FEATURES OF DEFORMATION OF THE «DEBRIS – EXTERNAL CORE APPARATUS» MODEL IN THE CASE OF USING STRUCTURES WITH DIFFERENT STRUCTURAL GEOMETRY

Abstract

Objective. To study the linear and angular displacements of the "fragments" during their connection with an external rod apparatus manufactured by HB ORTHO (Ukraine) or Orthofix (USA) under different variants of the geometry of the "fragments – apparatus" structure in order to clarify the mechanical principles of its rational construction. Methods. The model was rigidly fixed at one end in a horizontal position, and a transverse force was alternately applied to the opposite end using weights of 1, 2, 3, 4, 5 kg. The experiment involved the study of the magnitude and nature of the displacement of the fragments depending on the following parameters of the rods: the number of rods in the fragment (2 or 3); diameter (5, 6 mm); length of the rod section from the bone to the support (100, 50 mm); length of the bone section between the extreme rods (150, 100 mm); the presence of a multi-plane arrangement of the rods and, in particular, when they formed a 45° angle between them, the number of external supports: one or two, located in parallel or side by side. Results. The first most important parameter that influenced the amount of displacement of the distal "fragment" was the distance from the bone to the support. In the case of a distance of 50 mm, the amount of displacement of the fragment is 2–4 times less than in the case of 100 mm. The second parameter that influenced the displacement of the fragments was the length of the bone section between the extreme rods screwed into the fragment. If it is reduced by 30 %, the displacement increases by 64% and almost does not depend on whether 2 or 3rods were used. It is possible to significantly reduce the displacement of the distal fragment (at least twofold) by inserting rods in different planes, in particular, by positioning the rod so that in the proximal fragment near the fracture in a plane that is 45° to the frontal plane. With a gradual transverse load, the deformation of the structure at the initial stages (1, 2, 3 kg) is elastic in nature and with an increase (up to 4–5 kg), residual deformation occurs due to: movement of the clamp on the cylindrical support; plastic deformation of the rods, which is inherent in HB ORTHO devices (Ukraine).