Dealing with the deficient acetabulum in revision hip arthroplasty: the importance of implant migration and use of the jumbo cup.

The complexity of acetabular anatomy makes operative planning for revision difficult. Plain radiographs do not provide as much detail about structural damage to the acetabulum as the surgeon would like in order to decide on the need for special components and bone grafts. The authors have found that analysis of acetabular implant migration has been a reliable surgical planning technique to indirectly assess residual acetabular bone stock. Superior shift on the anterioposterior radiograph implies posterior superior acetabular roof loss, seriously compromising the major weight-bearing area of the acetabulum. Although this may require a structural allograft, the authors have found that bone stock at the level of the true acetabulum is much better than in the false acetabulum created by the migration. Therefore, rather than a small cup placed high on the ilium, a large jumbo cup can frequently restore better hip biomechanics and take advantage of all of the remaining host bone for component stability and support, without the need for structural grafts. Medial shift implies medial superior migration leaving the physiologically strong rim of the acetabulum intact and therefore, less structurally damaging than posterior superior migration. Because the acetabular rim is anatomically designed for load bearing, a rim fit cup restores physiological weight transfer across the pelvis. Bone graft behind such a cup is protected from full weight-bearing forces and has proven to be very reliable.