Zirconium dioxide based dental restorations : studies on clinical survival and fracture behaviour
Abstract: ABSTRACT The loss of teeth can affect a person’s appearance and functions such as eating and speaking. There is thus a need for prosthetic re-habilitation to improve quality of life. For many patients, a fixed dental restoration is preferred, and a common restoration is a por-celain-fused-to-metal bridge retained by teeth or implants. Metal-based restorations can potentially cause adverse reactions though, and this is indication for the search for alternative materials. All-ceramic materials are characterized by strong atomic bonds that make them reluctant to react with the environment, and thus unlikely to cause adverse reactions. All-ceramic materials have other attractive material properties and excellent aesthetic proper-ties and have been successfully used in dentistry, mostly for smaller anterior restorations. Ceramics do not withstand tensile forces as well as metals though, and are susceptible to brittle fractures. The connector area is especially prone to fracture. During later years, a new type of ceramic material, based on zir-conium dioxide, has been developed. Yttria-stabilized tetragonal zirconia polycrystals, Y-TZP, has a unique ability to resist crack propagation by being able to transform from one crystalline phase to another, and the resultant volume increase stops the crack and inhibits it from propagating. This material has the potential to be used for larger restorations and in the molar area. Not enough in-formation, however, is available on clinical follow-up of zirconia-based restorations, especially long-term, and information on all-ceramic restorations supported by implants is lacking. The over-all aim of this thesis was to evaluate how to design zir-conia-based restorations to achieve increased fracture resistance and evaluate the clinical performance of implant-supported zirco-nia-based restorations. In paper I implant-supported all-ceramic fixed partial dentures of two different ceramic materials were compared; a zirconia-toughened alumina material (group 1) and a densely sintered Y-TZP material (group 2). Eighteen patients were randomly divided between the two groups. At the one-year follow-up, all restorations were in function and no complete fractures were noted. Fractures of the veneering material were noted, however. There was a signifi-cant difference between the two materials; 54% of the restorations in group 2 showed veneer fractures compared to 8% of the restora-tions in group 1. Paper IV is a five-year follow-up of the same patient groups. All restorations were still in function without complete fractures, but an increase in veneer fractures was noted; 69% of the restorations in group 2 showed veneer fractures compared to 17% in group 1. In paper II the fracture strength was evaluated for 4-unit Y-TZP fixed dental prostheses frameworks with different connector di-mensions; 2.0, 2.5, 3.0, 3.5 and 4.0 mm. The results showed a sig-nificant increase in load at fracture for each increase in connector diameter and recommendations for clinically relevant connector dimensions were suggested. In paper III implant-supported Y-TZP fixed full-arch mandibular dentures were evaluated. At the three-year follow-up all restora-tions were in function and no complete fractures were noted. Frac-tures of the veneering material were however noted in nine of the ten patients, at 34% of the units. In paper V the fracture strength of crowns with different types of Y-TZP core materials and different core design and veneering ma-terials was evaluated. The crowns were cemented onto tooth-like abutments, except for one group cemented onto implant-like tita-nium abutments. The results showed significantly higher loads at fracture and different fracture mode for crowns with anatomically shaped cores compared to simple cores with even thickness. Crowns supported by implant-like titanium abutments showed sig-nificantly higher loads at fracture than did those supported by tooth-like abutments. Type of core-material and veneering material did not influence the results.
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