Intracapsular and atmospheric pressure in the dynamics and stability of the hip. A biomechanical study.

A cadaver study was undertaken to evaluate (a) the relations between the rotation around the axis of the neck of the femur, intracapsular effusion, and intracapsular pressure; and (b) the importance of the atmospheric pressure in stabilizing the hip joint. The following conclusions were reached: (1) There is no increase in intracapsular pressure within the normal range of rotation ("flexion") around the axis of the neck of the femur. (2) Intracapsular fluid (e.g., blood, pus, synovial edema and/or free synovial fluid) decreases this pressureless range of rotation, a likely cause of pain and subsequently flexion contractures. (3) Joint stability is primarily maintained by the atmospheric pressure within the normal range of rotation. The joint capsule is tightened only in extreme extension or flexion and contributes to stability only in these positions. (4) The traction force needed to overcome the stabilizing effect of the atmospheric pressure and thus subluxate the adult joint is approximately 200 N, in a child less, proportional to the square of the diameter of the femoral head. We also propose that intracapsular fluid makes the joint potentially unstable, a prerequisite for unfavorable mechanical cartilage load. These findings have clinical implications in synovitis, septic and juvenile arthritis, congenital dislocation of the hip, arthroplasty, and trauma.