Biomechanical analysis of the integration behaviour of cementless stems in total joint replacement

The total joint replacement presents one of the most important and wide-spread treatments of various diseases of bones and joints. Due to the normal bone reaction to any changed loading, the remodelling processes are triggered in the bone by the implantation of an artificial joint. In particular for the cementless implants, the bone remodelling stimuli may lead to the osseointegration of the implant as well as to an implant failure by implant loosening, which is caused by the atrophy of certain bone regions and loss of the secondary stability reached through the bony integration in a short or in a long term. The preoperative simulation of the mechanical loading of the bone and bony integration of the implant and the prognosis of the longevity of the implant has high clinical and socioeconomic impact. The finite element (FE) method provides a precise tool for the analysis of the stress distribution in the bone with and without stem under given loading conditions. In order to describe the physical behaviour of the skeletal system accurately the boundary conditions, especially the muscular loading, must be physiological. The present thesis focuses on the biomechanical investigations of the osseointegration of cementless titan endoprostheses on the example of the hip-joint replacement. In the present work, new principles for the determination of the muscle forces acting on bones are suggested and applied to two cases of human locomotion: gait and sitting down. The new principles are based on the widely accepted view that biological systems are optimised light-weight structures, in which the amount of unloaded material is minimised and evenly distributed loading throughout the structure is reached. Even though there is no practical way to validate in vivo the outcome of any computational model that predicts muscle forces, the muscle forces can be validated indirectly using the fundamental property of living tissue to functional adaptation. Thus, the muscle forces for a human femur for peak hip joint force during normal gait and sitting down are obtained based on the bending-minimisation principle. As a result, under the physiological loading the bone is loaded predominantly in compression and the stress distribution corresponds to the material distribution in cancellous as well as in cortical bone. Physiological stress levels for the different types of bone are determined.