Comparative in vivo and in vitro studies on the biomechanics of maxillary partial dentures. A methodological and experimental study

Abstract: Biomechanical aspects are generally agreed to be significant, particularly during the planning of restorative treatments and the design of prosthetic appliances. However, comparative in vivo/in vitro studies show wide differences in the magnitudes and patterns of functional strains in prosthetic devices. The general aim of this study was to investigate some aspects of the biomechanical behavior of maxillary partial dentures in vivo and in vitro. This study also verified the validity, precision and accuracy of in vivo reflection photoelasticity and bite force sensors based on force sensing resistors. Conical crown retained dentures of 7 subjects were prepared for reflection photoelasticity and strain gauge recordings. Three in vitro models with simple, intermediate and advanced anatomical simulation features were manufactured for each subject. In vivo and in vitro loading experiments were performed using bite force sensors. Several variables were tested in this study such as the occlusal loading position, sex, distribution of abutment teeth, palatal height index, in vitro models, and denture structural design. The results indicate that the bite force sensor was able to accurately detect bite force levels in the range of 50 N to 300 N. The bite forces registered with the new sensor were dependent on the loading position, sex and test subject. The reliability of the new sensor for submaximum bite forces was calculated to be 93%. Strain gauge results showed that the new sensor generated strain patterns of low variance during biting tests. In vivo reflection photoelasticity experiments showed that coated areas accessible to normally incident light produced valid, reliable and accurate data of the stress/strain distribution in prosthetic appliances. Comparative in vivo/in vitro data showed that all the studied models failed to reproduce the in vivo functional strain patterns. Anatomical simulation of in vitro models is insufficient to allow for accurate mechanical analyses of maxillary RPDs, therefore, only simple verifications of the strain levels in prosthetic appliances can be attained in vitro. The magnitudes and distribution of stress fields in dentures with different structural design revealed that the palatal major connector and the denture bases mucosal contacts contribute significantly to the rigidity and stability of RPDs retained by conical crowns. Patients with a high palatal height index are less dependent on major connectors for denture stability and rigidity than patients with a low palatal height index.