Knock knee - Causes of knock knee

Knock knee is fairly common in healthy children under six or seven years old. It occurs because a child’s weight falls to the outside of their knee joint, which is a normal part of their growth and development.

In most cases, knock knee corrects itself without the need for treatment. A child's body undergoes many changes as they grow – this includes their legs, which gradually change shape and become straighter.

If knock knee does not correct itself and the distance between your child’s ankles is greater than 10cm (around four inches), or if their knees are not symmetrical (even), your child may have an underlying health condition or growth problem.

In most cases of knock knee the cause is unknown, although childhood conditions that can be responsible include:

• rickets – a rare condition that affects a child's bone development and is usually caused by a vitamin D deficiency (sunlight is a source of vitamin D and is also found in oily fish, eggs and fortified spreads and breakfast cereals)
• nutritional conditions – such as scurvy (a vitamin C deficiency), which can adversely affect the development of bones and joints (although this is rare) 
• rare genetic conditions – examples include Cohen syndrome, which causes a number of problems, such as moderate to severe learning difficulties, progressive visual problems and unusually bendy joints 

A family history of skeletal abnormality can also be an underlying cause of knock knee in children.

Adults

In adults, knock knee can occur following an injury, such as an uncorrected fracture of the shin bone (tibia), or as a result of a joint condition, such as osteoarthritis or rheumatoid arthritis.

Arthritis in the knee joint can sometimes cause knock knee to develop. Having knock knees can also place the knee joint under extra pressure, which can lead to arthritis in later life.