In Vitro Study on Passive Fit in Implant-Supported 5-unit Fixed Partial Dentures

Purpose: Fabrication and retention methods have an influence on the passivity of superstructure fit. The objective of the study was to quantify the strain development of various cemented and screwretained fixed partial dentures (FPDs). Materials and Methods: Forty samples of 4 different types of FPDs (10 of each type) were investigated. Each sample had 3 ITI implant abutments and 2 pontics. The 3 implants were anchored in a straight-line configuration in a measurement model simulating a real-life patient situation. Strain gauges were mounted close to the implants and on the pontics. The developing strains were recorded during cement setting and screw fixation. For statistical analysis, multivariate 2-sample tests were performed, with the level of significance set at P = .1. Results: All FPDs investigated revealed a considerable amount of strain, with no significant difference between cement and screw retention. Furthermore, no significant difference was found between the conventional fabrication modes for screw-retained FPDs. The lowest strains were found in prostheses that were intraorally bonded onto gold cylinders. Discussion: Because bonding of the superstructure in the oral cavity may compensate for impression and laboratory variables, restorations with the best possible passive fit can result from this retention technique. Before this technique can be recommended, the long-term stability of the adhesive layer should be investigated. Conclusions: As an absolute passive fit of superstructures is not possible using conventional clinical and laboratory procedures, and as clinical fit-evaluation methods often do not detect "hidden" inaccuracies, the more sensitive straingauge technique should be utilized for an objective accuracy test. Reference strain values from implant-supported prostheses that have served without complications could help define a "biologically acceptable fit."