Abstract
Acetabular protrusio occurs from migration of the femoral head medial to Kohler's line. This occurs in inflammatory arthritis, osteoarthritis with coxa vara deformities, previous acetabular fracture, and in metabolic bone diseases such as osteomalacia, Paget's disease, Marfan's syndrome, and osteogenesis imperfecta. Total hip replacement in this situation is difficult due to the requirement to place the acetabular component opening at the level of the normal rim or the patient will be at risk for component-on-component or bone-on-bone impingement, resulting in dislocation or component loosening. The deficient medial wall doesn't resist cup subsidence and the deficient peripheral rim may provide poor initial cup stability.
Many management options have been described including using cement, bulk bone graft, and particulate graft to support the cup medially, and use of a reinforcement ring cage to provide better rim support. Gates reported on a series of 36 primary total hip replacements with acetabular protrusio treated with cemented cups and medial particulate autograft with a mean follow-up of 12.8 years with 6 definitively loose, 3 probably loose, and 22 possibly loose. The technique that provides initial porous cup stability and potential for long-term biological fixation is preferred. Mullaji and Shetty reported 90% good and excellent results and no loosening or migration at a mean 4.2 years in 30 primary total hips with acetabular protrusio treated with oversized porous cups for rim support and medial particulate bone grafting. Forty percent of their cases had protrusio greater than 15 mm medial to Kohler's line. Hansen and Ries also reported no revisions using this same technique in 19 revision total hips with an average follow-up of 2.8 years. However, they emphasised that this technique should only be used if the peripheral rim is intact, and if inadequate, to use a reconstruction cage. In revision total hips with large medial acetabular defects this is more likely to be the case. However, use of a reconstruction cage doesn't provide biological fixation. Ilyas reported a 15.1% loosening rate using cages for revisions with medial defects at a follow-up of 6 years.
I have alternatively used a porous protrusio shell when rim support is poor and the medial defect is greater than 10 mm. The technique is to perform a cylindrical peripheral ream and a medial hemispherical ream. This provides greater host bone to shell contact for stability and greater biological fixation, and fills much of the medial defect. I used this technique in 43 cases with an average follow-up of 3.7 years. There were no revisions, no apparent cup migrations, and no progressive component bone radiolucencies. For primary total hips with protrusio, when good rim support can be achieved with a few millimeters of peripheral over-ream, a standard porous cup and medial particulate autografting is preferred. However, in many primary cases with greater than 10 mm of protrusio, the peripheral rim may be significantly stress shielded and thus, may have poor rim support unless the rim is significantly over-reamed. Because of my excellent results using protrusio shells in revision cases, I will consider also using a protrusio shell in primary total hips in elderly patients with >10 mm of protrusio. I have experience in 10 primary cases with an average follow-up of 4.1 years. One failed for infection. The other 9 have been successful with no apparent cup migration and no progressive component bone radiolucencies.
