The effects of different wire and screw combinations on the stiffness of a hybrid external fixator.

Hybrid external fixators that use tensioned wires in the metaphysis and screws in the diaphysis combine the advantages of both unilateral and circular fixators and provide considerable flexibility in frame construction. This study aims to determine the effects of wire and screw combinations on the mechanical characteristics of such systems. Acrylic rods were used to simulate a long bone with a transverse fracture gap. A standard hybrid fixator design was used to stabilize the fracture in accordance with modern concepts with wires being used on one side of the fracture and screws on the other side. Twelve different wire and screw combinations were tested in axial compression and four-point bending in two planes. All the fixators tested demonstrated stiffening under increasing axial load, the extent of which appeared to correlate inversely with the number of wires used. Increasing the number of wires by one increased the axial stiffness by 7-16 N/mm (average 11 N/mm). The bending stiffness was largely controlled by the number of wires and increased by 0.42-1.03 N m/mm (average 0.74 N m/mm) for each additional wire. The axial stiffness of fixators with three screws was 4-7 N/mm (average 6 N/mm) higher than those with two screws if the offset screw was not in use. When the offset screw was used no stiffness advantages were found for three screws over two screws. The use of an offset screw substantially increased axial stiffness (by 12-26 N/mm, average 20 N/mm) and is recommended for unstable fractures. The two-ring hybrid fixators with four wires in one ring and three screws in the other had stiffness characteristics similar to the conventional four-ring Ilizarov fixator. Knowledge gained regarding the effects of wire and screw combinations on overall stiffness will be helpful both in the construction of frames and in destabilization during fracture healing.