Tarsal coalition is a condition in which one or more bones are abnormally joined together (fused or coalesced) in the back of the foot. Normal development of any joint requires that the bones on either side of the joint move freely.
A child born with tarsal coalition has bones that do not develop normally because the joint and its cartilage lining never fully develop.
For this reason, the two bones that are involved in the coalition are stuck together and the foot does not move in and out (inversion and eversion) adequately.
As the foot develops, the child and family will notice that the foot is flat, and often getting progressively flatter as the child gets older.