelain veneer crown. In accordance with the method described in this specification a pre-formed crown form is fitted over a stump of a tooth and impression material is introduced into the crown form in order to make an impression of the stump.
The external surface of the crown form is then ground using a grinding stone until it matches the surrounding teeth in the mouth.
An investment casting moulding may then be produced from the crown form, from which a crown can be cast in the investment casting mould.
According to one aspect of the present invention there is provided a method of making a crown for a tooth such as a gold crown comprising;
- forming a sheet of formable material into a crown form having the desired shape of the crown;
- producing an investment casting mould from the crown form and burning out the crown form; and
- casting the crown in the investment casting mould.
According to another aspect of the present invention there is provided a method of making a crown form for a tooth such as a gold crown, for use in an investment casting technique comprising;
- (a) forming a first sheet of formable material to a shape which approximates to the desired shape of the crown;
- (b) disposing the formed shape around a layer of deformable hardenable material;
- (c) applying a compressive force to the formed shape to deform the underlying hardenable material to the desired shape of the crown;
- (d) hardening the hardenable material to produce a crown form precursor;
- (e) removing the formed shape from the hardened crown form precursor; and
- (f) utilising the hardened crown form precursor to form a second sheet of formable material into a crown form of the desired shape of the crown.
Preferably in step (b) the hardenable material is applied between a cast model of the tooth stump to which the crown is to be fixed and the formed shape.
Desirably the assembly of the cast model of the tooth stump, hardenable material and formed shape is disposed in a model of at least the teeth surrounding the tooth to be crowned and the compressive force of step (c) is applied to the assembly by means of a model of the occluding tooth or teeth in the jaw opposite to that in which the tooth to be crowned is situated to deform the underlying hardenable material to the desired shape.
Preferably the method further includes the steps of:
- (a) producing an investment casting mould from the crown form of step (f) and burning out the crown form; and
- (b) casting the crown in the investment casting mould.
Conveniently, the first sheet is formed by means of a deep-drawing technique. The second sheet may also be formed by a deep-drawing technique.
Advantageously, additional material is added to the second formed sheet in order to produce the final desired shape of the form. Preferably the additional material is wax.
In one method according to the present invention a plaster model of the teeth in one jaw and a plaster cast of the stump of the tooth to be crowned is produced in the conventional way. An artificial tooth is then chosen from a selection of stock teeth to match approximately the remaining teeth in the mouth. Supplies of such stock teeth are readily available as they are used in the preparation of dentures. When a stock tooth of the desired shape is selected it is used to make a first plastics impression in a sheet of formable plastics material. This can be effected by a process known as deep-drawing in which the sheet is heated to a temperature at which it is formable and the stock tooth pushed into it. Usually a backing of a deformable material which will control the forming of the plastics sheet is employed for this step.
The first plastics impression, which is a female mould of the approximate shape of the desired tooth, is then cut from the sheet of plastics material filled with hardenable material and pressed on to the plaster cast of the stump. The stump is then inserted in the model of the teeth in that jaw and a model of the teeth in the other jaw impressed upon it so that the occluding tooth on the opposite jaw can depress contacting areas of the plastics impression. The hardenable material is then allowed to harden on the plaster cast of the stump.
The plastics impression is then removed and discarded and any excess of hardenable material around the margin of the stump is removed to give a smooth surface. The smooth hardened material at this stage is in the shape of the desired crown but is smaller by an amount governed by the thickness of the material of the first plastics impression, and is referred to herein as the crown form precursor.
The crown form precursor is then used to produce a second plastics impression by a method of deep-drawing as described above. The plastics impression so formed will have the desired shape of the crown to be produced as it has been derived by way of the crown form precursor from a selected stock tooth and has been adjusted for occlusion.
This impression is then cut from the sheet placed over the cast of the stump from which the hardenable material has been removed and the margins filled with wax of a type which is normally used for the investment casting process. Thus, in the region of the margin there will be a wax coating but in the remaining region of the impression there will be layer of shaped plastics material which will be spaced from the casting of the tooth at least in certain areas thereof. The plastics impression together with the wax margin is then removed from the cast of the stump and provided with a wax sprue and reservoir in the conventional manner. The sprued crown form of wax and plastics material is then used to produce a conventional investment casting mould and the gold crown cast in the normal way.
The process described above has two marked advantages over the conventional method of producing gold crowns. Firstly, the amount of time taken to produce the crown is considerably reduced particularly in view of the fact that it is not necessary to carry out any carving of a wax coating on the cast of the stump; this carving is both skillful and very time consuming.
Secondly, the thickness of the gold crown is governed only by the thickness of the plastics material which is used to form the impression. Thus, a much thinner crown can be achieved with the attendant saving in the amount of gold employed. It will be seen therefore that the process provides a much cheaper product which when cemented in position on the stump is indistinguishable from a conventional cast gold crown.
The use of the wax in the marginal portion will ensure that the crown has a very close fit in the important marginal areas of the stump.
The materials which are employed in producing the plaster cast and models may be those normally employed for this purpose by dental technicians. Suitable materials other than plaster may of course be used. Similarly any known material may be employed for the investment casting process which is carried out in a conventional manner.
