Stiffness and laxity of the knee--the contributions of the supporting structures. A quantitative in vitro study.

Thirty-five normal cadaver knees were tested manually in six positions of the knee using apparatus designed to measure the moment-rotation responses for both varus-valgus angulation and torsion of the tibia, as well as the force-displacement responses for anterior-posterior movement of the tibia. The responses of all knees to all modes of loading were non-linear, reflecting increasing stiffness. With the knee at full extension, stiffness was maximum and laxity was minimum. Hence, it was in this position that changes in stability (laxity and stiffness) were best demonstrated when ligament structures were sectioned. Differences in laxity were observed between right and left knees of intact paired specimens. Torsional laxity and internal rotation stiffness were most affected by sectioning the medial collateral ligament, while external rotation stiffness was only affected by division of both the lateral collateral ligament and the posterior capsule. Varus-valgus laxity was relatively unaffected by removal of the menisci or section of the cruciate ligaments but increased greatly when either collateral ligament was cut. The medial collateral ligament was the main contributor to valgus stiffness, whereas the lateral collateral ligament had no measurable effect on varus stiffness. Anterior-posterior stability was affected to some extent by virtually every sectioning procedure. Isolated section of the anterior cruciate ligament produced the greatest increase in anterior-posterior laxity at full extension and section of the posterior cruciate, the greatest increase at 90 degrees of flexion. Large increases in anterior-posterior laxity were also observed when the medial collateral ligament and posterior capsule were sectioned in combination.