Clinical and laboratory stages of dental bridge making
Fixed partial dentures are better known as ‘crown and nbridges. Bridges usually consist of one or more abutment (neighboring nteeth/tooth) and one or more pontic (the artificial tooth which is placed ithe edentulous area). The fixed bridge always requires preparation of the nneighboring teeth/tooth. The reshape of the neighboring tooth presents the nspace for the fabricated artificial abutments to be fitted over the natural nteeth/tooth perfectly, and obtain retention which increases the bridge’s nstability in the mouth. Before the fixed denture is positioned over the shaped nneighboring teeth/tooth, the retention of the denture is supported by applying nsuitable cement between the artificial unit and natural teeth/tooth. Types of nfixed bridges are classified according to the number of the abutments. When the nbridge has one abutment and is supported from one end, it is called cantilever nbridge, but if it has two abutments it is called a conventional fixed bridge. nIn some cases, where patient has a lot of caries all over the tooth, the ndentist will remove most of the natural tooth. The tooth can be fully covered nwith an artificial tooth called single crown. Fixed bridges and single crowns ncan be made of different kind of materials such as: gold, cobalt-chromium nalloys, nickel-chromium alloys or porcelain, or even a combination of more thaone of those materials. When patients have the choice, they often prefer to ncover the prosthesis with ceramic (porcelain) in order to improve the nappearance of the teeth/tooth.
Advantages of Conventional Bridges (fixed partial dentures):
Very good appearance.
It is fixed in the mouth (not removable) and lasts nlonger than the RPD. In
addition, it makes the patient feel like he/she has npermanent teeth.
It evenly distributes the chewing nforces.
Disadvantages of Conventional Bridges (fixed partial ndentures):
It is necessary to prepare the abutments in order nto fix the artificial teeth.
Usually, the neighboring tooth preparation leads to ndestroying the anatomy of
natural and healthy teeth since it requires a significant reduction.
The prepared teeth (abutments) are at risk of nrecurrent caries.
Expensive to remake if a replacement is required nfor any reason.
The unnatural appearance of the denture (betweethe bridge and gum).
Very accurate fabrication required.
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 npresence of distal support.
4. Absence of the 3rd next confronting teeth the presence of ndistal support (to consider the standing of supporting teeth and antagonists)
CONTRA-EVIDENCE:
1. Included defects within 4-5 teeth.
2. Mobility of the teeth (atrophy of bone hole down 1/2 and nmore).
3 Deformations of a bite and dental numbers with the partial nloss of teeth.
4. Production of cantilever prosthesis in the region of nmolars.
There are three main types of dental bridges:
1. Traditional fixed bridge
This is the most commonly used type of bridge and consists nof a pontic fused between two porcelain crowns that are anchored oneighbouring teeth or implants. The pontic is usually made of either porcelaifused to metal or ceramics. These are fixed and cannot be removed.
2. Resin-bonded bridges or Maryland-bonded bridges
These are chosen when the gap to be filled is in between the nfront teeth, or when the teeth on either side of the missing tooth are strong nand healthy without large fillings. The false tooth is made of plastic and is nfused to metal bands that are bonded to the adjacent teeth using resin that is nhidden from view.
3. Cantilever bridges
These are opted for in areas such as the front teeth that nare susceptible to lower stress. Cantilever bridges are used when there are nteeth present on only one side of the space, where the false tooth is anchored nto one or more adjacent teeth on one side.Depending on the material used, two ntypes of dental bridges are most often used: (#1) Porcelain Bridges and (#2) nPorcelain-Fused-to-Metal Bridges. The two types are indicated in different nsituations because of their respective advantages and disadvantages:
Porcelain Bridges
Porcelain Bridges are dental bridges made completely of nporcelain. Porcelain is an excellent material, since it is both extremely nstrong and aesthetic. Porcelain’s color and translucency closely matches enamel n(the outer layer of a tooth), making porcelain bridges very natural looking.
Learn more about porcelain bridges, including their advantages nand disadvantages
Porcelain-Fused-to-Metal Bridges
Porcelain-Fused-to-Metal Bridges have metal inside and nporcelain outside. The metal inside adapts perfectly to the teeth and provides nstrength and resistance to biting forces. The porcelain outside provides the nbeautiful appearance, matching the color and shape of your other teeth.
