The purpose - to study structural changes in knee articular cartilage and synovial membrane during locking intramedullary osteosynthesis of femoral fractures. Material and methods. The study tested structural changes in the articular cartilage of the femoral condyles and synovium during femoral fracture repair under the conditions of intramedullary locked nailing. Transverse diaphyseal femoral fractures were modeled in 12 adult mongrel dogs. Synovitis severity was assessed with a V. Krenn scale. Wilcoxon Test was used for testing hypothesis. Results. In the first group (n = 6) fractures were fixed with locked intramedullary nails ILoc (Biomedtrix, USA), while the same type of fixation in group 2 (n = 6) was performed on 4th day postoperatively after reduction by skeletal traction. It was revealed that in the first group a unified medullary cavity was seen by day 70 and the cortical layer at the fracture site approximated to the condition of the uninjured bone in intensity of bone formation and thickness. These featured were observed in the second group only by day 100. The histological study showed that the structure of the femoral condyle articular cartilage was regular in the first group but its changes were reduced thickness and reduced volumetric density of chondrocytes that might result due to disturbed trophical condition. In the second group the articular cartilage fibres tended to disintegrate on the surface that was accompanied by disturbed integrity of the basophil line and penetration of the vessels into the cartilage. Moreover, the cartilage destruction progressed as the experiment continued and chondrocyte proliferation sharply decreased. Synovitis was considerably expressed. The data obtained in the second group proved high risk of developing osteoarthrosis.