THE EFFECT OF A KNEE BRACE ON HIP FLEXION AND ADDUCTION

Abstract

Introduction: Posterior dislocation of replacement hip joints may occur during hip flexion and adduction. A hip brace is commonly used for recurrent dislocations in patients awaiting revision surgery or when unfit for it. However, these hip braces are cumbersome and have a low patient compliance.

Knee braces are more comfortable to wear, and they also restrict hip movement by tightening the hamstrings. With this background we investigated the effect of a knee brace, applied in full extension, on hip flexion and adduction.

Methods: The movement of 20 normal hips in 20 healthy volunteers aged 25–62, were assessed using a magnetic tracking system (Polhemus Fastrak). One tracking sensor was attached near the anterior superior iliac spine and another one on the lateral aspect of the thigh at a fixed distance from the knee joint. Subjects were then asked to lie on a couch and flex and adduct their hip three times each with the knee bent and then with their knee braced in extension. Two sets of three readings were recorded. During each movement the tracker recorded hip flexion and adduction angles, with a measurement accuracy of 0.15 degrees.

Results: With a flexed knee, the mean hip flexion angle was 66.0 degrees (CI95 = 61.1, 70.8). With the knee braced, the mean hip flexion angle was 35.3 (CI95 = 28.5, 42.1). Hence the knee brace reduced hip flexion by 46% (30.7 deg). A paired t-test found this highly significant, with P < < 0.001.

With a flexed knee, the mean hip adduction angle was 23.7 degrees (CI95 = 20.6, 26.9). With the knee braced, the mean hip adduction angle was 21.6 (CI95 = 19.2, 24.1). Hence the knee brace reduced hip adduction by 9% (2.1 deg). A paired t-test found this was not significant with P = 0.3.

Conclusion: The results indicate that a knee brace can restrict hip flexion by almost 50%. This information may be useful for patients in whom restriction of hip flexion provides hip stability. As the knee brace is more comfortable than the hip brace, a better patient compliance is expected.