Definition of Knock knees:

Knock knees is a condition in which the lower legs are positioned at an outward angle. When the knees are touching, the ankles are separated.

Causes, incidence, and risk factors:

Most children develop a slight knock-kneed stance by the time they are 2 or 3 years old, often with significant separation at the ankles when the knees are touching. This is part of normal development and often persists through age 5 or 6, at which time the legs begin to straighten fully. By puberty, most children can stand with the knees and ankles touching (without forcing the position).

Knock knees can also develop as a result of a disease, most often one that has already been diagnosed.