Aesthetic resin onlay restorations: ‘rationale and methods’

Adhesive onlay restorations (metallic, ceramic and resin onlays)

The use of resin composite as a material for the fabrication of adhesive onlay restorations versus the use of metallic alloys offers the distinct advantages of:

However, resin-based onlays do not offer the same level of strength in thinner sections as do metallic-based materials. Therefore, a greater level of tooth reduction is required to accommodate a resin onlay restoration, so as to attain an adequate level of structural integrity to withstand functional, occlusal loads. According to Yap,7 the occlusal clearance required for an adhesive onlay fabricated from a Co-Cr alloy (which offers a high level of rigidity) is in the range of 0.7 to 1.0 mm. In contrast, an occlusal clearance of at least 1.5 mm should be provided for a resin composite onlay.

The type of resin composite used to fabricate onlay restorations may also be a critical factor to their long-term success. Wear and bulk fracture, respectively, have been reported to be areas for concern, in particular where micro-filled materials have been used to fabricate both direct and indirect resin onlays for the management of posterior toothwear.10 The use of hybrid formulations for the fabrication of direct resin onlay restorations (or the management of toothwear), however, appears to be associated with a more superior prognostic outcome.11

When compared to ceramic-based onlay restorations, resin-based onlays are less abrasive towards antagonistic tooth surfaces and offer a greater degree of flexural strength which, in contrast to ceramic materials, are vulnerable to brittle fracture, or may cause wear or chipping of the opposing hard tissue surfaces.12

Wear of resin lutes used to place ceramic onlays also appears to be a concern.12 The latter may be due to the lower level of elasticity offered by ceramic restorations13 (which may lead to greater force transmission to the lute when occlusal loads are applied than would perhaps be the case were a more flexible, stress absorbing restorative material, such as composite resin, may be used instead).

The superior ability of resin composite onlays to absorb flexural forces in response to occlusal loading14 may also account for the high rate of success reported with the application of composite resin onlays applied (either directly15 or indirectly16) for the management of posterior teeth presenting with incomplete fractures.

The application of direct resin onlays without any, or minimal, tooth preparation (placed in supra-occlusion) has recently been described in the dental literature, as being a possible means for treating cracked tooth syndrome in an ultra conservative manner, utilizing the principles of the well documented ‘Dahl phenomenon’,17 hence the Direct Coronal Splint as described by Banerji, Mehta and Millar in 2010.6 The latter form of restoration may have the potential to provide an immediate relief of the symptoms commonly associated with cracked tooth syndrome, serve as a useful diagnostic tool, and be quick and easy to apply with minimal financial cost, however careful case selection is mandatory.

The superior shock absorbent properties offered by resin onlays may also provide a further indication for their application to restore brittle, endodontically treated teeth, or periodontally involved teeth where there is a need for cuspal coverage, but a desire to reduce the transmission of masticatory forces to the underlying coronal or alveolar tissues, respectively.9

Whilst it is possible to repair ceramic-based materials with resin composite, the latter is achieved more readily to existing resin restorations. This advantage offered by resin composites may be particularly relevant where the pulpal status of a given tooth requiring an onlay restoration may be in doubt; should subsequent root canal therapy be required, resin composite can easily be added to an existing resin onlay to restore the access cavity through which endodontic treatment is carried out.6

Adhesive resin onlays – the fabrication method: direct vs indirect approach

The ‘direct’ chairside application of resin composite to form onlay restorations alleviates the need for impressions, provisional restorations, additional laboratory costs and the unnecessary removal of any healthy hard tissue undercuts. Direct onlay restorations may also be fabricated in a single visit.

However, the technique for the placement of direct resin onlays is considerably time and operator skill demanding, where there is a pre-requisite for the operator to have a very good working knowledge of fundamentals of occlusion and the appropriate manual skills in order to attain an acceptable anatomical form. The establishment of patent, inter-proximal contact areas may also prove to be difficult.

The ‘indirect’ approach, where the restoration is fabricated extra-orally, allows the dental technician to establish the desired occlusal and aesthetic prescription with a greater degree of ease. Furthermore, as these restorations are cured extra-orally, it is possible to ‘post-light cure the resin based material’ (beyond what is attainable directly), so as to increase the overall level of polymerization conversion,18,19 and thus, in theory, improve the mechanical properties of the restoration, such as wear resistance, hardness and the elastic modulus (stiffness).

Post-curing is generally achieved either by the application of dry heat (at 120°C for a period of 15 minutes),13 or by extending the time of light curing, or by either adopting a slower rate of curing (which is thought to give the potential for the greater movement of molecular chains, and thus present a higher potential for energizing activation sites) or, indeed, a combination of heat and light. The process of post-curing is also thought to provide a higher degree of stress relaxation within the matrix of the material.13

Additionally, and of paramount importance as the indirect restoration is cured extra-orally, the well known problem of polymerization contraction (associated with the application of resin-based materials)20 occurs extra-orally, thereby lessening the undesirable after effects of this phenomenon such as post-operative sensitivity and marginal leakage, which is frequently associated with subsequent secondary caries and marginal staining.

