THE MECHANICAL PERFORMANCE OF THE STANDARD HOFFMANN-VIDAL EXTERNAL FIXATION APPARATUS

A standard Hoffmann-Vidal quadrilateral fixation system for tibial fractures was analyzed using different geometric and material variations of the basic configuration. The fixation stiffness properties were quantitated to provide objective comparison. The bone-pin interface is the least-stiff link in the entire structure, particularly under the anterior-posterior bending mode. Rigidity of the device can be substantially improved by increasing the number of pins, using full threaded pins with a larger diameter, decreasing side connecting-rod distances, and increasing pin-separation distances in each pin group. Symmetrical tightening of the compression screws by hand is sufficient to produce compression of bone at the fracture site. The use of titanium pins tends to reduce stiffness, but using a frame made of titanium can significantly decrease the weight of the apparatus without decreasing its stiffness. In clinical use of external fixation apparatus, the rigidity of fracture reduction can vary substantially according to the configuration of application. Guidelines are provided for increasing sugh rigidity, not only for the standard Hoffmann-Vidal device, but also for other designs of similar geometry.