Rotation in the interphalangeal thumb joint in vivo

To investigate rotation at the thumb interphalangeal (IP) joint in vivo to optimize the position of fusion of this joint.

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.

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).

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.