[The stabilizing effect of atmospheric pressure (AP) on hip joint subject to traction force--an experimental study]

We aimed to prove the stabilising effect of atmospheric pressure (AP) on the hip joint experimentally.

In the experiment, model joints of 28 mm, 32 mm und 36 mm diameter were subjected to increasing traction force. The acting force and the resulting dislocation distance were measured both with the capsule hermetically sealed, as well as with the capsula open.

For the hermetically sealed capsule we measured maximum resistances of 7.6 kp for the 28 mm joint, 10.4 kp for the 32 mm joint and 12.4 kp for the 36 mm joint. With the capsule open we found resistances from 0.4 kp to 1 kp. Our experimental results exceeded the predicted resistances of 6 kp, 7.8 kp and 9.9 kp. Increased amounts of synovial fluid reduced the stability.

Our measurements confirm the continual stabilising effect of AP on the hip joint, which can be quantified as the resting potential of stability (RPS) or luxation work (LW). The RPS is calculated by multiplying the difference of AP and saturated vapour pressure of synovial fluid with the cross-sectional area of the femoral head. It represents the force, necessary for luxation of the joint against the resistance of AP. The RPS is proportional to the square of the joint diameter. The LW, calculated by multiplying RPS with the luxation distance, is proportional to the joint diameter cubed. That is why a small increase of joint diameter leads to a significant increase of stability, while the rate of the increase of range-of-motion decreases. To achieve stability of a total hip arthroplasty the size of the joint components should depend on the size of the resected femoral head. Also the hermetically sealed capsule should be reconstructed carefully.