The plantar calcaneonavicular (spring) ligament spans the floor of this synovial cavity, enlarging the compartment and producing a functional anastomosis of sorts between the anterior portion of the sustentaculum tali and the navicular, known as the acetabulum pedis.214 This unique space occupied by the talar head and neck is reinforced by the bifurcate ligaments laterally and the deltoid ligament medially. The head of the talus and its large socket are enclosed by the same capsule that houses the anterior and middle facets of the subtalar joint. The capsule anatomically joins the subtalar joint and the talonavicular joints into the TCN joint.