Fatigue properties of the Hoffmann-Vidal external fixation apparatus

Fatigue tests of the standard Hoffmann-Vidal (H-V) external fixator were performed. After 2,500,000 cycles, fractures occurred in the post attached to the pin bracket. The screws on the pin fixation bracket tend to become loose after cyclic loading test, and periodic lightening of all the fixation screws on a H-V apparatus is recommended under in vivo application. In repeated lightening of individual joints, screws on the pin brackets and adjustable connecting rods fractured. Deformation of the retaining set screws in the universal ball joints caused joint stiffness reduction. Severe abrasive wear of the joint surfaces was observed after repeated lightening. It is concluded that this apparatus may be safely used for four to five six-month clinical applications, provided that the critical parts are replaced between sequential applications. Suggestions for clinical management and recommendations for component modifications are made.