PURPOSE OF THE STUDY The authors in their study compare the knee joint kinematics in patients before and after medial unicompartmental knee arthroplasty (UKA) and in the group of healthy volunteers. This study aims to confirm or reject the hypothesis that UKA preserves the physiological knee joint kinematics. MATERIAL AND METHODS In the course of 2015 and 2016 a total of 20 patients with grade III-IV medial knee joint osteoarthritis and 20 healthy volunteers were included in the study. In the first group of patients the standard Oxford (TM) unicompartmental knee prosthesis was implanted using kinematic navigation and the reference data were collected before and after the knee joint replacement. In the group of healthy volunteers an arthroscopic examination and subsequent data collection were performed in order to conduct a kinematic analysis. Subsequently, a statistical analysis of data was carried out and the groups were compared. RESULTS In our study two parameters were compared. The first was the maximum rotational movement of the tibia relative to the femur (rotational stability) in each of the degrees of flexion (0 degrees, 30 degrees, 60 degrees, 90 degrees, and 120 degrees). In this case, the knee joints prior to UKA showed laxity at all examined degrees. At 0 degrees flexion the preoperative range was -14.3 degrees to 8.2 degrees and it improved to -3.4 degrees to 7.3 degrees postoperatively, at 30 degrees it was -15.3 degrees to 15.8 degrees preoperatively and -13.3 degrees to 8.2 degrees postoperatively. At 90 degrees flexion the rotation of the knee joint before the UKA was -24.2 degrees to 13.2 degrees, while after the implantation the rotation improved to -19.3 degrees to 11.7 degrees. The second monitored parameter was the position of tibia with respect to the femur during the passive flexion test. The position in full extension before the implantation is 5.6 degrees internal rotation compared to 2.7 degrees external rotation after the implantation. The knee joint before the implantation shows minimum flexion/internal rotation throughout the entire flexion range. The postimplant values of flexion get close to the values obtained in a healthy knee joint. The data collected from the healthy knee joints were used as reference data. DISCUSSION The outcomes of our study correspond with the conclusions arrived at by other authors. The arthrotic process affects the knee joint kinematics by causing joint laxity and relative ACL insufficiency. This results in further progression of joint damage, the loss of the screw-home mechanism. In our study we benefit from the use of navigation in the intraosseous fixation, which compared to the other techniques enables more accurate data collection. CONCLUSIONS The arthritic process affecting the knee joint results in altered kinematics and biomechanics of the affected knee joint. This is manifested at both the axial and sagittal plane, when the kinematics fails to equal that of the healthy joint. The implantation of the medial unicompartmental knee arthroplasty improves the kinematics of the knee joint and helps achieve the requested values and stabilises the knee joint.