Background. Understanding the normal kinematics of the joints is important for reconstructive surgery. However, only a few extensive studies have been done on medial and lateral laxity of the normal knee. Methods. Radiographs of 50 normal knees were obtained under varus and valgus stress in both extension and flexion and the relative angle of the articular surface was measured. Results. In extension, the mean angle was 4.9° in varus stress and 2.4° in valgus stress. In flexion, the mean angle was 4.8° in varus stress and 1.7° in valgus stress. Lateral laxity was significantly greater than medial laxity in both extension and flexion. Conclusions. Lateral laxity may be necessary for the medial pivot movement of the normal knee. There is some disagreement regarding the importance of pursuing the perfect rectangular gaps during total knee arthroplasty (TKA). The methods for measuring the tension of soft tissues during the operation are not accurate and do not always reflect the postoperative tensions of dynamic phases, such as walking and standing. Slight lateral laxity can be accepted with TKA, and further studies are necessary to determine whether prosthesis lift-off occurs in the replaced knee with slight lateral laxity similar to that in the normal knee.
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