The fabrication of indirect resin restorations in an inert atmosphere of nitrogen has also been described, so as to reduce the entrapment of air voids and overcome the concept of air inhibition curing of resin composites.13

However, it has been postulated that, since the restorative materials used for either the direct or indirect technique are chemically of the same variety, the actual expected gain in mechanical properties of the indirect resin restoration versus that of the direct counterpart would be minimal.21

The above has been reflected by the observations of several studies, which appear to suggest that the gain in mechanical strength of heat-cured indirect resin composite restoration over that of direct restorations is largely short-lived.22 In an 11-year clinical analysis of the performance of directly and indirectly fabricated resin inlay/onlay restorations, respectively, Van Dijken23 reported the difference in clinical performance to be hardly significant, where factors such as occlusal wear, fracture and secondary caries formation were assessed.

Similarly, Wassell et al24 have reported no significant superiority of one method of resin restoration fabrication over the other, where restorations were observed for a period of 5 years. Wear and shrinkage of the resin-based lute used to place the indirect resin restoration appears to be a concern with such restorations.

However, with time, as more data are gathered about the polymerization patterns of resin composites, the application of novel post-curing methods, or indeed combining differing methods of post-curing such as the dual application of heat and light,25 it may be possible to fabricate indirect adhesive onlay restorations which offer significantly superior qualities to justify further their use over direct resin onlays.

Discussed below are two case reports describing the application of resin onlays, fabricated with the use of each of the techniques described above. These have been carried out as part of the clinical case requirements for the MSc in Aesthetic Dentistry, King's College London.

Case 1: Indirect approach

A 28-year-old female patient presented with sensitivity, initiated by thermal stimuli localized to her upper left quadrant. The symptoms had been present for the duration of the past 2 weeks. A detailed clinical examination revealed the presence of a fracture on the distal aspect of a large pre-existing MOD resin composite restoration sited at her maxillary left first molar26 (Figure 2).

A vitality test via an electronic pulp tester (DIgitest, Parkell, USA) elicited a positive response, and subsequent radiographic analysis indicated that the existing restoration was in close proximity to the dental pulp. A diagnosis of reversible pulpitis was made. The treatment aims were to alleviate the patient from sensitivity, maintain the vitality of the dental pulp and to restore the aesthetics, form and function of the affected tooth. Given the size and nature of the existing restoration, restoration via cuspal coverage was proposed as a means of restoring the tooth's inherent fracture resistance.

After careful consideration and communication with the patient, the decision was taken to prepare the tooth to receive an aesthetic indirect resin composite onlay (Gradia, GC Corporation), which would also provide cuspal coverage. This choice would be minimally invasive in comparison to a full coverage restoration. Edelhoff and Sorenson26 have reported that, on average, the preparation of a tooth to receive an onlay restoration would entail the removal of approximately 40% of coronal tooth structure; this is in contrast to approximately 60–70% coronal structure loss for a tooth to receive a full coverage preparation. Teeth that have been prepared to receive full coverage indirect restorations have been reported to have a heightened risk of subsequent pulp tissue death, as reported by Saunders and Saunders.27 Burke and Lucarotti28 have also reported higher complication rates amongst patients where full coverage restorations have been prescribed and placed in patients under the age of 30 years.

The merits and indications of indirect adhesive resin onlays have been discussed above.

Pre-operative shade selection was determined with the use of a Spectrophotometer (Vita EZ Shade). Characterization of the onlay restoration was discussed with the patient, who opted not to include any surface/fissure staining. A detailed occlusal assessment was also performed. Centric stops were marked pre-operatively using articulating paper (DEHP). Following the administration of a local anaesthetic, the distal fracture portion was provisionally ‘repaired’ with a Resin Modified Glass Ionomer Cement (Fuji II LC, GC Corporation). The correct occlusal form/fissure pattern was then carved into the latter restoration and the centric stops re-checked. Once complete, the occlusal and axial morphology was accurately recorded with the aid of two pre-operative silicone indices (VPS Hydro Putty, Henry Schein).

Tooth preparation to receive the indirect adhesive resin onlay involved an overall occlusal reduction of 1.5 mm, and a further 0.5 mm was reduced overlying the functional palatal cusp. A shoulder margin was prepared circumferentially, just apical to the occlusal reduction. Precision tooth preparation was guided with the aid of a sectioned putty index and depth reduction grooves. A digital calliper was used to measure the depth of bur (856 FG, 368 FG, Diatech) (Figures 3 and 4), so as to attain the desired level of tooth reduction.

Supragingival margins were prescribed to facilitate the visualization of the margins, simplify the process of impression taking, maximize bonding of the definitive restoration and to promote good oral hygiene. Special care was taken to ensure that the circumferential margin was finished onto sound enamel tissue, to optimize adhesion.