The plastics material which is used for the first and second impressions should be capable of being formed to the desired shape when heated and should retain that shape when cooled to ambient temperature. A further requirement for the second impression is that the plastics material should be capable of being “burned-out” in the investment casting step. Poly unsaturated hydrocarbon sheets such as polypropylene and, preferably, polyethylene have been found to be suitable but it will be appreciated that other formable materials can be employed provided that they exhibit the desired characteristics outlined above.
The hardenable material which is used for producing the crown form precursor should be capable of being shaped whilst in a plastic state and preferably hardens to form a resilient material which can be distorted at least sufficiently to enable removal of the plastics impression and will subsequently regain substantially its original shape. Hardenable elastomeric materials may be employed for this purpose, particularly cold-curing elastomeric materials. One example of such a material is available commercially under the Trade Mark “IMPREGUM”.
The deformable material which may be used as a backing in the deep-drawing steps may be any material which has the desired resistance to deformation at the deep-drawing temperature so as to control the plastic deformation of the heated plastics sheet to the desired degree. Such a material is available commercially and known in the art as MASTIC.
Reference is now made to the accompanying drawing which is a schematic representation of the steps involved in a preferred method according to the invention for producing a crown form suitable for use in producing an investment casting mould.
Stage I shown in the drawing is an illustration of the deep-drawing of a first sheet of plastics material 21 to the shape of a stock tooth 11 which has been selected to match the teeth surrounding the tooth to be crowned. The plastics sheet is preferably polyethylene.
The deep-drawing technique is carried out by heating a sheet of the plastics material in a frame until it is in a formable state. The stock tooth 11 is then pressed into the sheet 21 over a supporting body of a “Mastic” material which serves to support the plastics sheet 21 during the forming operation. The resulting first plastics impression 10 is shown in step I.
A plaster cast of the stump of the tooth which is to be crowned is produced in a conventional way. The first plastics impression 10 is trimmed from the remainder of the plastics sheet 21 and is filled with hardenable cold-curing elastomer such as “IMPREGUM” and pressed on to the cast model stump 13 as shown in step I1. This produces a layer 12 of hardenable material between the model stump 13 and plastics impression 10 and, the excess elastomer is expelled in the region of the margin 14 as can be seen in step III.
When the dentist makes an impression of the patients mouth he makes an impression of the teeth in both the upper and lower jaw as well as a specific impression of the stump of the tooth which is to be crowned. Plaster models of the teeth in the upper and lower jaw and the stump to be crowned are then made from the impressions taken by the dentist.
The assembly of the cast model stump 13, the layer of hardenable elastomer 12 and first plastics impression 10 is inserted in the plaster model of the appropriate jaw and the model of the other jaw pressed against it so that the occluding tooth 15 can bear upon the first plastics impression 10 and suitably shape the underlying layer of hardenable elastomer 12.
After the elastomer has hardened the first plastics impression 10 is removed and the excess hardened elastomer removed from the region of the margin 14 in step III and then smoothed down to give a smooth marginal area 17. This step produces the crown form precursor 16 as shown in step lV. The crown form precursor 16 is then utilised in a second deep-drawing step similar to the one employed for producing the first plastics impression 10. Again it is preferred to use a sheet of polyethylene for this purpose.
The impression is then trimmed from the plastics sheet to produce a second plastics impression 18 which will have the shape of the crown form precursor 16 which in turn has the general shape of the original chosen stock tooth 11 modified to suit the occluding tooth 15. Thus, the second plastics impression 18 will correspond to the desired shape of the crown but when it is assembled on the cast model stump 13 there will be a spacing over at least a portion thereof.
In order that the crown should fit the stump accurately further material in the form of wax is added in the marginal portions as shown in step V. The wax is smoothed down to the desired external configuration and then the second plastics impression 18 together with the wax 19 is removed from the cast model stump 13 to produce the crown form 20 shown in step VI.
This crown may then be provided with a conventional wax sprue and wax reservoir and utilised to produce the investment casting mould. Both the polyethylene and the wax are burned out to leave the mould cavity which is subsequently employed to mould the crown material such as gold to produce the crown.
It will be apparent from the above description, and particularly with reference to step V, that the crown which is produced is considerably thinner than the crown which would be produced by a conventional technique in which crown material would extend from the stump 13 to the outer profile of the second plastics impression 18. It has been found that the production of such thin crowns does not produce an inferior product as when the dentist applies the crown to the stump the space therebetween is filled with cement and the important marginal area is made to fit the stump precisely by virtue of addition of the wax 19 as shown in step V.
In certain circumstances the sheet formed in accordance with step (a) is sufficiently close to the desired shape of form that steps (b) to (f) can be eliminated. Also, where a thick crown is required this form can be used with the hardened material in place to produce an investment casting mould.
The sheet of formable material may be formed by using a convenient forming technique other than the deep-drawing technique described above. Vacuum forming could be used and in this case it would be possible to vacuum form a plurality of forms in a single sheet of formable material.
