TOTAL HIP REPLACEMENT IN PATIENTS WITH ACETABULAR DYSPLASIA

Abstract

Patients with hip dysplasia that are to be managed with total hip replacement constitute a special group of arthroplasty candidates. Each patient will need to be treated according to his or her anatomical pattern. The important point to remember is that dysplasia in itself is not a contraindication to cementless hip replacement. However, there are certain points that need to be taken into account in the management of this special patient population.

The surgical technique must be appropriate; in particular, cup medialisation is an important feature. The implant system used should be modular so as to permit a large number of combinations, and it also should provide cups for hard bone and cups for soft (osteoporotic) bone, as well as anti-dislocation inserts. Such a system will also allow limb length to be corrected, usually without reference to the centre of rotation of the hip joint. The primary objective is the positioning of the cup in the native bone stock at a site that will ensure optimum primary stability. If an adjunctive shelf procedure is considered necessary, any buttressing done will have to be performed after the cup itself has been stably implanted.

In our centre between 1993 and 1995, 122 were patients with dysplastic hips. One hundred and eleven of these hips were clinically and radiologically followed-up by the author. Mean follow-up was 4.1 years (range 2.1 to 6.1 years). Typically, there were more women than men: 88 cups were implanted in female patients and 23 in male patients. The average age at surgery was 53 years. The youngest patient in the series was 19 years of age and the oldest was 77. Coxa vara osteochondritica (25 cases) was treated as a separate diagnostic and management group. In 17 cases previous acetabuloplasties had been done, usually a Chiari osteotomy. In terms of severity of the dysplasia, 67 hips were Grade I, 36 were Grade II, and eight were Grade III according to Randelli.

All patients were managed with our biconical threaded cup made of commercially pure titanium. In 91 cases the Standard version, designed for use in normal or in hard bone, was employed. The Porosis type of cup, with a 46% greater thread surface area, was chosen in 20 cases. Antidislocation inserts were used in two cases. Conical cups are inserted with prestress and do not therefore require additional fixation with screws or lugs. The most important instrument for the achievement of optimum implant positioning is the medialising reamer with an aggressive front-cutting action. The use of this instrument allows sufficient bony coverage to be obtained to allow fixation of the titanium cup in the host bone with a good primary stability.

The following standard radiographs were used in the analysis: (1) AP view of the operated hip and the contralateral hip; (2) AP and axial views of the operated femur; (3) monitor-controlled AP view of the cup. In 110 cases the position of the cup had not changed during the follow-up period and cranial migration was seen in one case. This case and an additional one were judged to be “at risk” since the patients were pain-free and the position of the implants had remained unchanged for the past two years. These two patients also had more than 2 mm Zone III lucencies. In both cases a superior (Zone I) sclerotic area was found to have developed postoperatively. This pattern differed from the one observed in patients with stable implants, in whom the extent of preoperative sclerosis was significantly reduced following arthroplasty. This regression of sclerosis around a stable implant suggests that the implantation of the titanium cup results in an improvement of the periacetabular stress pattern. In 84 cases there was evidence of increasing integration of the implant; in 22 cases no bony response could be detected, i.e., there was neither apposition of new bone nor loss of existing bone stock. In one case part of the rim of the titanium cup was found to have broken off superiorly. This patient has been recently revised with an exchange of the cup. At revision the cup was found to be so soundly osseointegrated to the extent that it could be removed only after being cut up with a diamond tool. None of the other patients have required revision to date, and none are scheduled for revision as a result of follow-up.