Strain situation after fixation of three-unit ceramic veneered implant superstructures

The passive fit of superstructures used in implant prosthodontics is affected by several variables. The objective of this study was to quantify the strain development in various fixed partial dentures (FPDs), both in the condition as cast and after ceramic veneering. Five different types of three-unit FPDs (cementable/repositioning technique impression; cementable/pick-up technique impression; screw-retained/plastic cylinder; screw-retained/gold cylinder; screw-retained/bonded) with 10 samples each, representing commonly used FPD-types, were investigated before and after ceramic veneering. Two ITI implants were anchored in a measurement model simulating a real-life patient situation and strain gauges were mounted close to the implants. The strain development was recorded during cement setting and screw fixation. For statistical analysis, multivariate two-sample tests were performed with the level of significance set at P = 0.1. All FPDs revealed measurable amounts of strain. Neither the impression technique nor the fabrication modes for conventional screw-retained FPDs had a significant influence on strain development. Ceramic veneering caused an increase in strain development for the conventional bridge types. Furthermore, cementing appears to be able to compensate fabrication inaccuracies better than screw retention. The lowest strains were found in FPDs bonded to gold cylinders on the measurement model for metal frames and ceramic-veneered FPDs. Conventional procedures are unable to produce superstructures with an absolute passive fit. The technique of bonding superstructures to prefabricated components in the oral cavity seems to compensate for various shortcomings in superstructure fabrication.