Axial characteristics of circular external skeletal fixator single ring constructs

Objective - Evaluate the effects of varying ring diameter, wire tension, an d wire-divergence angle on the axial stiffness characteristics of circular external skeletal fixator single-ring constructs. Study Design - Biomechani cal evaluation using circular fixator components and a Delrin cylinder bone model. Methods - Single ring constructs using two 1.6 mm diameter Kirschner wires to secure a 19 mm Delrin cylinder centered within the ring were examined. C omponent variables evaluated were ring diameter (50 mm, 66 mm, 84 mm, and 1 18 mm), wire-divergence angle (30 degrees, 60 degrees, and 90 degrees), and wire tension (0 kg, 30 kg, 60 kg, and 90 kg). A total of 48 constructs wer e examined. Rings were rigidly mounted on a universal testing system and th e cylinder loaded in axial compression (7.4 N/s) to 220 N. Load/displacemen t curves were analyzed to determine the following: the displacement (mm) th at occurred before the slope of each load/displacement curve became linear, the stiffness (N/mm) of the linear portion of each load/deformation curve, and the total displacement (mm) produced at maximal load. Least-squares li near regression was used to model response variables as linear functions of ring diameter, wire divergence angle, and wire tension. Three-way interact ions and 2-way interactions among independent component variables were eval uated first in the modeling process and included in a best model if respons e variables were found to have statistically significant regression coeffic ients. The regression coefficients and corresponding standard errors and co variances were used to estimate the maximal effect and standard error attri butable to wire divergency angle (change from 30 degrees to 90 degrees) and wire tension (change from 0 to 90 kg) for each ring diameter. Results - All load/deformation curves had an initial exponential increase i n stiffness, with the slope becoming linear at higher loads. The exponentia l phase was more pronounced in larger-diameter ring constructs and was miti gated by tensioning the wires. Ring diameter had the greatest influence on displacement that occurred before the curve became linear (semipartial r(2) [sp-r(2)] =.89), stiffness (sp-r(2) =.94). and total displacement (sp-r(2) =.93). Wire tension exerted a smaller influence on displacement that occur red before the curve became linear (sp-r(2) =.06), stiffness (sp-r(2) =.03) , and total displacement (sp-r(2) =.05). Wire divergence angle had a nomina l effect on displacement that occurred before the curve became linear (sp-r (2) =.0001), on stiffness (sp-r(2) =.004), and on total displacement (sp-r( 2) =.003). Conclusions - Ring diameter had a profound effect on the axial stiffness ch aracteristic of single ring constructs. Tensioning of the fixation wires ca n improve the axial stiffness characteristics of these constructs. particul arly in larger diameter ring constructs, by mitigating the initial exponent ial phase of the load/deformation curve. Wire divergence angle had only a n ominal differential effect on axial stability. Clinical Relevance-Understan ding how individual component variables and their interactions influence bo ne segment stability should help surgeons to optimize interfragmentary stra in. Tensioning fixation wires is probably unnecessary in 50 mm diameter rin g constructs, but assumes greater importance as ring diameter increases. (C ) Copyright 2001 by The American College of Veterinary Surgeons.