The main advantage of Porcelain-Fused-to-Metal Bridges is ntheir strength and durability. Due to the high force on back teeth while nchewing, Porcelain-Fused-to-Metal Bridges are typically recommended when a ndental bridge is needed in the back of your mouth.
The main disadvantage of a Porcelain-Fused-to-Metal Bridge nis that the metal can sometimes be seen through the porcelain close to the gum nline, creating a grey line or grey shadow near the gums. This disadvantage is nan aesthetic issue and does not affect your gums or the longevity of the nbridge.
A special type of dental bridge is an Implant Bridge. Aimplant bridge is a dental bridge that is placed on top of implants to replace nmissing teeth. Dental implants are titanium screws that are placed in the njawbone in the area of missing teeth. Then, a bridge that looks and feels like nnatural teeth is placed on top of the implant screws to replace the missing nteeth.
First visit:Preparation of supporting teeth under selected nconstruction of supporting parts crowns, half-crown’s, pin teeth and so forth nthe removal of the anatomical of vises.
Second visit:Adjustment of crowns (or other supporting nelements) the determination of central occlusion, the removal of occlusioimpression.
Third visit:Adjustment (fitting of bridge-shaped prosthesis. nIdentification of the color of plastic for the facets.
Fourth visit:Fitting and the fixation of bridge-shaped prosthesis nby hanging-fop- cement.
LABORATORY STAGES
1. Obtaining model, making of supporting parts.
2. Casting of models to articulator (or casting occlusioimpression), modeling of the intermediate part of the bridge-shaped prosthesis nfrom wax, casting. Soldering immediate part to supporting crowns.
3. Polishing, chromium-plating, the simulation of facets nfrom wax, the replacement of wax down the plastic.
Preliminary nAlginate Impression
After na patient has been examined and treatment planned for a crown or bridge nprocedure, a preliminary alginate impression for study models can be made. nSubsequently, working casts can be created from the preliminary impression, nwhich will be used to fabricate custom trays. The philosophy behind making the nfinal impression with custom trays is to provide a well-fitting impression tray nthat is comfortable for the patient, contains an adequate amount of impressiomaterial to obtain an impression that reproduces fine details, minimizes waste nof expensive impression materials, and assures proper intraoral seating of the nimpression tray during the impression process.
Anesthetic nApplication
Applying ntopical anesthetic to the injection sites after drying the mucosa will provide npatient comfort during the injections. For patients, the administration of the nlocal anesthetic injection is probably the most nerve-racking part of the ndental visit, no matter what procedure will be performed.
Shade nSelection
Shade nselection can be done while the dental team is waiting for the local anesthetic nto take effect. Using a shade guide under natural lighting conditions, the ndentist, dental assistant, and the patient should determine which shade matches nthe adjacent teeth best.
Preparatioand Evacuation
After nthe teeth to be prepared have been anesthetized, the dentist will use burs in a nhigh-speed handpiece to reduce and shape the teeth. Simultaneously, the dental nassistant will use the high-velocity evacuation system and the air/water nsyringe to keep the oral cavity clear of debris and maintain a clear path of nvision for the dentist. When tooth preparation is complete, the dental nassistant will examine the patient’s oral cavity to verify that it is clear of ndebris.
Tissue nManagement
There nare various methods available for tissue management, the most common being the nuse of retraction cord. Retraction cords may be twisted or braided and are navailable in different sizes to accommodate various depths of sulcus. Using a nBalshi packer or another of gingival cord-packing instruments available, gently npush the end of the gingival retraction cord under the gingival margin. nContinue advancing around the prepared tooth, packing the gingival retractiountil arriving at the beginning end of the cord. Leave a piece of the cord nsticking above the gingival margin for removal immediately proceeding placement nof the wash or light- final impression material. Some retraction cords are ntreated with a chemical agent to enhance the contraction of the tissue it ncontacts. Aluminum chloride, potassium aluminum sulfate, ferric sulfate, or nzinc chloride cause the collagen fibers in the tissues around the capillaries nto swell. The expansion of the collagen around the capillaries induces pressure non them, which causes them to constrict. This contraction of the tissues allows nthe impression material to enter the sulcus to capture the margins of the tooth npreparation. Astringedent®, Astringedent® X, ViscoStat®, ViscoStat®Wintermint, nViscoStat® Clear (Ultradent Products, Inc, www.ultradent.com), Gingi-BRAID+ n(DUX Dental, www.duxdental.com), and Hemodent™ Cord, (Premier Dental Products nCo, www.premusa.com) are examples of tissue management products. Make sure you nare familiar with the patient’s medical history before selecting a gingival nretraction method. Astringedent X should be used on patients who are on the nanticoagulant warfarin or aspirin therapy, or who are hemophiliacs to control nbleeding. Also, gingival retraction cord should not be left in the sulcus for nan extended period of time as this may cause gingival recession, especially if na periodontal condition is present.