The method of the invention enables a high quality crown to be produced in a relatively short time and requires less skill than the conventional method involving a carving technique. The method can be employed for producing crowns of any castable material. The thickness of the crown is, of course, predetermined by the thickness of the formed sheet, unless a crown of other than constant thickness is required, in which case material such as wax, can be added in selected areas.
The first sheet of formable material may be preformed and supplied, for example, to a dental laboratory as part of a kit for making crowns. The preformed sheet may include a number of shapes of teeth from which the technician can select a suitable form for his purpose. This avoids the technician having to carry out step (a) of the method and also he does not need to carry a selection of stock teeth for this purpose.
It will be appreciated that the method described above is a particularly attractive commercial method for producing crowns of a precious metal such as gold, as the amount of precious metal used is considerably less than would be used in producing an equivalent crown by a conventional technique.
Dental-crown alloys: High noble (precious), Noble (semiprecious), Base (nonprecious).
When making plans to have a crown placed, your dentist may ask you to make a decision about what type of metal alloy is used when it is fabricated. (This is a decision that needs to be made for all-metal and porcelain-fused to metal dental crowns.)
In general, there are 3 basic types of dental alloys that can be used. They are: high noble, semiprecious, and nonprecious (this classification system based upon the metal’s composition). Each type has its own advantages and disadvantages, including: cost, insurance plan coverage, color (gold or “white”), as well as general physical properties.
Related
This page discusses each of the above considerations. However, if cost is not a factor, the alloy having the highest precious metal content typically makes the best choice.
What types of metals are used to make crowns?
Crowns (all-metal and porcelain-fused-to-metal) are made using specific types of dental alloys. No pure metals are used for crowns, not even gold. This is because the physical properties of dental alloys are superior.
The classification of dental alloys.
Here’s the formal classification system that is used to categorize dental alloys.
1) High noble alloys (Precious metals)
This group of alloys has a composition that is over 60% noble metal (gold, palladium and/or platinum), of which more than 40% must be gold.
These metals constitute the “gold standard” of dental alloys; all others are compared to them. High noble alloys are the easiest type of metal to work with (for both the dentist and dental laboratory) and create the most predictable bond with porcelain.
2) Noble alloys (Semiprecious metals)
These alloys have a noble metal content that is, at minimum, over 25%.
3) Non-noble (Nonprecious metals)
These alloys are also referred to as base metals. Their noble metal content is less than 25%. They often contain large percentages of nickel, chromium or beryllium.
Why should I care what metal is used to create my dental crown?
There are several reasons why the type of dental alloy that is used to fabricate your dental crown should be important to you. Some of these reasons will affect you directly. Others will be more of a concern to your dentist, or the dental laboratory that makes it.
A) Color – Dental alloys can be white or yellow.
In those cases where an all-metal dental crown is being placed, you might have a preference as to whether it should have a yellow (like gold) or silver (“white”) coloration. The alloy’s composition determines its color.
B) Costs – High noble metal alloys cost more.
The “noble” dental metals are gold, platinum and palladium. These metals are pricey. And the greater the percentage of them found in the composition of an alloy, the greater its cost will be. With some applications, the overall cost between using a high noble or base metal alloy might be small. But in the case of an all-metal crown for a large molar, it might be a consideration.
C) Dental plan and insurance policy limitations.
If some type of dental plan is paying a part of your bill, you might check to see if there are any limitations as to the type of metal that can be used for crowns. The policy might state that they do not cover the cost of high noble alloys. Or the level of coverage might change based on the type of alloy that is used.
D) Some people have metal allergies.
Studies report that about 10% of the female population and 5% of the male have an allergic response to nickel, chrome and/or beryllium alloys. These metals are often found in the composition of nonprecious dental alloys.
E) The physical properties of the alloy are an important consideration.
Dentists and dental laboratories often have a set opinion about which types of dental alloys they will consider working with. This is because their goal is getting the job done right, the first time. They know that any difficulties or problems experienced will just end up costing them money. So, if choosing a certain type of alloy makes getting a positive result more likely, then that’s the one they are probably going to want to work with.
Advantages of precious dental alloys.
In general, dentists and dental labs prefer to work with high noble alloys. These metals are easiest to cast, provide the most accurate fit on the tooth, offer some degree of malleability (so the fit of the metal can be adjusted, if needed), and offer the most predictable bond with porcelain.
Finishing lines:
Is the final margin that separate between the prepared axial tooth surface and the remaining unprepared tooth surface.
Requirements of finishing line:
1. It must be clear, well defined and smooth, so it can be reproduced on working model.
2. It must be continuous from one surface to another.
3. Whenever possible the finishing line should be placed on sound tooth structure.
Position of finishing line:
1. With the level of free gingival margin.
2. Supra gingival finishing line,: its better to place the finishing line supragingivally for the following reasons:
A. Easily to be prepared without trauma to the soft tissues.
B. Easy to be prepared and finished by dentist.
C. Patient can keep it clean easily.
D. Impression is easily made and can be removed without tearing or deficiency.
3. Subgingival finishing line: indicated in
A. Esthetic.
B. Caries or filling at the area of finishing line.
C. To increase retention of short teeth.
Types of finishing line:
1. Feather edge (knife edge).
2. Chamfer.
3. Shoulder.
4. Bevel shoulder.
The selection of certain type of finishing line depends on:
1. The materials used to construct the restoration.
2. The position of the tooth.
3. The tooth aspect to be prepared.
Feather edge (knife edge):
In this type all convexities coronal to the margin are removed only, its mostly unacceptable but it was advocated already before the development of high speed cutting instruments and improvement of impression materials and techniques, this type of margins lack strength, difficult to locate on the cast and difficult to fabricate the wax pattern, however it provide the best marginal seal and it’s the most conservative type.