A provisional restoration was fabricated in a bis-acryl material (Luxamtemp, DMG) using the second putty index. A working impression was taken using an addition cured vinyl poly-siloxane material (VPS Hydro, Henry Schein) (Figure 5). An opposing, full-arch alginate impression was also taken (Dust Free Alginate, DEHP). Stock trays were used for both impressions (Neotray, Poland). The provisional restoration was then luted with a non-eugenol containing temporary cement (Temp Bond NE, Kerr, USA).

A rubber dam was applied pre-cementation. The preparation was cleaned using pumice and the restoration tried-in to assess shade and marginal adaptation. At the cementation stage the onlay preparation was etched with 40% phosphoric acid (DEHP). The fit surface and preparation were coated with a bonding system and the onlay was cemented with dual-cured, fluoride-releasing, resin-luting cement (Calibra, Dentsply). Excess cement was removed and margins were refined and polished. The desired occlusal prescription was verified using articulating paper and any undesired contacts were eased, so as to conform to the existing occlusal scheme (Figures 6 to 9). The presence of centric stops were confirmed with the use of Shimstock articulating paper (Roeko) and the patency of the inter-proximal contact point confirmed with the use of dental floss.

Case 2: Direct approach

This 63-year-old patient presented with pain from her upper left first premolar. Clinical and radiographic examination suggested a diagnosis of irreversible pulpitis caused by secondary caries. The treatment aims were to alleviate pain and aesthetically restore the tooth via root canal therapy and to provide cuspal coverage with a minimally invasive approach. Treatment options were discussed with the patient, who ultimately opted for root canal therapy followed by restoration of the tooth with a direct composite onlay. The merits of a direct composite resin onlay restoration have been discussed above.

A pre-requisite for successful adhesive dentistry is the need for adequate isolation and moisture control. Abdalla and Davidson 29 have described the phenomenon of marginal leakage to occur as early as 2 weeks, following the placement of Class II resin composite restorations in the absence of rubber dam isolation. Polymerization shrinkage is also likely to be more of a concern, when comparing to an indirect approach, as discussed above.

Treatment commenced with caries removal and pulpal extirpation (Figure 10), following the administration of local anaesthetic and rubber dam isolation. Impressions were then taken using an addition to cured vinyl poly-siloxane material (VPS Hydro, Henry Schein) and alginate (Dust Free Alginate, DEHP) in stock trays (Neotray, Poland) to permit the fabrication of a diagnostic wax-up (Figure 11). This maintained a three-point contact (tripodization) with the functional cusps of the opposing dentition. This was then indexed with VPS, to guide the process of tooth preparation accurately.

The subsequent appointment included the completion of endodontic therapy. This was followed by tooth preparation for the direct resin onlay, guided by the pre-operative silicone index. Tooth preparation involved an overall occlusal reduction of 1.5 mm and a buccal/palatal bevel preparation (25–30 degrees) (Figures 12 and 13).

Once refined, the tooth surface was etched with 40% phosphoric acid, followed by application of bonding agent (Optibond Solo, Kerr). The bonding agent was then gently air-dried to permit solvent evaporation and light-cured for 20 seconds. The tooth was then restored with a direct resin composite onlay (Venus, Heraeus Kulzer) (Figure 14). This was achieved under rubber dam isolation using an incremental technique to ensure depth of light curing and minimization of the C-Factor. After an initial 1 mm increment was cured, a translucent enamel shade was used to create the mesial contact point. A dentine shade was then utilized to recreate the cuspal anatomy, whereby increments were light-cured for a few seconds to prevent displacement (Translux Power Blue 5W LED polymerization lamp, Heraeus Kulzer). The enamel shade was then placed in one increment and the restoration was polymerized. Following this, the occlusion was checked to ensure the absence of any unfavourable contacts.

The restoration was left for 15 minutes to allow the majority of dark polymerization to take effect.30 Refining and polishing took place using polishing stones (Shofu Dental) and abrasive discs (Soflex, 3M ESPE). A high surface shine was achieved with a polishing brush impregnated with silicone carbide (Astrobrush, Ivoclar Vivadent). This was crucial to maximize the optical refractive properties of the chosen resin composite, allowing a satisfactory aesthetic outcome. Finally, the restoration was re-polymerized for 40 seconds. Figure 14 provides a post-operative view of the restoration.

Conclusion

Consensus opinion would suggest that gold alloys remain the most successful material of choice for the fabrication of crowns and indirect intra-coronal restorations.31 However, it would appear that resin composite restorations offer a formidable, aesthetic alternative to the use of Type III and Type IV gold alloys for the construction of minimally invasive onlay restorations, for a variety of differing clinical applications.

The decision to prescribe an indirect resin onlay versus that of a direct resin onlay is largely dependent on the skills and knowledge of the operator, time and financial constraints imposed by the patient, and the desire to avoid impressions and provisional restorations.

At this point in time, there is little evidence to suggest the superiority of one method of restoration fabrication over the other. However, with advances expected in resin technology, it would be reasonable to speculate that the indirect technique may offer a more effective outcome, as more knowledge is acquired about how to post-cure available resins optimally to attain superior mechanical and aesthetic outcomes.