Musculoskeletal ImagingProtrusio acetabuli
the intrapelvic displacement of the medial wall of the
acetabulum. This abnormality may occur in a number of pathologic conditions, including
rheumatoid arthritis,
ankylosing spondylitis,
septic arthritis,
osteoarthritis,
osteomalacia,
Pagets disease and
sickle cell anaemia. It may also be seen in patients with
neoplasm and after
trauma or irradiation.
In rheumatoid conditions and calcium pyrophosphate dihydrate crystal deposition disease, axial migration of the femoral head with respect to the acetabulum along with symmetric loss of joint space is characteristic, whereas in osteoarthritis the migration tends to be superior or medial as a consequence of the more focal destruction of cartilage. An idiopathic (primary) or familial protrusion of the acetabulum is known as Otto pelvis. This variety predominates in women and is of unknown cause (Fig.1). It is usually bilateral. Ultimately the femoral head may extend toward the pelvis, and joint space narrowing may occur. In adults, protrusio acetabuli is diagnosed radiographically with a distance of 6 mm or more exists between the acetabular line (medial) and ilioischial line (lateral). The teardrop, shadow on radiographs may be used to help diagnose acetabular protrusion.
The acetabular protrusion occurring after irradiation may result from the processes involved in remodeling and revascularization in weakened bone. Some patients may also have calcification in the peritoneum.
DR/RB
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AP radiograph of the pelvis demonstrates idiopathic protrusion of the medial walls of the acetabula, which has been termed "Otto pelvis". Secondary arthritic changes are present.
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Protrusio acetabuli, Fig.1 | |