Dr. Christensen: The research question is: Do zirconia and alumina frameworks and their layered or pressed veneer ceramics perform the same, better, or worse than traditional PFM restorations clinically? To determine this, we are monitoring 293 posterior prostheses in a number of areas, such as overall durability, esthetics, periodontal health, fit, retention, surface smoothness, breaks, recurrent caries, need for endodontic therapy, wear of the test prostheses and opposing dentition, and the effect of occlusal adjustment. Patients’ perceptions of “feel” and esthetics are monitored as well.

Dr. Christensen: Each year, the 116 participating dentists are asked to collect data, which consists of clinical images of the bridge from the buccal, lingual, and occlusal as well as images of the opposing dentition. Polyvinyl silicone impressions of both the opposing dentition and the prosthesis are made. From those impressions, special polyurethane dies are poured and gold-sputtered to prepare them for scanning electron microscope imaging of each tooth. The clinicians also fill out a computerized grading form to evaluate a number of clinical characteristics, such as color match, condition of interproximal contacts, margin fit, gingival health, etc. This information is processed at our lab. The special gold-sputtered dies and clinical and scanning electron microscope images are used for definitive grading by two researchers evaluating surface smoothness, various types of breaks, wear on the prosthesis and opposing dentition, and presence and amount of occlusal adjustment.

Dr. Christensen: Let’s talk about frameworks first. We looked at three types of frameworks: metal, zirconia, and alumina. At two years, zirconia’s performance is equal to metal with no breakage of either. Importantly, we are seeing no difference clinically between the four zirconia brands (Cercon, Everest, e.max, and Lava). Alumina frameworks did not fare as well. At two years, 26% of the alumina frameworks have broken. When the veneer ceramic was pressed, the frameworks broke sooner, but the alumina bridges with layered veneer ceramic caught up in numbers broken after one year of service. The bottom line is, the data indicate alumina frameworks are inappropriate for posterior multi-unit restorations.