Protrusio acetabuli is identified on anteroposterior (AP) radiographs of the pelvis with an acetabular line projecting medial to the ilioischial line. We documented this radiographic sign and additional radiographic parameters in 19 patients (29 hips) with protrusio and compared the parameters to those of 29 older patients (29 hips) with advanced primary osteoarthritis (OA) but no protrusio and 12 younger patients (22 hips) with protrusio but no advanced OA. A negative acetabular roof angle and particularly large acetabular fossa were more apparent in younger patients; these hips suggest the destruction of a protrusio hip begins less in the medial joint area and more in the posteroinferior joint, and the mechanism is driven less by excessive medially directed forces but by a pincer impingement. While the indication for joint-preserving surgery currently consists primarily of a valgus femoral osteotomy based on AP radiographs and patient age, modern decision making also relies on cartilage evaluation and requires advanced surgical techniques. We conclude joint-preservation surgery must be tailored to the individual hip morphology.