What's the Diagnosis - Case 27

History 80 year old woman 2 years status post left THR with fall 2.5 weeks ago and radiating left low back pain.

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Findings Radiographs demonstrate radiolucency of the left greater trochanter. Bone scan demonstrates increased flow and pool activity of the left greater trochanter with intense radiotracer uptake of the left greater trochanter and left sacral ala on the delayed images. MRI shows intense edema of the left sacral ala and moderate edema of the intertrochanteric region of the left femur. Low signal linear bands are seen at the sites of marrow edema. A posterior fluid collection is seen containing prominent internal debris.

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Diagnosis: Periprosthetic left femur fracture eliciting a small adjacent fluid collection containing post traumatic/synovial debris. Additional sacral alar insufficiency fracture.

In the older patient population the presence of osteoporosis makes the bone more predisposed to insufficency type stress fractures. These fractures occur when normal stress is transmitted to abnormal and in this case weakened bone. In this case the patient recalls focal trauma but at times a defined, inciting event is difficult to recall.

Evaluation of arthoplasties is limited for various reasons in different modalities.

Radiographs as in this case may show radiolucency which may represent stress shielding or a focal area of osteolysis. Bone scans are often helpful to locate the site of abnormality but are frequently non-specific. Given the intensity and focal nature of the uptake in this case, the findings would be most consistent with a fracture. MRI is hampered by field inhomogeneity, difference in magnetization, and mismapping artifacts yielding areas of signal void and spurious high signal. Multiple technical parameters are employed to overcome these limitations. The linear bands representing fractures and the surrounding edema and fluid collection can be difficult to perceive and quite subtle.

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Resources

• Resnick. Diagnosis of Bone and Joint Disorders. 4th Ed. 2002.

• Magnetic resonance imaging of joint arthroplasty. Potter HG, Foo LF. Orthop Clin North Am. 2006 Jul;37(3):361-73, vi-vii.

• Magnetic resonance imaging of joint replacements. Naraghi AM, White LM. Semin Musculoskelet Radiol. 2006 Mar;10(1):98-106.

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