Intrapelvic protrusion of the acetabular component following total hip replacement.

Protrusion of the acetabular component into the true pelvis following total hip replacement has occurred in 5 patients, 4 with severe rheumatoid arthritis and 1 with a destructive type of degenerative hip disease. Preoperatively all hips had severe protrusio acetabuli, a markedly thin acetabular medial wall and advanced osteoporosis. Four had a McKee-Farrar prosthesis, a metal-to-metal device with high frictional torque, particularly when the contact is equatorial, and no damping capacity against marginal impingement in the extreme range of motion. In order to reduce the incidence of intrapelvic protrusion, extreme care should be given to preserve the medial bone stock of the acetabulum, more so when it is already damaged or defective. If anchoring holes are used they should be restricted to the superior ilium, pubis and ischium and should not perforate the medial wall. Once loosening is present, reoperation is indicated to avoid progressive bone reabsorption by the abrasive motion of the loosened prosthesis, that might lead to irreparable bone loss. To reduce the stress transmitted to an already weakened acetabulum, select a total prosthetic device with low friction; fix it with acrylic cement in order to distribute the stress over a large surface; carefully orient both components to avoid marginal impingement; be certain to preserve the medial wall as much as possible and if it is already defective reinforce it by bone grafting and/or wire mesh.