Thumb function and appearance in thrombocytopenia: Absent radius syndrome

Purpose: To evaluate thumb function and appearance in patients with thrombocytopenia absent radius (TAR) syndrome. Methods: The size and shape of the 14 thumbs in 7 patients with TAR syndrome were quantified and compared with age-matched normals. Function was assessed using a series of standardized tasks. Results: The thumb length averaged 73% relative to the length of the index finger proximal phalanx (normal, 70%) and 36% relative to the length of the entire index finger (normal, 32%). The thumb/index nail width and girth ratios measured 138% and 101%, respectively (normal, 133% and 105%, respectively). The relative thumb/index thumbnail width-to-girth ratio was 1.36, which was significantly greater than the normal average of 1.27, suggesting the TAR thumbs are relatively wide and flat compared with normal thumbs. The thumb interphalangeal joint was held in a neutral posture and did not show active motion in any patient; the metacarpophalangeal joint was held in a position of flexion and had an average of 32 degrees of extension lag. Patients had difficulty with all activities tested, especially grasping large objects. Conclusions: The thumb in TAR syndrome patients is of relatively normal size and shape; however, the thumb is held in a position of metacarpophalangeal flexion in most patients and function is impaired.