A 15-year-old boy presents to the office for a routine sports physical. During the examination his mother mentions that the patient has unusual fingernails and bony deformity of the elbows, both findings present since birth. His mother believes that a nutritional deficiency may be causing the abnormalities. He denies any other musculoskeletal deformities, including of the lower extremities and chest wall. The patient reports no medical history and, despite the deformity, is very active in sports, with no limitations. On further questioning it is reported that several family members on the mother’s paternal side have similar nail findings. His mother and his brother have nail involvement solely of their thumbs.

On physical examination, bilateral radial head subluxation is evident. The patient’s arms are deviated laterally (Fig 1). He has limited extension at the elbows, with some limitation of pronation and supination (Fig 2). The patellae are...

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