Generally, nchemically treated retraction cord can be left in place for 5 to 7 minutes and nuntreated cord for 10 to 15 minutes. Gingival retraction products containing nepinephrine should be avoided in patients who have hypertension, heart disease, ndiabetes, or hyperthyroidism. In addition to the use of cords for gingival nretraction, retraction caps (Roeko Comprecap, Coltène Whaledent Inc, nwww.coltenewhaldent.com; GingiCap™, Centrix, www.centrixdental.com), retractiogel (GelCord® Tissue Management Gel, Pascal International, Inc, nwww.pascaldental.com), and gingival retraction paste (Expasyl™, Kerr nCorporation, www.kerrdental.com; Traxodent® Hemodent®, Premier Dental nCorporation, www.premusa.com) can be used. Roeko Comprecaps should be used iconjunction with an expanding polyvinyl siloxane material to press the gingival ntissues away from the margins of the preparation. If the gingival tissues are ntoo bulky and esthetics will not be compromised, the dentist can create a ntrough around the preparation using electrosurgery to open the sulcus.
Final nImpression
After nthe tissues have been managed, the final impression materials can be prepared. nThe selection of the impression material depends on the dentist’s preference. A nsingle-viscosity, monophase impression material is favored by many dental nprofessionals because there is only one material to mix and it can reproduce nfine details. A combination of medium-, heavy-, or putty along with a wash-type nimpression material can be used with a two-step technique. When selecting aimpression technique and materials, consider how many hands will be available nfor mixing the materials and loading them into a syringe and/or impressiotray. An option is to use materials that are prepared using an automixer.
After nthe master impression is removed from the patient’s mouth, the impressioshould be disinfected and then examined by the dentist to ensure all necessary ndetails were captured. A clear reproduction of the margins is needed because nthe dental laboratory technician must be able to read the margins on the dies nproduced from the impression to fabricate a well-fitting permanent prosthesis n(if the necessary anatomic features were not reproduced, the impression would nhave to be repeated; this also may include reapplication of the gingival nretraction method).
Immediately nafter the impression is completed, a bite registration should be recorded with na high-viscosity elastomeric impression material. The bite registration also nneeds to be disinfected before packaging to be sent to the laboratory.
Provisional nRestoration Creation/Delivery
After nthe final impression has been completed, provisional coverage is provided. nOptions available for provisional coverage are: custom-made, preformed polymer, npreformed polycarbonate, and aluminum. Stainless steel crowns also can be used nas a form of provisional coverage in adults. Another possible option requires nsending the working cast to the dental laboratory a week before the preparatiovisit. Provisional restorations can be fabricated by the dental laboratory ntechnician from the pretreatment cast. These provisionals are reinforced with northodontic wire or Kevlar fiber. Using laboratory-fabricated provisionals casave chair time, but adds an expense. This option is especially useful if the nprovisional restorations are expected to be worn for an extended time.
Provisional nRestoration Cementation
Regardless nof the type of provisional used, it will be cemented onto the prepared teeth nusing a temporary luting agent. A well-fitting provisional needs a minimal namount of temporary cement. An excessive amount of cement inside a crown or nbridge may prevent proper seating onto the prepared tooth/teeth. After the ncement has set, the gingival margins should be examined and any excess cement nremoved. A piece of knotted dental floss should be carefully passed ninterproximally to remove residual cement. If the provisional is a bridge, nfloss should be passed under the pontic areas as well.
