Patient demographics and their demands are changing, with younger and more active patients. This is in conjunction with increased expectations, not only of patients but also of healthcare providers, clinicians and regulators, who typically demand a success rate of greater than 90% at ten years. Wear debris induced osteolysis and failure remains the major cause of failure of joint replacements.1–3
Pre-clinical testing can play a key role in assessing the performance of joint replacements and historically pre-clinical wear testing under a standard walking cycle simulation has been shown to compare well with clinical data for some designs and materials. However, testing under such conditions as those of the ISO standard walking cycle have not been able to predict wear related failure or wear mechanisms clinically, neither have they been able to consistently differentiate the performance of different designs.
In order to enhance the safety and reliability of joint replacements, pre-clinical testing should include a wider range of clinically relevant conditions that are investigated in a systematic and rigorous fashion. This stratified approach to simulation should include changes in the prosthetic device, different patients activities, different types of patients or patient conditions and different surgical delivery and component positioning. Some examples of these conditions have been presented in this article but more research is needed to define the effect of different design variables on wear under a wider set of adverse conditions and a wider range of daily activities. Such a stratified approach to simulation will inform future design by defining the performance envelope of device, identifying failure mechanisms, informing clinicians and industry of conditions where a device might not be suitable and enhancing future product development for both design and material selection.
The first part of this article reported results under standard baseline test conditions for various bearing materials for historical and current joint replacements from our existing data set of over 5 billion test cycles. When combined with clinical experience, these results have defined indicative levels of acceptable performance in terms of wear rates for different materials. The differing bio-reactivity and cellular response meant that there appears to be a lower clinical tolerance to metal compared to polyethylene.
The second part of this article has introduced the concept of a new stratified approach for enhanced reliability (SAFER) pre-clinical simulation testing of joint prostheses, with many examples of how different clinically relevant conditions affect the wear rates and performance of various prostheses, and most importantly shown how different types of prostheses respond differently to the different conditions. The results from these wider sets of conditions have been compared with those obtained from the baseline standard walking cycle testing.
In the polyethylene hip there remain concerns around deterioration of the metallic head causing elevated wear and potentially increasing the risk of osteolysis. The scratch resistant ceramic head may lessen this risk, particularly if coupled with cross-linked polyethylene, which has shown reduced wear under standard conditions.43,44
In the hip, edge loading can occur when the femoral head contacts the rim of acetabular cup. This has been shown to produce a characteristic stripe on the femoral head for hard-on-hard bearing materials; indeed stripe wear was first observed on ceramic-on-ceramic retrievals by Nevelos et al.17
This stripe wear was not seen in vitro
under standard testing conditions but can be produced in vitro
through rotational or translational mal-positioning depending on the bearing material and design. For example, in ceramic-on-ceramic bearing, edge loading due to rotational mal-position did not cause stripe wear and increased wear, whereas translational mal-position (microseparation conditions) resulted in increased wear, stripe wear mechanisms, and the generation of bimodal micron sized wear particles, as seen on retrievals.17,19,20,45
Under standard conditions, the current generation of ceramic-on-ceramic Delta material when compared to previous generations, showed no distinguishable difference. However under adverse translational mal-positioning conditions the improved Delta material showed a greater resistance to wear.26
This highlights an advantage of the stratified approach, which is the capability to differentiate between different devices. This can reflect improvements in material performance. The ability to compare wear rates with clinical experience and performance is a fundamental requirement.
The wear of metal-on-metal bearings has been shown to be sensitive to both rotational and translational mal-positioning as well as design (cup coverage) and head size.27
When compared to ceramic-on-ceramic the absolute level of wear is much higher, and when combined with the increased reactivity, indicates a greater potential for clinical damage.
Under standard walking conditions, knee prostheses generally have low wear. In-vivo fluoroscopic studies have demonstrated significant variability in knee kinematics between patients, particularly the incidence of femoral condylar lift off from the tibial bearing.46
increased kinematics, through both increased anterior posterior translation and condylar lift off, produced elevated wear. Moderately cross-linked polyethylene has been shown to reduce wear under both standard conditions and higher kinematic conditions. Given the significant variability in knee kinematics between patients it is important to begin to test the performance of current knee designs under a wider range of clinically relevant kinematic conditions.
There is a need to consider other patients activities such as stair climbing, squatting, chair-rise and descent for example. However these activities may only impact on less than ten percent of the tribological cycles, whereas a mal-positioned prosthesis leading to edge loading in the hip or instability in the knee leading to femoral condylar lift off can cause high kinematic demand and increased wear on every step.