Simultaneous force measurements in 3 dimensions on oral endosseous implants in vitro and in vivo. A methodological study.

A novel piezo-electric force transducer was developed that is mounted directly on ITI-implants and allows for simultaneous force measurements in 3 dimensions. The reliability of this measuring method in vitro and in vivo was evaluated. For test and calibration measurements 5 edentulous patients with 2 mandibular implants were selected and master casts obtained with original implants in situ. Preload of the abutment screw was measured with a torque controller by repeated tightening and loosening. The introduction of additional preloads when bar connectors or telescopes were mounted was examined on the casts and in the mouth. Tests were carried out to identify the influence of screw tensions on force measurements by loading the implants in vertical and transverse directions. Calibration measurements in vivo and in vitro were carried out by means of a spring balance. A regression model was calculated to analyze the influence of the applied preload on measured forces. The mean preload value was 315, ranging from 200 to 480N. A significant correlation of applied loads to measured forces was found (r = 0.99). It was independent of the preload in vertical direction but dependent in transverse direction. No additional tension of the abutment screw was ever observed when bars were mounted. Calibration measurements with the spring balance showed that bars contribute to load sharing between the implants. Comparable results were found for all measurements in vitro and in vivo. It was concluded that the new piezo-electric transducer is an adequate instrument to measure forces in 3 dimensions on ITI-implants.