Comparing in vivo kinematics of unicondylar and bi-unicondylar knee replacements.

Preserving both cruciate ligaments in unicondylar knee arthroplasty likely provides more normal knee mechanics and contributes to enhanced patient function. It follows that preserving both cruciate ligaments with total knee arthroplasty should provide functional benefit compared to arthroplasty sacrificing one or both cruciates. The purpose of this study was to compare knee kinematics in patients with optimally functioning cruciate-preserving medial unicondylar and bi-unicondylar arthroplasty to determine if knee motions differed. Eight consenting patients with seven medial unicondylar and five bi-unicondylar arthroplasties were studied using lateral fluoroscopy during treadmill gait, stair stepping, and maximum flexion activities. Patient-specific geometric models based on CT and CAD data were used for shape matching to determine the three-dimensional knee kinematics. Tibiofemoral contact locations were computed for the replaced compartments. Maximum flexion in kneeling was 135 degrees +/-14 degrees for unicondylar knees and 123 degrees +/-14 degrees for bi-unicondylar knees (p=0.22). For 0 degrees -30 degrees flexion during the stair activity, the medial condyle translated posterior 3.5+/-2.5 mm in unicondylar knees and 4.7+/-1.9 mm in bi-unicondylar knees (p>0.05). Lateral posterior translation was 5.0+/-2.3 mm in bi-unicondylar knees for 0 degrees -30 degrees flexion. From heel-strike to mid-stance phase, there was little tibial rotation, but unicondylar knees showed 1.5+/-1.6 mm posterior translation of the medial condyle, while bi-unicondylar knees showed 5.1+/-2.2 mm (p<0.05). The bi-unicondylar knees showed 3.8+/-3.4 mm posterior lateral condylar translation. Preserving both cruciate ligaments in knee arthroplasty appears to maintain some basic features of normal knee kinematics. Knees with bi-unicondylar arthroplasty showed kinematics closer to motions observed in total knee arthroplasty, slightly less weight-bearing flexion, and greater dynamic laxity in gait than unicondylar knees. Despite kinematic differences, knees with unicondylar and bi-unicondylar arthroplasty can provide excellent functional outcomes in appropriately selected patients.