A coalition is a technically a connection between two bones that should not be present. Coalitions can be congenital or acquired. When acquired, it is usually due to trauma. But when the diagnosis of a coalition is given around the foot and ankle (the tarsal bones), it infers the congenital variety.
A tarsal coalition is a congenital or developmental deformity whereby there is failure of separation of bones that should be separate. This connection between the bones blocks full formation of the joints, so to speak, and thereby will limit motion. This ‘tether’ can consist of a bony or a fibrous tissue and it tends to affect the joints and bones that are involved in side to side motion.
The result of this limitation in side to side motion is an intrinsic difficulty in navigating uneven ground, or performing rapid pivots or rotations on the involved foot. Consequentially, that energy goes into the ankle joint, straining the ligaments and causing ankle sprains. The problem, however, is not at the ankle joint.
Teenagers and young adults have difficulty being active. They limit participation in team sports and athletics. If the coalition is a large bone tether, even a simple activity such as hiking or prolonged walking for exercise can be challenging and painful.
The classic presentation is a teenager or younger adult with a rigid flat foot and a history of recurrent ankle sprains. Depending on the location and size treatment can range from orthotic support, to excision of the tether, to fusion of the joints.