[Dynamic stabilization of a total hip arthroplasty. Apropos of 100 total prostheses of which 52 dislocations].

This paper outlines the principal factors which stabilise a normal hip, and describes the modifications which occur following total joint replacement. Deficiencies in muscle function and mechanical restraints due to the prosthesis may be present. A biomechanical mathematical model has been devised which examines the variations in length of the middle gluteal muscle during movement, particularly in hip flexion, when there is the greatest risk of dislocation. This enables a critical evaluation of the muscle forces surrounding the hip, and allows identification of patients with a high risk of dislocation. These observations have been applied to a group of 100 patients, 52 of whom had experienced a dislocation and 48 with stable prostheses. Patients at high risk according to the statistical model tended to dislocate from one month after operation onwards, and are to be distinguished from those who dislocate early due to malposition of the prosthesis.