Effects of hip joint position and intra-capsular volume on hip joint intra-capsular pressure: a human cadaveric model

BACKGROUND Increase in hip intra-capsular pressure has been implicated in various hip pathologies, such as avascular necrosis complicating undisplaced femoral neck fracture. Our study aimed at documenting the relationship between intra-capsular volume and pressure in various hip positions. METHODS Fifty-two cadaveric hips were studied. An electronic pressure-monitoring catheter recorded the intra-capsular hip pressure after each instillation of 2 ml of normal saline and in six hip positions. RESULTS In neutral hip position, the control position for investigation, intra-capsular pressure remained unchanged when its content was below 10 ml. Thereafter, it increased exponentially. When the intra-capsular volume was 12 ml, full abduction produced a 2.1-fold increase (p = 0.028) of the intra-capsular hip joint pressure; full external rotation and full internal rotation increased the pressure by at least 4-fold (p < 0.001). Conversely, there was a 19% (p = 0.046) and 81% (p = 0.021) decrease in intra-capsular hip joint pressure with flexion of the hip joint to 90-degree and 45-degree, respectively. CONCLUSION Intra-capsular pressure increases with its volume, but with a wide variation with different positions. It would be appropriate to recommend that hips with haemarthrosis or effusion should be positioned in 45-degree flexion.