Ankle Anatomy and Biomechanics

Introduction

The ankle joint is composed of bones, ligaments, and tendons that provide an inherent balance between structure and function. Ligamentous structures connect the bones of the ankle to create a strong foundation for transmission of forces during weight-bearing activities. Traversing tendons work synergistically to create motion and generate power during ankle movement. Nerves and blood vessels provide sensation, proprioceptive feedback, and oxygen to all structures of the ankle. Each of these vital components work together biomechanically and play a critical role in the gait cycle. Understanding anatomy and biomechanics is important when evaluating and treating athletic injuries involving the ankle joint. In addition, gender-related differences including structure, kinematics, laxity, and neuromuscular control should also be addressed when managing injuries in the female athlete. This chapter will review the ankle bony, soft tissue, and neurovascular anatomy; the biomechanics underlying the gait cycle; and gender-related differences within the ankle joint.

Bony Anatomy

The distal tibia, distal fibula, and talus articulate to form the bony structure of the ankle joint. The distal tibial articular surface, also known as the tibial plafond, is a quadrilateral surface that is wider anteriorly. This surface is concave in the sagittal plane and slightly convex in the transverse plane. This allows smooth articulation with the dorsal surface of the talus, also called the trochlea, which is convex in the sagittal plane and approximately 4 mm wider anteriorly. ^, The distal end of the tibia has an inferomedial projection, the medial malleolus, which articulates with the medial articular surface of the talus during ankle motion. The distal aspect of the medial malleolus has two rounded projections, the anterior colliculus and the posterior colliculus. The anterior colliculus projects more distally. The distal, lateral tibia contains a notch for the fibula, also known as the incisura fibularis, and is surrounded by strong ligaments that make up the ankle syndesmosis, which will be discussed later. The distal aspect of the fibula projects past the tibial plafond and is referred to as the lateral malleolus. The medial aspect of the lateral malleolus articulates with the lateral surface of the talus. The tibial plafond, medial malleolus, and lateral malleolus together are known as the ankle mortise. With an intact syndesmosis, the ankle mortise acts to contain the talus and prevent its medial and lateral translation. The concavity of the tibial plafond in the sagittal plane creates a posterior, distal protrusion known as the posterior malleolus. This structure differs from the medial and lateral malleoli by not providing a vertical bony wall to prevent talar translation.

Soft Tissue Anatomy

The lateral ankle joint is stabilized by the lateral ligamentous complex, which is composed of the anterior talofibular ligament (ATFL), the calcaneofibular ligament (CFL), and the posterior talofibular ligament (PTFL). The ATFL extends from the anterior tip of the lateral malleolus proximally to the anterolateral surface of the talus distally ( Fig. 12.1 ). The CFL runs from the middle aspect of the tip of the lateral malleolus, courses deep to the peroneal tendons, and attaches to the lateral surface of the calcaneus. The PTFL courses from the posterior part of the lateral malleolus to the posterolateral talar surface.

The medial ankle joint is stabilized by the medial ligamentous complex, also referred to as the deltoid ligamentous complex, which connects the medial malleolus to the navicular, talus, and calcaneus. The deltoid ligamentous complex functions to limit lateral, anterior, and posterior talar translation and prevents talar abduction. It is composed of a superficial layer containing four ligaments and a deep layer containing two ligaments ( Fig. 12.2 ). The superficial and deep layers are separated by a thin layer of adipose tissue and all six ligaments are named for their attachment sites. The four ligaments that compose the superficial layer include the tibionavicular ligament, the tibiocalcaneal ligament, the superficial posterior tibiotalar ligament, and the tibiospring ligament. The deep layer consists of the deep anterior tibiotalar ligament and the deep posterior tibiotalar ligament. The entire deltoid ligamentous complex sits deep to the tendons and neurovascular structures that traverse the medial ankle.

