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Objective. To research biomechanical particularities of knee joint flexion defomation in rheumatoid arthritis patients. Methods. The work is based on the analysis of the preoperative examination of 23 patients with RA with multi-plane deformations of the lower limb, who were treated in our clinic in the period from 2021 to 2024. The average age of the examined patients was (49.14 ± 6.37) years old. The quantitative assessment of contractures and computed tomography (CT) of the knee were performed on the patients. Results. As can be seen from the obtained results, the stresses in the contact zone on the lateral condyle of the tibial plateau increase by 36.71 % in conditions of flexion contracture of the knee 30°, on the medial condyle of the tibial plateau -- by 36.64 %. It was established that the load on the condyles of the tibial plateau increased by 12.8 % in forced passive flexion of the knee joint by 6°, as a result of passive extension by 3° -- the load on the condyles of the tibial plateau increased by 95.2 %, which is a critical and can lead to the degradation of bone tissue in the contact area and increase the risks of secondary arthrosis. Conclusions. The simulated solid 3D-model of the knee joint was developed and demonstrates that in the knee joint flexion position of 30º, according to the radius of curvature of the mediaal and lateral condyle, the contact area on the medial condyle of the tibial plateau is 2 times larger than that on the lateral one and is shifted slightly backwards. The standing with one support under the weight of a body 60 kg (600 N) in conditions of flexion contracture in the knee joint in a position of 30°, there is an increase in contact stresses on the external condyle of the tibial plateau by 36.71 %, on the internal condyle -- by 36.64 %, which contributes to the progression of the clinical picture of RA with an increase in the phenomena of osteoarthritis specifically in the back parts of the knee joint. [ABSTRACT FROM AUTHOR]

Background. Rheumatoid arthritis (RA) is an immunomodulatory, chronic inflammatory disease accompanied by the proliferation and articular cartilage destruction that cause disability. Biomechanical factors in RA can play an important role in the onset and progress of the joint degenerative processes, secondary to the inflammation process. The biomechanical factors in RA can play an essential role in the start and progress of the degenerative processes within the joint that are secondary to the inflammatory process. Materials and methods. A solid simulation 3D-model of the knee joint was created that contained both tibia and fibula, the femur bone, femoral condyle cartilage and tibial plateau cartilage, menisci. It was done for further analytical calculations and finite element modeling calculations. Analytical calculations are based on the data of previous studies of quantitative evaluation of the knee joint contracture (compliance of contracture to the external corrective action) and on calculations data of the joint forces that manifest in ambulation of the patient with RA. Results. The created simulation computer 3D-model of a knee joint with its flexion contracture at late stages of RA shows that the forced passive flexion in the knee joint to 6°, the load on the condyles of the tibial plateau increased by 12.8 %, and as a result of forced passive flexion in the knee joint to 3°, the load on the condyles of the tibial plateau increased by 95.2 %, which is critical and may cause degradation of the bone tissue in the contact area. Conclusions. Increase of load on the lateral areas of the tibial plateau and, correspondingly, the tensions on the contact areas of the femur bone condyles with the tibial plateau, may contribute to the progress of the clinical picture of RA with the increase of arthrofibrosis and osteoarthritis events, particularly in the lateral areas of the knee joint. [ABSTRACT FROM AUTHOR]