Direct composite resin onlays: rationale and clinical application.

Ceramic full crowns can restore lost tooth structure, strength, and esthetics but remove about twice as much tooth as an onlay design. Onlay restorations can maintain facial enamel for long-term esthetics, preserve the dentogingival junction for optimum periodontal health, and use supragingival margins for predictable adhesive dentistry. Metallic onlays can create fracture strength 3 times that of a natural tooth and can be designed to be nonvisible, but this solution may not be esthetically acceptable to all patients. Porcelain and composite resin onlays are a solution for these patients. Porcelain onlays are fabricated indirectly at a dental laboratory or milled in the office, whereas composite resin onlays can be fabricated with either of the aforementioned techniques, created in-office on flexible polyvinyl siloxane models, or placed directly. Porcelain is a brittle material with a high modulus of elasticity, similar to that of enamel, and composite resin has a low modulus of elasticity, similar to that of dentin. Composite resin deforms more during function and transfers more compressive stress to dentin than does ceramic, which absorbs more of the masticatory stress. Magne & Knezevic found that indirect composite resin onlays—whether fabricated conventionally or via computer-aided design and computer-aided manufacturing (CAD/ CAM)—had higher resistance to loading than CAD/CAM porcelain onlays, despite a lower elastic modulus. They concluded that the choice of composite resin versus porcelain may not be critical for average bite forces, as both materials withstood those, but may be relevant in the event of high forces from food trauma. Composite resin is more easily repaired if a fracture does occur and is less abrasive to the opposing tooth; porcelain retains its gloss better, accumulates less plaque, and is more wear resistant. Bottacchiari et al found that composite resin onlays were a durable restoration over 10 years. Direct composite resin onlays have been recommended in preference to crowns for posterior teeth undergoing comprehensive restoration. Indirect composite resins are composed of materials with the same chemistry as direct composite resin but are processed extraorally with light, heat, and pressure to improve their polymerization and wear resistance. It was anticipated that these indirect restorations would exhibit better marginal seal and less polymerization stress, since only the small cement volume is polymerized intraorally. However, multiple authors have found similar clinical results for wear resistance and marginal integrity with direct and indirect composite resins of inlay and onlay designs. Manhart