Measurement of knee stiffness and laxity in patients with documented absence of the anterior cruciate ligament

Abstract

Thirty-five patients with documented absence of the anterior cruciate ligament were tested on the University of California, Los Angeles, instrumented clinical knee-testing apparatus and we measured the response curves for the following testing modes: anterior-posterior force versus displacement at full extension and at 20 and 90 degrees of flexion; varus-valgus moment versus angulation at full extension and 20 degrees of flexion; and tibial torque versus rotation at 20 degrees of flexion. Absolute values of stiffness and laxity and right-left differences for these injured knees were compared with identical quantities measured previously for a control population of 49 normal subjects with no history of treatment for injury to the knee. For both the uninjured knees and the knees without an anterior cruciate ligament, at 20 and 90 degrees of flexion the anterior-posterior laxity was greatest at approximately 15 degrees of external rotation of the foot. The injured knees demonstrated significantly increased total anterior-posterior laxity and decreased anterior stiffness when compared with the uninjured knees in all tested positions of the foot and knee. The mean increase in paired anterior-posterior laxity for the injured knees in this group of patients at ±200 newtons of applied anterior-posterior force was 3.1 millimeters (+39%) at full extension, 5.5 millimeters (+57%) at 20 degrees of flexion, and 2.5 millimeters (+34%) at 90 degrees of flexion. The mean reduction in anterior stiffness for injured knees was also greatest (-54%) at 20 degrees of knee flexion. Only slight reduction in posterior stiffness (-16%) was measured at 20 degrees of flexion, and this probably reflected the presence of associated capsular and meniscal injuries. In the group of anterior cruciate-deficient knees, the patients with an absent medial meniscus showed greater total anterior-posterior laxity in all three positions of knee flexion than did the patients with an intact or torn meniscus. Varus-valgus laxity at full extension increased an average of 1.7 degrees (+36%) for the injured knees, while varus and valgus stiffness decreased 21% and 24%. Absence of the medial meniscus (in a knee with absence of the anterior cruciate ligament) increased varus-valgus laxity at zero and 20 degrees of flexion. Torsional laxity increased by an average of 10% in knees without an anterior cruciate ligament and internal rotation stiffness decreased 16%. There was no difference in internal and external isometric torque-generating capacity between the injured and uninjured knees. Clinical relevance: The clinical drawer test is most commonly performed at 90 degrees of knee flexion. Our results show clearly that measured right-left differences in anterior stiffness and total anterior-posterior laxity are greatest at 20 degrees of knee flexion. Differences in anterior-posterior laxity are best observed when high (200-newton) anterior-posterior forces are applied to the tibia and stiffness changes are best sensed at low levels of force (100 newtons or less).