Bipolar revision arthroplasty for failed threaded acetabular components: radiographic evaluation of cup migration

We studied the radiographic results of bipolar revision hip arthroplasty in 25 patients (25 hips). These patients had suffered from osteoarthrosis of the hip due to acetabular dysplasia and had undergone total hip replacement using smooth threaded acetabular components. In the revision surgery, these components were replaced by bipolar cups; autografts were implanted in 23 patients and hydroxyapatite grains were added in 7. At the time of follow-up, the severity of migration was radiographically graded from I to IV: grade I, 5 mm or less; grade II, 6 to 10 mm; grade III, 11 to 15 mm; and grade IV, 16 mm or more. Five to 11 years (average, 7.0 years) after surgery, 9 patients were classified as grade I, 6 as grade II, 4 as grade III, and 6 as grade IV. Among the 6 individuals with grade IV migration, hydroxyapatite grains had been used in 5. There was a significant negative correlation between the distance of migration and the increase in size of the threaded acetabular components used in the primary total hip replacement and the bipolar cups in the revision surgery. Because of the tendency to migrate, we concluded that this procedure has a limited role in revision surgery.