The Biomechanics of Functional Knee Braces on the Anterior Cruciate Ligament Deficient Knee During Non-weightbearing and weightbearing

B.D. Beynnon, D.G. Brown, R.S. Labovitch, B.C. Fleming, and D.L. Churchill

Department of Orthopaedics and Rehabilitation, McClure Muskuloskelatal Research Center, University of Vermont College of Medicine, Burlington, Vermont USA

Introduction

Previous studies have shown that functional knee braces can reduce anterior-posterior (A-P) displacement of the tibia relative to the femur (e.g. A-P laxity) in the non-weightbearing knee, and improve knee function. The purpose of this investigation was to study the effect of functional knee braces of the biomechanics of the anterior cruciate ligament (ACL) deficient knee while subjects were weightbearing as well as non-weightbearing.

Methids

A new device (called the Vermont Knee Laxity Device or VKLD) was used to measure A-P laxity while subjects were either weightbearing or non-weightbearing. The inter-observer reliability and day-today repeatability of the VKLD have been evaluated by us previously and found to be similar to planar stress x-ray and the KT-l000 arthrometer (Churchill et al). A-P laxity of the knee was defined as the displacement of the tibia relative to the femur produced by 130N (anterior) and -120N (posterior) directed loads. Nine subjects (7 males and 2 females) with chronic ACL-deficient knees and normal contralateral knees were studied. The protocol was initiated by positioning the subject in the VKLD with their knees at 20 degrees of flexion. A-P laxity of both knees was measured while the joint was non-weightbearing and then weightbearing (40% of body weight on each leg. The subject exited the VKLD, was fitted with a functional knee brace, and then repositioned in the VKLD for A-P laxity measurements, again with the legs non-weightbearing and then weightbearing. This was repeated three times, each with a randomly selected functional knee brace, these included the Softec (Bauerfeind inc.), the Donjoy Legend (Smith and Nephew Donjoy Inc.) and Townsend Rebel (Townsend Inc.). The protocol was finalized by repositioning the subject in the VKLD without a brace and measuring A-P laxity of both knees, this assured repeatability of the initial A-P laxity measurement. A complete block experimental design with blocking on subjects was used to compare the A-P laxity values (dependent variable) between brace conditions. The data were analyzed with analysis of varience.

Results

With the lower extremity non-weight-bearing, average A-P laxity of the ACL-deficient knees (19.5 mm) was significantly greater than the normal knees (12.0mm) (p<.05). Bracing the non-weightbearing ACL-deficient knee significantly reduced abnormal A-P laxity as compared to the same leg without a brace (p<.05). These values were 11.25, 11.5, and 12.5 mm for the Donjoy, Townsend, and Softec braces respectively. A-P laxity values of the braced ACL-deficient leg and unbraced normal leg were not significantly different. With the lower extremity weightbearing. average A-P laxity values of ACL-deficient knees (10.0mm) were significantly greater than the normal knees (5.5mm) (p<.05). Bracing the weight-bearing ACL-deficient knee significantly reduced abnormal laxity values as compared to the same knee without a brace (p<.05). These values were 6.5, 6.6, and 6.6mm for the Donjoy, Townsend, and Softec braces respectively.

Discussion

Athletes with disrupted, injured, or reconstructed ACLs depend on functional knee braces to protect their knees when substantial compression and shear loads are transmitted across the knee. This study revealed that functional knee bracing reduces abnormal A-P laxity of the ACL-deficient knee in both the non-weightbearing and weightbearing conditions. This reduction in A-P laxity may provide a reduction in abnormal shear stress at the tibiofemoral joint surfaces, stress that is thought to initiate the progression of degenerative joint changes. This study provides a biomechanical rational for future studies to determine if functional bracing of the ACL-deficient knee can also reduce the shear stress produced at the tibiofemoral joint.

References: Churchill DL, et al 45th Annual Meeting. Orthopaedic Research Society, February 4, 1999. Anaheim, California

Abstracts, XVIIth ISB Congress, Calgary, August 3 - 13th 1999