All-Ceramic inlays and onlays for posterior teeth

Direct intra-coronal restorations for posterior teeth

The impact of the loss of tooth tissue on the strength of that which remains can be significant. Vale in 1956 was one of the first investigators to describe the weakening of the tooth resultant from Class II cavity preparations.¹ This has been verified by numerous researchers over the years, but it cannot easily be related to clinical outcomes. The point at which the risk of fracture becomes likely has never been clearly defined, although it is evident that root-treated posterior teeth are particularly at risk unless the final coronal restoration does something to protect the remaining tooth tissue.² The continued development in adhesive materials and the decline in dental caries have encouraged a more conservative approach to cavity preparation which has both pulpal and structural benefits. The ability to produce adhesion between a restorative material and tooth structure is potentially a key element in developing or retaining strength in what remains of the tooth. However, this has not been quantified clinically, but there can be little argument that the principle is correct.

Where the loss of tooth structure has been significant, there may be benefits in considering the use of an indirect restoration in order to protect the tooth structure that remains. Under these circumstances it is clearly highly desirable that minimal further loss of tooth structure should take place whilst good tooth preparation requires high quality manual skills in order to create the best conditions for the final restoration. However, research has failed to link the quality of the performance of the dentist and the dental technician to the outcome but there is an inescapable logic in doing so.

Two relatively recent systematic reviews report that success rates for silver amalgam restorations remain higher than those for direct resin composite when used to restore posterior teeth.^3,4 However, continued materials' development and increasing usage leading to improvements in clinical techniques have produced change, with studies now reporting higher success rates for direct composite restorations than for silver amalgam.^5,6 This trend is likely to continue with the increasing emphasis on minimally invasive techniques and the reduction in the use of silver amalgam. The material of choice for restoring small to moderate-sized cavities in posterior teeth is direct resin composite, notwithstanding the need for good isolation and careful operative technique. There is no absolute definition or description of what constitutes an extensively damaged posterior tooth, and the term remains one dependent on clinical opinion. The damage is often described in terms of loss of marginal ridges or cuspal elements. However, perhaps a more significant descriptor is the amount of dentine remaining at the base of the cusps. It is the cuspal bases that provide the support for the coronal parts of the tooth or the restoration. This is one major reason for recommending that when a tooth is considered to be so damaged that an indirect restoration is required, it is good practice to remove the existing coronal restoration in order to be able to evaluate the quantity of tooth remaining at the cuspal bases. This is also important when a tooth requires a core to support a crown as it allows retention for the core to be created in areas that will be less affected by the crown preparation itself.

The principles of restoration for the extensively damaged posterior tooth should be to:

Preservation of tooth structure

This means keeping as many reasonably intact surfaces as possible. It seems reasonable that a tooth with relatively small amounts of missing tooth structure should be restored with an adhesive direct composite resin restoration. This can apply even to teeth where a significant amount of tooth structure has been lost internally, ie following root canal treatment. It is the adhesive nature of restoration that is important, despite there being only weak data to support this approach^7,8Figure 1 shows a maxillary first premolar which had been root-treated and where there was a disto-occlusal cavity that required restoration. Given the amount of tooth tissue evident externally, it would appear grossly destructive to provide a crown for this tooth even though that is what the literature might recommend as best practice.

However, the ability of the operator, the size of the cavity and the long-term performance of the restorative material have relevance in reaching this decision. Whilst Figure 1 shows a reasonable approach because the direct restoration is relatively small, the situation is not the same as the restoration becomes larger. Under these circumstances greater requirements are placed on the adhesive, the restorative material and the clinical expertise of the operator. Figure 2a shows a large MO cavity in a root-treated maxillary first molar which was then restored with a directly placed composite resin (Figure 2b). This might seem like a reasonable approach but, as the form of the restoration shows, the result is less than ideal. The proximal contact is too broad, with inevitable consequences for the interdental papilla,⁹ whilst the occlusal form is poor. The relevance of the latter has frequently been ignored. The reasons for needing good occlusal form are to provide stability, ie tooth contacts that do not change very much with time and also to ensure that the likelihood of the development of occlusal interferences can be minimized. These criteria are linked with the intention that loads between opposing teeth in any mandibular-maxillary relationships are minimized. This also requires stability in the restorative material, no wear of the antagonist tooth and effective occlusal contacts from the time of placement. It is evident from clinical observation that this is more difficult when the size of a direct restoration made in composite resin increases.

