In vivo six degrees of freedom kinematics during step ascent in three total knee arthroplasty designs: A randomised clinical trial

Background: Total knee replacement (TKR) is generally successful in alleviating joint pain, and restoring function, however, select patients experience continuing discomfort and unsatisfactory functional outcomes postoperatively. These deficits are particularly evident in highly demanding activities such as a step-up motion. The aim of this study was to compare the six degrees of freedom kinematics of three common TKR designs during step-up. 

Methods: Sixty-eight participants were randomised to receive one of three TKR implants: cruciate-retaining fixed bearing (CR-FB), cruciate-retaining rotating platform (CR-RP) or posterior-stabilised fixed bearing (PS-FB). Participants performed a step-up task which was imaged via single-plane fluoroscopy at 12 and 24 months postoperatively. Implant models were registered to the fluoroscopy, generating kinematic data. Linear regression models were used to test the effect of implant design on kinematics during step-up. 

Results: Fifty-four participants were included in the final analysis. Key kinematic findings were that the CR-FB designs started and remained more posterior on the tibia throughout step-up. Furthermore, CR-RP designs consistently demonstrated greater degrees of external femoral rotation. Interestingly, there were no differences in rotation or translation between designs. 

Conclusions: The findings indicate that there may be kinematic advantages when selecting a particular implant design. However, the CR-FB design demonstrated unique kinematics which may favour a more efficient step-up. Further investigation of the relative biomechanical efficiency of the CR-FB implant would be an interesting next step in this research.