Intra-operative joint gap kinematics in unicompartmental knee arthroplasty

The use of an offset type tensor for total knee arthroplasty that can be set with patellofemoral joint reduction and femoral component placement enables surgeons to assess soft tissues in the physiological postoperative knee condition, showing different kinematic pattern of soft tissues in varus osteoarthritic knees between cruciate-retaining and posterior-stabilized total knee arthroplasty. However, gap kinematics in unicompartmental knee arthroplasty is unclear.

Using a newly developed tensor that is designed to assess soft tissue balance throughout the full range of motion with femoral component placement, we assessed the intra-operative joint gap measurements of unicompartmental knee arthroplasties performed at 0, 10, 30, 45, 60, 90, 120 and 135° of flexion in 20 osteoarthritic patients. In addition, the kinematic pattern of unicompartmental knee arthroplasty was compared with those of cruciate-retaining and posterior-stabilized total knee arthroplasty that were calculated as medial compartment gap from the previous series of this study.

While the joint gap measurements of unicompartmental knee arthroplasties increased from full extension to extension (10° of flexion), these values remained constant throughout the full range of motion. Of note, the gap values of cruciate-retaining total knee arthroplasty were significantly smaller from midrange to deep flexion compared with posterior-stabilized total knee arthroplasty, and furthermore unicompartmental knee arthroplasty showed a significantly smaller gap from extension to midrange flexion compared with cruciate-retaining total knee arthroplasty.

Accordingly, we conclude that the intra-operative joint gap kinematic pattern in unicompartmental knee arthroplasty differs from the pattern in total knee arthroplasty.