Unilateral fusion of the scaphoid and the trapezium.

Fusion of the carpal bones is a rare anatomical abnormality, caused by a defect in the separation and cavitation during embryonic development. It has a prevalence of about 0.1% and most of the time, patients do not present with symptoms. Symptoms are usually caused by wear and tear of the ligaments attached to the fused carpal fragments. We present a case in which a woman aged 36 years experiences pain caused by a lunato-triquetral instability which in turn is caused by a unilateral complete fusion of the scaphoid and the trapezium. A unilateral complete fusion of the scaphoid and trapezium has not been reported before.