Stiffness characteristics and inter-fragmentary displacements with different hybrid external fixators

Objective. To determine how the different approaches of constructing a hybrid external skeletal fixator affect the mechanical environment at the fracture site. Design. A comparative biomechanical analysis of external fixators that represent a spectrum of designs from unilateral to circular fixators. Background. The most pertinent parameter that affects fracture healing is the inter-fragmentary displacement. Most studies on the mechanical performance of external fixators have concentrated on the overall stiffness of the fixators. Methods. Mechanical testing was performed on four types of hybrid fixators in which the overall stiffness of the fixator as well as the axial, transverse shear and angular displacements at the fracture site were measured. Results. The Ilizarov hybrid fixator with one wire and one screw on each ring behaved more like a unilateral fixator than a circular fixator. The bar-ring hybrid fixator that simply connected a unilateral fixator body to a wire/ring assembly was too flexible. Reinforcing the bar-ring hybrid system with diagonally placed struts eliminated the deformation associated with the unilateral body and the resulting construct performed considerably better. The two-ring hybrid fixator most closely resembled the Ilizarov fixator. Conclusion. Depending on the construction characteristics hybrid fixators possess a spectrum of mechanical characteristics of both unilateral and circular fixators. The constructions using two rings in which at least one bone segment is supported entirely by wires appears to have the best mechanical characteristics.