Effects of ring diameter and wire tension on the axial biomechanics of four-ring circular external skeletal fixator constructs.

OBJECTIVE: To determine relative effects of ring diameter and wire tension on axial biomechanical properties of 4-ring circular external skeletal fixator constructs. SAMPLE POPULATION: 4-ring circular external skeletal fixator constructs and artificial bone models. PROCEDURE: 4-ring constructs were assembled, using 50-, 66-, 84-, or 118-mm-diameter rings. Two 1.6-mm-diameter fixation wires were attached to opposing surfaces of each ring at intersection angles of 90 degrees and placed through a gap-fracture bone model. Three examples of each construct were loaded in axial compression at 7 N/s to a maximum load of 400 N at each of 4 wire tensions (0, 30, 60, and 90 kg). Response variables were determined from resulting load-displacement curves (construct stiffness, load at 1 mm of displacement, displacement at 400 N). RESULTS: Ring diameter and wire tension had a significant effect on all response variables and had a significant interaction for construct stiffness and displacement at 400 N. Significant differences within all response variables were seen among all 4 ring diameters and all 4 wire tensions. As ring diameter increased, effect of increasing wire tension on gap stiffness and gap displacement at 400 N decreased. Ring diameter had a greater effect than wire tension on all response variables. CONCLUSIONS AND CLINICAL RELEVANCE: Although effects of wire tension decrease as ring diameter increases, placing tension on wires in larger ring constructs is important because these constructs are inherently less stiff. The differential contribution of ring diameter, wire tension, and their interactions must be considered when using circular external skeletal fixators.