Knock knee (also called "knock-knee deformity," "knock-knee syndrome," "knocked knee" or "genu valgum") is an incorrect alignment around the knee that can affect people of all ages. If left untreated, it can lead to knee pain and joint degeneration. Correction of the deformity will improve knee mechanics and walking capability, reduce pain and prevent a rapid progression of damage to the knee.

What is knock knee?

Knock knee is a condition in which the knees bend inward and touch or "knock" against one another, even when a person is standing with their ankles apart. This places excessive force on the outer side of the knee, which can cause pain and damage over time.

Knock knee is usually bilateral – affecting both legs – but in some cases, it may only affect one knee.

What causes knock knee?

Knock knee can be caused by an underlying congenital or developmental disease or arise after an infection or a traumatic knee injury. Common causes of knock knees include:

Being overweight or obese can also put extra pressure on the knees and contribute to knock knee.

What are the symptoms of knock knee?

The most prominent symptom of knock knee is a separation of a person’s ankles when their knees are positioned together. Other symptoms, including pain, are often a result of the gait (manner of walking) adopted by people with knock knees. These symptoms may include:

A person may also have other symptoms from an underlying condition that is causing the knock-knee syndrome. In people of all age groups who have knock knees, one or both knees is abnormally overloaded. This excessive force can lead to pain, further bone deformity, knee instability and progressive degeneration of the knee joint. In particular, adult patients who have been knock-kneed for many years often overload the outside (lateral compartment) of the knee, and stretch the inside (medial collateral ligament – MCL). These forces can cause pain, knee instability (including complex patellofemoral instability) and arthritis.

How is knock knee diagnosed?

An orthopedic specialist will review the patient’s medical and family history, any pre-existing conditions and current health. They will also do a physical examination of the legs and gait. Standing-alignment X-ray or EOS images will help confirm the diagnosis. These are radiological images of the leg from hip down to the ankle, which help the doctor locate the exact location and mechanical axis of the deformity.

How is knock knee treated?

For mild cases of knock knee in children or adolescents, bracing may reposition the knees. When this does not work, or if the patient is an adult at the time of diagnosis, a knee-realignment osteotomy is done to prevent or delay the need for knee replacement.

If knock knee is caused by an underlying disease or infection, that condition will be addressed before any orthopedic correction begins. Treatment for mild cases of knock knee in children or adolescents may include braces to help bones grow in the correct position.

If a gradual correction does not occur, surgery may be recommended. In the growing child, guided-growth minimal-incision surgery may be used to encourage the leg to gradually grow straight.

In severe cases and in skeletally mature adolescents and adults (any person whose bones are no longer growing), an osteotomy – in which bone is cut and then realigned – may be needed to straighten the leg. In most cases, the femur (thighbone) is treated. This is known as a distal femoral osteotomy (DFO). Some patients may benefit instead from a tibial osteotomy, in which the tibia (shinbone) is realigned. Certain cases may call for both a femoral and tibial osteotomy. (Find a surgeon who treats knock knee.)

Knock-knee surgery animation: Distal femoral osteotomy

X-ray or EOS images are used to show the location and magnitude of the deformity, and so that the orthopedic surgeon can determine which operative treatment is most appropriate. When the deformity is moderate, the osteotomy is typically stabilized with internal fixation (plates or rods). When the malalignment is more severe, the surgeon may also place an external fixator to gradually realign the leg. With external fixators, pins are inserted into the bone and protrude out of the body to attach to an external stabilizing structure. The pins are then removed after the straightening period ends.

Physical therapy is an important part of knock-knee treatment, especially in cases where surgery has occurred.