[Fatigue performance of Hoffmann-Vidal external fixator].

The question concerning reusability of the Hoffmann-Vidal frame has been widely discussed, but no definite recommendation can be made on scientific data. The present study was designed to investigate the change of frame stiffness and fatigue life of the components in a Hoffmann-Vidal quadrilateral fixator. Failures were found at the base of the pin fixation bracket rod. They can be classified into 3 types: complete rod fracture, incomplete rod fracture and loosening of the rod. Only complete fractures of the rod caused a significant reduction of frame stiffness. Strength and loosening torque were proportional to the initial tightening torque for both the universal ball joint and articulation coupling. The mean reduction of screw fixation strength was 16% after 2.5 million cycles. As a result of repeated tightening, two retaining screws from three universal ball joints were severely deformed and the threads flattened. Two fixation screws on the pin bracket and two square-headed screws on the adjustable connecting rods fractured at the 5th to 6th tightening sequence. Significant wear debris were observed during repeated tightening of the articulation coupling. No gross material defects were observed on the fracture surfaces of the failed components. The Hoffmann-Vidal external fixation apparatus may be safely used for four to five sequential applications provided that all critical components be carefully inspected and the fixation screws replaced after each application. The screws should be tightened appropriately using only the T-wrench in order to prevent overloading. These screws should be retightened periodically to maintain incorporated to minimize the screw and rod fractures and to avoid the wear problem after repeated tightening of the screws.