Mechanical evaluation of the effect of the rod to rod distance on the stiffness of uniplanar external fixator frames.

Abstract

Purpose: To investigate the effect of the rod-to-rod distance on the mechanical stability of single-rod and double-rod external fixator frames.

Methods:  Four different constructs, one single-rod and three double-rod constructs with different rod-rod distances, were subjected to the axial, bending, and torsional forces. The stiffness of different configurations was calculated.

Results:  Single-rod configuration had statistically the lowest stiffness when subjected to the axial, bending, and torsional forces. Maximum stiffness against the axial and anterior-posterior bending forces was achieved when the rod-rod distance was adjusted to 50 mm (halfway between the first rod and the end of the Schanz pins). There was no statistically significant difference in lateral bending stiffness among different double-rod configurations (p value: 0.435). The maximum stiffness against torsional forces was achieved when the rod-rod distance was adjusted to 100 mm (the second rod at the end of the Schanz pins).

Conclusion:  Double-rod uniplanar external fixator frames are significantly stiffer than the single-rod constructs, and however, the rod-rod distance can significantly affect the construct stiffness. We found that a frame with 50 mm rod-rod distance was the optimum fixator among tested configurations that allowed a balance between axial, bending, and torsional stiffness of the construct.