Experimental substantiation of the layout of external fixation apparatus for lengthening the tibia in children
Achondroplasia is a genetically caused disease, which is accompanied mainly by a violation of the growth of the skeleton and limbs in length. Most authors came to the conclusion that it is advisable to carry out the elongation simultaneously on the symmetrical segments, and begin with the legs. At the same time, the authors note a significant number of complications, among which the most severe are the appearance of secondary deformations of the segment during distraction. Objective: to determine the stabilizing capabilities of external fixators (EF) with their various configurations in the experiment on physical models. Methods: an experimental study was carried out on physical models of plastic tibia that was cut in the upper third of the diaphysis. In the experimental group, EF were fixed on the model according to the author’s scheme with a V-shaped arrangement of rods in the proximal tibia (3 models). As a comparison group, we used models with EF, which were fixed according to the «classical» scheme with the location of all the rods in one plane (3 models). All models were tested under axial compressive loads. Bending loads were performed in two planes: parallel and perpendicular to the support beam of the EF. Results: at compression loads of up to 50 N, the displacement of the proximal fragments of the tibia does not have statistically significant differences (p = 0.066). At loads of 100 N or more, the EFs, which were fixed according to the V-shaped scheme, provided smaller displacements of the proximal fragment of the tibia in the diastasis zone compare to EFs with an in-line layout of the rods. At loads of 50 and 150 N, the in-line arrangement of the EF provides a statistically significantly less (at the level of p = 0.001 and 0.019, respectively) displacement of the proximal tibia fragment in the area of EF application of the load than the EF according to the V-shaped pattern. At loads of 100 and 200 N, there is no statistically significant difference between the displacements of the fragments in the both models. (р > 0.2). Under bending loads in the plane perpendicular to the supporting beam of the EF, devices with a V-shaped arrangement provide more stable fixation of the proximal fragment of the tibia than comparative model. Conclusions: the V-shaped arrangement of the EF allows statistically significantly stabilize fixation of fragments of the tibia than the EF with the in-line layout of the rods in almost all the studied loading methods.
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