The effects of variation in tissue properties of an idealised synovial joint : a finite element study
The first aim of this study was to construct idealised models of synovial joints in order to investigate a systematic and efficient method of varying the parameters within the models. These models were used to examine the effect on stresses within the tissues of changes in the material properties, thicknesses and curvatures of the tissue layers comprising the joints. An additional aim of the study was to investigate what effect, if any, elastic stress were propagation during impact loading might have on stresses within the joint. The Taguchi method of fractional factorial design was identified as an efficient tool for varying systematically the parameters of a finite element model. In order to test and fully understand the power of the Taguchi approach, a simplified model of a beam was constructed and the model’s material properties and dimensions were varied using a standard Taguchi design. The results from this analysis demonstrate that an experimental design requiring less computational time and effort may be used since interactions between factors are seldom important. Idealised models of synovial joints were then constructed using finite element analysis. The shape, thickness and material properties of the tissues were varied across ranges obtained from a search of the literature. It was found that for a given joint shape, changes in properties of the articular cartilage, most notably its stiffness, were primarily responsible for determining the state of stress in the joint. The highest stresses in the joint were always to be found in the subchondral bone plate, whilst the lowest stresses were found in the cancellous bone. Four different joint curvatures were modelled. A smaller area of contact between the upper and lower components led to increased stresses throughout the joint. The highest stresses occurred in model C (hemisphere-on hemisphere), whilst the lowest stresses occurred in model B (completely conforming). Stresses arising from the incongruent contact (model D) were found to be more dependent on the properties of the cartilage than those stresses arising from conforming contact (model B).