Answer

Radiographic examination of the left knee reveals a small bony object in the center of the joint. In the absence of trauma, this object may be a “joint mouse” resulting from osteochondritis dissecans. The next step is to refer the patient for consultation with an orthopedic surgeon.

Discussion

Osteochondritis dissecans occurs when small intra-articular bone fragments become loose in the joints, most commonly the knee. Although the cause of osteochondritis dissecans is unknown, localized bone ischemia is thought to play a role. Symptoms typically include pain, stiffness, and swelling of the affected area as well as clicking, locking, and/or crepitus. Physical examination often reveals limited range of motion and a joint effusion. The lack of joint line tenderness would rule out a torn meniscus (Table).

Table. Joint Mouse Associated With Osteochondritis Dissecans

Description• Loose, bony fragment in the joint
Symptoms• Pain
• Clicking/locking of the joint
• Stiffness, swelling, and effusion
Differential Diagnosis• Torn meniscus
• Osteoarthritis of the knee
• Joint infection
Treatment • Refer to an orthopedist for surgery

Radiographic imaging may show a single or multiple small bony fragments within the joint accompanied by joint effusion. Concomitant conditions such as osteoarthritis may be present.  Unless a more severe condition such as joint infection is suspected, additional testing is rarely indicated.

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Treatment of osteochondritis dissecans involves surgery with removal of the bone fragment or fragments, as well as drilling holes into the involved areas of bone in an attempt to stimulate growth of new blood vessels. Conservative management with rest, nonsteroidal anti-inflammatory medication, and possibly bracing the knee may be used prior to or as an alternative to surgery.

Brady Pregerson, MD, is an emergency physician at Cedars-Sinai Medical Center in Los Angeles and at Tri-City Medical Center in Oceanside, California.

Reference

Pregerson DB. Emergency Medicine 1-Minute Consult Pocketbook. EMresource.org; 2017;5.