[Luxation of total hip prosthesis. Statistical validation of a modelization, apropos of 52 cases]

A previous modelization of the Total Hip Dislocation phenomenon enabled us to separate two main forms: The malposition dislocation, secondary to an error in the position of one of the prosthetic pieces, and mainly of the acetabular cup; the dislocation by muscular discoaptation, where the arthroplasty has discompensated a precarious muscular balance. The authors present a statistical validation of the model regarding 52 dislocations which occurred during the follow-up of 1196 hip arthroplasties. A "control" group was composed of 50 prostheses taken at random within the non-dislocated hips. The total rate of dislocation was 4.30 per cent. Every second one showed to be of malposition origin with a significant limitation of the theoretical intra-prosthetic amplitudes. These dislocations--all appearing early--were, for half, of the anterior type, and in that case, were mainly due to an excess of acetabular anteversion (average 35.2 degrees), the other half was of posterior type, with, in particular, a low value of acetabular anteversion (average 11.4 degrees when it was 23.4 degrees in the non-dislocated group). The other dislocations (all posterior and more delayed) corresponded to a defect of muscular coaptation in the "high-risk" patients (previously operated hip, weight loss, necrotic etiology, neuromuscular etiology, old age). Thus, in almost 90 per cent of the cases, the clinical study brought a validation of our model giving a different explanation from the too classical "cam-acting", often held responsible for the dislocation phenomena.