Ankle Joint Anatomy 

The ankle joint is a hinged synovial joint with primarily up-and-down movement (plantar flexion and dorsiflexion). However, when the ranges of motion (ROMs) of the ankle and subtalar joints (talocalcaneal and talocalcaneonavicular) are taken together, the complex functions as a universal joint (see the following image).

The combined movement in the dorsiflexion and plantarflexion directions is greater than 100°; bone-on-bone abutment beyond this range protects the anterior and posterior ankle capsular ligaments from injury. The anterior and posterior ankle capsular ligaments are relatively thin compared with the medial and lateral ankle ligaments.

Type I collagen tissue constitutes the bulk of the capsule and supporting ligaments of the ankle joint. The fiber density and orientation are arranged dynamically according to the typical mechanical stress experienced by the joint. Within limits, the greater the excursion of the joint capsule and ligaments, the less likely sprains are to occur; with increased motion, the muscles absorb the mechanical force energy without exceeding the tensile limits of either the joint capsule or the ligaments.

General anatomy

The lateral complex of ligaments has 3 components (as shown in the image below): (1) anterior talofibular ligament (ATFL), (2) calcaneofibular ligament (CFL), and (3) posterior talofibular ligament (PTFL).

The subtalar joint is defined differently by various groups. The anatomic subtalar (talocalcaneal) joints refers structurally to the articulation between the talus and the underlying calcaneus. Orthopedic surgeons, however, refer to the functional subtalar joint as consisting of the anatomic subtalar joint plus the talocalcaneal part of the talocalcaneonavicular joint; it is not possible for the 2 joints to function independently.

When referring to the anatomic subtalar (talocalcaneal) joint, the lateral complex is composed of the following 4 ligaments:

• CFL (Note that the CFL spans the tibiotalar and talocalcaneal joints.)
• Lateral talocalcaneal ligament (LTCL)
• Cervical ligament (CL)
• Interosseous talocalcaneal ligament (IOL) (This ligament provides an axis of rotation about which movement occurs in the talocalcaneal joint.)

Biomechanical function

In addition to the general anatomy of the ankle, note the biomechanic function of each component in stabilizing the joint. In dorsiflexion, the ATFL is loose, and the CFL is taut. This is reversed in plantarflexion, in which the ATFL is taut, and the CFL is loose. The PTFL is maximally stressed in dorsiflexion.

The CFL is cordlike and is thicker and stronger than the ATFL, and it runs from the tip of the lateral malleolus to the lateral aspect of the calcaneus directly below the fibula. The CFL prevents adduction and acts virtually independently in neutral and in dorsiflexed positions.

The ATFL has a lower load to failure than the CFL; that is, the maximum load to failure of the CFL is roughly 2-3.5 times greater than that for the ATFL.^[1] However, the ATFL can undergo the greatest amount of deformation (strain) before failure and allows for internal rotation of the talus during plantarflexion, in contrast to the CFL and PTFL. The ATFL primarily restricts internal rotation of the talus in the mortise; when in plantarflexion, the ATFL also inhibits adduction.

The PTFL is the strongest of the 3 portions of the lateral ankle. It runs almost horizontally from the fossa in the inner aspect of the tip of the lateral malleolus to the posterior tubercle of the talus. The PTFL inhibits external rotation with the ankle in dorsiflexion. Note that medial ligaments are the primary restrictors of dorsiflexion (see the following image), and that the PTFL only assists in this function. The short fibers of the PTFL can also restrict internal rotation after the ATFL has been ruptured. After disruption of the CFL, the PTFL inhibits adduction with the ankle in dorsiflexion.

General anatomy

The deltoid ligament is divided into 2 portions: (1) the superficial deltoid ligament and (2) the deep deltoid ligaments, as seen in the image below.

The superficial deltoid ligament originates from the anterior colliculus (an anterior bony prominence) of the medial malleolus. The tibionavicular portion inserts onto the navicular and is the most anterior part; the tibiocalcaneal portion begins at the anterior colliculus and inserts onto the sustentaculum tali; and the final component of the superficial deltoid is the posterior tibiotalar ligament (PTTL).

The deep deltoid ligament originates from the intercollicular groove and the posterior colliculus. It is shorter and thicker than the superficial portion and is contiguous with the medial capsule of the ankle joint and the medial portion of the interosseous talocalcaneal ligament (IOL).

The greatest mechanical forces across the ankle joint are directed medially in the normal external rotation of the foot in walking and running. This is reflected in the strength and thickness of the deltoid ligament.