The distal articulation between the tibia and fibula is stabilized by the syndesmotic ligament complex and is composed of three main parts: the anteroinferior tibiofibular ligament (AITFL), the posteroinferior tibiofibular ligament (PITFL), and the interosseous tibiofibular ligament (IOTFL). The AITFL originates on the anterior tubercle of the distal tibia approximately 5 mm proximal to the articular surface and runs in a lateral and distal orientation to its attachment site on the anterior margin of the lateral malleolus ( Fig. 12.3 ). Often the AITFL can appear to be divided into fascicles by perforating branches of the peroneal artery. The PITFL is composed of superficial and deep layers. The superficial layer originates from the posterior tibial tubercle and runs distally and laterally to attach to the posterior margin of the lateral malleolus. This layer is homologous to the AITFL. The deep layer of the PITFL, also known as the transverse ligament, originates on the posterior edge of the distal tibia as far medial as the medial malleolus and projects laterally to attach to the proximal aspect of the lateral malleolar fossa. The transverse ligament is cone shaped owing to its broad attachment to the posterior tibia and can aid in ankle joint stability by acting as a labrum preventing posterior talar translation. The IOTFL is a dense network of short fibers spanning directly between the distal tibia and fibula. It is the distal continuation of the interosseous membrane at the level of the syndesmosis and helps provide stability to the syndesmotic ligamentous complex.

The tendinous portions of muscles that cross the ankle joint act in synchronicity to provide movement. Anteriorly, from medial to lateral, the tibialis anterior, extensor hallucis longus, extensor digitorum longus, and peroneus tertius tendons cross over the ankle joint. They all work to dorsiflex the ankle joint and have varying functions distally within the foot. Posteromedially, from medial to lateral, the tibialis posterior, flexor digitorum longus, and flexor hallucis longus tendons cross the ankle and aid in plantar flexion of the ankle, with varying functions distally within the foot. Directly posterior, the Achilles tendon crosses the ankle and provides the majority of plantar flexion strength. Posterolaterally, the peroneus brevis and peroneus longus tendons cross the ankle just posterior to the lateral malleolus. The peroneus brevis runs anterior to the peroneus longus and together they assist in plantar flexion of the ankle.

Neurovascular Anatomy

Neurologic structures provide sensation to the skin overlying the ankle and innervate muscles that create motion at the ankle joint. Sensation about the ankle, as in all parts of the body, can be described in either dermatomal or peripheral nerve distributions. The anterior ankle dermatomes are supplied by the L4 nerve root medially and L5 laterally. The posterior ankle is supplied by S1 laterally and S2 medially. L5 supplies an area of skin between S1 and S2 over the posterior ankle. Sensation to the ankle can also be described by the individual peripheral nerves supplying skin branches. The superficial peroneal nerve provides sensation to the anterior ankle, the saphenous nerve supplies the medial and posteromedial ankle, and the sural nerve supplies the lateral and posterolateral ankle. The distal tibiofibular joint receives innervation from the deep peroneal, tibial, and saphenous nerves. The ankle joint is innervated by the deep peroneal and tibial nerves. The muscles that cross the ankle are innervated by three peripheral nerves that are supplied by the L4-S2 nerve roots. The anterior compartment muscles are all innervated by the deep peroneal nerve, the peroneal muscles are innervated by the superficial peroneal nerve, and all three posteromedial muscles are innervated by the tibial nerve.

Blood supply to the ankle is provided by three main arteries: the anterior tibial artery, posterior tibial artery, and peroneal artery. ^, Around the ankle, the anterior tibial artery runs from proximal to distal just lateral to the tibialis anterior tendon and deep to the extensor hallucis longus, which crosses superficially from lateral to medial. This main artery provides the anterior medial and anterior lateral malleolar branches, which course perpendicularly toward those structures they are named for. The anterior tibial artery continues distally into the foot as the dorsalis pedis artery. The peroneal artery courses longitudinally along the posterolateral ankle. It provides a branch named the peroneal perforating artery that pierces the distal tibiofibular syndesmosis 5 cm proximal to the tip of the lateral malleolus to anastomose with the anterior lateral malleolar artery. The peroneal artery also gives off a posterior lateral malleolar artery, which courses perpendicularly, deep to the peroneal tendons, to anastomose with the anterior lateral malleolar artery and perforating artery to create a vascular network around the lateral malleolus. A communicating branch arises from the peroneal artery 6 cm proximal to the tip of the lateral malleolus to anastomose with the posterior tibial artery. Lastly, the peroneal artery branches into the lateral calcaneal artery, which supplies the heel pad and calcaneus distally. The posterior tibial artery crosses the ankle joint between the flexor digitorum longus tendon medially and flexor hallucis longus tendon laterally and courses next to the tibial nerve. This artery gives rise to the posterior medial malleolar artery, which courses perpendicularly, deep to the posterior tibialis and flexor digitorum longus tendons, to anastomose with the anterior medial malleolar artery and create a vascular network in this area. Distally, the posterior tibial artery provides a branch named the medial calcaneal artery, which anastomoses with the lateral calcaneal artery over the posterior calcaneus. ^,

