Evaluation of stiffness and stress of external fixators with curved acrylic connecting bars

The use of acrylic connecting bars in external fixators has become widespre ad in veterinary orthopaedics. One of the main advantages of an acrylic con necting bar is the ability to contour it into a curved shape. This allows t he surgeon to place the transcortical pins according to safety and convenie nce considerations, without being bound by the requirement of the standard stainless steel connecting bar, that all transcortical pins be in the same plane. The purpose of this study was to evaluate the stiffness of unilateral and b ilateral medium-sized external fixator frames with different curvatures of acrylic connecting bars. Finite element analysis was used to model the vari ous frames and obtain their stiffness under four types of load: Axial compr ession, four-point medio-lateral bending, four-point antero-posterior bendi ng and torsion. The analysis also provided the maximal pin stresses occurri ng in each frame for each loading condition. Based on the results of this study, curvatures of acrylic connecting bars o f up to a maximal angular difference between pins of 25 degrees will result in very similar stiffness and maximal pin stresses to those of the equival ent, uniplanar stainless steel system. In both unilateral and bilateral sys tems the stiffness decreases slightly as angulation increases for axial com pression and medio-lateral bending, increases slightly for torsion and incr eases substantially for antero-posterior bending.