Applying tribological principles to improve the performance of the Hoffmann external fixator's universal joint

As part of a continuing investigation of sources of slippage in the universal joints of external fixation devices, a study was conducted to determine if selected interfaces of the Hoffmann external fixator (rod/clip torsional, cheek/bowl and clip/cheek) could be improved by moving the point of contact between the wing-nut clamp and cheek closer to the centre of the wing-nut clamp. It was felt that the movement of the point of contact would reduce the magnitude of the frictional torque resisting the tightening of the wing-nut clamp. The point of contact was changed by the addition of a Belleville washer between the interface of the wing-nut clamp and the cheek. Increased slippage torques of approximately 100 per cent were noted in all interfaces at low values of tightening torque (6 and 8 N m) of the wing-nut clamp and improvements of not less than 50 per cent were obtained at higher tightening torques (10 and 12 N m) on the wing-nut clamp.