Our 13-year old son had a serious leg injury while at soccer camp. The X-ray showed a piece of bone between the two bones in the lower leg. The doctor said this wasn't normal. What could cause something like this to grow in a child?

The bridge of bone between the tibia and fibular bones of the lower leg seen on X-ray is called a synostosis. It is usually a sign of trauma causing an overgrowth of bone. Sometimes a synostosis forms in the ankle after an ankle ligament is torn from a severe sprain.

In children, a synostosis can be congenital, meaning your child was born with it. This does not mean it is genetic or inherited. Congenital synostosis occurs if there's been trauma or infection while the child is in the mother's womb. The synostosis forms during the healing process.

In adults, synostoses can occur after bone graft in the lower leg or if absorbable rods are used for ankle fractures. Any kind of growth delay or abnormality in child or adult can result in synostosis between the two lower leg bones.

It may not be possible to tell if your child was born with this feature, or if a former soccer injury could have led to its formation. Synostoses do grow in size as a result of repeated, tiny fractures occurring called microfractures.

Your son may need to take some time off from sports to allow the healing process to complete itself before further fracture(s) occur.