Patient nInstruction
After nthe provisional is in place, the patient should be instructed about the nimportance of plaque control and any modifications to his or her oral hygiene nroutine. Diet modifications while the provisional restoration is in place nshould be discussed. Equally important is informing the patient to notify the ndental office immediately should the provisional restoration come off the npreparation before the next scheduled appointment.
Final nCementation Visit Steps
Removal nof Provisional Restoration
The nnext visit is scheduled to permanently cement the prosthesis onto the prepared ntooth/teeth. When the patient arrives, the dental assistant will remove the nprovisional restoration. The prepared tooth/teeth should be examined and any nremaining temporary cement removed. The preparation is carefully rinsed and ndried.
RestoratioCementation
The npermanent restoration is tried in. If the dentist and patient are satisfied nwith the fit and appearance of the prosthesis, it can be cemented with the npermanent cement designated by the dentist. After isolating the preparation, nthe dental assistant will need to mix the permanent cement according to the nmanufacturer’s directions and then coat the inner surface of the abutment teeth nof the prosthesis with enough cement to ensure adequate adhesion of the nprosthesis to the patient’s teeth. Again, excessive amounts of cement may nprevent proper seating. The prosthesis is seated onto the preparation by the ndentist and the patient is instructed to bite continuously until the cement has nhardened. After the cement is set, the dental assistant should examine the area nand remove any excess cement as was done with the provisional restorations.
Steps to clinically preparing the FPD
Before the treatment initiates, the dentist first examines nthe situation of the patient to determine if the fixed partial denture is nnecessary as well as carrying out an X-ray examination. If the dentist agrees nwith the patient that he or she really does require fixed prosthesis, the dentist nwill then begin the first steps of the fixed crown or bridge. After the nagreement of applying the fixed partial denture, the dentist then has to start npreparing the abutment teeth/tooth. During the patient’s first visit, the ndentist will reshape the tooth/teeth in such a way which allows the denture to nbe placed over the natural tooth/teeth. Normally, there are standard ncircumstances for the fixed partial denture preparation by making a reductioon all around the tooth/teeth; this reduction will create the required space nfor the new prosthesis to be positioned over the prepared units. When the nreduction is complete, the tooth/teeth must then be tapered into shape. If nthere is any undercut on the surface of the tooth, the restoration will not be nable to be removed from the die. The prepared units require specific nmeasurements of two to three degree of taper. Clinically, there are special ninstruments to aid the dentist to getting that specific degree, and allows the nfixed bridge or crown to be placed properly in the mouth. It is also important nto note that if the tooth has been tapered too much it will affect the nrestoration stability as there will not be enough retention for the denture to nbe held into place. That means, a six-degree of taper all around the tooth is nneeded in order to give a total of twelve degrees of taper; any more than this ncould negatively affect the stability of the denture. During the tooth npreparation process, it is also important to make the margin ideal. The margican be described as the line which is present around the tooth at the junctiobetween the artificial tooth and the natural tooth. The fixed denture cannot nsurvive without a good margin, and even if the artificial tooth is perfectly nconstructed, it will still not last for a long time if the margin experiences nany problems. The margin must be smooth and strictly adapted to the finish nline. If there is an opening in the margin it will affect the denture nstability; moreover, it will also allow food to stick in the openings, which ncan ultimately lead to poor oral health. The margin is normally located betweethe natural tooth and the artificial tooth, and so it might be seen when the nrestoration is in the posterior area; in that case, dentists always try to take ncare when they draw the margin simply because it must work sufficiently and not nbe too visible to the naked eye.
There are different kinds of margins, which each depend opreparation. The first type of margins is called a ‘chamfer’. This margin type nis very popular with full gold prosthesis. It provides the best strength which nthe crown/bridge needing to be adapted perfectly, and is very effective iremoving a very small amount of the tooth structure, which allows the dentist nto work with the smallest detail. The second type of margin is called the n‘shoulder margin’. This particular margin is popular with the PFM 27(PorcelaiFused to Metal) prosthesis, and it is also useful when preparing the tooth for nall-ceramic restorations. After the tooth preparation, an impression of the ntooth/teeth is taken and sent to the dental laboratory in order to fabricate nthe artificial prosthesis. Because of the improvement of the dental technology, nthere are many different ways of fabricating the dental crowns/bridges.
