**2. Anatomy**

Understanding the anatomy and articulations of the calcaneus is crucial for diagnosing and treating calcaneus fractures (**Figure 1**) [8, 9]. The unique threedimensional shape can make evaluation and conceptualization of surgical techniques difficult for the novice. The superior calcaneal surface contains three facets. The posterior, middle/sustentacular, and anterior facets articulate with the talus's anterior, middle, and posterior articular surfaces, forming the talocalcaneal (or subtalar) joint. The larger posterior facet of the calcaneus is convex, articulating with the talus above. The posterior facet is the major weight-bearing surface of the calcaneus. Due to the

**Figure 1.**

*Anatomic landmarks of the calcaneus.*

high loads placed through the posterior facet, calcaneus fractures from axial loading often cause articular fragmentation. Anatomic reduction and fixation of the posterior facet are paramount to reducing the incidence of post-traumatic arthritis [10]. The calcaneus's smaller anterior and middle facets are concave in shape and separated from the posterior facet by the interosseous calcaneal ligament. However, a wide range of anatomical variations in the anterior and middle facets have been reported, ranging from a single ovoid-shaped facet to completely separated facets [11]. The middle facet lies on the superior surface of the sustentaculum tali. A posteromedialdirected groove called the calcaneal sulcus runs between the posterior and middle facets. A corresponding groove on the talus, called the sulcus tali, also runs between the posterior and middle facets of the talus. Together, these grooves make up the talocalcaneal joint articulation and form the sinus tarsi (tarsal sinus/talocalcaneal sulcus) between the sulcus tali and calcaneal sulcus. Within the sinus tarsi lies an anastomosis between the peroneal and posterior tibial arteries and the cutaneous dorsolateral nerve of the superficial peroneal nerve [12]. Five ligaments are also contained in the sinus tarsi, the interosseous talocalcaneal ligament, the cervical ligament, and the medial, intermediate, and lateral root of the inferior extensor retinaculum. The primary function of these ligaments is to stabilize the talocalcaneal joint.

The medial surface of the calcaneus contains the sustentaculum tali (A.K.A. constant fragment). Superiorly, it sits against the middle talar articular facet. Inferiorly, this surface contains the groove for the flexor hallucis longus (FHL) tendon. The sustentaculum tali is bound to the talus by the interosseous talocalcaneal ligaments, spring ligament, and deltoid ligament. Fractures through the sustentaculum tali are rare and highly associated with intra-articular calcaneus fractures [13–15]. Due to the ligamentous connections on the sustentaculum tali, fractures are commonly nondisplaced [16], earning this portion of the calcaneus the term "constant fragment".

The lateral surface of the calcaneus has a bony prominence known as the peroneal tubercle. This structure lies in close relation to the tendons of the peroneus brevis and peroneus longus. The peroneus brevis passes anterior to the peroneal tubercle, and the peroneus longus travels inferior to it. The calcaneofibular ligament attaches proximally to the peroneal tubercle.

The posterior (nonarticular) surface of the calcaneus has three facets. The superior facet contains the retrocalcaneal bursa. The middle facet serves as the attachment for the Achilles tendon. The inferior facet forms the calcaneal tuberosity [17]. The anterior surface of the calcaneus contains the articular facet for the calcaneocuboid joint and the bifurcate ligament, which attaches the calcaneus to the cuboid and navicular.
