THE EFFECT OF A KNEE BRACE ON HIP FLEXION AND ADDUCTION

Abstract

Posterior dislocation of replacement hips may occur during hip flexion and adduction. Whilst hip braces can restrict hip movement, they are cumbersome and have a low patient compliance. Knee braces are more comfortable to wear and also restrict hip movement by tightening the hamstrings. This study investigated the effect of a knee brace on hip flexion and adduction.

The movement of 20 normal hips in 20 healthy volunteers aged 25–62, were assessed using a magnetic tracking system (Polhemus Fastrak). Tracking sensors were attached over the iliac crest and lateral thigh. Subjects were asked to lie on a couch and flex and adduct their hip three times with their knee bent. A knee brace was then applied and the hip movements were repeated with the knee extended. During each movement the tracker recorded hip flexion and adduction angles with an accuracy of 0.15 degrees.

When the knee was flexed, the mean hip flexion angle was 66.00 (CI95 = 61.1, 70.8). When the knee was braced, the mean hip flexion angle was 35.30 (CI95 = 28.5, 42.1). Hence the knee brace reduced hip flexion by 46 % (30.70). A paired t-test found this highly significant (P < 0.001).

When the knee was flexed, the mean hip adduction angle was 23.70 (CI95 = 20.6, 26.9). When the knee was braced, the mean hip adduction angle was 21.60 (CI95 = 19.2, 24.1). Hence the knee brace reduced hip adduction by 9 % (2.10). A paired t-test found this was not significant (P = 0.3).

These 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 can be expected.