Calcaneonavicular fusion (sometimes called calcaneonavicular coalition) refers to the fusing of two bones in the foot: the calcaneus (heel bone) and the navicular bone. The navicular is an important bone because it joins with many other bones in the foot and ankle. It is located on the medial side of the foot (side closest to the other foot). It articulates (moves against) the four and sometimes five other ankle bones.
When calcaneonavicular coalition occurs, the affected individual (usually a child between the age 8 and 12 years old) reports ankle pain with loss of motion. They are no longer able to point the foot down all the way. Turning the foot and ankle inward can also be limited. The loss of these motions makes it difficult to walk, run, and participate in daily activities at school.
It appears that the bones are fused or held together with a thick, binding cartilage. The child was probably born with this condition but rapid growth during the pre-pubescent years makes the problem more apparent.
Left untreated, besides being painful and limiting, the joint eventually develops degenerative arthritis. The current standard of care is to remove the cartilage bridging the two bones. The hole that is left is filled with muscle from the extensor digitorum brevis (EDB) in the foot. Some surgeons have tried using fat as a filler instead. Both methods have advantages and disadvantages.
After surgery, the parents and child should be prepared for the fact that the bone might grow back. If that happens, a second surgery may be needed. No one is quite sure why regrowth happens in some children, but not others.
There has been some speculation that damage to the nearby joints and soft tissue structures might be a factor. When surgeons first tried using fat instead of a rolled up tendon to fill in the hole, they hoped it would eliminate the need for revision because of regrowth. But it didn’t, so other factors must be at work. Further research is needed to find the optimal treatment for this problem.