Fixed prosthodontics

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Fixed prosthodontics is the branch of prosthodontics that focuses on dental prostheses that are permanently affixed (fixed). Crowns, bridges (fixed dentures), inlays, onlays, and veneers are some examples of indirect dental restorations. Prosthodontists are dentists who have completed training in this specialty that has been recognized by academic institutes. Fixed prosthodontics can be used to reconstruct single or many teeth, spanning tooth loss areas. The main advantages of fixed prosthodontics over direct restorations are improved strength in big restorations and the possibility to build an aesthetic-looking tooth. The concepts utilised to select the suitable repair, as with any dental restoration, include consideration of the materials to be used, the level of tooth destruction, the orientation and placement of the tooth, and the condition of neighboring teeth.

Contents

Preparation techniques

A large amount of tooth structure is removed irreversibly during crown preparation. All restorations have impaired structural and functional integrity when compared to healthy, natural tooth structures. As a result, if a dental practitioner does not recommend it, crowning a tooth is probably not a good idea. Dentists trained worldwide at different institutions in different times may have varied ways of treatment planning and case selection, resulting in varying treatment recommendations.

Traditionally, crown and bridge work requires more than one visit, and the extra time necessary for the procedure can be a drawback; nevertheless, the improved benefits of such an intervention will generally outweigh these factors.

Dimensions of preparation

When preparing a tooth for a crown, the preparation should, ideally, exist entirely in the enamel. As elaborated below, the amount of tooth structure required to be removed will depend on the material(s) being used to restore the tooth. If the tooth is to be restored with a full gold crown, the restoration need only be .5 mm in thickness (as gold is very strong), and therefore, a minimum of only .5 mm of space needs to be made for the crown to be placed. If porcelain is to be applied to the gold crown, an additional minimum of 1 mm of tooth structure needs to be removed to allow for a sufficient thickness of the porcelain to be applied, thus bringing the total tooth reduction to minimally 1.5 mm. For porcelain or ceramic crowns the amount of tooth reduction is 2 mm. For metal, it is 1 mm.

If there is not enough tooth structure to properly retain the prosthetic crown, the tooth requires a build-up material. This can be accomplished with a pin-retained direct restoration, such as amalgam or a resin-like fluorocore, or in more severe cases, may require a post and core. Should the tooth require a post and core, endodontic therapy would then be indicated, as the post descends into the devitalized root canal for added retention. If the tooth, because of its relative lack of exposed tooth structure, also requires crown lengthening, the total combined time, effort, and cost of the various procedures, together with the decreased prognosis because of the combined inherent failure rates of each procedure, might make it more reasonable to have the tooth extracted and opt to have an implant placed.

Taper

The prepared tooth also needs to possess a certain degree of taper to allow for the restoration to be properly placed on the tooth. There can be no undercuts on the surface of the prepared tooth, as the restoration will not be able to be removed from the die or fit on the tooth (see explanation of lost-wax technique below for an understanding of the processes involved in crown fabrication). Conversely, too much taper will severely limit the grip that the crown has while on the prepared tooth, thus contributing to the failure of the restoration. Generally, 3° of taper around the entire circumference of the prepared tooth, giving a combined taper of 6° at any given sagittal section through the prepared tooth, is appropriate to both allow the crown to fit yet provide enough grip.

Margin

The most coronal position of untouched tooth structure (that is, the continual line of original, undrilled tooth structure at or near the gumline) is referred to as the margin. This margin will be the future continual line of tooth-to-restoration contact and should be a smooth, well-defined delineation so that the restoration, no matter how it is fabricated, can be properly adapted and not allow for any openings visible to the naked eye. An acceptable distance from the tooth margin to the restoration margin is anywhere from 40 to 100 nm. However, the R.V. Tucker method of gold inlay and onlay restoration produces tooth-to-restoration adaptation of potentially only 2 nm (confirmed by scanning electron microscopy), less than the diameter of a single bacterium.

The tooth-to-restoration margin is an unsightly thing to have exposed on the visible surface of a tooth when the tooth exists in the aesthetic zone of the smile. In these areas, the dentist places the margin as far apical (towards the root tip of the tooth) as possible, even below the gumline, though problems may arise when placing the margin too subgingivally (below the gumline). There may be issues in terms of capturing the margin in an impression to make the stone model of the prepared tooth (see stone model replication of tooth in photographs, above). Another important consideration is biological width. Biologic width is the mandatory distance to be left between the height of the alveolar bone and the margin of the restoration, and if this distance is violated because the margin is placed too subgingivally, serious repercussions may follow. In situations where the margin cannot be placed apically enough to provide for proper retention of the prosthetic crown on the prepared tooth structure, the tooth or teeth involved should undergo a crown lengthening procedure.