Indirect restorations

It is not possible to specify when a tooth should be restored with an indirect restoration. It has been recommended that when a cavity becomes too large for directly-placed composite resin to be advisable, an indirect composite resin inlay might be used. Construction of the restoration away from the oral cavity gives the opportunity to control the form of restoration better and to increase the degree of cure of the composite material. With the composite resin being cured before cementation, the stresses affecting the tooth produced by polymerization shrinkage are limited to the composite luting agent used to cement the restoration. The use of indirect inlays requires a cavity without undercuts, although modest amounts of blocking out with a resin-modified glass ionomer cement would seem to be appropriate if necessary. The challenge in using indirect composite inlays is that there is little data which suggest that they perform better than those placed directly.^10,11 Their occlusal form will undoubtedly be better, which improves the prospects for control of the occlusal contacts and therefore loading, but there appears to be no translation from the theoretical benefits into improved outcomes. Consequently, it is difficult to recommend indirect composite inlays for regular use in larger intra-coronal cavities in posterior teeth. Figures 3a–c show two maxillary premolar teeth where worn MOD direct composite resin restorations were replaced by indirect composite inlays.

A more suitable material for a non-metallic indirect restoration for an extensively damaged tooth may be dental porcelain. It offers better physical properties than resin composite and can be fabricated to provide excellent fit and good aesthetics. Porcelain inlays were described as early as 1857;¹² these inlays were made of feldspathic porcelain fired on a platinum foil burnished into a replica of the cavity preparation. Construction was challenging, and good fit was difficult to achieve. Historically, these restorations would have been cemented with a silico phosphate cement which retained the restoration mechanically rather than adhesively. The subsequent development of adhesion, the ability to etch the fitting surface of the restoration with hydrofluoric acid, improved fabrication techniques and developments in dental ceramics have made these restorations useful and predictable.

Etching of porcelain with hydrofluoric acid was first described by Calamia in 1983 to facilitate the cementation of porcelain veneers with composite resin.¹³ A major development in the available porcelain materials suitable for use as inlays and onlays for posterior teeth has been the introduction of leucite-reinforced ceramic that makes use of either a lost-wax technique and pressing of the ‘green’ ceramic or through milling and CAD-CAM processes. Other porcelains are available, including traditional feldspathic porcelain, which now has little indication for use in posterior teeth due to its limited physical strength: zirconia is a further option with good mechanical properties but presents challenges in achieving a predictable bond with composite-luting agents.

Inlay or onlay

The principle for the restoration of an extensively damaged posterior tooth is protection of the remaining tooth structure whilst preserving as much useful tooth structure as possible. In traditionally cemented fixed prosthodontics, this has required reduction of the cuspal elements of the tooth and their coverage by the restorative material. Such extra-coronal restorations have been demonstrated to control stresses within the remaining tooth structure more effectively than when an inlay is used.¹⁴ However, these principles were developed before adhesion to tooth structure and the ceramic restoration could be achieved with composite resin.

An onlay is not only able to limit stresses but permits full control of the form of the occlusal surface, which is only partially possible when an inlay is used. However, there may be instances where much of the occlusal surface and the resultant tooth contacts are satisfactory and simply to replace what is missing becomes an attractive conservative option when the restoration can also be bonded to the cavity. Figure 4a shows a maxillary first molar which had developed a reversible pulpitis and transillumination (Figure 4b) followed by removal of the amalgam restoration revealing a crack mesially (Figure 4c). Given the reasonable occlusal form and the acceptable occlusal contacts, both in the intercuspal position and on excursive mandibular movements, it was decided to restore the tooth with an indirect pressed ceramic inlay. The cavity was modified to allow a path of withdrawal although the axial wall taper was maintained at a low value of less than ten degrees. There was already sufficient depth to allow adequate strength for the porcelain with a minimum of 2 mm in the area of the central fossa being considered appropriate. The proximal boxes were provided with flares in order to release undercuts present cervically. The completed preparation shown in Figure 4c demonstrates good retention and resistance form: this might seem unnecessary when the mode of attachment of the inlay to the tooth is adhesive. However, the presence of retention and resistance may reduce the stresses placed on the adhesive bond. Whilst the latter has proved generally predictable and long lasting, there is evidence that the adhesive bond undergoes degradation with time,^15,16 therefore minimizing stresses at the restoration-tooth structure interface would represent good practice. Following fabrication of the leucite-reinforced ceramic inlay and try-in, the fitting surface of the inlay was etched with hydrofluoric acid, cleaned and then silanated before being cemented under rubber dam isolation using a multi-stage etching and bonding procedure with a composite luting agent. Very minor finishing was required following a small amount of occlusal refinement and the surface of the restoration was re-polished using abrasive rubber points under a water spray. The final restoration is shown in Figure 4d.