FPD fabrication procedure in the laboratory
The chosen method of fabrication purely depends of the type nof material used with the restoration, for example, whether it is full ceramic, nfull metal or Porcelain Fused to Metal. When the dental laboratory receives the nimpression, the dental technician pours and mounts the cast by using different ntypes of dental stone or plaster, and when the stone model has dried when the nthermal heat disappears the cast is then ditched and trimmed into the required nshape. After this stage, the pins are then cemented to the bottom of the cast nand another mixture is made in order to mount the base of the cast and attach nit to the articulator, which is a device used to attached the upper (maxillary) nand lower (mandibular) casts. The articulator provides the link between the nupper and lower jaw, as it is in the patient’s mouth which ultimately allows nthe dental technologist to see how the upper and lower jaw meet with each other nand subsequently aids him/her to properly fabricate the tooth to do its function. nFrom this stage, the casts are then taken out of the articulator before they nsplit the cast from the base and split the prepared teeth/tooth from the rest nof the cast into individual dies by using a saw. The finished line of every nsingle die (of the prepared tooth) is then exposed using the wax knife and the nteeth/tooth fabrication is made using one of the following techniques:
All-Wax technique
The all-wax technique is used when the denture if made ncompletely from metal. There are different materials which could be used in the nfull metal restoration, and gold is one of the best kinds of material. The gold nalloys consist of various different elements, such as gold, palladium, platinum n(noble materials), tin, silver and copper (based materials). Although the full ngold prosthesis does not look good cosmetically, they do nevertheless have the nadvantage of being very good quality, as they are considered as high-noble nmaterials, according to the American Dental Association because it contains 75% nnoble metal. Full metal dentures irrespective of whether it is gold or any nother suitable dental material are manufactured with the implementation of a ntechnique called the ‘lost wax technique’. In order to get the final shape by nusing the lost wax technique, the artificial teeth must first undergo many more nstages before getting the final result. Firstly, a die spacer should be applied nover the die/dies in order to provide the space between the artificial nteeth/tooth and the prepared tooth. This thin space will allow place for the ncementation which will be applied clinically before fixing the denture in the npatient’s mouth. After this stage, a die lubricant must then be applied so that nthe wax pattern can be easily removed the die lubricant prevents the wax npattern from sticking to the die after the wax build-up is completed. Secondly, nthe dental technologist will then start to build-up the wax pattern by copying nthe die. The coping procedure is focused on creating a thin layer of wax over nthe die. Normally, the coping is made from wax but it is also possible to make nthe coping from a heated resin sheet. After this stage, the wax is then applied nover the copying layer be using a hot wax spatula. Some other instruments might nalso be used, for example, a wax knife and wax carver, in order to build-up the nwax pattern. At some point, the die should be returned to the cast (which is nfixed to the articulator) in order to manipulate the wax until the wax patterappears to be similar to the dimensions of the original tooth, and does not ninterfere with the opposing teeth (in the opposing jaw).Thirdly, when the wax ntooth/teeth appear to be in a good shape (final shape) the wax pattern is theremoved from the die and attached to a special kind of wax stick called a n‘sprue former’, which is a small diameter tube usually mad of wax; the ideal ntube width used with crown/bridge is 2.6 mm. The sprue is attached to the wax npattern from one end, while the other end is attached to a conical rubber base. nThe sprue with the wax pattern is inserted into a special ring while the rubber nbase works as a base to that ring. After this stage, the investment material is nthen mixed and poured into the ring. When the investment material has ncompletely set, the rubber ring is then removed, and the base will then form a ncrucible shape with a hollow in the middle. This funnel-shape will force the nmetal to go into the mold during the casting procedure while the hole in the nmiddle of the ring will allow the molten wax to get out the mold. It is highly nrecommended that the sprue is attached with its angle to the bulkiest area of nthe wax pattern. Attaching the sprue in angle with the wax pattern will allow nthe molten metal to flow into the mold easily. It is also important to consider nthat the wax pattern should be 6 mm away from the end of the ring; if it is too nclose to the end of the ring, the