Causes / typical injury mechanism: Osteoarthritis of the knee can be divided into primary and secondary types. Primary osteoarthritis implies that the articular cartilage degeneration occurs without a specific underlying cause. Secondary osteoarthritis is due to structurally abnormal cartilage, such as that in rheumatoid arthritis, or due to abnormal force distribution across the joint, as seen in patients with a prior injury or trauma. Osteoarthritis is a slow, progressive joint degradation process that typically occurs over 10-15 years, if not longer.

Classic history and presentation: Patients often present due to long-standing pain and stiffness in one or more joints. Further examination can reveal crepitus, bony enlargement or osteophyte formation, joint effusion, and point tenderness. Classically, the patient will have joint pain that worsens with activity, as opposed to an inflammatory arthritis that will improve with activity. Imaging is often used to diagnose the extent of joint arthritis and follow the joint's degenerative progression.

Prevalence: About one-third of people older than 65 years have knee osteoarthritis, so this disease has a very high prevalence among the general population. Recent estimates have shown that the prevalence will only increase, due to more people becoming overweight and obese.

Risk factors: There are many risk factors for knee osteoarthritis, including female sex (45% increased risk for women compared to men), older age, genetics, overweight or obese body mass index (BMI), previous joint injury / trauma (eg, intra-articular fracture, ligamentous injury, meniscal injury), joint malalignment, and repetitive joint damage.