There are a number of different types of margins that can be placed for restoration with a crown. There is the chamfer, which is popular with full gold restorations, which effectively removes the smallest amount of tooth structure. There is also a shoulder which removes slightly more tooth structure but allows for a thickness of the restoration material, necessary when applying porcelain to a PFM coping or when restoring with an all-ceramic crown (see below for elaboration on various types of crowns and their materials). When using a shoulder preparation, the dentist adds a bevel; the shoulder-bevel margin serves to effectively decrease the tooth-to-restoration distance upon final cementation of the restoration.

Ferrule effect

The most important consideration when restoring with a crown is, the incorporation of the ferrule effect. As with the bristles of a broom, which are grasped by a ferrule when attached to the broomstick, the crown should envelop a certain height of tooth structure to properly protect the tooth from fracture after being prepared for a crown. This has been established through multiple experiments as a mandatory continuous circumferential height of 2 mm; any less provides for a significantly higher failure rate of endodontically treated crown-restored teeth. When a tooth is not endodontically treated, the remaining tooth structure will invariably provide the 2-mm height necessary for a ferrule, but endodontically treated teeth are notoriously decayed and are often missing significant solid tooth structure. Contrary to popular belief, endodontically treated teeth are not brittle after being devitalized according to the following study -CM Sedglay & Messer 1992 Journal of Endodontics. Contrary to what some dentists believe, a bevel is not suitable for implementing the ferrule effect, and a beveled tooth structure may not be included in the 2 mm of required tooth structure for a ferrule.

Restoration types

Fabrication of feldspathic porcelain crowns on the dental model, then cementing them on the upper central teeth using a luting agent Feldspathic VM9 Porcelain Crowns -side view.jpg
Fabrication of feldspathic porcelain crowns on the dental model, then cementing them on the upper central teeth using a luting agent
An inlay fitted into the model and ready to be cemented clinically on the natural tooth Emax Press fabricating an inlay.jpg
An inlay fitted into the model and ready to be cemented clinically on the natural tooth
A sample of feldspathic onlay made with refractory die technique using VM9 vita porcelain Feldspathic onlay.jpg
A sample of feldspathic onlay made with refractory die technique using VM9 vita porcelain

Crown

A crown is used to cover a tooth and may be commonly referred to as a "cap." Traditionally, the teeth to be crowned are prepared by a dentist, and records are given to a dental technician to construct the prosthesis. The records include models, which are replicas of a patient's teeth, and the impressions used to make these models. There are many different methods of crown fabrication, each using a different material. Some methods are quite similar and utilize either very similar or identical materials. Crowns may be made of gold or other similar metals, porcelain, or a combination of the two. Crowns made of Zirkonia Oxide are being made more popular due to their high translucency and durability as opposed to the chipping disadvantages of porcelain crowns.

Bridge

A bridge is used to span, or bridge, an edentulous area (space where teeth are missing), usually by connecting to fixed restorations on adjacent teeth. The teeth used to support the bridge are called abutments. A bridge may also refer to a single-piece multiple-unit fixed partial denture (numerous single-unit crowns either cast or fused together). The part of the bridge which replaces a missing tooth and attaches to the abutments is known as a "pontic". For multiple missing teeth, some cases may have several pontics.

Inlay

An inlay is a restoration that lies within the confines of the cusps. These restorations are considered to be more conservative than onlays or crowns because less tooth structure is removed in preparation for the restoration. They are usually used when tooth destruction is less than half the distance between cusp tips.

Onlay

An onlay is a method of tooth restoration, that covers, protects, or reinforces one or more cusps. Onlays are methods for restoring teeth in an indirect way. Onlays are often used when teeth present extensive destruction due to caries or to trauma.

Veneer

A veneer is a thin layer of restorative material placed over a tooth surface, either to improve the esthetics of a tooth or to restore a damaged tooth surface. Materials used for veneers may include composite and porcelain. In some cases, removal of tooth structure is needed to provide sufficient space for the veneer, whereas sometimes a restoration may be bonded to a tooth without preparation of the tooth.

Dental implants

Screw-retained restoration

The main benefit of screw retention is the retrievability of the restoration.

This does not exist in common fixed prosthodontics on teeth. As a result, any complication with the restoration is easily addressed. The screw-retained restoration can be easily removed which allows to repair or examine the soft tissue and direct visualization of the implant. This also negates the need to remake the restoration if an abutment screw or prosthetic screw loosens. This eliminates the potential complications associated with excess residual cement—often difficult to completely remove with a cement-retained crown. The screw-retained restoration lacks glue and hence is preferable for the health of the gingiva and the implant.