The decision whether to make a porcelain inlay or onlay should be based on information derived from the literature, although at present this is difficult to do. The majority of the published papers do not differentiate between ceramic inlays and onlays in terms of their survival and complications. For both types, survival is approximately 90% at 10 years. The major causes of failure are fracture of the ceramic and marginal discoloration whilst the incidence of recurrent caries is low.^17,18 Therefore, the choice between an inlay and an onlay is clinically based and relates to the factors outlined earlier.

Ceramic onlays

The requirements for a ceramic onlay preparation are the creation of sufficient occlusal reduction which, for pressed ceramic, should be a minimum of 2 mm, but is increased to 2.5 mm over the supporting (functional) cusp. The finish line on the axial surface adjacent to the supporting cusp should be a heavy chamfer of just under 1.0 mm width and located beyond the site of any occlusal contact. The finishing line on the axial surface adjacent to the non-supporting cusp should be as close to a right angle as possible whilst still maintaining the form of the cuspal inclines. The palatal/lingual and buccal finish lines should blend into the proximal boxes which should present flat floors and buccal and lingual flares. There should be no sharp angles in the preparation, although good parallelism in the isthmus and internally within the proximal boxes is advisable. Figures 5a–c show an early preparation for a ceramic onlay. The patient had presented with fracture and loss of the palatal cusp of the maxillary second premolar (Figure 5a) and the tooth had previously been restored with an MOD amalgam restoration: the tooth was pulpally vital and free from dental caries whilst the occlusion appeared stable and there was no evidence of parafunctional activity. The options for treatment were to place a core and make a partial coverage gold veneer or a porcelain fused to metal crown. The patient did not wish for any display of metal, whilst the operator wished to avoid a metal-ceramic crown preparation which would have removed most of the intact buccal wall of the tooth, leaving little coronal dentine. The decision was made to make a porcelain onlay and this procedure took place in the early 1990s with feldspathic porcelain being the only ceramic possibility. The completed preparation is shown in Figure 5b. The amalgam restoration was removed and the buccal and palatal cuspal inclines on the occlusal surface reduced to create 2 mm of clearance from the opposing tooth. The patient had group function on a working mandibular movement and, in order to provide sufficient strength for the onlay, 2 mm of space was created over the buccal cusp. Palatally, a supporting cusp bevel was made giving 2.5 mm of inter-occlusal space before creating a wide chamfer of 1 mm at the termination of the palatal fracture line: the palatal chamfer blended mesially and distally into the proximal boxes. The proximal boxes were modified to provide a path of insertion and some resistance form before flaring their buccal and palatal margins and blending these into the palatal and buccal finish lines. Any undercuts in the isthmus area and parts of the proximal boxes were blocked out with glass ionomer cement, although currently resin-modified glass ionomer cement is the material of choice for this purpose as its physical properties are better. In Figure 5b showing the completed preparation, it should be noted that the junctions between the reduced buccal cusp and the mesial and distal boxes are sharper than would be currently recommended if stresses are to be minimized and the laboratory fabrication not made more difficult. Figures 5 c and d show the etched and cemented onlay, the buccal finishing line is just visible on close inspection.

Given the lack of clinical data linking preparation design to outcome, it is not surprising that variations are described. The most significant of these is based on the principle that, because retention is achieved by the composite luting agent, stresses will be minimized and fabrication facilitated by making preparations which have greatly reduced resistance form. An example of such a preparation of a mandibular right first molar is shown in Figures 6 a–d. In this instance, there is appropriate occlusal reduction and the functional cusp bevel has been created, but there is no isthmus and the proximal boxes are rounded in form. These blend buccally into the 1 mm chamfer at the termination of the functional cusp bevel and lingually into the reduced lingual cusp where the finishing line was created by simply reducing the cuspal height. Such a design of preparation is attractive as it is relatively simple to execute and also facilitates the fabrication process for the dental technician. However, as there is little geometric retention and resistance form, it places great reliance on the bond between the composite-luting agent, the etched porcelain and the tooth structure which is predominantly dentine. Given the questions remaining over the stability of the bond to dentine, the use of a preparation design with greater resistance form seems to be prudent.