Chamfer finishing line:
This type is prepared with a tapered round ended fissure diamond bur, its regarded as the line of choice for most veneer cast metal restorations and lingual margins of porcelain fused to metal restoration. It has been shown to exhibit the least stress.
Shoulder finishing line:
This is the best choice for jacket crowns; the wide ledge provides resistance to occlusal forces and minimizes stresses that might lead to fracture of the restoration, and its less conservative. This finishing line is prepared with flattened end tapered diamond fissure bur. Its very well defined finishing line so it’s easily detected on the cast.
Shoulder with bevel:
In this type we create a bevel on the end margin of unprepared tooth structure, this lies between the prepared and unprepared tooth structure and is very critical area. This type of finishing line is recommended for extremely short walls, since the axial walls of this type is nearly parallel to each other so enhances retention. This type of finishing lines is used for porcelain fused to metal and full cast veneer with acrylic facing.
Types of crowns:
1. Full metal (veneer) crown:
This provides better retention and resistance because all the axial surfaces of the teeth are included in the preparation.
Indication:
1. Posterior abutment teeth with excessive caries.
2. As retainer on tooth receive clasp (posterior teeth).
3. High caries index.
4. Necessity of maximum retention and strength.
Contraindication:
1. Teeth located in the appearance zone.
2. Low caries index.
Advantage:
1. Strong.
2. More conservative and easy to prepared.
3. Provide more retention and resistance compared to partial veneer crowns.
Disadvantage:
1. Poor esthetic.
2. Tarnish and corrosion, so it needs prophylactic measures.
3. Difficulty to test the vitality of the abutment teeth.
Steps of preparation:
Depth orientation grooves must be prepared on the surface of the tooth to act as guide or reference to determine when sufficient amount of tooth structure is removed, without these grooves we may remove much or less tooth structure or we loss time in repeated checking.
1. The preparation for a full veneer crown is begun with the occlusal reduction. By accomplishing this step first, the occlusso gingival length of the preparation can be determined. The potential retention of the preparation can then be assessed, and auxiliary features can be added if necessary there should be 1.5 mm clearance on the functional cusps and about 1.0 mm on the non functional cusps. A No. 170 taper fissure bur or a round-end tapered diamond is used to place the grooves on the ridges and in the primary grooves of the occlusal surface. If there is already sonic clearance with the opposing tooth because of malpositioning or fracture of the tooth being prepared, do not make the grooves as deep.
The tooth structure remaining between the orientation grooves is removed to accomplish the occlusal reduction then smooth any roughness left by the grooves. Keeping the occlusal surface in the configuration of the geometric inclines that make up the occlusal surface of any posterior tooth after that a wide bevel is placed on the functional cusp again using the No. 170 buy or rounded tapered diamond. The functional cusp bevel placed on the buccal inclines of mandibular buccal cusps and the lingual inclines of maxillary lingual cusps after completion of occlusal surface preparation we should check the occlusion of the patient in centric and eccentric positions of jaw relationship.
2. Buccal surface: because of the anatomy of the buccal surface of the lower posterior teeth, this surface should be divided into two parts: gingival two thirds and occlusal one third for the gingival two thirds we should place a (DOG) in the center of this surface parallel to the long axis of the tooth and by moving the bur mesially and distally following the inclination of the surface so this surface prepared. For the occlusal one third a (DOG) is placed in the center of this area by placing the bur 45 degree with the long axis of the tooth and by moving the bur with the curvature of the surface to be prepared. This type of preparation is called two plane preparation or two steps
preparation The two plane preparation is done on the buccal surface of the lower molar and the palatal surface of the upper molar.
3. Lingual surface :the (DOG) is placed in the middle parallel to the long axis of the tooth and by moving the bur mesially and distally so we complete the reduction, this type of preparation is done in one plane as it is indicated for the lingual lower and buccal upper molar and premolar teeth.
4. Proximal surfaces: we start with a fine tapered diamond fissure bur (needle type) to open and remove the contact area carefully without touching the adjacent tooth because caries will be developed in the damaged surface later on, because we are going to create a rough surface in addition to removing the outer layer of enamel which is saturated with fluoride. The bur should be rested on the prepared tooth itself and by moving the bar up and down the contact will be removed, finally any sharp angle should be removed to prevent fracture due to stress concentration, sometimes seating groove is placed in the buccal surface of the lower and the palatal surface of the upper molar teeth which act as a guide during placement of the crown, to prevent the rotation of the restoration, increase the surface area of preparation so it enhance the retention and the resistance, finally it improves the seating of crown as it let the escape of the excess cement during cementation.
