This is a condition where there is erosion of the inner wall of the socket (acetabulum) and the femoral head migrates into the pelvis. It is more common in females and can be associated with inflammatory arthritis such as rheumatoid arthritis. They are often very symptomatic and can require high levels of painkillers.
Figure 1: x-ray showing significant protrusio on both sides
These can be technically difficult hip replacements. The hip can be difficult to dislocate and the inner wall very thin and prone to fracture. Depending on the bone quality, the socket may need bone grafting which has a significant effect on post-operative rehabilitation. The femoral side is usually straightforward, but care needs to be taken to restore the original anatomy to ensure optimal function.