What's the Diagnosis - Case 29

History 66 year old woman with increasing left hip pain.

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Findings AP view of the pelvis demonstrates a sclerotic band along the roof of the acetabulum or at the acetabular sourcil. On the MR images, a focal low signal intensity line is seen corresponding to this site of sclerosis on the radiographs. This is surrounded by a marked amount of marrow edema pattern of the left acetabulum. Posterior images demonstrate no high signal of the sacrum. At the left femoral neck, a mild degree of edema pattern is seen with a low signal band extending across the femoral neck.

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Diagnosis: Stress fractures are typically related to repetitive force applied to bone. They are typically classified as overuse where there is increased load transmitted to normal bone or insufficiency fractures where there is normal load transmitted to abnormal bone. Insufficiency fractures are frequently seen in the setting of osteoporosis, either postmenopausal, senile, or related to other factors. In the pelvis, the classic sites for insufficiency fractures are the sacral ala, superior acetabulum, pubic symphysis, superior/inferior pubic rami, and the subcapital femoral neck.

Stress fractures can be diagnosed on radiographs by either a thin lucent line in the cortex typically perpendicular to the long axis of the bone, periosteal bone or cortical thickening, or by bands of sclerosis in the medullary cavity. The sensitivity of MRI is vastly superior to radiographs and allows earlier establishment of a diagnosis. On MRI, the fracture line is seen as a low signal band typically surrounded by a prominent amount of increased T2 weighted signal representing marrow edema. Alternative means of diagnosis are on a bone scan where focal, typically oval or band like areas of increased radiotracer uptake are seen at the fracture site.

In this case, there is a clear stress fracture at the superior acetabulum. The low signal band at the femoral neck and mild edema pattern at this location likely represent sequelae of a more remote stress fracture.

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Resources

• http://emedicine.medscape.com/article/309106-overview

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