2-Full metal crown with facing
It is a full metal crown having the labial or buccal surface covered by a tooth colored materials (acrylic, Porcelain), it combines the strength of full metal crown and the cosmetic effect of the tooth colored material, and it is not a conservative type of crown since it includes excessive tooth preparation to provide enough space for the metal and the facing material in addition to that there is excessive contact with the gingival tissue when the margin of the crown is placed close or below the gingival margin ,it can be used on anterior and posterior teeth
Indication:
1. Improvement of esthetics (carious teeth, malposed teeth, peg shaped lateral incisor, discolored teeth).
2. Fracture of tooth without pulp exposure.
3. Teeth with large filling.
4. As a bridge retainer especially in long span bridge.
5. Endodontically treated teeth with sufficient remaining tooth structure.
Contraindications:
1. Teeth with large pulp.
2. Teeth with short crown.
3. Patient with poor oral hygiene.
Advantage:
1. It combines the strength of full metal crown and the cosmetic effect of the tooth colored material.
2. Natural appearance can be closely matched by good technique and if desired through characterization of the restoration with internally or externally applied stains.
Disadvantages:
1. Requires significant tooth reduction to provide sufficient space for the restorative materials.
2. Increases the potential for periodontal disease.
3. Because of the glasslike nature of the veneering material, a metal-ceramic crown is subject to brittle fracture (although such failure can usually be attributed to poor design or fabrication of the restoration).
Preparation
1. Preparation for posterior teeth
We should follow the same principles as in full metal crown as in the full metal crown with one exception only, by doing a deep reduction on the buccal surface to provide enough space for the metal and the facing material and also to gain bulk for proper shade of the final crown The finishing line is shoulder on the buccal surface and chamfer all around the remaining tooth aspects.
2. Preparation for anterior teeth
A. Incisal edge reduction:
We started by basing a depth orientation groove of 2 mm in the center of the of the incisal edge and by using a fissure bur placed with the palatal inclination of the incisal edge, this edge will be reduced (in the lower anterior teeth the bur should be placed with the labial inclination to follow the anatomy of the tooth) the 2mm reduction of the incisal edge is to provide a space for the facing material and metal to get a better translucency of the incisal edge.
B. Preparation of the labial surface:
This surface should be divided into two parts, gingival and incisal, for the gingival part a DOG of 1.5 mm is placed in the gingival part parallel to the longitudinal axis of tile tooth and by moving the bur mesially and distally this part will be reduced, while for the incisal reduction we place a DOG with the inclination of this area since the preparation if done without inclination we may have pulp exposure.
C. Lingual surface preparation:
For the cingulum area a DOG of 0.5 mm depth is placed in the center of the cingulum area parallel to the longitudinal axis of the tooth and by following the inclination of the tooth this area will be reduced The remaining axial lingual surface should he reduced using a wheel diamond bur, we must keep in mind not to remove or over reduce the junction between the cingulum and the axial and the remaining part of the lingual axial surface if not we may create a conical shape preparation which will lead to lack of retention and resistance, finally we should smooth and round the lie angle to facilitate the next steps of crown construction.
Why we do the two steps preparation on the lower buccal, upper palatal surfaces of the posterior and labial surfaces of anterior teeth:
1. To follow the anatomy and the inclination of the tooth and not to disturb the surface geometry.
2. To increase the surface area this will give increase in retention and resistance of the final restoration.
3. To avoid hitting of the pulp chamber during preparation.
4. To give enough space for the restorative material so this will enhance the structural durability other vise we will have bulky restoration, bulky facing or poor shade of the tooth.
The classification and formulation of crown and bridge gold alloys (used for all-gold restorations)
Prior to the introduction of Porcelain Fused to Metal (PFM) restorations, gold based alloys were virtually the only castable alloys used in dentistry. There were four types:
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Type
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Hardness
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yield strength (MPa)
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Percent elongation
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I |
Soft |
<140 |
18 |
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II |
Medium |
140-200 |
18 |
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III |
Hard |
201 |
12 |
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IV |
Extra-Hard |
>340 |
10 |
Type I
Type I was hard enough to stand up to biting forces, but soft enough to burnish against the margins of a cavity preparation. It was used mostly for one surface inlays.
Type II
Type II was less burnishable, but hard enough to stand up in small multiple surface inlays that did not include buccal or lingual surfaces.
Type IV
Type IV was used for partial denture frameworks, and was not used in fixed prosthetics.
Type III
The most commonly used type of gold for all-metal crowns and bridges is Type III. It is still used whenever a patient requests an all gold restoration such as an all gold crown, inlay or onlay. A typical type III gold alloy has approximately the following formula:
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Gold |
75% |
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Silver |
10% |
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Copper |
10% |
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Palladium |
3% |
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Zinc |
2% |
The purpose of each component is as follows:
Gold
Gold is a “noble metal”. In other words, it resists tarnish and corrosion and will participate in very few chemical reactions, which means that it is non toxic and hypoallergenic. It is also highly ductile and malleable and has a relatively low melting point, which are major factors accounting for its use by people in early historical periods. Gold’s long civilizational lineage and incorruptibility made it a natural first choice for use in dentistry. It forms the bulk of the composition of the alloy.