molten metal might blast through the ninvestment material and, if it is too far from the end of the ring, that may ndisallow the gases from escaping from the mould during the casting .Fourthly, nthe investment ring is placed upside down in an oven at a temperature of 600F nfor thirty minutes. During that stage, the wax will burn out from the mould. At nthis point, the temperature should then be increased up to 1,200F for one more nhour, after which the ring is then removed from the oven and placed into the ncasting machine (the casting temperature is vary depending on the type of the nmetal alloy). It is very important to note that when the investment ring is nremoved from the oven, the molten metal should be permitted to flow into the nmould within thirty seconds because if the ring loses the heat, the mould will nthen contract and the dimensions of the mould will changed. After placing the nring in the cast machine, a sufficient amount of metal is then placed into the nmachine whereupon the casting will begin. The metal will be quickly melted and nthen shot through the opening into the mould. The metal will take the same nshape and dimensions of the disappeared wax, which is then called the Lost Wax ntechnique.At this point, the ring is finally removed from the casting machine nand placed at room temperature until it is properly cooled. Then, the ring is nde-invested and the crown/bridge is separated from the sprue, which can then be nrecycled. The crown/bridge is then finished and polished until it shines before nit is sent to the dentist to try in the patient’s mouth during the second nappointment. Little adjustment might be applied upon the final ncementation.30Porcelain Fused to Metal is made similarly, except that after the ncoping procedure, the wax pattern is not waxed up completely; instead, it just nneeds to have a suitable wax adjustment, which allows the porcelain to be built nup over the metal (after transforming the wax into metal with same steps).
Empress
The Empress technique uses the same basic idea and principle nof the lost wax technique, only there is a hole in the investment ring. The nempress system works in a slightly different way as it has a special design, nwhich is the pressure injection. The main principle of this particular system ninvolves leucite-reinforced ceramic, whereupon the ceramic is pressed into the nmould of the artificial tooth in a unique pressable porcelain oven [19]. This nmachine does the work of both the oven and the casting machine together, which nmeans the restoration is melted then pressed into the mould at the same time. nIn addition, the Empress machine can do any job the lost wax may require. It ncan also make any prosthesis for the all-ceramic restoration, such as, single ncrowns and laminated veneers, which is made only from ceramic and did not have nany metals.
In-Ceram
In-ceram has been introduced by Vita; it is also one of the nfamous systems which hasbeen used in full ceramic restoration. Unfortunately, nall-ceramic crown prosthesis requires too much tooth removal. For in-ceram, the ntooth preparation must be 1.5mm arterially and from 1.5mm to 2mm from the nocclusal side; although the allceramic restoration has the best tooth nappearance, it still needs a significant amount of tooth reduction, which most nof people consider being a negative thing.
Dental bridges – construction that are used to substitute nsmall and medium-sized defects of dentition, bounded from both sides by the nteeth. Structurally they consist of crowns with the help of which the denture nis fixed on the abutment teeth, and placed between them the intermediate part nthat replaces the lost teeth. Number of abutment teeth varies (2, 3 and more), ndepending on their masticatory efficiency, condition of periodontal tissues and nsize of replaced defect. In some cases, it is possible to use dental bridges of nconsole type, i.e. with unilateral support, but their functional value is nlower. However, the use of this type of denture is justified in case of the nsmall degree of defect and reluctance of patient to use the removable nconstruction.
Classification:
• nby the way of fixation: removable, non-removable n.
• nby the type of pivot points: crown, portiocrown, stump crown, telescopic crown, pivot crown, inlay, abutment and abutment ncomprising clammers.
• nby the number of pivot points: one-abutment n(cantilever), two-abutment, multiple-abutment.
• nby the location of pivot points: point n(cantilever), line (on one line), polygonal.
• nby the material: metal (chromium-cobalt, nchromium-nickel, silver-palladium alloys, gold of 900 fineness), plastic, nphotopolymer, ceramic, composite (metaloplastic, metal-photopolymer, nmetal-ceramic).
• nby the method of laboratory manufacture: nsoldered, integrally cast, solderless.
• nby the peculiarities of clinical manufacture: nwith preparation of abutment teeth, without preparation of abutment teeth.
• nby the constructional peculiarities: simple, ncomplex (non-disposable).