Onlays can appear to be aggressive preparations but it is important to bear in mind that it is the dentine at the base of the cusps which is important in maintaining strength of the tooth. If the diagrams in Figures 7a and b are compared, it is easy to conclude that the full crown preparation removes more tooth structure, both axially and cervically, and that that is likely to have an adverse effect on the strength of the remaining coronal tissues, particularly when there is a core present. In the onlay preparation more dentine is retained cervically in the region of the cuspal bases. Ceramic onlays are of particular use for premolars which require an indirect restoration and success rates are higher for these teeth than for molars. Figures 8a and b show the maxillary and mandibular arches of a patient who required a full reconstruction at an increased occlusal vertical dimension. The patient was content with the proposal that gold veneers were used for the molar teeth but requested that no metal be visible on the premolar teeth. Full metal-ceramic or all-ceramic crowns would have been unnecessarily destructive so pressed ceramic onlays were used. All the premolars were restored with leucite-reinforced ceramic onlays, the post-restoration occlusal views of both arches are shown in Figures 8c and d. The right maxillary premolars are shown in Figure 9a, the onlay preparations are illustrated in Figures 9b and c. The occlusal and buccal views of the onlays are shown in Figures 9d and e.

The clinical limitations of porcelain onlays are not completely established. The risk of fracture of the porcelain is clear but it is a benefit that, under these circumstances, re-restoration of the tooth is likely to be feasible. Figure 10a shows a maxillary first molar following root canal treatment. The intra-coronal amalgam restoration is large and a final restoration which protects the remaining tooth structure and controls the occlusal contacts is required. In making an assessment of the type of restoration indicated, it was necessary to remove the amalgam restoration to evaluate the quantity and location of the remaining tooth structure. Figure 10b shows the tooth with the restoration removed, if the imaginary outline form of a metal-ceramic crown preparation is superimposed on the tooth. It can be concluded that virtually nothing will remain of the buccal cusps whilst, not only are the proximal surfaces extensively damaged, but the disto-palatal cusp has also been lost. This tooth was clearly extensively damaged, and the presence of a mesio-distal crack was noted: it was concluded that a crown preparation would remove a significant amount of the useful remaining tooth structure so that the tooth was prepared for a pressed ceramic onlay. The gutta percha in the pulp chamber was sealed with resin-modified glass ionomer cement and the weakened buccal wall had been reduced in height until it was considered to have sufficient thickness to minimize the chances of fracture (Figure 10c). Palatally, occlusal reduction and a supporting cusp bevel were made and, apical to the supporting cusp bevel, a wide chamfer finishing line was placed with the buccal and palatal finish lines being blended into the proximal boxes. Retention and resistance form were developed by maintaining minimal taper between the buccal and palatal walls internally. The completed preparation is shown in Figure 10d. The vulnerability for the final restoration in this tooth must be the floor of the distal box which is no longer in enamel but in dentine/cementum. This constitutes a risk factor for adhesively retained restorations.^15,16 This risk is not mitigated by building up the floor of the proximal box with composite resin or a resin-modified glass ionomer cement as the interface with the dentine/cementum will still be vulnerable

Temporization of teeth prepared for ceramic onlays is challenging as a good temporary restoration is required for patient comfort and stability both occlusally and proximally whilst its form should facilitate plaque control procedures. The clinician would like the temporary to be both stable and easy to remove. In addition, temporary cements containing eugenol should be avoided otherwise these will contaminate the tooth structure and will interfere with the curing of the composite luting agent. A decision must also be made about the need for dentine sealing prior to making the impression.¹⁹ A stable temporary restoration can be made from a bisacrylic temporary crown and bridge material as long as a silicone matrix made either from a diagnostic wax-up or directly from the tooth prior to preparation is used. The temporary onlay is trimmed, adjusted and polished prior to cementation. It can be cemented with an eugenol-free temporary cement, although sometimes this may not provide sufficient retention. An alternative is to cement the temporary restoration with flowable composite resin on the unetched and unsealed dentine and enamel. This method retains the temporary onlay effectively and, on removal of the restoration, the flowable composite comes away cleanly from the tooth being better attached to the bisacrylic.

