Protrusio acetabuli is an uncommon defect of the acetabulum. The acetabulum is the socket that receives the femoral head to make the hip joint. It is composed of three bones, the ilium, the ischium and the pubis. The socket is too deep and may protrude into the pelvis. 
Limitation of joint range of movement is the earliest sign
The protrusio may progress until the femoral neck impinges against the pelvis.
Diagnosis is made on the basis of an anteroposterior radiograph of the pelvis that demonstrates a center-edge angle greater than 40 degrees and medialisation of the medial wall of the acetabulum past the ilioischial line.
Joint replacement surgery is indicated for severe pain or substantial joint restriction
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^ Van de Velde S, Fillman R, Yandow S (2006). "Protrusio acetabuli in Marfan syndrome. History, diagnosis, and treatment". J Bone Joint Surg Am88 (3): 639–46. doi:10.2106/JBJS.E.00567. PMID 16510833.
McBride MT, Muldoon MP, Santore RF, Trousdale RT, Wenger DR (2001). "Protrusio acetabuli: diagnosis and treatment". J Am Acad Orthop Surg9 (2): 79–88. PMID 11281632.