Cement-retained restoration

The cement-retained restorations ensure maximum aesthetics but have two downsides.

One, the restoration is cemented to an abutment that is screw retained. If the abutment screw becomes loose the final restoration cannot be removed without destroying it in many instances. This results in a remake and increased cost. Two, excess cement along the implant surface can potentially act as a medium for colonization by bacteria and can jeopardize the attachment, ultimately resulting in implant failure. In certain instances, cement retention is the only option.

See also

Related Research Articles

Cosmetic dentistry is generally used to refer to any dental work that improves the appearance of teeth, gums and/or bite. It primarily focuses on improvement in dental aesthetics in color, position, shape, size, alignment and overall smile appearance. Many dentists refer to themselves as "cosmetic dentists" regardless of their specific education, specialty, training, and experience in this field. This has been considered unethical with a predominant objective of marketing to patients. The American Dental Association does not recognize cosmetic dentistry as a formal specialty area of dentistry. However, there are still dentists that promote themselves as cosmetic dentists.

<span class="mw-page-title-main">Bridge (dentistry)</span> Dental restoration for missing teeth

A bridge is a fixed dental restoration used to replace one or more missing teeth by joining an artificial tooth definitively to adjacent teeth or dental implants.

Dental restoration, dental fillings, or simply fillings are treatments used to restore the function, integrity, and morphology of missing tooth structure resulting from caries or external trauma as well as to the replacement of such structure supported by dental implants. They are of two broad types—direct and indirect—and are further classified by location and size. A root canal filling, for example, is a restorative technique used to fill the space where the dental pulp normally resides.

<span class="mw-page-title-main">Dental technician</span> Technician working on dental appliances

A dental technician is a member of the dental team who, upon prescription from a dental clinician, constructs custom-made restorative and dental appliances.

<span class="mw-page-title-main">Dental implant</span> Surgical component that interfaces with the bone of the jaw

A dental implant is a prosthesis that interfaces with the bone of the jaw or skull to support a dental prosthesis such as a crown, bridge, denture, or facial prosthesis or to act as an orthodontic anchor. The basis for modern dental implants is a biological process called osseointegration, in which materials such as titanium or zirconia form an intimate bond to the bone. The implant fixture is first placed so that it is likely to osseointegrate, then a dental prosthetic is added. A variable amount of healing time is required for osseointegration before either the dental prosthetic is attached to the implant or an abutment is placed which will hold a dental prosthetic/crown.

<span class="mw-page-title-main">Crown (dental restoration)</span> Dental prosthetic that recreates the visible portion of a tooth

In dentistry, a crown or a dental cap is a type of dental restoration that completely caps or encircles a tooth or dental implant. A crown may be needed when a large dental cavity threatens the health of a tooth. Some dentists will also finish root canal treatment by covering the exposed tooth with a crown. A crown is typically bonded to the tooth by dental cement. They can be made from various materials, which are usually fabricated using indirect methods. Crowns are used to improve the strength or appearance of teeth and to halt deterioration. While beneficial to dental health, the procedure and materials can be costly.

<span class="mw-page-title-main">Inlays and onlays</span> Restoration procedure in dentistry

In dentistry, inlays and onlays are used to fill cavities, and then cemented in place in the tooth. This is an alternative to a direct restoration, made out of composite, amalgam or glass ionomer, that is built up within the mouth.

<span class="mw-page-title-main">Veneer (dentistry)</span> Layer of material placed over a tooth

In dentistry, a veneer is a layer of material placed over a tooth. Veneers can improve the aesthetics and function of a smile and protect the tooth's surface from damage.

A dental emergency is an issue involving the teeth and supporting tissues that are of high importance to be treated by the relevant professional. Dental emergencies do not always involve pain, although this is a common signal that something needs to be looked at. Pain can originate from the tooth, surrounding tissues or can have the sensation of originating in the teeth but be caused by an independent source. Depending on the type of pain experienced an experienced clinician can determine the likely cause and can treat the issue as each tissue type gives different messages in a dental emergency.

<span class="mw-page-title-main">Luting agent</span>

A luting agent is a dental cement connecting the underlying tooth structure to a fixed prosthesis. To lute means to glue two different structures together. There are two major purposes of luting agents in dentistry – to secure a cast restoration in fixed prosthodontics, and to keep orthodontic bands and appliances in situ.

