Biomechanical study on the fixation effect of circular external fixators for oblique fractures of long bones / 中华骨科杂志

ABSTRACT

Objective:To compare the biomechanical properties of different circular external fixators for the fracture site of oblique long bone fractures.Methods:15 polyethylene (PE) plastic rods with the same batch were selected to make the model of oblique fractures in the middle of long bones. According to the connection between PE rods and external fixators, the PE rods were randomly divided into the group that was simply used kirschner wires (group kirschner wires), the group that was simply used olive wires (group olive wires), the group that was simply inserted half pins (group half pins), the group that was inserted single cortical fixed half pins at either side of the fracture site after being fixed by the tensioned olive wires (group olive wires+ half pin), the group that was inserted tensioned trans-fracture olive wires after being fixed by the tensioned olive wires (group olive wires+ olive wires). The axial compression tests were carried out, and the interfragmentary displacements under axial loads of 200 N, 400 N, 600 N and 800 N were measured and the axial stiffness was calculated. Then, the torsional tests were carried out, and the interfragmentary torsional angles were measured under torsional loads of 4 N·m, 7 N·m and 10 N·m, and the torsional stiffness was calculated.Results:With the increase of axial load from 200 N to 800 N, the axial interfragmentary displacement in each group gradually increased. The interfragmentary axial displacement of each group in ascending order was group olive wires+ olive wires, group olive wires+ half pins, group half pins, group olive wires, group kirschner wires. The axial stiffness of each configuration under 800 N axial load in descending order was group olive wires+ olive wires [863.93 (824.32, 875.87) N/mm], group olive wires+ half pins [119.92 (113.16, 123.58) N/mm], group half pins [81.92(79.42, 82.40) N/mm], group olive wires [76.83 (72.45,79.47) N/mm], group kirschner wires [70.80 (67.49, 71.59) N/mm]. The pairwise comparisons of the axial stiffness data of each configuration had statistical significance (all P <0.05). With the increase of the torque load from 4 N·m to 10 N·m, the interfragmentary rotational angle in each configuration gradually increased. The interfragmentary torsion angle of each group in ascending order was group half pins, group olive wires+ olive wires, group olive wires+ half pins, group olive wires, group kirschner wires. The torsional rigidity of each configuration under 10 N·m torsional load in descending order was group half pins [1.80 (1.63, 1.85) N·m/°], group olive wires+ olive wires [1.05 (1.02, 1.07) N·m/°], group olive wires+ half pins [0.99 (0.98, 1.03) N·m/°], group olive wires [0.81 (0.78, 0.82) N·m/°], group kirschner wires [0.75 (0.74, 0.76) N·m/°]. The pairwise comparisons of the torsional rigidity data of each configuration had statistical significance (all P < 0.05). Conclusion:The axial stiffness and torsional stiffness of circular external fixators can be increased by using tensioned olive wires or half pins at the fracture site. Due to the insufficient support between oblique fracture site, when the load is applied, the axial displacement and torsion angle of the fracture site will still be fairly large after being fixed the fracture site with half pins. Treating with tensioned trans-fracture olive wires after being fixed by the tensioned olive wires at either side of the fracture site can effectively control the interfragmentary shear and displacement, thus providing an ideal mechanical environment for fracture healing.