What Are the Biomechanical Effects of Half-pin and Fine-wire Configurations on Fracture Site Movement in Circular Frames?

Abstract

Background Fine-wire circular frame (Ilizarov) fixators are hypothesized to generate favorable biomechanical conditions for fracture healing, allowing axial micromotion while limiting interfragmentary shear. Use of half-pins increases fixation options and may improve patient comfort by reducing muscle irritation, but they are thought to induce interfragmentary shear, converting beam-to-cantilever loading. Little evidence exists regarding the magnitude and type of strain in such constructs during weightbearing. Questions/purposes This biomechanical study was designed to investigate the levels of interfragmentary strain occurring during physiologic loading of an Ilizarov frame and the effect on this of substituting half-pins for finewires. Methods The ''control'' construct was comprised of a four-ring all fine-wire construct with plain wires at 90°-crossing angles in an entirely unstable acrylic pipe synthetic fracture model. Various configurations, substituting half-pins for wires, were tested under levels of axial compression, cantilever bending, and rotational torque simulating loading during gait. In total three frames were tested for each of five constructs, from all fine-wire to all half-pin. Results Substitution of half-pins for wires was associated with increased overall construct rigidity and reduced planar interfragmentary motion, most markedly between all-wire and all-pin frames (axial: 5.9 mm ± 0.7 vs 4.2 mm ± 0.1, mean difference, 1.7 mm, 95% CI, 0.8-2.6 mm, p \ 0.001; torsional: 1.4% ± 0.1 vs 1.1% ± 0.0 rotational shear, mean difference, 0.3%, 95% CI, 0.1%-0.5%, p = 0.011; bending: 7.5°± 0.1 vs 3.4°± 0.1, mean difference, À4.1°, 95% CI, À4.4°to À3.8°, p \ 0.001). Although greater transverse shear strain was observed during axial loading (0.4% ± 0.2 vs 1.9% ± 0.1, mean difference, 1.4%, 95% CI, 1.0%-1.9%, p \ 0.001), this increase is unlikely to be of clinical relevance given the current body of evidence showing bone healing under shear strains of up to 25%. The greatest transverse shear was observed under bending loads in all fine-wire frames, approaching 30% (29% ± 1.9). This was reduced to 8% (±0.2) by incorporation of sagittal plane half-pins and 7% (±0.2) in all half-pin frames (mean difference, À13.2% and À14.0%, 95% CI, À16.6% to 9.7% and À17.5% to À10.

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“…However, such simplified wire placements, particularly with smooth wires alone, may generate behavior that does not directly replicate the clinical scenario, as seen under bending loads. Nonetheless, this approach, with the resulting low levels of variability we observed between samples, allows greater confidence in analyzing and drawing conclusions from the small sample size used in this study, a commonplace practice in such mechanical studies where conditions are almost entirely controlled and variability is low [20,25,27,31]. Small sample sizes also may create difficulty in statistical analysis, particularly in the clinical scenario; however, in a comparative mechanical analysis such as this, with a highly controlled testing environment and almost identical samples, simple comparative statistics, and specifically the t test, which has been shown to be accurate in small sample analysis, allows comparison to be made between two different samples each with their own mean and distribution of results to determine if significant variability may exist [7].…”

“…An acrylic tube with an outer diameter of 32 mm and a 4-mm wall (Clear Plastic Supplies, Chesterfield, UK) was used as a mechanical substitute for bone. This symmetric, uniform model was chosen to minimize any variability between testing cycles that may be caused by minor differences in wire placement in more anatomic models or by bone density variation in cadaveric samples, a common practice in similar mechanical studies [20,24,25,27,28,30,31]. While not attempting to reproduce a directly clinically comparable environment, the reproducibility of such mechanical modeling allows greater confidence in reporting observed differences with small sample sizes, a finding confirmed by the low variance observed in preliminary testing and which was used to guide the sample size for this study.…”

“…To maximize reproducibility and isolate the effect of the frame construct, in keeping with previous studies [6,20,27,31], bone models were mounted centrally in each ring with fixation elements placed at theoretically "ideal" crossing angles of 90°. To prevent bone apposition during loading, simulating an entirely unstable fracture or corticotomy, bone models were mounted with a 20-mm fracture gap.…”

“…The data met assumptions for parametric testing using the D'Agostino and Pearson omnibus test and QQ-plot analysis and therefore Student's t-test and ANOVA testing with post hoc analysis using Tukey's method were performed. This methodology is validated in small sample-size studies and commonly applied to such mechanical modeling studies where conditions are almost entirely controlled and comparison between two samples, with low variance, is required [7,20,25]. While recognizing the potential for type II error with small sample sizes, a p value less than 0.05 was considered statistically significant throughout.…”

“…This allows simultaneous correction of multiplane deformity and distraction and compression in a much simpler manner than would be possible using traditional Ilizarov equipment and with several reports of good clinical and functional outcomes after such treatment [1,19,35]. Numerous studies exist in the literature investigating and describing the potential effects of altering any one of the modular parts of a traditional Ilizarov circular frame on the biomechanical environment at a fracture site [2,20,23,24,27], however there are almost no studies examining the biomechanical behavior of a hexapod construct [21] and none, to our knowledge, directly investigating the potential effect that substituting the longitudinal rods of an Ilizarov fixator for the six struts of a hexapod may have on the overall behavior of a fixator construct, and therefore on the mechanical environment of a fracture site. Given the existing body of evidence comparing in vitro biomechanical behavior of circular frames with clinical outcomes, and the ever-expanding understanding of the complexities of bone healing biomechanics, a biomechanical comparison was planned to better understand the behavior of the TSF in isolation, but also to see whether any observed differences in the behavior of the frame components in isolation persisted once the effect of bone-fixation elements in the simulated fracture model were added, and therefore whether any clinically important effect was likely to result from substitution of a TSF for a traditional Ilizarov frame.…”