Osteoarthritis of the knee is a degenerative condition that is almost universal by old age. It is commonly seen over the age of 50 years. Early development is associated with genetic factors including the shape and angle of the knee.
There is good evidence that early onset of knee osteoarthritis is caused by obesity with a substantially increased risk, probably by a factor of at least ten times.
There is good evidence that occupational factors cause early onset, particularly heavy physical loading of the knee, and loading in a squat or kneeling for prolonged periods, and these factors may double the risk.
There is good evidence that trauma to the knee, particularly meniscal damage and instability from ligament damage, causes early onset.
There is no evidence that sporting activity alone increases the risk of knee osteoarthritis other than elite sports.
There is good evidence that some physical activity is protective, including individuals who already have some knee osteoarthritis.
There is no evidence that occupation in the military, fire service, police, or healthcare increases the risk of knee osteoarthritis.