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What’s your diagnosis?

In this feature, AAOS Now publishes a series of images, challenging readers to diagnose the condition depicted. The images for this month’s challenge were submitted by Stuart J. Fischer, MD, who provided the following information:

The patient is a 63-year-old man, who had a total hip arthroplasty 18 years ago. He was seen in the emergency department after experiencing a sudden snap in his hip, inability to bear weight, and a feeling that his hip was giving way. Prior to the episode he reported having mild pain but no mechanical symptoms and was able to walk without difficulty.

A radiograph taken in the emergency department showed the femoral head to be eccentric in the acetabulum. A previous radiograph had shown the femoral head in a stable position. What’s your diagnosis?

Do you have a challenging case you’d like to submit for publication? Email a short case description and any accompanying images to aaoscomm@aaos.org

Find the answer to this month’s challenge below

The radiograph taken in the emergency department (A) showed the femoral head to be eccentric in the acetabulum, although a previous radiograph (B) had shown it in a stable position.

Open larger version of images (PDF)

Answer
According to Stuart J. Fischer, MD, who submitted the case, the patient was diagnosed with a fracture of the polyethylene liner. Dr. Fischer reports that polyethylene fractures are unusual but can result from progressive, long-term wear. They most often occur at the superior edge of the acetabulum. Mechanical factors believed to contribute include the following: vertical cup position placing increased stress on the superior edge; femoral neck impingement; failure of the locking mechanism causing the polyethylene to loosen in the metal shell; and liner design with thin polyethylene at the rim.

Do you have a challenging case you’d like to submit for publication? Email a short case description and any accompanying images to aaoscomm@aaos.org

Although not common, a fracture of the polyethylene liner can result from progressive long term wear, most often at the superior edge of the acetabulum.

AAOS Now
December 2011 Issue
http://www.aaos.org/news/aaosnow/dec11/clinical11.asp

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