According to Children's Hospital Boston, a standing child whose knees touch but ankles do not is said to have knock-knees. Knock-knees is also referred to as genu valgum and may occur in one or both legs. Knock-knees are normal early in development and usually persist until age 5 or 6 when the legs straighten. By age 10, most knock-knees are corrected without treatment. Children who are overweight commonly develop knock-knees as a complication of leaning inward to support their weight. Knock-knees have many causes, including vitamin D deficiency, bone infections or rare genetic diseases.
Rickets
Rickets is a disease of bone that occurs when an infant or child does not have sufficient vitamin D, which is required for absorption of calcium and phosphorus from food.
Dr. Clifford Clarke, of Russells Hall Hospital, in the West Midlands region of the U.K., once called rickets "the old English disease," because for centuries it caused bone deformities in children in the U.K. It was due to a combination of insufficient vitamin D in milk and lack of sunshine, which forms vitamin D in skin by the action of ultraviolet light. Rickets is still a problem in developing countries.
According to "Davidson's Principles and Practice of Medicine," rickets causes extension and widening of the growth plates in bone; this results in a flask-shaped enlargement of the lower ends of the leg bones. The problem gets worse in severe cases of rickets when the child begins to walk due to deformities of the shafts of the leg bones. Knock-knees is the end result of this process.
Bone Infection
Bacteria can enter bone via the bloodstream, from a nearby infection or from trauma. According to "Harrison's Principles of Internal Medicine," infection usually occurs in a single long bone---the tibia or fibula, for example---and primarily in children. During infancy, if not treated with antibiotics, infection may spread through the growth plate into the joint causing knock-knees.
Genetic Disease
A rare genetic disease called metaphyseal dysplasia, or Pyle's disease, causes deformities of the lower femur and upper tibia, resulting in knock-knees. Inheritance is autosomal resessive, meaning each parent is a carrier of the gene and a child has a 25 percent chance of receiving a gene from both parents and developing the disease.
References
- Chidren's Hospital Boston: Knock-Knees
- "Davidson's Principles and Practice of Medicine"; John Macleod; 1977
