More than 20 million Americans have osteoarthritis (OA), which affects the knee more often than any other joint and is the most common cause of long-term disability in persons older than 65 years. Knee OA is common, particularly in older patients and especially the obese. Knee injury, depending on the severity and type, can increase the risk of developing and the rate of progression of OA. In assessing impairment for knee OA, the evaluator must obtain a thorough history and physical examination and identify all potential risk factors. To opine that an injury caused OA, the evaluator should demonstrate that the traumatized knee shows significant arthritis but that the contralateral uninjured knee is radiographically normal; for an evaluator to suggest that pre-existing arthritis was aggravated, the involved knee should show significantly more advanced OA than the contralateral joint. Joint space widths (cartilage intervals) of both knees must be measured carefully on anteroposterior films obtained standing with a film-to-camera distance of 90 cm (36 in) and the beam at the level of and parallel to the joint surface. Taking into account all of the data (history, physical findings, and radiographic measurements from both the involved and contralateral joint), the rating physician can assess causation, estimate impairment, and apportion the latter to one or more etiologies.