Cementation is an important part of the procedure and good isolation is essential, with the use of rubber dam being necessary. Figure 10e shows the maxillary first molar after removal of the temporary onlay: it can be seen that the interdental papilla between the first and second molars is inflamed. It is hard to see how the restoration can be bonded and cemented effectively unless a rubber dam is placed. The tooth is cleaned with a pumice and water slurry prior to cementation to ensure that any biofilm is removed. The fit, contour, proximal contacts and the occlusion will all have been verified prior to placement of the rubber dam and the fitting surface of the leucite-reinforced onlay etched with hydrofluoric acid and silanated prior to cementation. The tooth structure is etched and the adhesive applied, preferably using a multi-stage system. The etched and silanated fitting surface of the onlay is coated with the luting cement and the restoration carefully seated, with an instrument being used occlusally to avoid coronal rebound. The excess cement is removed with a disposable brush or sponges before initiating the final curing process with the manufacturer's recommendation being followed. The presence of the rubber dam makes removal of the cured cement easier from all areas as the excess cement remains on the oral side of the rubber dam. Once the excess cement has been removed, the rubber dam is also removed and the occlusion verified, together with any further minor finishing being carried out. The final pressed ceramic onlay is shown in Figures 10f and g; the occlusal form is broader than would be considered optimal which leads to a flatter occlusal surface.

The full crown

The ceramic onlay represents a suitable method for restoring both moderately and more extensively damaged teeth but has limitations when there is insufficient tooth structure for adhesion, and when higher levels of loading are anticipated, such as those patients who demonstrate parafunctional activity or where contacts between opposing posterior teeth are significantly reduced. There can be no clearly defined point when a crown becomes a better option: research has been reported which seeks to develop a tooth restorability index,²⁰ but no useful correlation has been demonstrated between the amounts of tooth structure remaining and clinical outcomes. However, it is important to bear in mind that it is the tooth structure that provides support for the restoration. As a core beneath an indirect restoration becomes larger, it has to take on an increasingly structural role in supporting the final restoration. Clinicians have a tendency to assume that, once a core has been placed, the tooth is essentially ‘as good as new’. This is clearly not the case as all materials used for cores have inferior physical properties when compared with tooth structure.

The full crown, whilst providing protection for what remains, causes loss of additional tooth structure and increased risk of pulpal damage. The traditional, by some considered old-fashioned, three-quarter gold veneer is a demanding preparation but is able to fulfil more completely the requirements for a satisfactory protective restoration as compared with a full coverage crown, particularly those made of metal-ceramic or all-ceramic. Despite all the advances in alternatives to metal restorations, it remains the case that, if maximum predictability is required, cast gold restorations providing either partial or full coverage continue to be the restoration of choice from the perspective of oral health.²¹ However patients' demands and dentists' inclinations have resulted in the partial metal veneer becoming virtually obsolete.

The principles of preparation for crowns are well established.²² The clinician should bear these in mind, but just as importantly must remember that the support for the restoration is derived from the remaining tooth structure. A crown is broadly only indicated for teeth that are extensively damaged and therefore will usually have relatively large cores which are used to replace the missing tooth structure and to provide a foundation for the final restoration. The design of the crown preparation should take into account the amount and location of the remaining tooth structure and also where retention for the core may be obtained. The use of silver amalgam as a core material has declined, with composite resin becoming the popular choice: with composite resin, use should be made of adhesion, but it should be borne in mind that the majority of the tissue which remains once the preparation is completed will be dentine. Adhesion to dentine is less predictable than to enamel,^15,16 it is therefore advisable to provide mechanical retention for the core as well as making use of adhesive techniques.

Crowns receive loading in function and therefore stresses within the restoration, the tooth structure and the cement are inevitable. One particular location for the concentration of stress is at the margins of restorations and this will include both the core and also the finishing line of the crown preparation. It is therefore important that the margin of the crown preparation is placed apical to the termination of the core (Figure 11). This was termed the ferrule effect by Sorensen in 1990.²³ A similar finding was made in a lesser known study by Hoag and Dwyer²⁴ where, with large cores, placement of the margin of the crown 2 mm apical to a core resulted in greater resistance to failure in their laboratory tests on extracted extensively damaged teeth. The larger the core, the more important this principle, as it better transfers the stresses to the tooth structure rather than allowing them to be concentrated within the core.

Conclusions

The reduction in dental caries, developments in dental materials and the increasing requirements of patients for aesthetic restorations have changed the pattern of dental delivery. There is a range of options for the restoration of more extensively damaged posterior teeth. The use of indirect ceramic partial coverage restorations provides a predictable restoration which is able to restore form, is conservative of structurally important dentine and will protect the remaining tooth structure whilst satisfying patients' aesthetic requirements.