The influence of a weight-bearing platform on the mechanical behavior of two Ilizarov ring fixators: tensioned wires vs. half-pins

Abstract

BackgroundA weight-bearing platform applied at the distal end of an Ilizarov external frame allows patients with hindfoot transfixations, foot deformities or plantar skin lesions to bear weight. This leads to an indirect loading of the fracture or osteotomy site. However, the effect on the fracture/osteotomy site's motion or compressive loads is unknown. The aim of this study was to analyze the mechanical effects of a weight-bearing platform on the traditional all-wire, four-ring frame in comparison to a two-ring frame consisting of half-pins.MethodsTwo frame configurations, with either anatomically positioned wires or half-pins, were analyzed with and without a weight-bearing platform applied underneath the distal ring. Composite tibiae with a mid-diaphyseal osteotomy of 3.5 mm were used in all the experiments. An axial load was applied with the use of a universal test machine (UTS®). Interfragmentary movements, the relative movements of bone fragments and movements between rings were recorded using displacement transducers. Compressive loads at the osteotomy site were recorded with loading cells.ResultsIndirect loading with a weight-bearing platform altered the force transmission through the osteotomy. Indirect loading of the tibiae decreased the extent of the axial micro-motion by 50% under the applied weight load when compared to direct weight loading (p < 0.05). The half pin frame was 25% stiffer than the wire frame under both direct and indirect loading of the tibiae (p < 0.05). Compressive loads under indirect loading were reduced by 67% in the wire frame and by 57% in the half-pin frames compared to direct loading of the bones (p < 0.05). While axial loading in the wire frames resulted in plain axial movements at the site of the osteotomy, it was coupled with translational movements and angular displacements in the half pin mountings. This effect was more apparent in the case of indirect loading.ConclusionsA weight-bearing platform has substantial influence on the biomechanical performance of an Ilizarov external fixator. Half-pins induce greater stiffness to the Ilizarov external fixator and allow the usage of only one ring per bone segment, but shear stresses at the osteotomy under axial loading should be considered. The results allow an estimation of the size and direction of interfragmentary movements based on the extent of weight bearing.

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

“…Likewise, substitution of half-pins for fine-wires under axial load also produced results in keeping with those described by Gessmann et al [13], with reduced axial interfragmentary strain and increased shear strain generated by cantilever bending [32]. Despite this increase in axial stiffness, the half-pin construct still generated 4 mm of mean interfragmentary compression, and with only 1.9% shear, falling well within the greater than 0.5 mm axial strain and up to 25% shear strain reported to induce good bone healing [2,8,12,19].…”

“…The pattern of observed interfragmentary strain and construct rigidity under axial loading in an all fine-wire frame correlated closely with previously published data; tensioned fine-wires behaved in a ''self-stiffening'' nonlinear manner with minimal shear strain observed [1,13,32]. Likewise, substitution of half-pins for fine-wires under axial load also produced results in keeping with those described by Gessmann et al [13], with reduced axial interfragmentary strain and increased shear strain generated by cantilever bending [32].…”

“…Developments on the original Ilizarov method of fine-wire fixation have introduced elements, such as threaded half-pins, which allow more versatility in frame application and with reduced muscle transfixion. Although purported to improve patient comfort and tolerance, this has raised concerns that, by altering the mechanics of the frame construct through cantilever bending and introducing increased shear, such elements may have a detrimental effect on the biomechanical environment at a fracture or osteotomy site [4,6,13] This study has limitations reflecting its experimental nature and methodology, which must be taken into consideration when interpreting the results. Attempts were made during design to limit the number of variables between frames to focus on the specific elements under investigation and improve reproducibility between constructs and testing cycles.…”

“…This opens wider avenues of fixation and largely avoids muscle transfixation. Some surgeons advocate the use of half-pins on the basis that this will increase patient comfort and compliance [4,13]; however, the introduction of half-pins in a construct potentially leads to cantilever bending during loading, which is theorized to increase shear at the fracture [13]. For this reason some surgeons avoid the use of half-pins wherever possible.…”

What Are the Biomechanical Properties of the Taylor Spatial Frame™?

“…The pattern of results observed was broadly similar to those obtained when testing the frames alone, although with less marked differences between the Ilizarov and TSF constructs. Likewise, the increased rigidity and more-linear load deformation properties of half-pin compared with fine-wire constructs, as seen throughout our testing, is well recognized [15,20,24]. Axial loading of bone-frame constructs followed these recognized trends, with half-pin constructs of the Ilizarov and TSF frames showing greater rigidity than with use of fine wires, and Ilizarov constructs proving more rigid than the TSF when comparing constructs using the same bone-fixation element.…”