The sustentaculum tali is a facet of the calcaneus, or heel bone. Also known as the talar shelf, it is found on the medial side of the calcaneus, the same side of the foot as the big toe. The sustentaculum tali forms a joint with a portion of the talus, a bone of the ankle, and is an insertion point for three ankle ligaments: the plantar calcaneonavicular ligament, the tibiocalcaneal ligament, and the medial talocalcaneal ligament. As it projects from the main bone mass at a 90-degree angle, it is important as a weight-bearing structure.
An oblong, irregularly shaped bone, the calcaneus presents several surfaces for articulation with adjacent bones, including the talus above as well as with the cuboid bone in front. The sustentaculum tali is one such surface. It is an almost cylindrical prominence that emanates from the top inside aspect of the calcaneus like a rounded shelf. The top side of this shelf, however, is not flat but concave, and this is where the medial side of the talus forms a joint with the calcaneus.
On the underside of the sustentaculum tali shelf is a groove running front to back. This is where the tendon of the flexor hallucis longus muscle crosses from behind the ankle en route to the underside of the big toe. When this muscle, whose belly is located deep in the calf, contracts, its tendon pulls down and back on the underside of the big toe, causing the toe to curl.
The side of the shelf, which is smooth, flat from above to below, and rounded from front to back, is where the three ligaments attach. Inserting along the front edge of the shelf is the plantar calcaneonavicular ligament, which as its name suggests links the calcaneus to the navicular bone, a bone of the hindfoot located in front of the heel bone and alongside the cuboid bone. Along the inside edge of the sustentaculum tali, two more ligaments attach: the tibiocalcaneal ligament and the medial talocalcaneal ligament. The tibiocalcaneal ligament, a portion of the triangular deltoid ligament of the inner ankle, vertically connects the heel bone to the inner side of the tibia bone just above the ankle joint. Angling backward and slightly upward from the sustentaculum tali is the medial talocalcaneal ligament, which links the calcaneus to the back inside edge of the talus bone.
As a weight-bearing apparatus, the sustentaculum tali not only is essential to standing upright and locomotion but also is susceptible to injury. Since it supports the inside edge of the talus, the bone upon which the leg bones stack at the ankle joint, it helps keep the ankle from rolling inward when weight is placed upon the foot, thus it is key to maintaining one’s center of gravity. If excessive forces are directed upon the lower leg from an outward direction, however, such as a blow to the outside of the leg or a misstep that causes an ankle roll, the ligaments attaching to the tali may be damaged. Less common is a fracture of this shelf, an injury that tends to be accompanied by damage to other foot and ankle structures and that is typically caused by an intense trauma, for example a fall or automobile accident.