Comparison of mechanical performance in four types of external fixators

The Hoffmann-Vidal triangular apparatus for fracture fixation of the femur, the Volkov-Oganesian device with Steinmann pins in tension, the Kronner device with a plastic elliptic frame, and the Roger-Anderson apparatus (both standard and compression-distraction designs) were studied based on loading and analytic methods established previously. The results were compared with those for the standard Hoffmann-Vidal quadrilateral configuration. The Kronner device, with five connecting bars, was the stiffest under most loading modes, closely followed by the Hoffmann-Vidal quadrilateral apparatus. The standard Roger-Anderson device demonstrated average rigidity; its new design with compression-distraction capability was relatively weak. All devices showed an apparent weakness in antero-posterior bending. The Volkov-Oganesian design demonstrated low overall stiffness but was the most effective apparatus in resisting anteroposterior bending. External fixators provide effective compression at the fracture site, and the amount of compression is proportional to the stiffness of the apparatus.