<span class="mw-page-title-main">CAD/CAM dentistry</span> Computer-aided design and manufacturing of dental prostheses

CAD/CAM dentistry is a field of dentistry and prosthodontics using CAD/CAM to improve the design and creation of dental restorations, especially dental prostheses, including crowns, crown lays, veneers, inlays and onlays, fixed dental prostheses (bridges), dental implant supported restorations, dentures, and orthodontic appliances. CAD/CAM technology allows the delivery of a well-fitting, aesthetic, and a durable prostheses for the patient. CAD/CAM complements earlier technologies used for these purposes by any combination of increasing the speed of design and creation; increasing the convenience or simplicity of the design, creation, and insertion processes; and making possible restorations and appliances that otherwise would have been infeasible. Other goals include reducing unit cost and making affordable restorations and appliances that otherwise would have been prohibitively expensive. However, to date, chairside CAD/CAM often involves extra time on the part of the dentist, and the fee is often at least two times higher than for conventional restorative treatments using lab services.

<span class="mw-page-title-main">Crown lengthening</span> Dental procedure

Crown lengthening is a surgical procedure performed by a dentist, or more frequently a periodontist, where more tooth is exposed by removing some of the gingival margin (gum) and supporting bone. Crown lengthening can also be achieved orthodontically by extruding the tooth.

A post and core crown is a type of dental restoration required where there is an inadequate amount of sound tooth tissue remaining to retain a conventional crown. A post is cemented into a prepared root canal, which retains a core restoration, which retains the final crown.

<span class="mw-page-title-main">Root canal treatment</span> Dental treatment

Root canal treatment is a treatment sequence for the infected pulp of a tooth which is intended to result in the elimination of infection and the protection of the decontaminated tooth from future microbial invasion. Root canals, and their associated pulp chamber, are the physical hollows within a tooth that are naturally inhabited by nerve tissue, blood vessels and other cellular entities. Together, these items constitute the dental pulp.

A resin-retained bridge is a bridge replacing a missing tooth that relies for its retention on a composite resin cement. It is one of many available dental restoration methods which is considered minimally invasive and conservative of tooth tissue. The resin-retained-bridge has gone through a number of iterations. Perhaps the best known is the Maryland bridge and other designs used in the past include the Rochette bridge. The five-year survival rate is around 83.6% and the ten-year rate at 64.9%. The case selection is important and as with any dental prosthesis, good oral hygiene is paramount for success. In recent years, the indications for the use of resin-retained-bridges have diminished significantly and there have been changes in the principles underpinning their design. Resin-retained-bridges should be considered when a fixed prosthesis retained by natural teeth is required. The use has been driven by the advent of evidence-based dentistry showing the benefits to patients of reduced tooth preparation and the importance of an intact enamel structure for the long-term health of the teeth. The bridge is currently in favour in the United Kingdom for these reasons. Indeed, recent contemporary research shows resin retained bridges have better success rates than implants and are a cheaper alternative.

<span class="mw-page-title-main">Dental prosthesis</span> Intraoral device for reconstructing missing teeth

A dental prosthesis is an intraoral prosthesis used to restore (reconstruct) intraoral defects such as missing teeth, missing parts of teeth, and missing soft or hard structures of the jaw and palate. Prosthodontics is the dental specialty that focuses on dental prostheses. Such prostheses are used to rehabilitate mastication (chewing), improve aesthetics, and aid speech. A dental prosthesis may be held in place by connecting to teeth or dental implants, by suction, or by being held passively by surrounding muscles. Like other types of prostheses, they can either be fixed permanently or removable; fixed prosthodontics and removable dentures are made in many variations. Permanently fixed dental prostheses use dental adhesive or screws, to attach to teeth or dental implants. Removal prostheses may use friction against parallel hard surfaces and undercuts of adjacent teeth or dental implants, suction using the mucous retention, and by exploiting the surrounding muscles and anatomical contours of the jaw to passively hold in place.

In dentistry, an abutment is a connecting element. This is used in the context of a fixed bridge, partial removable dentures and in implants. The implant fixture is the screw-like component that is osseointegrated.

A crownlay is a type of dental restoration.

An Endodontic crown or endocrown is a single prostheses fabricated from reinforced ceramics, indicated for endodontically treated molar teeth that have significant loss of coronal structure. Endocrowns are formed from a monoblock containing the coronal portion invaded in the apical projection that fills the pulp chamber space, and possibly the root canal entrances; they have the advantage of removing lower amounts of sound tissue compared to other techniques, and with much lower chair time needed. They are luted to the tooth structure by an adhesive material. The ceramic can be milled using computer-aided techniques or molded under pressure. Endocrowns can be an alternative to conventional crown restorations.

<span class="mw-page-title-main">Overdenture</span> Removable dental prosthesis

Overdenture is any removable dental prosthesis that covers and rests on one or more remaining natural teeth, the roots of natural teeth, and/or dental implants. It is one of the most practical measures used in preventive dentistry. Overdentures can be either tooth supported or implant supported. It is found to help in the preservation of alveolar bone and delay the process of complete edentulism.

References