Effects of Transfixation Pin Positioning on the Biomechanical Properties of Acrylic External Skeletal Fixators in a Fracture Gap Model.

Abstract Objective  The aim of this study was to evaluate the biomechanical effects of transfixation pin positioning in acrylic columns of external skeletal fixators (ESF). Study Design  Twenty-four type I acrylic ESF were built simulating a fracture gap-model. Transfixation pins were placed centric ( n  = 12) or eccentric at ¼ of the column diameter ( n  = 12) in the acrylic columns. Six constructs from each group were subjected to axial compression and four-point bending tests. Stiffness, yield load and mode of failure were recorded. Results  Stiffness was not influenced by centring ( p  = 0.373), but it was higher in four-point bending than in axial compression ( p  < 0.001). Pin positioning had no influence on the yield ( p  = 0.535) and failure loads ( p  = 0.715) in axial compression, nor on the yield load in bending ( p  = 0.135). Eccentric pin positioning decreased failure loads by 28% in bending ( p  < 0.001). Conclusion  Eccentric position of transfixation pins within the acrylic columns alters the biomechanical properties of type I ESF constructs. While acrylic offers several advantages, when forming the columns, frame strength will be optimized if pins are centrally located.