Isolated popliteus tendon injury does not lead to abnormal laxity in posterior-stabilised total knee arthroplasty

The popliteus tendon is crucial to postero-lateral stability and prone to iatrogenic injury intra-operatively. Its role in the stability of the replaced knee remains contentious. The aim of this study was to use computer navigation to quantify the effect of popliteus sectioning on the 'envelope of laxity' (EoL) offered by a posterior-stabilised (PS) total knee arthroplasty (TKA) and compare with that of the native knee. Loaded cadaveric legs were mounted on a purpose built rig. EoL was measured in 3 degrees of freedom using computer navigation. Knees were subjectively stressed in varus/valgus, internal/external rotation and anterior draw. This was performed preoperatively, during TKA and after sectioning of the popliteus tendon. Real-time data were recorded at 0A degrees, 30A degrees, 60A degrees and 90A degrees of flexion as the operating surgeon stressed the knee in 3 degrees of freedom to its subjective endpoint. Mixed-effect modelling was used to quantify the effects of intervention on degree of laxity. In all conditions, there was an increase in laxity with knee flexion. Insertion of a PS TKA resulted in increased constraint, particularly in rotation. Sectioning of the popliteus did not result in a significant increase in knee laxity to 90A(0) of knee flexion. However, at deeper flexion angles, tendon sectioning overcame the constraints of the implant resulting in a significant increase in rotatory and varus/valgus laxity towards the native condition. These findings support the view that certain current designs of PS knee replacement can constrain the knee in flexion in the absence of postero-lateral deficiency. For this implant, isolated sectioning of the popliteus tendon did not substantially generate abnormal knee laxity.