The study of the stress-strain state in the “implant-bone” system on the model of the allocomposite endoprosthesis of the proximal femur
Keywords:bone tumor, endoprosthetic replacement, implant
Background. The restoration of massive post-resection defects of long bones is one of the main challenges facing surgeons in the treatment of patients with bone tumors. Therefore, the deve-lopment of optimal implants to replace defects of bones and joints has been going on for many decades. To select the “ideal” implant, which must meet the requirements of biocompatibility, mechanical stability and be safe, you need to conduct many different studies and tests. It is the allocomposite endoprosthetic replacement method that is of interest for in-depth researches. The purpose was to substantiate the technique of allocomposite endoprosthetic replacement based on mathematical modeling by the finite element method. Materials and methods. The finite element method was used to study a mathematical model of the femur with a “tumor” endoprosthesis. We studied the stress-strain state of the model when performing a femoral resection with a direct section and a section in the form of a step. The situation was simulated at different times after surgery, in 3 and 6 months. For each type of resection, cement and cementless versions of fixing the endoprosthesis stem were modeled. Results. With transverse osteotomy and a cementless method of fixing the stem, the maximum stresses (92.3 MPa) arise in the osteotomy zone. In the diaphyseal femur, the zone of maximum stresses is located on the medial side, they are 10.5 and 10.1 MPa in the distal and proximal parts, respectively. The use of stepwise osteotomy in combination with cementless fixation of the endoprosthesis stem in the immediate postoperative period reduces the level of stress in the osteotomy zone to 59.1 MPa. In the process of increasing the strength of the bone regenerate after using transverse osteotomy in combination with cementless fixation of the stem, a decrease in stress level in the osteotomy zone is observed — 75.6 MPa. In the diaphyseal part of the femur, stresses range from 9.5 to 10.0 MPa. When using stepwise osteotomy and cementless fixation of the endoprosthesis stem, the stress range in the diaphyseal part of the femur is from 7.9 to 13.8 MPa. The use of cement in combination with transverse osteotomy in the postoperative period of 3 months does not lead to fundamental changes in the stress-strain state of the model. The maximum stress value (77.1 MPa) is determined in the osteotomy zone. When using stepwise osteotomy, the presence of a cement layer between the stem of the endoprosthesis and the wall of the bone marrow canal reduces the level of stress in the osteotomy zone to 31.5 MPa. Conclusions. As a result of mathematical modeling, it was found that stepwise osteotomy of the femur allows reducing the level of mechanical stresses in the osteotomy zone compared to models with transverse osteotomy. The use of bone cement for fixing the endoprosthesis stem in a theoretical model also allows one to significantly reduce the level of stresses in all variants of the studied models, due to the formation of a damper layer between the metal and bone tissue. An increase in the strength of the bone regenerate over time leads to equalization of stress values, both between the control points of each model and between models with different types of femoral osteotomy. Thus, as a result of the study, a mathematically sound technique of allocomposite endoprosthetic replacement was deve-loped, which we improved. The best results were obtained with the use of stepwise osteotomy and fixation of allobone with the recipient’s bone according to the type of “Russian castle” and intramedullary fixation of the entire system.
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