acetabular protrusion

Acetabular protrusion, also known as protrusio acetabuli, is intrapelvic displacement of the acetabulum and femoral head, so that the femoral head projects medial to the ilioischial line. It should be differentiated from coxa profunda.

Pathology

Acetabular protrusion is divided into primary and secondary types.

Primary

Primary acetabular protrusion is characterized by a progressive protrusion in middle-aged women. The condition may be associated with osteoarthritis and may be familial.

Secondary

Mnemonic

Radiographic features

  • intrapelvic displacement of the medial wall of the acetabulum

Different diagnostic criteria have been suggested on an anteroposterior radiograph of the pelvis:

  • center-edge angle greater than 40 degrees and medialisation of the medial wall of the acetabulum past the ilioischial line (Kohler's line) 
  • acetabular fossa greater than 3 mm beyond the ilioischial line in men and greater than 6 mm in women
  • projection of the femoral head medial to the ilioischial line

A useful trick is that the acetabular cavity will often, but not always, be seen protruding medially to the iliopectineal line.