Development of clinically relevant testing methods for the fatigue performance for tibial trays
This thesis investigates whether the testing method proposed by the International Standards Organisation (ISO) for determining the fatigue performance of total knee tibial trays is clinically relevant, and if so what would be the appropriate load for testing. Fracture of tibial trays occurs for a number of reasons. These include poor design, insufficient support beneath the tray, inadequate manufacturing and material properties, malalignment, incomplete coverage of the tibial plateau, and overloading due to patient weight and/or activity level. The literature identified a number of trays that had a propensity for fracture. These tray designs are now no longer used. A survey was conducted of current designs from manufacturers' data. Examples of trays that had a history of clinical failure and those that are regarded as clinically successful were analysed using finite element analysis (FEA) and mechanical testing under the ISO test conditions for a range of load values. The results showed that using a physiological value of 2000N (peak force during the normal walking cycle) all trays that had some history of failure and some of the clinically successful trays failed. Further analysis revealed that a load level of 900N distinguished between clinically successful and unsuccessful trays. The ISO test also showed that trays always failed with a crack propagating in the anteroposterior direction (across the length of the tray). Examination of retrieved trays using light microscopy and scanning electron microscopy revealed that crack propagation occurred in a number of other ways as well as across the length of the tray. FEA of a tibial tray placed in a realistic bone model showed that support conditions play a large part in the way in which crack propagation occurs and the likelihood of fatigue fracture. Further fatigue testing with a variety of support conditions validated this result. Hence an alternative testing method was developed based on clinical findings. This showed that multiple crack formation occurred for a tray that had some history of clinical failure due to fatigue. Using this method, a load of 2000N can be used to distinguish between clinically successful and unsuccessful trays. The test was also repeatable. However the results of this testing method always revealed the same design faults as those predicted by the ISO test protocol. Based on these factors, it is the final conclusion of this study that the ISO test protocol is clinically relevant when using a load level of 900N.