Zirconium dioxide based dental restorations. Studies on clinical performance and fracture behaviour

Loss of teeth can affect a person's appearance and functions such as eating and speaking. There is thus a need for prosthetic rehabilitation to improve quality of life. For many patients, a fixed dental restoration is preferred, and a common restoration is a porcelain-fused-to-metal bridge retained by teeth or implants. Metal-based restorations can potentially cause adverse reactions though, and this is cause 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 properties and have been successfully used in dentistry, mostly for smaller anterior restorations. Ceramics, however, do not withstand tensile forces as well as metals, and are susceptible to brittle fractures with the connector area being especially prone to fracture. More recently, a new type of ceramic material, based on zirconium dioxide, has been developed. Yttria-stabilized tetragonal zirconia polycrystal, 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 prevents it from propagating. This material has the potential to be used for larger restorations and in the molar area. Not enough information, however, is available on clinical follow-up of zirconia-based restorations, especially long-term, and information about all-ceramic restorations supported by implants is lacking. The aim of this thesis was to evaluate designs of zirconia-based restorations in relation to achieving increased fracture resistance and evaluate the clinical performance of implant-supported zirconia-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 fully-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. However, fractures of the veneering material were noted. There was a significant difference between the two materials: 54% of the restorations in group 2 showed veneer fractures compared to 8% of the restorations 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 prosthesis frameworks with different connector dimensions: 2.0, 2.5, 3.0, 3.5 and 4.0 mm. The results showed a significant 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 restorations were in function and no complete fractures were noted. However, fractures of the veneering material were noted in nine of the ten patients, affecting 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 different veneering materials was evaluated. The crowns were cemented onto tooth-like abutments, except for one group cemented onto implant-like titanium abutments. The results showed significantly higher loads at fracture and less severe veneer fractures for crowns with anatomically shaped cores compared to simple cores of even thickness. Crowns supported by implant-like titanium abutments showed significantly higher loads at fracture than did those supported by tooth-like abutments. The type of core-material and veneering material did not influence the results.