The kinematics and stability of single-radius versus multi-radius femoral components related to mid-range instability after TKA

There continues to be some dissatisfaction with the function of total knee arthroplasties (TKA). "Mid-range instability" has been linked to multi-radius femoral components allowing transient ligament slackness and instability during knee flexion. Single-radius designs have been introduced to avoid this. We compared the kinematics and stability of eight natural knees versus multi-radius and single-radius TKAs in vitro. The loading conditions imposed across the range of active knee extension were anterior-posterior drawer forces, internal-external rotation torques, and varus-valgus moments. Significant differences were not found between the biomechanical behavior of the two TKAs. Both were significantly different from the natural knee in allowing greater anterior drawer laxity near extension, probably caused by excision of the anterior cruciate ligament, but no difference occurred beyond 30° flexion. No differences were found for any of the other degrees-of-freedom of movement. A geometric analysis suggested that the multi-radius design may tense the MCL more than the single-radius in mid-flexion, contrary to expectation. These kinematic and stability tests did not find mid-range instability of the knees, and so they could not demonstrate enhanced mid-range stability of the single-radius TKA over the older multi-radius implant. This suggests that mid-range instability may relate to unrecognized ligament laxity during surgery, rather than being inherent to a specific feature of implant design.