Biomechanical function

Biomechanically, the deltoid ligament primarily prevents abduction. After division of both components of the deltoid ligament, anterior instability of the ankle does not increase. Once the lateral ligaments are cut, the deltoid ligament acts as a secondary restraint against anterior translation. The fibular ligament primarily inhibits lateral translation of the talus. The deep deltoid ligament provides the greatest restraint against lateral translation. In order to tilt the talus in valgus within the mortise, the superficial and deep deltoid ligaments must be completely ruptured.

General anatomy

The syndesmosis of the ankle refers to the membrane connecting the tibia to the fibula. The tibia and fibula are connected throughout their length by an interosseous membrane. However, there are 3 definable ligaments at the ankle, as follows:

• Anterior (anteroinferior) tibiofibular (AITF) ligament: The is the most commonly injured ligament in syndesmotic sprains
• Posterior (posteroinferior) tibiofibular (PITF) ligament: This ligament has a deep portion, called the transverse tibiofibular ligament, and a superficial portion
• Interosseous tibiofibular ligament: This ligament is located between the nearly congruent distal surfaces of the tibia and fibula and is the primary bond holding the bones together; superiorly, the interosseous tibiofibular ligament is contiguous with the interosseous membrane, which provides some additional strength to the syndesmosis

Biomechanical function

Biomechanically, a certain amount of motion is allowed in all planes with respect to the distal ends of the tibia and fibula. When the ankle goes from full plantarflexion to full dorsiflexion, the distance between the lateral and medial malleoli increases by approximately 1.5 mm. Rotation of the tibia on the talus can also occur while a person is walking. This rotation can be as much as 6°.

The AITF ligament provides approximately 35% of ankle stability; the deep PITF provides 33%; the interosseous PITF provides 22%; and the superficial PITF provides 9%.^[2] Ogilvie-Harris et al experimentally demonstrated the importance of the syndesmotic ligaments to ankle stability by sectioning the ligaments.^[2] Rasmussen demonstrated that the ligaments of the syndesmosis play little role in the stability of the ankle as long as the other ligamentous structures are intact.^[3] No study exists in which a purely ligamentous injury to the syndesmosis has been produced through externally applied stress (ie, external rotation and abduction).

General anatomy

The subtalar joint (talocalcaneal joint) can be divided into anterior and posterior articulations that are separated by the sinus tarsi and the tarsal canal, as seen in the image below. The anterior subtalar joint (talonavicular) is formed by the anterior portion of the talus, the posterior surface of the navicular, the anterior part of the calcaneus, and the calcaneonavicular ligament and the fibrous capsule. The posterior talocalcaneal portion is formed by the posterior facet of the inferior surface of the talus and the corresponding posterior facet of the calcaneus.

The primary ligaments of the subtalar joint (talocalcaneal joint) are the calcaneofibular ligament (CFL), lateral talocalcaneal ligament (LTCL), cervical ligament (CL), and the interosseous talocalcaneal ligament (IOL). The CL is believed to be the strongest bond between the talus and the calcaneus.

Harper categorized the ligamentous structures of the subtalar joint (talocalcaneal joint) in layers, as follows^[4] :

• The superficial layer consists of the lateral root of the LTCL and CFL
• The intermediate layer consists of the CL
• The deep layer contains the medial IOL

Biomechanical function

From a biomechanical standpoint, the motion of the subtalar joint (talocalcaneal joint) is flexion-supination-adduction or extension-pronation-abduction. The motion occurs via talar ovoid surfaces moving over calcaneal ovoid surfaces.

Sectioning studies by Kjaersgaard-Andersen et al showed that sectioning the CL resulted in a 10% increase in rotation, and sectioning of the IOL produced a 21% increase in rotation.^[5] In earlier studies, the same authors found a 77% increase in adduction at the subtalar joint (talocalcaneal joint) after sectioning the CFL.^[6] The investigators' findings emphasize the importance of the CFL in providing lateral stability to the subtalar joint (talocalcaneal joint). This is contrary to a study by Cass et al that demonstrated no such influence on subtalar motion.^[7] The discrepancy among the studies may be explained by the variability of orientation of the CFL.

The previous sections described the natural anatomy and biomechanics of the lateral complex of ligaments, medial ligaments, ankle syndesmosis, and subtalar joint and ligament. However, there are also 3 primary anatomic variants, as identified by Trouilloud. These variants include the following^[8] :

• Type A (35%): A lateral talocalcaneal ligament (LTCL) blends with or reinforces the calcaneofibular ligament (CFL)
• Type B (25%): A distinct LTCL is present just anterior to the CFL
• Type C (42%): The LTCL is absent

When the CFL is injured in a type A or C anatomic variant, increased subtalar motion is expected.