Purpose: This work developed a novel preclinical test of total knee replacements (TKR) in order to explain TKR instability linked to patient dissatisfaction. It was hypothesised that stability tests on the isolated moving prostheses would provide novel comparative data on the stability and kinematics among TKR designs.
Methods: Three TKR designs: DePuy Synthes Attune MS, Stryker Triathlon and Zimmer Biomet Persona MC, were assessed using a robotic arm while flexing-extending 0-140°. Tests imposed 710N body weight combined with three tibial loads: no AP force, 90N anterior or 90N posterior force. Other load effects were minimised and the kinematics recorded. Each implant was tested 6 times to investigate the repeatability of the method. Data were analysed using statistical parametric mapping with one-way ANOVA. If significance was found (p<0.05), post-hoc t-tests with Bonferroni correction were used to contrast groups.
Results: Significant differences were found throughout flexion-extension. Femoral rollback, AP stability, coupled internal-external rotation and AP position (roll-back) were all influenced by implant design. AP stability of the TKRs reduced with flexion reaching Attune 15mm, Persona 13mm and Triathlon 21mm at 140O flexion. Tractive rolling significantly affected kinematics in the less-congruent Triathlon design, with 6mm different paths between flexion and extension motion (p<0.05 across 5-100O). Paradoxical anterior femoral sliding in early flexion (0-40O) occurred in Persona and Triathlon designs. Conclusions: The novel testing technique provides, for the first time, comparative data on the inherent stability and kinematics of the TKR implants themselves across the arc of flexion-extension, independent of variables including soft tissue behaviour and surgical technique. The data show how much each prosthesis can contribute to the stability and motion of the implanted knee. Similar data from a wider range of designs will enable more informed decisions regarding implant design choice, aiming to reduce the prevalence of TKR instability in patients.