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Knock knees describes an unusual angle of the lower legs. Most commonly it is a normal shape as children's legs develop, but occasionally in children or adults it can cause problems or have an underlying cause. Most children who have knock knees grow out of the condition.

The medical term for knock knees is 'genu valgum' - genu meaning 'knee' in Latin, and valgum meaning 'bending outwards'. In fact with knock knees, if the knees are together it is the lower leg which seems bent outwards. So if you have knock knees you are unable to stand with your knees AND your ankles together.

As children grow up, they tend to start off 'bow-legged'. This means if their feet are together, their knees are apart, leaving a diamond shape between the legs. This is normal in babies and toddlers until the age of about 2 years. As the child's legs grow and develop, they then tend to take on a 'knock-kneed' shape. Now if their knees are together, their feet and ankles are apart. It tends to be most obvious around the age of 3 or 4 years, usually gradually improving after that. Knock knees are normal up to the age of 7 or 8 years, although the condition may last longer.

Bow Legs and Knock Knees

Diagram of bow legs and  knock knees
Image provided for by Dr Mary Harding

The normal knock knees of childhood do not cause problems. If the condition is severe, or caused by an underlying problem, it may cause pain or difficulty walking.

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In most cases, knock knees are purely caused by the normal phase of growth and development. Occasionally - in either children or adults - there can be an underlying problem causing the condition. For example:

You do not need to see a doctor if your child is at an age where having knock knees is normal (aged more than 2 years and less than 8 years) and if the deformity is not very severe. If your child lies on their back with their knees together, you can measure the distance between their ankles. If it is less than 10 cm around age 4 years, this is likely to correct itself as they grow. If you take photos of your child every 3-6 months, standing with their knees together, you should see that the legs gradually become straighter from the age of 4 years or so.

See a doctor if:

  • Only one leg is affected.
  • Your child has knock knees before the age of 2 years or after the age of 8 years.
  • There is a larger gap between the ankles than that described above.
  • Your child has pain in the lower legs or knees.
  • Your child has problems walking or walks with a limp.
  • You develop knock knees as an adult.

The diagnosis of knock knees can be made by just looking at a child. If it is particularly severe, or if it occurs outside the normal age range, then further tests may be needed. This might include X-rays or blood tests. The severity of the knock knees can be assessed by measuring the distance between the ankles when the child is standing with knees together. or by measuring other angles of the legs.

In most cases no treatment is needed. For children who develop knock knees as a normal part of their childhood growth, the condition corrects itself as they continue to grow. They do not need any special shoes or braces, and do not need to avoid any sporting activities. They do not need any physiotherapy or other special treatment.

If the condition is severe or caused by an underlying problem, then treatment may be needed. This will then depend on the underlying condition. Occasionally an operation to correct the deformity is needed.

Further reading and references