Knock Knee

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?
Are knock knees normal?
What causes knock knee?

What are the symptoms of knock knee?
How is knock knee diagnosed?
How is knock knee treated?

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.

Are knock knees normal?

Temporarily knocked knees are part of a standard developmental growth stage for most children. This usually corrects itself as the child grows. Knock knees that persist beyond six years of age, are severe or affect one leg significantly more than the other may be a sign of knock-knee syndrome.

Most children experience normal angular changes in their legs as they grow. Children are typically bowlegged until they begin walking at around 12 to 18 months. By about 2 to 3 years of age, their legs have usually begun to angle inward, making them knock-kneed. During normal growth phases, the child's legs will straighten out by age 7 to 8.

Knock knees that remain outside of these normal developmental growth patterns may be caused by disease, infection or other conditions. If the angle of the legs from hip to foot falls outside normal patterns, worsens over time, or is present on only one side of the body, this suggests a person has a more serious form of knock knees, and further evaluation by an orthopedic specialist may be necessary. Surgery may be needed to treat the condition.

X-ray images of knock knees and then straightened legs after corrective osteotomy.
X-rays: (Left) preoperative image showing knock knee. (Right) postoperative image showing leg alignment after surgical correction and removal of metal plates.

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:

metabolic disease
renal (kidney) failure
physical trauma (injury)
arthritis, particularly in the knee
bone infection (osteomyelitis)

rickets (a bone disease caused by lack of vitamin D)
congenital (inborn) conditions
growth plate injury
benign bone tumors
fractures that heal with a deformity (malunion)

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:

knee or hip pain
foot or ankle pain
feet not touching while standing with knees together
stiff or sore joints
a limp while walking

reduced range of motion in hips
difficulty walking or running
knee instability
progressive knee arthritis in adults
patients or parents may be unhappy with aesthetics

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 and arthritis.

Video: Knock-knee realignment surgery

View a multimedia presentation on knock knee and the surgical methods to correct it.

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.

HSS patient video: Knock-knee correction surgery helps high school athlete get back in the game

Watch this story of Maggie, a teenager who had recurring knee sprains and other injuries before learning she had knock knee, which was causing these problems.

In severe cases and in skeletally mature adolescents and in adults, an osteotomy – in which bone is cut and then realigned – may be needed to straighten the leg. X-ray or EOS images are used to determine the location and magnitude of the deformity. In most cases, the femur (thighbone) is treated. In some situations, the tibia (shinbone), or both the femur and tibia are treated. 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 gradualy 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.