A tarsal coalition is a fusion (coalition) between two or more bones in the tarsal bones of the foot and is cased by a failure of separation of the bones during fetal development.
During early development of the fetus, the “bones” of the foot are just one single mass of cartilage and then with development this cartilage differentiates into the different bones of the foot. For some reason, on a rare occasion that process does not happen for two or more bones and they develop as just one bone (or two bones with a bony bridge between them, so they are essentially one bone).
Just what causes that failure of the bones to develop properly is not clear. There may be a genetic or hereditary factor or there may be something else that occurs during the pregnancy (eg environmental; smoking?) that causes this proper bone develop to fail.
(Additionally, in later life a coalition between tarsal bones may develop as the result of trauma, such as a fracture involving two or more bones. During the healing process the bone fuse.
