Abstract
Introduction
To meet the demands of younger more active patients more robust pre-clinical wear testing methods are required, in order to simulate a wider range of activities. A new electromechanical simulator (Simulation Solutions, UK) with a greater range of motion, a driven abduction/adduction axis and improved input kinematic following has been developed to meet these requirements, as well as requirements of the relevant international standards. This study investigated the wear of a fixed bearing total knee replacement using this new electromechanical knee simulator, comparing with previous data from a pneumatic simulator.
Materials/Methods
The wear of six Sigma CR fixed bearing TKRs (DePuy, UK) with curved moderately cross-linked polyethylene inserts (XLK) was determined in pneumatic and electromechanical Prosim knee simulators (Simulation Solutions, UK). Standard gait displacement controlled kinematics were used, with a maximum anterior-posterior displacement of either 10mm (high) or 5mm (intermediate) [1]. The output profiles from the simulators were obtained and compared to the demand input profiles. The lubricant used was 25% new-born calf serum and wear determined gravimetrically. Statistical analysis was performed using the one-way ANOVA with 95% confidence interval and significance was taken at p<0.05.
Results
The electromechanical and pneumatic knee simulators both achieved the demanded maximum axial load although the pneumatic simulator did not achieve the initial peak on heel strike. The maximum delivered AP displacements from the electromechanical knee simulator were 2.8 (3.5mm input) and 9.6 (10mm input) [mm] compared to 1.7 and 9.2 [mm] from the pneumatic simulator during the stance and the swing phases respectively. The corresponding values for the IE rotation angle were ±4.9 (5 degrees input) and ±4.1 [degrees] from the electromechanical and pneumatic simulators respectively (both stance and swing phases) (Figure 1). The electromechanical knee simulator produced a mean wear rate of 2.7 ±0.9mm3/MC (mean ± 95% CI) under intermediate kinematics, compared to 2.6 ±0.9mm3/MC from the pneumatic simulator (p=0.99). The corresponding mean wear rates under high kinematics were 5.6 ± 2.3 and 6.7 ±1.5 [mm3/MC] from the electromechanical and pneumatic knee simulators respectively (p=0.59).
Discussion
The wear rates from the electromechanical and pneumatic knee simulators were not significantly different. However, the output kinematic profiles followed the input kinematic profiles more closely on the electromechanical simulator than the pneumatic simulator. This electromechanical knee simulator can be used for a wider range of conditions, including high-flexion, due to it's improved capability and performance over the pneumatic simulators.
Conclusion
The electromechanical knee simulator showed improved performance and capability compared to the pneumatic knee simulator, and can therefore meet higher current and future testing demands. The wear trends, from the two simulators, were however not significantly different under standard gait conditions.
Acknowledgements
This research work was supported by EPSRC, Innovate UK and BBSRC [IKC Medical Technologies], the Leeds Centre of Excellence in Medical Engineering, WELMEC, funded by the Wellcome Trust and EPSRC, WT088908/Z/09/Z and the Leeds Musculoskeletal Biomedical Research Unit (LMBRU), funded by NIHR. JF is an NIHR Senior Investigator. DePuy Synthes, UK, supplied the TKRs.
