Summary: Intraoperative assessment of knee kinematics during passive flexion and extension of the unloaded knee fails to adequately replicate the kinematics of the loaded knee during a functional activity.
Introduction: Intraoperatively, the alignment and stability of the prosthetic knee are assessed by observing the motion of the articular surfaces during passive flexion/extension. However, this examination is performed via a medial arthrotomy with limited visibility of the articular surfaces and with the joint unloaded. In view of these limitations of the intraoperative exam, this study was conducted to determine whether unloaded knee motion observed on the operating table is predictive of the motion of the knee during a loaded functional activity.
Methods: Six cadaveric knees were tested:
in a simulator which reproduced the manual intraoperative manipulation of the knee during unloaded passive range of motion (PROM), and
in a functional activity simulator which recreated a loaded squatting maneuver.
Standard 14cm midvastus medial arthrotomies were performed on each knee, and the PROM and squatting simulations were repeated. A laser scanner was used in conjunction with CT models to recreate the three-dimensional position of the knee and allow calculation of medial and lateral femoral rollback and tibial rotation.
Results: With PROM, the femoral condyles translated posteriorly (medial: 6.8±2.2mm, lateral: 15.2±1.3mm) and the tibia rotated internally (13.8±2.0°). A similar motion pattern was observed during squatting with slightly less medial (5.2±0.7mm; p=0.57), and lateral (12.8±0.9mm; p=0.06) rollback and rotation (10.7±1.54°; p=0.30). Interestingly, paradoxical anterior translation of the femur (>
2mm) and external rotation of the tibia (>
2°) were observed in 30% of knees during a loaded squat; however, this motion was not predicted by the PROM test.
Discussion: Similar knee kinematics are observed during unloaded flexion/extension and a physiologic squatting activity. However, the unloaded intraoperative test was unable to predict the occurrence of paradoxical motion during functional loading. Therefore, passive intraoperative testing of the knee is of limited value as a predictor of functional knee biomechanics.
