[The significance of the acetabular labrum for hip joint stability--an experimental study]

Aim of our study was to create artificial labra for joint models of different diameters (28, 32 and 36 mm) and to measure their stability potential at rest (SPR). The experiment was performed under the same conditions as a previous one with capsulated joint models. Our target was to prove the hip stabilising effect of the atmospheric pressure (AP) as well as the parameters for joint stability and to test the function of the artificial labra.

Uncapsulated joint models having 28 (A), 32 (B) and 36 (C) mm diameters were sealed with moulded preformed silicone labra. An increasing traction force was applied under water on the fixed joint. Using specially designed software, the exerted force and the dislocation distance were simultaneously and continuously recorded on a computer. The SPR of the examined joint models was calculated as the difference between the maximal exerted force and the weight of the ball-neck component.

Statistical analyses showed that SPR had mean values of 58.12 +/- 2.23 N for the 28 mm joint model (n = 118; A), 75.66 +/- 2.75 N for the 32 mm model (n = 88; B) and 99.91 +/- 1.30 N for the 36 mm model (n = 82; C).

The traction force required for dislocation agreed closely to the expected precalculated SPR values of 58.6 N (A), 76.4 N (B) and 96.7 N (C), which proves the joint stabilising effect of AP in the presence of the essential prerequisites (spherical ball articulating in a hemispherical socket, hermetically closed joint capsule and/or labrum, which contains a small amount of fluid and excludes air). The measured SPR was directly proportional to the square of the joint diameter. Indirectly, it was concluded that the dislocation work at rest is directly proportional to the joint diameter cubed. Consequently, the risk of dislocation after total hip arthroplasty (THA) can be reduced by applying bigger, size-adapted hip balls, whose diameter grows according to the outer diameter of the cup. The increasing range of motion is a favourable side-effect. With careful reconstruction of the capsule and insertion of an intracapsular drain, the risk of dislocation in the early postoperative period could be furtherly reduced. Theoretically, artificial labra could be a useful alternative to augment joint stability in THA with a high dislocation tendency instead of constrained liners.