• nin relation of the washing part to the mucous nmembrane of the alveolar process: washing – with washing pontic (for washing nfood) – side areas; tangent – front area; saddle-shaped.
Stamp-soldered dental bridge with facet intermediate part – nnon-removable orthopedic construction that is used for replacement of included ndeffects in the frontal and lateral areas. Structurally it consists of metal or ncombined metaloacrylic crowns (by Belkin or Borodyuk), between which is placed nthe intermediate part, lined with plastic (standard or cast individually). It nis more aesthetic than integrally-metal, so has wider indications for the use. However, nit is necessary to remember that the crown by Belkin is structurally weaker nthan the metal, and the crown by Borodyuk has worse fixation. Therefore, with nthe large size of the gap between abutment crowns should be preferred the nmetal. Veneering of the intermediate part can be accomplished only from the nvestibular surface, from the vestibular and chewing or completely from all nsurfaces. The degree of veneering is dictated by aesthetic requirements and the nneed to reduce the pressure of chewing on the abutment teeth and nteeth-antagonists due to the fact that plastic has a lower specific mass and nlower hardness than metal. However, the plastic chewing surface is quickly nerased that smooth its relief and reduce interalveolar height. Therefore, this nfeature should be considered in orthopedic treatment of excessive abrasion of nteeth and deformations of bite. On the other hand, the plastic chewing surface nis easy to correct to the downside and to the upside of interalveolar height nthat can be used for gradual normalization of interocclusive relations.
After the end of teeth preparation imprints are removed from nboth jaws. One of them is working, the other – subsidiary, may be working both. nWorking imprint must clearly display the teeth, their cervix, cutting edges and nchewing surfaces, alveolar portion in defect area. Supporting imprint should ncontain imprints of the dentition, especially cutting edges of the front and nchewing surface of the side teeth. With getting imprints is completed the first nstage. According to the imprints are cast models, they are made up in the nposition of central occlusion according to peculiarities specific to each type nof bite, or using wax patterns. When the central occlusion is known, the gypsum nmodels are fixed in such position by binding with thick thread or clueing using na match, filling them with boiling wax.
Models are fixed in occluder, moreover the top model is nfixed to the upper arch, and the lower – to the bottom. Remnants of gypsum are ncut from the models so that the pin of occluder height would bear against the narea. After preparing of models the gypsum is mixed, it is placed on a smooth nsurface and immersed in it the lower frame of the occluder. Then is added a nsmall layer of gypsum and fixed model are situated on them. With the spatula nthe gypsum is smoothed down, and then layers of gypsum are put on the model of nthe upper jaw and lower the upper frame of the occluder. After hardening of ngypsum its remnants are removed, the thread is taken, the occluder is opened nand the modeling of the dentures begins. Then are made modeling of stump of all nabutment teeth, manufacture of imprints, gypsum and metal, stamping of abutment ncrowns. This concludes the first laboratory stage. Crowns are sometimes nbleached, preferably in black form are sent to the hospital, where they nconclude the second or third clinical stages. The last consist of fitting the nabutment crowns, testing the central occlusion and getting the imprint along nwith crowns for the manufacture of intermediate part of dental bridge. After nreceiving the imprint are taken all the basic bits and sent to the laboratory.
If the imprint is of gypsum, it is carefully collected and nglued together. When gluing the imprint, the crowns are carefully established nin its bed, watching that closely adjoiot only to walls, but also in the nregion of the cell. Wax is not poured into the crown only when the crown is nlined. Model is cast and spared from pieces of imprint, compared with the model nof antagonist jaw and plasterer in the occluder. After fixation in the occluder nproceed to model the intermediate part of dental bridge.
The space between crowns is filled with roller made of nwax. Roller should be a little bit taller and wider than the crown. The roller nis installed; the models are closed, allowing getting the imprint of nantagonists on the roller. From the roller with the trowel are modeled teeth, nthen it is marked according to the number of missing teeth, and is started the nmodeling of each tooth, creating the appropriate anatomical shape on the nvestibular and chewing surfaces for premolars and molars for and vestibular, ncutting and oral surfaces for front teeth. When the side of the crown, facing nthe defect, is of small height, from the body of dental bridge on the tongue nside of the crown should be taken the process. This helps to increase the nsurface of connection the crown with the body of the bridge. Technician imodeling of the crown does not model the chewing surface – it is formed whemodeling the intermediate part and is poured together with crown. Thus there is na combination of metal with the crown.
