Objective — to reveal and analyze clinical characteristics, knee joint histopathology, and quality of life in patients at late stages of knee osteoarthritis (OA) triggered by age, trauma, metabolic syndrome, or their combination. Material and methods — We studied 120 subjects with knee OA (sensu Altman R.D., 1991) of Kellgren-Lawrence Grades 3-4. They were distributed among 4 groups (30 participants in each) based on the presence of age-related, post-traumatic, metabolic, or combined phenotypes. Clinical examination of patients with pain and their functional status evaluation (via Visual Analogue Scale, WOMAC, Lequesne index), quality of life assessment (MOS SF-36 questionnaire), along with histopathological study of medial tibial plateau cartilage and synovial membrane, were caried out, followed by statistical data processing. Results — Age-related OA phenotype was characterized by the latest clinical onset [59.5 (54-68) years of age] with the largest average patient age [72.5 (63-77) years], moderate to severe pain and knee dysfunction [total WOMAC score of 160 (127-190) points and Lequesne index of 20 (8-21) points], severe degenerative cartilage lesions [8.5 (6-10) points sensu Mankin] with high-grade synovitis [5 (3-8) points sensu Krenn]. Post-traumatic OA phenotype was distinguished by the lowest pain, stiffness and knee functional limitations [total WOMAC score of 129 (100-166), Lequesne index of 15 (14-19)], the highest quality of life in patients [physical component summary of 34.1 (30.5-36.1) points, mental component summary of 40.4 (32.9-43.8) points] against the background of local severe cartilage lesions [8 (6-8) sensu Mankin] with reparative pattern and synovial fibrosis. For metabolic OA phenotype, the typical traits included female-biased sex ratio (87%), high prevalence of clinical synovitis (77%), severe pain and functional knee disorders [total WOMAC score of 188 (162-207) points, Lequesne index of 20 (19-23) points], the worst quality of life [physical component summary of 28.0 (24.3-31.9) points, mental component summary of 30.9 (26.9-35.9) points], vascular invasion of cartilage, and high-grade synovitis [4 (3-5) points sensu Krenn]. Combined OA phenotype was characterized by variable clinical and histopathological features. Conclusion — Comprehensive comparative clinical and morphological analysis of late-stage knee OA of various origin was completed, and age-related, post-traumatic, metabolic and combined OA phenotypes were studied. The methodological basis for differential approach to treating different categories of OA patients was developed.