The other noble metals are:, palladium, silver, tantalum, platinum, iridium, osmium, ruthenium, and rhodium. The classification of noble metals is an ancient one and is rather loosely defined since silver certainly tarnishes, and copper is sometimes included in the list.
Copper
Copper is the principal hardener. It is necessary for heat treatment and is usually added in concentrations of greater than 10%
Silver
Silver lowers the melting temperature and also modifies the red color produced by the combination of gold and copper. It also increases ductility and malleability.
Palladium
Palladium, (another noble metal) raises the melting temperature, increases hardness and whitens the gold, even in very small concentrations. It also prevents tarnish and corrosion and acts to absorb hydrogen gas which may be released during casting causing porosity.
Zinc
Zinc acts as an oxygen scavenger and prevents the formation of porosity in the finished alloy. It also increases fluidity and reduces the surface tension in the molten state improving the casting characteristics of the alloy.
For more oearly all the individual metals used to formulate dental alloys, click here.
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Porcelain alloys
Until the mid 20th century, gold and amalgam were virtually the only materials available for the restoration and replacement of posterior teeth. Metal was the only game in town. Porcelain jacket crowns were available for front teeth, but they did not fit very well, and they were prone to easy fracture. In 1962 that all changed when Dr. Abraham Weinstein patented the first gold based alloy upon which porcelain could be baked. The metal substructure reinforced the porcelain and gave it the durability and the strength to resist fracture in the mouth. It made it possible for the first time to replace missing teeth with natural looking tooth colored fixed bridgework. In addition, due to the accuracy of the lost wax technique, the appliances could fit the tooth preparations exactly. Producing a metal framework that was compatible with a durable porcelain superstructure was not an easy task:
Porcelain will not chemically bond with gold by itself. There needs to be a mix of trace elements in the composition of the alloy to allow the formation of an oxide layer on its surface, which then bonds the porcelain to the metal. The three oxide-forming elements are iron, indium and tin. Porcelain is, itself, made of metal oxides. Thus it will bind with the oxides on the surface of the gold framework.
The necessity for the formation of metal oxides on the surface of the underlying casting means that ions from the metal casting will mix with the porcelain, potentially affecting the color, reflective properties and translucency of the finished product. Thus the porcelain must be formulated to overcome these effects.
Porcelain melts at high temperatures (between 850°C and 1350°C depending on the type of porcelain used). It is applied as wet powder over the metal framework, and baked, or fired in order to fuse the powder particles together. This means that the metal substructure upon which the porcelain is applied must resist sagging and deformation while being held at this high temperature for several hours while the porcelain is fused over it. Otherwise, the casting will not fit the teeth in the mouth.
The metal is opaque and generally has a gold or gray color. Porcelain must be translucent, or it fails the tests of esthetics. There must be a mechanism to “opaque” the underlying metal framework, or the finished appliance will have a gray cast and not look real.
The index of thermal expansion of the metal must be nearly identical to that of the porcelain. Otherwise, the porcelain will simply shatter off of the framework as it cools after being fired. If the metal shrinks less than the porcelain during cooling, the porcelain will “craze” (develop little cracks throughout its structure). If the metal shrinks too much more than the porcelain during cooling, the porcelain will “shiver” (the opposite of crazing, sort of like “puckering”, but having the effect of breaking the porcelain off of the framework.
Ideally, porcelain should be under slight compression in the final restoration. This objective is accomplished by selecting an alloy/porcelain combination in which the alloy contracts slightly more than the porcelain on cooling to room temperature. Compression of the porcelain reduces the likelihood that cracks will propagate throughout the structure during service.
All porcelains used to veneer metallic substructures contain leucite crystals. These crystals serve two functions in the porcelain. They act to limit the propagation of cracks in the porcelain veneer, and they serve to increase the index of thermal expansion of the porcelain. By carefully adjusting the proportion of leucite crystals in the glass, it can be made to “fit” the metallic substructure during the sintering and fusing phases of manufacture.
How porcelain is applied to a metal coping
In the image below, a cast metal coping is placed back on the die after the buccal gingival margin is removed. This is done in order to allow a butt porcelain margin so that no metal will show in the final crown: (Thanx to Bothell Dental lab for these images.)
Next, a thin layer of opaque porcelain powder (frit) is layered over the metal in order to mask the underlying darkness. Otherwise, the finished crown would always show a gray caste.
After the opaque layer is fused onto the metal coping, the first layer of overlying porcelain is applied with a wet paintbrush. Different shades of frit are applied over various parts of the crown in order to make the finished tooth look more natural:
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The coping, along with its “green” porcelain is removed from the die and placed in a vacuum kiln and fired at about 1700 degrees F:
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The green porcelain shrinks during its firing, so a second layer of porcelain frit is layered over the first bake.
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Step 1 – Cleaning Impressions
These are the impressions of the upper and lower teeth which are delivered to the lab. These can range from clean to gross! First they MUST be disinfected.
