The effect of knee prosthesis design on tibiofemoral biomechanics during extension tasks following total knee arthroplasty.

BACKGROUND: Determine whether the tibiofemoral motion and electromyographic activity of the knee differs in patients with a medial pivot implant, compared to those with cruciate-retaining and posterior-stabilised designs, during knee extension after Total Knee Arthroplasty (TKA). METHODS: An observational study was conducted on a cohort of patients that had undergone TKA for a minimum of 12 months prior. Three matched groups (n = 18) were categorised based on implant type: medial-pivot (MP), posterior-stabilised (PS) and cruciate-retaining (CR). Kinematics, with motion analysis (Vicon, USA) and surface electromyography (Delsys, USA) were assessed during step-ascent and walking tasks. RESULTS: All groups displayed a similar amount of knee extension in both tasks. They also paradoxically produced an average mean internal rotation movement during knee extension in both the step-ascent and walking tasks. The only significant difference was found in the step-ascent task, in which the MP group produced a larger absolute amount of rotation than the CR implant group (P = 0.007), but neither group differed from the PS implant group. The groups did not differ in rotation during the walking task (P > 0.05). The MP group displayed significantly (P < 0.01) greater knee extensor activation during the step-ascent than the PS group. CONCLUSION: The MP design was only significantly different to another implant design for the step-ascent task. Patients with either knee implant types were not strictly limited to producing the traditional "screw-home" mechanism, defined by external rotation during extension. Furthermore, comparison with the non-implant contralateral limb suggested that rotation is not necessarily dictated by implant design.