Kinematics of passive flexion following balanced and overstuffed fixed bearing unicondylar knee arthroplasty

Fixed‐bearing medial unicompartmental knee arthroplasty restores neither the medial pivoting behavior nor the ligament forces of the intact knee in passive flexion

“…This finding agrees with our results: internal rotation of 14° in the intact knee from 0° to 90° of flexion. Our model predicted 11.2° of tibial external rotation from 0° to 30°of flexion after UKA implantation in a balanced position, which also agrees with a previous cadaveric study …”

“…Similarly, the 2.8 mm RMS difference between predicted and experimentally measured AP translations of the lateral condyle was 2.2 times less than the predicted differences caused by installing the balanced UKA. The model of the intact knee also corroborated the AP condylar translations measured in a corresponding cadaver experiment, and, after UKA, the abnormal external rotation with passive flexion measured in a group of ten similarly tested knee specimens . Thus, the conclusions derived using the model are strong.…”

“…Therefore, UKA perturbs the constraint provided by the medial compartment thereby altering kinematics compared to the intact tibiofemoral joint (i.e., a knee with unreplaced articular surfaces and all passive restraints including cruciate and collateral ligaments, and menisci). For example, we and others previously showed that installing a fixed‐bearing, nonconforming UKA causes external rotation of the tibia relative to the femur and increased anterior–posterior (AP) translation of the medial femoral condyle compared to the intact knee joint . Thus, the screw home motion, a characteristic feature of intact knee function, was disturbed …”

The influence of polyethylene bearing thickness on the tibiofemoral kinematics of a bicruciate retaining total knee arthroplasty

“…Moreover, the chosen thickness of the polyethylene bearing can have important implications on implant wear, alignment, knee kinematics, and ligament tensioning, among other factors, for both total and unicondylar knee arthroplasty [16,30,31]. To the authors' knowledge, there have been no previous studies documenting the impact of bearing thickness on BCR TKA kinematics.…”

“…Generally, ligament balance is subjectively assessed by the surgeon [13] and can be modified by altering the thickness of the polyethylene bearing. If chosen improperly by using either an under-or oversized polyethylene insert, the resulting tibiofemoral kinematics, contact pressure, and wear properties can be altered [14,15].While previous studies have documented the effect of bearing thickness on unicompartmental implants [16,17], there have been no studies documenting the influence on BCR TKA.…”