Biomechanics of the Ankle-Gait Cycle

The anatomic structures of the ankle discussed earlier work together to power ankle motion, which plays an important role throughout the gait cycle. The gait cycle is a series of pelvis and lower extremity motions that allow humans to walk or run. One cycle of gait is described as occurring between the initial ground contact of one foot and continues until that same foot contacts the ground again. ^, This cycle can be divided into a stance phase and a swing phase. The stance phase is a continuum from the initial heel strike, the foot lying flat on the floor with the body passing over, the heel rising from the floor, and to the eventual toe rise. After toe rise, that foot goes through the swing phase. With slower speed gait cycles, such as during walking, there are periods where both feet are in contact with the ground and there is always one foot in contact with the ground throughout all time points of the cycle. As the speed of the gait cycle increases, such as during running, a float phase will start to be incorporated where neither foot is touching the ground. Also, with increasing speeds, the duration and percentage of the stance phase in the gait cycle decreases and the float phase increases. ^,

The various muscles that cross the ankle joint work together during the gait cycle to create a smooth transition between dorsiflexion and plantar flexion through both concentric and eccentric contractions. ^, The posterior (deep and superficial) and lateral lower leg compartment muscles provide plantar flexion force, whereas the anterior compartment lower leg muscles drive ankle dorsiflexion. During gait, the anterior compartment concentrically contracts at the end of toe-off phase to bring the ankle from plantar flexion to dorsiflexion. Dorsiflexion is maintained throughout the swing phase with continued anterior compartment muscle contraction. This contraction continues during heel strike and the concentric contraction then stops and eccentric contraction begins to control the rapid plantar flexion that occurs during the transition to foot-flat phase. At this point, the anterior compartment becomes dormant until the end of toe-off phase. The posterior compartment musculature first becomes active with eccentric contraction during the foot-flat phase to control the forward movement of the tibia over the foot. Then concentric contraction occurs to drive ankle plantar flexion during the heel-off and toe-off phases. This contraction ceases before full plantar flexion once the opposite limb begins weight-bearing and the terminal portion of plantar flexion occurs passively through gravity without muscle contraction. ^,

During walking, the vertical ground reaction force is 1.0–1.5 times the body weight, and during running, it is 2.0–2.9 times the body weight. These forces can be even greater during sports that require push-off, acceleration, or jumping. During walking, the ankle joint experiences a force of approximately five times the body weight and is the highest during the flat foot portion of the stance phase. With running, the force transmitted across the ankle joint can increase to 13 times the body weight. Approximately 83% of the total force is transmitted across the tibiotalar joint, whereas 17% is transmitted through the fibula. The load-bearing area of the tibiotalar joint is relatively large, approximately 12 cm ² , which aids in distribution of the high amount of force seen by this joint. Because of the major forces experienced by the ankle joint, the bony architecture and ligaments are pivotal in providing adequate stability to transmit forces, while being supple enough to help absorb the large forces experienced during impact.

The majority of plantar flexion and dorsiflexion of the ankle is thought to occur at the tibiotalar joint, with only a few degrees of motion occurring through the subtalar joint. Inversion and eversion have been shown to be distributed between the tibiotalar joint and the subtalar joint. The sagittal plane range of motion of the ankle joint during the gait cycle encompasses a 70-degree arc with 20 degrees of dorsiflexion to 50 degrees of plantar flexion. ^, This range of motion occurs twice during the normal gait cycle. Just before heel strike, the ankle is dorsiflexed, and at heel strike, rapid plantar flexion occurs. During flat foot, as the body moves over the foot, the ankle goes from plantar flexion to dorsiflexion. Then plantar flexion occurs again during heel rise and toe-off phase.

Gender-Related Differences in Ankle Anatomy and Biomechanics

Previous studies have demonstrated various gender-related differences in ankle anatomy and biomechanics that may attribute to the variability in the development of injuries about the foot and ankle ( Table 12.1 ). Females typically have proportionately shorter foot and arch lengths, wider forefoot widths, and shorter metatarsal lengths when compared with males. ^, Nozaki et al. conducted a study evaluating the morphologic differences in talar anatomy between males and females. The authors found that the female talus had a longer neck; a narrower head width, which was more twisted and elongated in the dorsoplantar direction; and a more lateral superiorly tilted trochlea. These factors can potentially alter the subtalar and talonavicular joint kinematics during gait.

Table 12.1

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