Rotation in the interphalangeal thumb joint in vivo.

PURPOSE: To investigate rotation at the thumb interphalangeal (IP) joint in vivo to optimize the position of fusion of this joint. METHODS: Standardized photographs were taken of 176 thumbs end-on (88 asymptomatic volunteers) placed on a custom-made splint with the IP joint at 40 degrees . Three blinded investigators measured rotation at the IP joint from these photographs as the angle between a line aligning the eponychial folds and a line aligning the proximal phalanx condyles. Gender, age, hand dominance, and type of occupation of the asymptomatic vounteers were recorded. RESULTS: The variable pronation at the IP joint of the thumb (range, 0 degrees to 12 degrees) was significantly greater on the left than right (p=.001), although the actual difference was only 1 degrees . In subjects who performed fine dexterous work, thumb IP joint pronation was significantly less than in subjects who performed administrative or manual work (p=.009), but we found no statistical difference between manual and administrative groups. There was no correlation between thumb IP joint rotation and hand dominance (p=.2), age (p=.4) or gender (p=.5). CONCLUSIONS: There is functional pronation at the IP joint of the thumb. We propose that this should be taken into account when performing arthrodesis on the joint or designing a joint replacement. The degree of rotation may be associated with occupation.