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

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 fine-wires.
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, -4.1°, 95% CI, -4.4° to -3.8°, 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, -13.2% and -14.0%, 95% CI, -16.6% to 9.7% and -17.5% to -10.6%, both p < 0.001).
CONCLUSIONS: Appropriate use of half-pins may reduce levels of shear strain on physiologic loading of circular frames without otherwise altering the fracture site mechanical environment at levels likely to be clinically important. Given the limitations of a biomechanical study using a symmetric and uniform synthetic bone model, further clinical studies are needed to confirm these conclusions in vivo. CLINICAL RELEVANCE: The findings of this study add to the overall understanding of the mechanics of circular frame fixation and, if replicated in the clinical setting, may be applied to the preoperative planning of frame treatment, particularly in unstable fractures or bone transport where control of shear strain is a priority.