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The treatment of knee osteoarthrosis represents a difficult task. Osteotomy is one of the treatment regimes which, in earlier times, was the only surgical procedure giving reliable mid-term results. The improvement was attributed either to the changing of the acting forces or to the so-called biological factor. The use of a photoelastic model allows the observation of the direction of the significant contact and internal stresses at every point in a single plane.In the present study, we investigated the stress-strain situation of the knee joints in models made from 1 cm thick Araldite plates. These models reproduce tracings from anteroposterior X-ray views of monopodal loading of human knee joints. The models represent: normal knee joints, normal varus or valgus knee joint, knee joints suffering osteoarthrosis in varus or valgus, and knee joints suffering osteoarthrosis, following corrective osteotomies. The models were first loaded along the longitudinal axis with the leg in full extension, with 70 kg (body weight) and then with 140 kg, representing double the normal body weight.The application of longitudinal force on a normal knee joint, in full extension, results in symmetrical arrangement of trajectories in the condyles. Loading of the leg in varus or valgus produces shifting of stresses towards the inclination side. Increased contact stresses are always greater in the tibial condyles. The concentration of strain is directed towards the cortices of the loaded area. In a knee joint with osteoarthrosis, deviation of the trajectories in the condyles towards the inclination of the leg axis is observed. Also, increased contact stresses appear in the articular surfaces, on the inclination side. Isoclinics have meeting points on the loaded cortices. Supracondylar femoral osteotomy, or high tibial osteotomy, restores the axis of the leg, redistributes internal stresses (strain) in the condyles, corrects the trajectorial lines, and redistributes contact stresses in the articular surfaces towards the normal values.Correction of the femorotibial axis results in redistribution of the isochromatics, isoclinics, trajectorial lines and contact stresses. The values of the above parameters are corrected and remain closer to these of the normal knee, but in no case does the correction of all the above factors attain normal values. The change of direction of the main stresses following osteotomies towards those of the normal knee probably represents the biomechanical explanation of the way the osteotomy relieves pain and improves function.