Biomechanical characterization and modeling of natural and alloplastic human temporomandibular joint

Abstract

To improve reduced functions of the temporomandibular joint, total replacement involves removing the non-functional joint and replacing it with an artificial one. Recent prostheses may lead to different cases of failure that require additional surgical procedures. Some solutions are available to improve the artificial joint survival rate. Materials and geometry play important key roles in enhancing the long-term life of the joint, but the biomechanics of the human masticatory system must also be well characterized. Forces applied to the mandible bymuscles, articular joints and teeth need to be determined to assess the strain and stress patterns of the whole mandible and particularly of the condyle area to control the effects of stress shielding. While the femur, for example, is a well-documented bone structure, this is not the case for the mandible bone; there seems to be little investigation in the literature into the biomechanics of the mandible. The first part of the study describes the characterization of the muscular actions, i.e. the forces exerted by the elevator muscles that were considered: deep and superficial masseters, pterygoid and temporal. In vivo electromyography and MRI contributed to quantifying force intensities when the mandible was loaded. This load between the teeth was recorded using a sensor which also adjusted the mouth aperture. The description of the articular surfaces and the calculations of the muscular insertion co-ordinates were obtained from four cadaver dissections. In the second part of the study, the synthetic mandible was digitized with a laser scanner device to build the finite element model. The solid model of the mandible was created with a modeling package after digitizing the surfaces. The study proved that the finite element model of the mandible can reproduce experimental strainswithin an overall agreement level of 10%. The model correctly reproduced bone strains under different load configurations. For this reason, it adequately reproduces the mechanical behavior of the mandible and has therefore become an essential tool for predicting biomechanical changes in the mandible and long-term failure. First, we describe mandible strains under natural loads. Using simulations we were able to define the worst boundary conditions for the condyle and the associated mandible behavior. Next, a plate implant was screwed onto one of the mandible ramus. The results were then used to determine the strains at the end of the plates and around the screws; the modified mandible strain patterns and mandible displacements are compared to demonstrate the influence of the implant on mandible behavior.