Re: ONLAYS

"Preparation Protocol To Ensure Predictable Aesthetic Restorations"

Abstract:

Advances in adhesive dentistry have created higher bond strengths. Higher bond strength has allowed for greater predictability in the utilization of ceramic-based restorations to restore damaged teeth back to their original pre-operative strength. Dentists have been slow to accept both direct and indirect posterior esthetic restorations despite improvements in flexural strength, esthetics, margin fit and wear to opposing dentition.
This article is meant to familiarize clinicians with the proper diagnostic and preparation principles for ceramic onlays, and to review finishing and polishing procedures to obtain an optimal clinical result. These are critical steps in creating functionally sound and esthetically pleasing restorations. If strict adherence to the protocol is not followed, clinical failure can occur for both direct and indirect restorations.

Indications and Contraindications

A healthy periodontium and a balanced occlusion are vital whenever the utilization of esthetic materials is considered to restore deteriorating posterior dentition. If complete isolation from blood and saliva cannot be achieved, adhesive dentistry is contraindicated. In cases where margins are supragingival or slightly subgingival and complete isolation is possible bonded procedures are an option. If margins are too far subgingival because of decay or previous restorations, the treatment of choice would be a conventional cemented gold inlay/onlay/crown restoration or a cementable all ceramic crown (esthetic inlay/onlays cannot be used with conventional cements).

Resistance, retention and optimal physical strength of bonded ceramic restorations are maximized by the adhesive process. Bonded restorations are said to restore the dentition to near pre-restoration strength. It is difficult to quantify the amount of bondable surface area, but it can be said that the greater the bonded surface area, the stronger the restoration is and the greater it’s ability to withstand normal functional occlusal loads. Facial margins need not be placed subgingivally to achieve esthetic success; proper lab and color communications ensure supragingival margins and are virtually undetectable.

Various Onlay Preparations Designs

Onlays are indicated when you are required to extend the preparation into more than 50% of both the buccal and lingual cusps. The cusp may be severely weakened with decay, fractured or have a wall thickness of less than 1 mm. It is critical to have a good balanced occlusion for onlay preparations. Four different preparation designs predominate. These classifications are the author’s opinion only, as he assessed each given clinical situation.

When a large restoration with an occlusal isthmus width extends approximately two thirds the way up the cusp inclines and does not have extensive undercuts present in the proximal box area (but still has a good sound 2mm of enamel at the cusp tip area), then the onlay will be performed without sacrificing the cusp tips. This scenario calls for the onlay to sit on top of the cusp incline and once bonded in place will restore the tooth back to ideal strength.
The second scenario presents itself when one or more of the cusp tips are severely weakened through decay or when the wall thickness is less than 1mm. In this situation, an onlay restoration is required which will allow for the proper reduction of the weakened tip that is necessary to create a sound restoration that utilizes no unsupported enamel.
The third scenario is present when the proximal box has extensive undercuts that extend more than two-thirds the gingival incisal height of either the buccal or lingual cusp. To restore this weakened state, blockout technique would be contraindicated, therefore the preparation design would need to be modified to conserve tooth while also properly supporting the weakened cusp with the restoration. The modification is to prepare from the proximal box area occlusally while reducing the amount of unsupported cusp tip needed to render a well-supported cusp.
The final onlay design applies if a patient presents with an isolated cusp fracture. Follow the onlay design principles but keep the onlay isolated to the compromised area. Abandon the traditional GV Black guidelines. The damaged cusp can be replaced by itself with no other tooth structure being removed.

Preparation Protocol

The design principles for bonded onlays are quite different from those of cemented gold onlay restorations. Bonded onlay preparation guidelines state that these restorations should have a 5o to 15o flaring of the axial walls. The gingival box should have a butt joint finish and all internal line angles must be rounded. All visible margins should be finished with a butt joint and a heavy chamfer. Beveled and feather edges should be avoided. The minimum isthmus width required is 2.5 – 3.0 mm; the minimum thickness of material from the opposing supporting cusp is 2.5 – 3.0 mm. For onlays, all cusps should be covered with 1.5 – 2.0 mm of material and the minimum wall thickness of 1 – 1.5 mm is required for optimal strength.

Discussion

Some clinicians still avoid using tooth colored direct and indirect restorations in the posterior dentition because of associated historical deficiencies. By following a strict protocol, a large portion of these deficiencies can be eliminated. Clinical unpredictability such as poor marginal fit and inadequate flexural strength, which inherently made many of yesterday’s ceramic restorations clinically unpredictable, has greatly been reduced with advances made in ceramic strength and esthetics, predictable seating protocol and advances with 5th generation adhesive systems. Highly esthetic restorations are now the norm. A pressed ceramic system was used in this case presentation. This was done to insure that like materials are used when restoring teeth from opposing arches, this safeguards against occlusion/contact issues that could arise as a byproduct of the use of different materials.

Conclusion

Today’s patients are educated consumers; they demand esthetic excellence and most often, metal free restorations. Esthetic onlays have been shown to satisfy even the most discerning patient. It is imperative not to forget there will be clinical situations where the new ceromer materials cannot be used because proper isolation cannot be achieved. In these situations rely on cementable restorations for predictable clinical results. A true understanding of current restorative materials and attention to detailed procedural protocols will enable clinicians the opportunity to provide metal free esthetic dentistry on a predictable basis.