Protrusio acetabuli is a developmental anomaly with a deep socket with the femoral head crossing the ilio-ischial line. It can develop with Rickets, Osteogenesis imperfecta, and JRA, but most protrusio is idiopathic.
Protrusio with normal medial coverage (MCE 25 degrees or more) and flat sourcil angle typically causes pincer type of impingement and develops arthritis by 40 years of age. Treatment before arthritis sets in can be extensive acetabular rim trimming to improve range of motion.
Protrusio with medial dysplasia / high acetabular fossa (MCE < 20 degrees) and negative sourcil angle causes medial osteoartrithis from foveal loading on the acetabular fossa and pincer impingement from acetabular over coverage. Treatment before arthritis sets in is reverse Periacetabular osteotomy to flatten the sourcil and improve medial coverage. This treatment also requires antero inferior rim trimming to decrease the prominence of the inferior acetabulum from medial rotation of the acetabular fragment.
Any amount of valgus cannot compensate for a significantly negative sourcil. Just like any amount of varus cannot compensate for severe acetabular dysplasia.
Bilateral protrusion with negative sloped sourcils and medial osteoarthritis on the left side.