Chapter 12 Inlays, onlays and veneers
Historically, gold was the material of choice for inlays and onlays. The accuracy and malleability of gold made it an ideal material to restore lost tooth structure and protect weakened cusps destroyed by caries and trauma. Although gold is versatile, its use has declined over recent years but is still used by those clinicians who recognize its unique qualities in areas of the mouth where appearance is not important. Modern attitudes to appearance and colour have seen a preference for tooth-coloured materials.
Modern materials utilize the properties of adhesive cements to reduce the necessity for extensive tooth preparation and restoration, so conserving tooth tissue. Both ceramic and composite-based materials are regularly used as an alternative to gold in restoring broken down teeth, and veneers have been designed to improve the appearance of minimally restored teeth.
The most common indication for inlays or onlays is extensively restored and weakened teeth. The distinction between the two designs is unclear but simply those restorations without cuspal coverage that remain within the body of the tooth would be considered inlays (Figure 12.1) whereas onlays replace tooth tissue including cusps (Figure 12.2).
Inlays are usually used when difficulty is anticipated or experienced in obtaining an acceptable contour, contact point and occlusion on a directly placed restoration. Repeated fracture of a directly placed restoration may also indicate the placement of an inlay, and in this situation a gold inlay may be advocated for its strength. Composite and ceramic inlays may also be chosen for their aesthetics in situations where strength is not a major requisite. This can be particularly relevant for the mesial surfaces of upper premolar teeth and the occlusal surfaces of lower teeth when appearance is important to the patient. Increasingly, patients are concerned about the placement of amalgam and request an alternative when the restorations need replacement and in rare situations amalgam replacement is necessary because of lichenoid reactions. In these situations, directly placed composites must be considered the most appropriate material to restore minimal cavities. The necessity of using an indirect material with associated elimination of undercuts and laboratory costs means their indications are limited to more extensive cavities.
The most commonly placed partial coverage extracoronal restoration would be an onlay where weakened tooth structure can be protected without further extensive tooth removal. A common indication for an onlay would be a root-filled posterior tooth where cuspal protection is required. Root canal treatment in posterior teeth is usually the result of caries and restorative procedures and, as such, these teeth are extensively broken down and have weakened cusps. The coronal access cavity needed for a root treatment removes the roof of the pulp chamber, weakening the tooth further, and can leave a limited amount of buccal and lingual tooth tissue which might be completely removed if prepared for a crown (Figure 12.3). Preservation of some part of the buccal and lingual tooth helps to retain the core and reduces the need to consider a post, especially in premolar teeth (Figure 12.4). The three-quarter gold crown preparation on a premolar tooth, particularly when only the buccal cusp remains, will preserve tooth tissue but the colour and appearance of gold within the smile line is considered by some to be a disadvantage (see Chapter 8 and Figure 12.5).
Figure 12.3 The upper left first premolar tooth has an extensive mesial-occlusal-distal (MOD) amalgam restoration; preparation of this tooth for a metal–ceramic crown might result in the loss of the entire buccal cusp, leaving the amalgam unsupported or poorly retained, with potential loss of the restoration. An onlay or inlay preparation would preserve the buccal tissue.
Figure 12.4 Onlays cemented to both premolars restoring teeth which were extensively destroyed by caries. By using an onlay rather than a crown the buccal tooth tissue is preserved, avoiding the need for a full coverage restoration.
Bonding materials (including gold) to a tooth, using adhesive cements, reduces some of the need for conventional principles of retention. Onlays can be considered when there is no or little intracoronal shape to the preparation and where retention is poor. Despite the retention provided by adhesive cement, conventional concepts of tooth preparation for retention should not be ignored and, where possible, should be incorporated into the design of the preparation (Figures 12.2 and 12.6); routine use of adhesive cements to achieve retention poses problems if retrieval is ever necessary.
Whilst gold is gradually being superseded by tooth-coloured materials for the provision of indirect restorations, its use should not be ignored as gold has some advantages over indirect composites and ceramics when used for inlays and onlays. It has high strength, ductility and has the ability to be cast accurately, especially into thin sections. A dentist would be unable to obtain full consent from a patient if gold (with its advantages and disadvantages) was not discussed as a treatment option.
Gold onlays need less occlusal and axial reduction than that needed for tooth-coloured materials, as the latter tend to be more brittle and require greater bulk for strength. The ductility of gold and its ability to be highly polished means that it is not as abrasive to the opposing dentition, especially for those patients with tooth wear. All intra- and extra-coronal restorations made from gold need tooth preparation with near parallel walls and an absence of undercuts (Figure 12.7).
The need for tooth-coloured inlays competes with directly placed composites for aesthetically pleasing tooth-coloured restorations. Laboratory-made indirect composites have a theoretical reduction in polymerization shrinkage as the curing is completed in the laboratory. The curing occurs by subjecting the composite to heat, pressure or intense light, and this higher degree of cure improves strength and reduces wear; the indirect composite inlays/onlays are often referred to as ‘super cured’. This super curing does, however, leave fewer reactive resin groups to bond to the resin luting cement. The accuracy of fit is significantly less than restorations made in either ceramic or gold and they can absorb dietary stains after a few years which significantly affects their appearance.
Ceramic restorations are indicated where the aesthetic demands of the patient are high. The colour matching of modern porcelains allows the margin between the tooth and material to be almost imperceptible. Where there is a slight difference in colour, the margin of a ceramic inlay/onlay will be obvious (see Figure 12.4) as there is a sudden transition from ceramic to tooth. Ceramic materials, unlike directly placed composite, cannot be placed into progressively thinner layers to blend and merge the colour to that of the tooth, due to their brittleness. This progressive transition with direct composite can camouflage the restoration. Ceramic is a hard material and has high wear resistance; however, it can be abrasive to the opposing dentition when used on the occlusal surfaces, especially if the glaze is lost and the restoration has been adjusted.
The manufacturing techniques used for porcelain and indirect composites generally cannot tolerate such parallel preparations as those produced for a gold inlay, and more tapered walls are needed. Undercuts in the tooth preparation can either be eliminated by further tooth preparation or be blocked out with an adhesive tooth-coloured material. All variants of the preparations should avoid sharp line angles, particularly intracoronally (Figure 12.8).