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.

You can assess the extent of your child’s knock knee by measuring the distance between their ankles when their knees are together.

To measure the distance between your child’s ankles, lie them on their back and straighten out their lower limbs. Place their knees together and measure the distance between the insides of their ankles.

If your child is between two and four years old and has a gap between their ankles of 10cm (around four inches) or less, it will usually correct itself as your child grows. If the gap is greater than 10cm, you should take them to see their GP. A gap of more than 10cm in older children should also be investigated to see if there is an underlying problem.

As well as the distance between your child’s ankles, your GP may also assess your child’s:

• height
• knee symmetry (how even the knees are)
• leg, hip and feet alignment
• gait when they walk (to check whether they can walk normally)

Your GP may refer your child to an orthopaedic surgeon (a doctor who specialises in muscle, bone and joint disorders or injuries). Your child may need to have an X-ray to help identify any underlying problems that are causing knock knee.

Knock knee is an abnormal curvature of the lower legs, resulting in a large gap between the feet and ankles when the knees are touching.

The medical name for knock knee is genu valgum.

Most children have bow legs until they are around three years old, as a result of the legs being folded inside the womb. They are “knock kneed” at around four or five years of age. The legs begin to straighten and align normally by the time they are six or seven.

However, in some children, knock knee continues, or it occurs at a later stage of development. In these cases, the cause is often unknown, although knock knee can be caused by bone conditions, such as rickets or scurvy. It may also be the result of an inherited bone deformity.

Read more about the causes of knock knee.

What are the symptoms of knock knee?

Most cases of knock knee do not cause any significant problems, and the child’s legs will straighten on their own by the time they are about six or seven.

If knock knee persists, or if there is a significant deformity, the child may have difficulty walking or walk awkwardly. Unnecessary strain on their knees may also lead to knee pain.

Read more about the symptoms of knock knee.

Treating knock knee

If your child’s knock knee is caused by an underlying condition, such as rickets or scurvy, the condition will need to be treated. Rickets can be treated with vitamin D and calcium supplements, and scurvy can be treated with vitamin C supplements.

Corrective surgery may be recommended in cases where knock knee is the result of an inherited bone deformity and is causing severe knee pain and problems walking.

There are several different surgical techniques that can be used, but a procedure known as an “osteotomy” is most commonly used. This involves cutting and re-aligning the lower part of the upper leg bone (femur) to correct the position of the leg and re-distribute the weight going through the knee.

Read more about how knock knee is treated.

If a child with knock knee stands with their knees together, their lower legs will be spread out so that their feet and ankles are further apart than normal.

A small distance between the feet and ankles is considered normal. In children up to four years old, a gap of up to 10cm (around four inches) is not a cause for concern.

However, you should take your child to a GP if:

• the gap between their ankles is greater than 10cm
• there is a big difference between the angle of their lower legs when standing straight
• there is an excessive inward or outward knee angle
• they have knee pain as a result of the angle of their knee
• they are having difficulty walking or they walk awkwardly

Read more about leg and foot problems in children.

Knock knee in adults

Knock knee can sometimes occur in adults. In older people, it is often associated with joint problems such as osteoarthritis or rheumatoid arthritis.

In severe cases of knock knee, poor posture may start to affect other parts of the body, such as the hip joints, back or feet. This can cause pain, a limp or problems walking.

In most cases, knock knee does not need to be treated, because the abnormal curvature of the lower legs corrects itself as a child grows.

In young children (up to four years old), a gap between the ankles of 10cm (around four inches) or less is not usually a cause for concern. It will often correct itself by the time the child is six or seven years old.

If the distance between your child’s ankles is greater than 10cm, you should take your child to see a GP so that the underlying cause can be investigated (read more about diagnosing knock knee).

Treating underlying conditions

If your child’s knock knee is caused by an underlying condition, such as rickets or scurvy, the condition will need to be treated.

Rickets, also known as osteomalacia (soft bones), is a rare condition that is often caused by a lack of vitamin D. This plays an essential role in the development of strong and healthy bones.

Rickets can be treated using regular (daily) vitamin D supplements or a yearly vitamin D injection. It also helps to eat a diet rich in calcium and vitamin D.

Read more about how rickets is treated.

Scurvy is also a rare condition that causes a variety of symptoms, including severe joint pain. It is caused by a lack of vitamin C in the diet and can be treated with vitamin C supplements and by eating a healthy, balanced diet.

Read more about how scurvy is treated.

Surgery

In rare cases, corrective surgery is used to treat severe cases of knock knee. It is usually only recommended when the gap between a child’s ankles is greater than 10cm and their lower legs are severely curved, resulting in knee pain or difficulty walking.

The surgical procedure used may be either:

• guided growth – placing a small plate on the inside of the knee that gradually corrects the growth of the knee over six to 12 months
• an osteotomy – cutting and realigning the leg bone to correct the angle of the knee and redistribute the weight going through it

Guided growth is usually preferred for children up to around 14-16 years old, as they are still rapidly growing.

What happens during an osteotomy?

The operation needs to be carried out on each leg at different times, so that the weight can be kept off the leg that has been operated on while it heals.

Before the surgery, a full-length X-ray of both legs will be taken so that the hips, legs, knees and ankles can be closely examined and the amount of realignment that is needed can be assessed.

In some cases, when the knee joint is thought to be severely damaged, further investigations such as a magnetic resonance imaging (MRI) scan or an arthroscopy (keyhole surgery) may also be required.

A distal femoral osteotomy is carried out in hospital, and your child will usually need to stay in for one or two days. The operation is carried out under general anaesthetic, which means they will be asleep throughout the procedure and unable to feel any pain.

During the operation, the lower end of the upper leg bone (the femur) is cut just above the knee joint, and a small wedge of bone is removed. This allows the leg to be realigned to create a more normal, straighter position. A plate and screws are used to fix the leg bone in its new position.

After the operation, the knee has to be kept still and straight using a splint (support), which will usually need to be worn for about five to six weeks. Crutches will also need to be used during this time to keep the weight off the leg that has been operated on.

It will usually take about three months for the leg to fully heal. During this time, physiotherapy may be recommended to help improve the mobility of the knee and aid recovery.

After around three months, any knee pain should be significantly reduced, and your child’s mobility should be improved.

Important advice for adults

If you need surgery and you smoke, it is very important that you do not smoke for at least two weeks before the operation and for a minimum of three months afterwards. This is because the effects of smoking restrict the blood supply to the bone, which can prevent or delay the wound from healing.

You should be able to drive six to eight weeks after having a distal femoral osteotomy.