The impression on the right has a section of differently coloured impression material where the crown is destined for. This gold material is runnier than the blue material and so is better at capturing detail.This is why it is being used for the area where the crown is to be fitted – it really needs to be accurate.
A PFM crown is a porcelain-fused-to-metal crown, consisting of a metal substructure in a cap shape (made to fit perfectly over the patient’s tooth) with a porcelain coating.
A patient requiring a PFM croweeds to have their damaged tooth ground down by a dentist to remove the damaged tissue and to create enough room to fit the metal and porcelain crown into.
The impressions above belong to a patient who did not have enough undamaged tooth tissue left to support a crown, so the whole tooth was removed and a tapered hole was drilled into their alveolar (gum) ridge. A metal post will be made to fit into this hole and this will support the PFM crown as the tooth would have done.
You might be able to see the gold spike in the impression shown above, where the gold impression material has been pushed into the drilled hole of the patient’s gum. This spike shape will be exactly replicated in metal to form a post.
Step 2 – Positioning the Impression
Plasticine is rolled out into a long strip and cut so that it just comes up to the slot in the back of the metal tray. This is so that a plastic plate can be slotted correctly onto the metal tray in the next step.
The ‘important’ impression i.e. the one containing the impression of the crucial tooth which requires the crown, is raised to make it level and centred, plus to keep it approx. 1-2cm below the plasticine strip.
Note that the impression has been trimmed with a scalpel to remove excess impression material and to make the impression flatter and more level on top.
Step 3 – Preparing the Mould
The plasticine is wrapped around the impression and some is used to fill the hole in the centre.
The top of the plasticine should be flat and level, and the top should lie in line with the bottom of the slot in the back of the metal tray.
The plasticine must have no holes or gaps.
Step 4 – Baseplate Preparation
Next, a white plastic baseplate is inserted into a drilling machine. The pointed metal rod at the top lines up with a drill that is underneath the baseplate.
Step 5 – Drilling the Baseplate
The silver metal tray is secured onto the black metal block containing the white baseplate. This will ensure that the impression lines up with the white baseplate below. The black surface can slide around.
The metal rod above the impression is in line with a drill beneath the white baseplate, and is used to direct the drill into the baseplate to create holes in the correct places. Later on in the process, the cast made from this impression will be split into sections – in this case, it will be 3; the crown section will need to be removeable so the cast will need to be split either side of the place where the crown will be fitted.
Therefore, the places to put the holes must be decided in this step. There needs to be at least 2 holes drilled into each section, depending on it’s size. Metal pegs will be put into these holes in the next step to help with relocation later.
Step 6 – Add the Metal Pegs
Metal pegs are inserted into the holes. Note that the crown will be positioned where there are 2 pegs close together.
Step 7 – Pour the Dental Stone
Dental stone powder is mixed with water in a vacuum mixer. The mixture is slowly poured into the impression whilst holding the tray on a vibrating table to prevent air bubbles being trapped. Air bubbles can ruin the resulting stone cast.
Some of the stone mixture is poured onto the white baseplate, to try and prevent air bubbles being trapped around the pegs.
Step 8 – Make The Opposing Cast In Plaster
The white baseplate is pressed into the slot in the metal tray, making sure it is pressed down onto the plasticine.
The other impression (of the lower teeth) is filled with plaster on a vibrating table, and a pile of plaster is put on the worktop. When the pile has set slightly so it’s no longer runny, the impression is turned over onto the pile, and the plaster is shaped around the impression tray shape. The impression must be level and centred.
The stone only needs to be used for the ‘important’ upper impression (rather than plaster), because this cast is the one that will be worked on and so needs to be more hard and durable.
Step 9
When the plaster and stone have set, the plasticine and impression trays are carefully removed to leave you with a plaster model of the lower teeth, and a stone model of the upper teeth attached to a white baseplate with metal pegs.
Step 10
This is the upper model from the top, and from the bottom when the white baseplate is removed.
The metal pegs allow the cast to be removed from the baseplate and then connected again in exactly the same position.
Step 11
Excess stone material is removed from inside and outside the tooth arch. The model is then sanded and smoothed on the sides to basically make it presentable and easier to handle.
Step 12
The model is put back onto the baseplate and it is sawn into 3 pieces with a cut either side of where the crown is needed. The gingival margin, which is the top edge of the gingiva (gum) surrounding the tooth crown – and is where the crown will need to meet the gum, is marked out in red.
The upper half of this stone section is sanded into a smooth tapered shape upto the red line.
Step 13
Surface sealer and lubricant is used to coat the crown section of the model so that wax will not stick to it.
Then a thin plastic post is used to help molten wax fill the hole in the model completely, add strength and keep the wax post together.
Step 14
A rough tooth shape is built up in wax on top of the post. Enough space is left on every side of the wax to fit the crown into. Note that the wax is not flat on top because a v shape means that the crown will have more support from the post and more surface area to attach the crown to. This will help the crown stay in place despite the strong biting/eating forces which will act in many directions on the crown.
