|Osteoarthritis (OA) is a disease with multifactorial provenience. All subjects who have developed OA are probably affected by both genetics and environmental factors with individually distributed predominance. It has previously been reported that the risk of post-traumatic OA after a meniscal injury of the knee is strongly affected by a family history of OA, by the presence of generalized OA (like the nodal osteoarthritis of the hand), by obesity, and by sex. Many subjects with OA in one joint have the disease at different stages in other joints. The condition is usually expressed as a mixture of different features including inflammation, osteophyte formation and cartilage loss. However, since several of the environmental risk factors such as obesity, joint injury and joint overload are of biomechanical nature, OA is primarily a mechanical problem. Joint changes result usually from either abnormal mechanical strains that affect healthy cartilage or from failure of pathologically impaired cartilage that degenerates under the influence of physiological mechanical strains. In the cases with mechanical origin, the pathology is focal and not generalized. In general, the disease occurs when the dynamic equilibrium between the destruction and repair of joint tissues is impaired. The degenerative process starts at joint cartilage level and is characterized by a discrepancy between cartilage production and degradation. The process is, however, not limited to cartilage alone but is considered as a chronic disease of all joint tissues including subchondral bone, menisci, ligaments, and per articular muscles. It consequently leads to joint instability and muscle weakness that nowadays can be regarded as early features of structural and functional joint failure. In the natural course of OA, one can distinguish the symptomatic phase, the radiographic stage and the end-stage that demands knee replacement surgery.