Bow leg and knock knee are very common and often normal (physiological) depending on the age of the child. It is generally reassuring if any perceived deformity is symmetrical.
Modest bow leg is normal in infants up to 18 months of age. The legs should straighten out between 18 months and 2 years. Some children will then ‘overshoot’ between 3 and 4 years and become knock kneed before straightening out at around age 7.
Most children who are still bow legged when 2 yrs old will be normal but it is prudent to exclude other conditions particularly if deformity is severe. The commonest pathological condition will be Ricketts. This can be diagnosed with X rays or blood testsand usually responds to vitamin D replacement.
Mild cases of bow leg and knock knee will have few long term implications other than cosmetic. More severe cases can predispose to osteoarthritis in adulthood or instability of the patella (kneecap).
Whilst still growing deformity can reliably be corrected by hemiepiphyseodesis. Once growth plates have closed the femoral bone has to be divided (osteotomy).
In older children knock knee is more common. As a ‘rule of thumb’ if there is 10cm or more between the ankles when the patient stands with the knees just touching then surgery may be indicated.