Knock knees

Knock knees, or genu valgum, describes the position in which the knees come together while the ankles stay apart with a gap between them. It is usually the parents of a three to five year old who notice it and take fright, although at that age this leg shape is an ordinary stage of growth that sorts itself out. Far less often knock knees are a sign of disease or the result of an injury, and then it matters to spot it in good time. What separates the two is not so much the number of degrees as the age, the symmetry, and which way things are moving over time.

What it looks like and how it is measured

Checking is simple. The child or adult stands up straight with the legs together, the knees touching and the kneecaps facing forwards. If a gap remains between the inner ankle bones, that is knock knees, and the width of that gap is called the intermalleolar distance; it is the measurement used to follow progress.

Most people have nothing else to report. The legs simply look different from other people's. When the deviation is marked, the rest follows: the gait becomes wide, the child catches one knee against the other and trips when running, the legs tire sooner, and shoes wear unevenly along the inner edge. Knock knees are often accompanied by flattening of the arch, as the foot rolls inwards following the knee. In teenagers and adults with obvious valgus the kneecap sits less securely in its groove, so partial and complete dislocations of the kneecap are more common in them.

Why in children it usually settles by itself

Legs change shape to a timetable, and it is worth knowing it so as not to treat the normal. A baby is born bow-legged, with the shins curved slightly outwards. By around eighteen months to two years they straighten, and the pendulum carries on the other way: at about three to four years the valgus reaches its maximum, and the gap between the ankles in a perfectly healthy child can measure several centimetres. The legs then gradually align, and by seven or eight years a small physiological valgus is left that stays with a person for life, usually a little greater in women because of the width of the pelvis.

Hence a simple rule: knock knees in a preschooler that are symmetrical, painless and slowly reducing are not a diagnosis but a phase. They need no treatment, only patience and one or two follow-up checks to confirm things are heading the right way.

When it is no longer about growth

A child should be seen by a doctor if any of the following applies.

• The deviation is visible in one leg only, or the legs are clearly asymmetrical.
• The legs have become more bent rather than straighter over the past six months.
• The child complains of pain in the knee or lower leg, limps, or refuses to walk far.
• The valgus persists, or has appeared for the first time, after seven or eight years of age.
• The child is noticeably shorter than classmates or growing more slowly than before.
• The legs are of different lengths.
• The gap between the ankles is large, as a rough guide more than eight to ten centimetres.
• There are skeletal conditions in the family, or a similar deformity in an adult relative.

The urgent case stands apart. A hot, swollen, sharply painful knee that the child guards and will not let anyone straighten, particularly with a fever, is nothing to do with leg shape. That is how joint infection and osteomyelitis present, and they need attention the same day, because pus inside a joint destroys the cartilage and the growth plate within days.

Causes other than growing up

When the valgus falls outside what is normal for the age, the search is for whatever is stopping the bone growing straight.

• Rickets and vitamin D deficiency. Softened bone deforms under body weight. Besides ordinary vitamin D deficiency there are inherited forms in which the kidney loses phosphate; these are not corrected by the vitamin alone and need their own management.
• Injury to the growth plate. A fracture near the knee can damage it, after which the bone grows unevenly. The reverse situation also exists: after a fracture of the upper shin in a small child the leg sometimes drifts into valgus even though the fracture has healed perfectly, and over time this partly corrects itself.
• Bone or joint infection in early childhood. It can damage the growth area and leave its mark for years.
• Joint inflammation. Juvenile arthritis alters the shape of the knees, and rheumatoid arthritis behaves the same way in adults.
• Inherited skeletal disorders. Multiple bony outgrowths, various bone dysplasias, storage diseases. They give themselves away by short stature, deformities in more places than the knees, and similar cases in the family.
• Chronic kidney disease. Disturbed calcium and phosphate handling in a growing child deforms the bones.
• Excess weight. It both drives the deformity and makes it less likely to resolve on its own, because load on the growing bone is distributed unevenly.

What gets investigated depends on what the examination finds. Usually it is a standing X-ray of the legs, which shows the whole axis of the limb and allows the angle to be measured, plus blood tests for calcium, phosphate, alkaline phosphatase, vitamin D and kidney function.

What doctors do

The great majority of children need nothing beyond observation. The doctor measures the gap between the ankles, records it and compares six months or a year later: if it is shrinking, the matter is closed.

If a cause is found, that is what gets treated: vitamin D and calcium are replaced in rickets, treatment is adjusted in the renal and inherited forms, inflammation is brought under control in arthritis. Often that alone is enough for the bone to start growing straighter.

Where the deformity is marked, is not reducing and is getting in the way, a child whose growth plates are still open has an elegant option: guided growth. A small plate with two screws is fitted to the inner side of the bone beside the knee, slowing growth on that side while the outer side carries on growing. The leg straightens gradually over some months or a year, after which the plate is taken out. It is a minor operation, needs no plaster, and walking is allowed almost at once. Its one condition is being in time, while the child is still growing.

If growth has already finished and the deformity is significant and painful, the axis is corrected by osteotomy: the bone is divided, set in the right position and fixed. This is a bigger undertaking with a long recovery, and it is used when there is pain and a clear prospect of it worsening, not for the sake of appearance.

What helps at home and what does not

It is worth knowing what not to spend money and time on. Orthopaedic shoes, insoles, arch supports, night splints and massage do not straighten bone: the shape of the leg is set by the growth plate, not by what the foot is wearing. An insole can be reasonable if the foot rolls inwards and that in itself is uncomfortable, but its job is comfort, not the axis of the limb. Promises to correct knock knees with exercises or devices belong in the same category.

What does work is this. Keeping weight within sensible limits is the one measure at home with a demonstrated effect on the load through the knee and on how fast it wears. Making sure of enough vitamin D and calcium, not as a cure for valgus but so that bone is built properly; where a deficiency is confirmed, the dose is set by a doctor. Not stopping a child running and playing, because growing bone needs movement. Teenagers and adults with obvious valgus benefit from exercises for the buttock and hip abductor muscles: these do not change the shape of the bones, but they improve control of the knee, reduce pain and lower the risk of the kneecap slipping out.

When knock knees appear in an adult

This always calls for an explanation, because adult legs do not bend out of shape for no reason. The commonest cause is osteoarthritis wearing away the outer part of the joint, with the leg gradually drifting into a knock-kneed position; it becomes a vicious circle, because the misalignment speeds the wear. Valgus also remains after fractures that healed out of position, after removal of a meniscus, and in rheumatoid arthritis.

What can be done: lose weight, strengthen the thigh muscles, agree pain relief with a doctor and, in advanced arthritis, discuss an osteotomy or a joint replacement. The point is not to leave it until no choice remains.

Online consultation

In an online consultation the doctor will judge, from your description and a photograph, whether your child's leg shape is within the normal range for the age or whether there are signs that call for an examination in person and an X-ray. They will explain how to measure the gap between the ankles properly at home and how to track it, which tests are worth doing if rickets or another cause is suspected, and help you read reports you already have. An adult whose legs have changed shape and whose knees hurt will be told which investigation will show the state of the joint and what can be done before surgery comes into the conversation.

This material is for information only and does not replace medical advice.