DOI: https://doi.org/10.22141/1608-1706.2.21.2020.202228

Study of the stress-strain state of models of bone fragment fixation with biodegradable polylactide plates

O.M. Khvisyuk, O.D. Pavlov, M.Yu. Karpinsky, O.V. Yaresko

Abstract


Background. In orthopedics and traumatology, polymeric materials that are absorbed and dissolved in biological fluids are used more often to make screws and fixing pins, plates and anchors, cages. Among these materials, L-polylactic acid (L-polylactide, PLA) implants are used more common, they are characterized by biodegradation, osteointegration, the ability to induce bone formation and high biocompatibility with the body. It is possible to increase the quality of polylactide biomaterials by including ceramic materials, in particular hydroxylapatite, into their composition. However, the strength of these implants has not been studied yet. Objective: using the method of mathematical modeling, to study the stress-strain state of the lower leg models with tibial fracture fixed with the help of plates made of biodegradable polylactide materials. Materials and methods. A mathematical model of the lower leg was developed, it consisted of the elements of shin and calf bone, heel and ankle bone, as well as boat-shaped and wedge-shaped bones. The mechanical properties of the plate were changed by simulating three types of materials: titanium, PLA, composite material consisting of PLA — 70 %, tricalcium phosphate — 20 % and hydroxylapatite — 10 %. The model had a rigid fixation on the base of the calcaneus and the section of the cuneiform bones. The model was loaded with a vertically distributed force of 500 N, which corresponds to the body of a person weighing 70 kg (700 N) in a single-leg stance. The bending load of the model was 50 N. The torsional load was carried out using torque force of 5 N • m. Results. Under the influence of axial compressive loads, the highest level of stress is observed on titanium plates (20.9 MPa), unlike PLA plates and PLA composites, where stresses reach values of 6.1 and 9.0 MPa, respectively. The rigid titanium plate also appropriately influences the bone tissue in the fracture zone, where the stress level reaches 7.0 MPa above the fracture line and 10.3 MPa below it, and also the fixing screws in the fracture zone are subjected to increased stresses in the fracture zone — 6.2 and 7 MPa, respectively, above and below the fracture line. In models with PLA plates, the stresses in the fracture zone are twice lower in bone tissue and three times lower — in fixing screws, compared to the model with titanium plate. In the diaphyseal and proximal parts of the tibia, the stress level for all models is almost the same. When bending, the largest stresses occur in the titanium plate — 26.4 MPa, in PLA plates the level of stresses is the same (13.6 MPa). In other parts of all models, the level of stress in bending loads has practically no differences between the models. At torsional loads, the highest level of stresses is observed in the titanium plate (15.3 MPa). The lowest stresses occur in a plate made of PLA (8.6 MPa). A plate made of PLA with an admixture of 20 % of tricalcium phosphate and 10 % of hydroxylapatite has an intermediate position in terms of maximum stresses (11.8 MPa). The titanium plate also causes increased stresses on the fixing screws around the fracture zone, where they reach values of 7.1 and 7.3 MPa above and below the fracture line, respectively, which is significantly higher than in similar sections of models with PLA plates. Conclusions. The results of the study showed that for all types of loads, the greatest stresses arise in a model with titanium plate. The zones of increased stresses are observed on the plate, as well as on the fixing screws and in the bone tissue around the fracture line. In models of the lower leg with tibial fracture in the lower third, with osteosynthesis using PLA plates, the stresses in the models are distributed more evenly both in the elements of the fixing structure and in the bone tissue.

Keywords


composite material; polylactide; tricalcium phosphate; hydroxylapatite

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