A functional subtalar joint consists of an anterior, middle, and posterior facet.

The subtalar joint is stabilized by a number of ligaments. The interosseous talocalcaneal ligament is probably the most important stabilizer. More peripheral stabilizers on the lateral side are the calcaneofibular ligament, the lateral talocalcaneal ligament, the cervical ligament, and the bifurcate ligament. On the medial side the superficial deltoid ligament is a secondary stabilizer of the talocalcaneal joint.

In case of a talocalcaneal coalition, the middle facet is predominantly involved. Limited movement between talus and calcaneus leads to excessive stress in the hindfoot joints and excessive stress on the (pseudo) coalition itself. Congenital coalitions appear to represent a failure of primitive mesenchymal differentiation; the mode of inheritance is thought to be autosomal dominant. Some coalitions may resolve spontaneously during childhood due to weight-bearing biomechanics. In general, timing of onset of symptoms is explained by ossification of the cartilaginous coalition, at around 12–16 years of age.

On history taking and physical examination, a differentiation should be made between patients with a posttraumatic condition like osteoarthritis of the subtalar joint and patients with a talocalcaneal coalition.

Painful talocalcaneal coalition is a recognized indication for talocalcaneal arthrodesis in skeletally mature patients. A talocalcaneal coalition is a congenital osteofibrous, cartilaginous, or osseous union of the talus and calcaneus. A talocalcaneal coalition ossifies either completely or incompletely between 12 and 16 years of age. This is when the abnormality usually becomes symptomatic. Patients usually present with nonspecific hindfoot pain. A forceful inversion trauma of the ankle may elicit complaints and history may reveal repeated ankle sprains.

Patients with posttraumatic (e.g., calcaneal fracture, chronic instability after inversion trauma(s)) osteoarthritis of the subtalar joint complain of chronic deep ankle pain and swelling, months to years after a trauma. Morning stiffness, night pain, and starting pain may be present. In patients with a talocalcaneal coalition, this pain may also occur after trauma, but in many cases the complaints develop spontaneously. Some patients report a limited range of motion.

On physical examination, there may be abnormal gait. Inversion and/or eversion is diminished or absent. In talocalcaneal coalitions, often some hindfoot valgus is present. A lump may be palpable under the tip of the medial malleolus indicating a prominence of the talocalcaneal coalition. The coalition itself may be painful to pressure, specifically after a recent ankle trauma. A tarsal tunnel syndrome may develop due to a large middle facet resulting in increased pressure on the median plantar nerve.

Conventional weight-bearing anteroposterior and lateral radiographs are made. In case of subtalar osteoarthritis, sclerotic bone, osteophytes, joint space narrowing, and bone cysts can be expected.

The C-sign is a reliable diagnostic criterion for talocalcaneal coalition. This is a C-shaped line formed by the medial outline of the talar dome and the inferior outline of the sustentaculum tali on lateral radiographs of the ankle. To confirm diagnosis and for preoperative planning (e.g., location and extend of the coalition), a CT scan is advised.

The main indication for arthroscopic subtalar arthrodesis includes persistent subtalar pain, secondary to degenerative osteoarthritis, rheumatoid arthritis, or posttraumatic arthritis (Tuijthof et al. 2010). Other indications include neuropathic conditions, gross instability, and paralytic conditions, secondary to poliomyelitis and posterior tibial tendon dysfunction. Factors that play a role in determining if arthroscopic subtalar arthrodesis is appropriate include the severity of the deformity and the amount of bone loss. As with open subtalar arthrodesis, patients must have failed conservative treatment to qualify for arthroscopic subtalar fusion.

Contraindications for isolated arthroscopic subtalar arthrodesis include degenerative changes in the ankle joint, talonavicular joint, and calcaneocuboid joint. Significant angular or rotatory deformity in the talocalcaneal joint and significant bone loss are also contraindications. One should also be careful considering this procedure for diabetic patients or patients with cardiovascular disease, although the risks of complications are reduced as compared to the open procedure. In heavy smokers there is an increased risk for nonunion.