A Possible Explanation for Perceived Knee Laxity in ACL-Deficient Athletes Wearing Braces

Abstract & Commentary

Synopsis: ACL braces were least effective in controlling abnormal knee laxity during the transition from nonweight bearing to weight bearing.

Source: Beynnon BD, et al. The effect of anterior cruciate ligament deficiency and functional bracing on translation of the tibia relative to the femur during non-weight bearing and weight bearing. Am J Sports Med. 2003;31(1):99-105.

Many authors report on the efficacy of functional bracing of the knee during unweighted or weight-bearing conditions. However, little is known about the transition between these positions. The purpose of this study was to quantify the amount of tibia motion in the anterior cruciate ligament (ACL)-deficient knee during nonweight bearing, throughout the transition to weight bearing, and during weight bearing. Nine subjects with chronic ACL tears were studied with and without knee braces. Anterior/posterior shear and compressive loads were applied to the knee, simulating weight-bearing, nonweight-bearing and transitional weight-bearing conditions. Anterior laxity of the tibia relative to the femur was measured with the Vermont Knee Laxity Device.

Anterior/posterior tibial motion was significantly reduced in the braced anterior cruciate ligament-deficient knee to a level within the limits of the normal knee motion during nonweight-bearing and weight-bearing conditions. However during the transition phase from nonweight bearing to weight bearing, the anterior translation of the tibia was 3.5 times greater than in the normal knee, and bracing did not significantly reduce the translation.

Beynnon and associates concluded that bracing the chronic ACL-deficient knee was effective in significantly reducing abnormal anterior/posterior laxity during nonweight-bearing and weight-bearing conditions. However, braces were not effective in reducing the abnormal translations produced during the transition phase.

Comment by James R. Slauterbeck, MD

It is important to understand if knee braces control anterior knee laxity in both loaded and unloaded conditions. However, the transition phase between the loaded and the unloaded condition is very dynamic and may be the point where high sheer forces are likely the greatest. Studying the knee during weight-bearing transition may identify higher than expected anterior tibia translation as a result of quadriceps and hamstring torques applied to their respective insertion sites. The knee likely experiences the high change in torque many times during running and cutting. Under these dynamic conditions, the effects of bracing could be diminished if the femur is significantly anterior to the expected position from a resulting anterior sheer force. This unexpected anterior position could lead to a sensation of abnormal laxity that would not be measured under static conditions of weight bearing or nonweight bearing.

The strengths of this study are that it is performed in humans in a well-controlled setting. This study uses a device, which applied a compressive and anterior-based force to the knee. This combination of applied loads simulates weight bearing, nonweight bearing and a transition phase between the 2 over a functional range of motion. This is a unique article and may help explain why some athletes perceive increased laxity in a properly fitting brace during running and cutting.

Dr. Slauterbeck is Associate Professor, Department of Orthopaedic Surgery, Texas Tech University Health Sciences Center, Albuquerque, NM