If the patient has enough tooth material to support a crown, these steps are obviously missed out as a post will not be needed. Instead, the ground down tooth will look similar to the wax shape shown above, and a crown will be built onto that (see later steps for the crown manufacture).
Step 15
The wax post is removed from the model and attached to a yellow cone shape via a sprue (a wax rod).
Everything in wax is going to be surrounded with a special investment material and the wax will be removed, leaving a cavity where the wax was. A sprue is a wax channel which, when removed, is designed to leave a tunnel from the outside of the investment material to the inside post shape.
The cone shape and the sprue are made to be smooth and rounded, to transport the molten metal used when casting to the post shape as smoothly and as quickly as possible – any obstructions may create turbulence and bubbles, and will slow the metal down. Slowing the movement of the metal down is not recommended because the metal cools quickly and it must not set at all before it reaches the shape to be cast.
The position of the sprue on the wax post is important. It must touch the wax post at the largest/thickest section and then direct the metal down the path of least resistance.
Step 16
A flexible tube mould is pressed down onto the yellow cone so that the wax post and sprue are enclosed near the centre of the tube. A small amount of debubbleiser (great name!) is sprayed onto the wax post, which is to make the investment flow easier over the wax to try and prevent bubbles.
Investment material is mixed in a vacuum mixer, and this is poured slowly into the tube.
Step 17
When the investment is set, it is removed from the tube. The cone is taken out and the cylinder is placed in a furnace for the wax to be burnt out to leave just a cavity in the investment where the wax previously was.
The investment cylinder is then put into a casting machine so that it lines up with a crucible (in which the metal is placed). The crucible and cylinder are then enclosed in a vacuum and the metal is melted (which you can see in the bottom right of the photo above). When the metal is at the right temperature (i.e. completely molten), the metal is poured into the cavity of the cylinder. The cylinder is removed from the machine with tongs and left to cool.
Step 18
A hammer is used to break the metal casting out of the investment material. Any excess investment is removed from the metal.
Step 19
A sandblasting machine is used to fire a stream of fine granules at the surface of the metal casting to clean and polish the surface.
Step 20
The excess metal is removed from the casting and a fine grade bur* such as a diamond bur is used all over the metal post to remove any bobbles/irregularities and to smooth the surface. The bur is not used to remove any metal because this may cause the post to be ill-fitting for the patient.
The post is checked to see that it fits snugly in the stone model.
*A bur is an abrasive attachment for an electric handpiece, which rotates fast like a drill. There are many available with different designs, different uses, and different grades.
Step 21
Now we move onto the manufacture of the crown.
First, the casting wax is built up into what the shape of the metal substructure will be. The wax must be melted before it is added to the crown, as it needs to be closely adapted. The margin around the bottom of the crown must go upto the red line which was drawn on previously. If the wax doesn’t go upto the red line, the resulting crown will not meet the patient’s gum, and the gap will allow bacteria and food to seep under the crown leading to infection.
The blue collar on the inside of the crown is a ledge for the porcelain layer to rest on.
Step 22
The wax crown is then cast in metal just as the post was in previous steps.
So now there is a post (on the left of the photo) and the metal subsructure of a jacket crown (on the right of the photo). The crown fits perfectly over the post, with a tiny, uniform gap between the two for the cement that will hold them together in the patients mouth.
It’s not shown in previous steps, but a spacer is used on top of the post before the wax is added so that there is enough of a gap between the crown and the post for this very thin layer of cement.
Step 23
The crown is ‘degassed’ in the furnace and sandblasted before the coat of opaque porcelain, which stops the metal colour from showing through, is added.
The lip at the back of the crown is not covered in porcelain. This metal lip is to support the porcelain, add strength and maintain a clear, clean and definite margin.
Step 24
Next, layers of porcelain are built up and fired in the furnace.
The porcelain mix is created by mixing porcelain powder with a liquid (often water), and this is added to the crown using a paintbrush. Tissue paper is used to absorb excess moisture.
The pink coloured porcelain turns yellow when fired in the furnace, and is used to represent the darker tooth material dentine, which is found underneath the enamel iatural teeth.The white porcelain mix turns into a light coloured and quite transparent porcelain when fired, which replicates how enamel looks in natural teeth.
The dental technician aims to make the porcelain coating the correct shape, but a larger size than the finished croweeds to be. This is because the porcelain shrinks slightly during firing.
The technician will use a photo to use the right shades of porcelain, and build up the layers in the correct way to give the same appearance as the patient’s adjacent teeth. You need a good eye for colour here!
Step 25
After firing, a bur is used to grind the porcelain into exactly the correct shape so that the sides are in line with the adjacent teeth, the curve of the tooth is smooth and matches the other teeth, and it is the correct size. The bur is also used to grind detail into the porcelain to match the same tooth (premolar) on the other side of the dental arch.
The lower teeth, which were cast in plaster earlier, are used to check that the crown is in the correct position in relation to these opposing teeth, and that they fit together correctly. There must be no premature contacts when biting/eating.
Step 26
The last furnace firing programme is used to glaze the crown and produce a shiny finish. The metal collar at the back of the crown is polished and it’s finished!
