Mechanical characteristics of an upper-extremity external fixator.

Abstract

Static and fatigue tests of the small Orthofix unilateral external fixator frame and its main components were performed. The use of six pins increased torsional rigidity, but this configuration failed at lower bending loads when compared to the four-pin configuration, reflecting the uneven holding strength of the pin clamp on three pins. The ball joint maximum torsional resistance was reduced by 27.2%, and the maximum bending resistance was decreased by 35.2% after first failure loading of a fatigue test. The strength of the ball joint returned to the normal level after replacement of the cam and bushing with new components. The locking strength of the fixator body telescoping mechanism was high and did not change during repetitive failure loading. Hence, the fixator should be applied under the four-pin configuration, placing pins in the outer slots of the clamps as recommended by the manufacturer. The frame stability is governed by the performance of the ball joint, and its locking cam should be tightened periodically. If the ball joint locking position starts to migrate from its initial position, the cam and bushing should be replaced.