[MATHEMATICAL MODELING OF OPTIONS FOR MOUNTING AN EXTERNAL FIXATION DEVICE ON THE TIBIA DURING ITS LENGTHENING]

The main problem in the treatment of axial deformities and limb shortening in pediatric orthopedics is the presence of incomplete growth of the patient, which gives an unfavorable basis for the achieved correction. The most advanced technology for the treatment of deformities and limb shortening is the use of external fixation devices (AVF). Target. To study the stress-strain state of the "tibia-AVF model with different variants of AVF mounting in conditions of tibia lengthening. Mathematical modeling of tibial osteosynthesis with external fixation devices in children with achondroplasia was carried out. The model of the distal end of the lower limb contained both lower leg bones and bony elements of the foot. Two variants of the AVF imposition were modeled: with the arrangement of all rods in the same plane, and according to the scheme with a V-shaped arrangement of the rods in the proximal tibia. The values of maximum stresses and values of relative deformations in the elements of the models were investigated. Sweat is a compressive load, the zone of maximum stress (66.8 MPa) in the bone tissue occurs around the upper rod. The smallest stresses (22.8 MPa) are observed around the fourth bar. On the second and third rods, the stresses are the same - 41.6 and 41.5 MPa, respectively. The V-shaped arrangement of the rods leads to a decrease in stresses around the first rod to 57.2 MPa. The stresses in the distal part of the proximal tibia fragment do not exceed 35.5 MPa. Under torsional loads in the model with parallel arrangement of rods, the zone of maximum stresses arises around the first rod (16.3 MPa). The stresses in the distal fragment of the tibia are determined in the range from 1.2 to 3.9 MPa on the lower and upper rods, respectively. The use of an AVF with a V-shaped arrangement of the rods causes stresses in the tibia at a level of 13.1 MPa on the first rod, a minimum 1.1 MPa on the fourth. When the model is compressed with a parallel arrangement of the AVF rods, the values of relative deformations in the bone regenerate are determined in the range from 62 to 85%. The use of the AVF according to the scheme with a V-shaped arrangement of the rods makes it possible to reduce the magnitude of the relative deformations to the level of 77%. In the model with a parallel arrangement of the AVF rods, the relative deformations in the bone regenerate under torsional loads are determined in the range from 3.0 to 6.0%. When using AVF with a V-shaped arrangement of rods, the level of relative deformations is determined in the range from 1.0 to 4.0%. The use of an AVF with a V-shaped arrangement of rods in the lower part of the proximal fragment of the tibia during its osteosynthesis allows reducing the level of stresses at all control points of the model for all types of loading, as compared to the model of an AVF with a parallel arrangement of rods. AVF with a V-shaped arrangement of the rods makes it possible to reduce the values of the relative deformations of the bone regenerate at all the studied control points of the models, both under compression and torsion loads.