From the point of view the hygiene to dental bridges are nimposed special requirements. Very important is the form of intermediate part nof the bridge and its relation to surrounding tissues of dental bed – mucosa of ntoothless alveolar process, lips, cheeks, tongue. In the tactile form, the nabsence of pressure on the mucosa is checked with probe. If its tip is easily ninjected under the body of the bridge, this means that the pressure on the gums nis absent and at the same time there is no visible crack, which has unaesthetic nlook during smile or conversation. Wash space is enough large, especially othe lower jaw, approximately on the thickness of a match. On the upper jaw the nwash space is a little smaller than on the lower, and in the area of premolars nand canines it can be built deep into the contact with mucosa. After modeling nthe vestibular, chewing and masticatory surfaces, begin to design the parts ndirected to the gums. For this with the sharp trowel is cut the bridge at aangle to the vestibular surface departing from the place of transition of the nchewing surface into the tongue of 2-4 mm. Then, when the wax is cooled, it is removed from the model. On cross-section the form of intermediate part of nthe bridge resembles a triangle.
Then begins the casting process for getting metal parts. This nprocess involves a series of successive operations:
• nmanufacture of wax models of parts;
• ninstallation of sprue making pins and creatioof gating system;
• ncovering of models with fire-resistant layer;
• nformation of the model with fire-resistant mass nin muffel;
• nmelting of wax;
• ndrying of form;
• nmelting of alloy;
• ncasting of alloy;
• nliberation of parts from fire-resistant mass.
All wax compositions and alloys during the transition from nliquid to solid stage are reduced in volume, i.e. shrink. Shrinkage of alloys nis compensated with special compensating masses with double coefficient of nexpansion. Shrinkage of wax compositions is reduced by the formation of ncompounds with the introduction of carnauba, montanic and other types of wax.
Before soldering, for fixation of the bridge is required nplastering. For this is applied gypsum with the addition of pumice, marble ndust, sand. For plastering steel bridges, Citrin mass is used sometimes. The nbridge is dipped in a small amount of fire-resistant mass with upper and nchewing surfaces down. Crowns are filled with mass and is cover the inside nsurface of the cast teeth, leaving open the places of soldering. Then begins nthe process of soldering. Soldering – a connection of metal parts by heating a nhomogeneous alloy with a low melting point. Connecting alloy is called solder.
Solder must meet the following requirements:
• nhave a melting point lower than in base metals non 50-100 degrees;
• nwell spill;
• nwell penetrate into the thickness of base metals n;
• nbe resistant to acids and alkalis;
• nmatch base metals with color;
• nbe resistant to corrosion;
• ndo not give shells and blisters.
Depending on the density and melting point, solders are ndivided into soft and hard. When soldering the connecting parts are solid and nsolders are molded. The metal surfaces to be soldered should be cleaned of noxides and contaminants, for which is used mechanical method of cleaning. Since nthe soldering takes place during heating in an open fire, on the surface of nmetal can be formed oxide cover that prevents the diffuse of solder.
Therefore it is necessary to counteract the formation of noxide cover to the moment of reaching operating temperature. This is achieved nby using different soldering materials, or fluxes. The largest spread has got nborax. When heated the borax absorbs oxygen, thereby preventing it from getting nto the metal and the formation of oxides on its surface. After soldering, the ndenture bridge is immersed with molding mass into cold water, cleaned from nfire-resistant mass, bleached and washed in boiling water. Then start grinding nand polishing.
For whitening products are immersed in the boiling solutioand boiled for about 1 minute. Then the bridge is removed from the solution, nwashed with water, cleaned of scale. After bleaching the bridge is grinded with nvarious circles, bevels, hard and soft brushes. Then polished using a variety nof pasta, depending on the material from which dental bridges are made. Facet nintermediate part is modeled of wax and then replaced with plastic. After npolishing the bridge is washed with water and soap and sent to the clinic for napplying and fixing the bridge in the oral cavity.
