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

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.