THE EFFECT OF SUPERIOR PLACEMENT OF THE ACETABULAR COMPONENT ON THE RATE OF LOOSENING AFTER TOTAL HIP-ARTHROPLASTY - LONG-TERM RESULTS IN PATIENTS WHO HAVE CROWE TYPE-II CONGENITAL DYSPLASIA OF THE HIP

A method for measurement of the true acetabular region and the approxi mate femoral head center as well as a classification consisting of fou r zones for assessment of the acetabular position of the acetabular cu p were used to analyze the results of primary total hip arthroplasty w ith cement in 117 patients (145 hips). All patients had Crowe type-II congenital dysplasia of the hip. The mean age at the time of the arthr oplasty was fifty-one years (range, fifteen to seventy-six years), and the mean duration of follow-up was fourteen years (range, two to twen ty-two years). The initial position of the acetabular cup outside of t he true acetabular region and outside of zone 1 (inferior and medial) was associated with an increase in the rates of loosening (p < 0.05) a nd revision (p < 0.04) of the femoral components. Cups that initially were more than fifteen millimeters superior to the approximate femoral head center, without lateral displacement, were associated with an in creased rate of loosening (p < 0.001) and of revision (p < 0.04) of th e femoral components as well as with an increased rate of loosening (p < 0.002) and of revision (p < 0.01) of the acetabular components. The se findings suggest that superior positioning of the acetabular compon ent, even without lateral displacement, leads to increased rates of lo osening of the femoral and acetabular components. An attempt should be made to position the acetabular component in or near the true acetabu lar region.