Foot and Ankle Anatomy and Biomechanics

Foot and Ankle Anatomy and Biomechanics

Marissa D. Jamieson, MD

T. Jay Kleeman, MD, FAAOS

Neither of the following authors nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this chapter: Dr. Jamieson and Dr. Kleeman.


It is essential to have a thorough understanding of normal anatomy and biomechanics of the foot and ankle to effectively diagnose and treat pathologic conditions. Whether correcting deformity, reconstructing the foot or ankle after trauma, or properly aligning a total ankle arthroplasty (TAA), an orthopaedic surgeon must understand the interaction of static and dynamic structures in maintaining stability and function. Weight-bearing CT is advancing the understanding of complex three-dimensional conditions including hallux valgus, cavovarus deformity, and progressive collapsing foot deformity. With increasing interest in minimally invasive surgery, comprehensive knowledge of structures at risk with percutaneous approaches is imperative for successful outcomes. An awareness of recent developments in applied anatomy and biomechanics as well as imaging of the foot and ankle is important.



There are 28 bones that make up the foot, as well as a variable number of common ossicles. The ankle is a mortise joint with inherent bony stability. It is composed of the distal tibial plafond, including the medial and posterior malleoli, as well as the lateral malleolus of the fibula, which houses the body of the talus. The foot can be visualized as a tripod composed of the calcaneus, first metatarsal head, and lesser metatarsals, with deviation from this structural base causing biomechanical issues. The foot can be divided into three main regions from proximal to distal: the hindfoot (talus and calcaneus), the midfoot (navicular, cuboid, and three cuneiforms), and the forefoot (metatarsals, phalanges, and sesamoids).

The foot’s unique anatomy and biomechanics allow it to transition from rigid to flexible throughout the gait cycle. During push-off, the foot inverts, causing the Chopart joints (calcaneocuboid and talonavicular) to lock and form a stable platform. During stance, the foot everts, unlocking the Chopart joints to allow for accommodation to the ground. Furthermore, the medial column, consisting of the talus, navicular, medial cuneiform, and first metatarsal, is stiff and creates the longitudinal arch of the foot, whereas the lateral column, consisting of the calcaneus, cuboid, and fourth and fifth metatarsals, is more flexible, allowing for accommodation on uneven terrain.

Proper alignment of the medial column continues to be a subject of debate in the treatment of hallux valgus. A better three-dimensional understanding of hallux valgus, through the increased use of weight-bearing CT, has shown that medial rotation or pronation of the first metatarsal plays an important role in the pathomechanics of the deformity by altering the directional pull of the dynamic structures of the great toe. A recent review summarizes the current understanding of axial first metatarsal rotation and the modifications to previous corrective osteotomies and fusions now designed to correct this rotational deformity1 (Figure 1).


With growing interest in minimally invasive surgical procedures for deformity correction, fracture fixation, and soft-tissue reconstruction, a comprehensive understanding of neurovascular anatomy is important to avoid iatrogenic injury to these structures. Minimally invasive Achilles tendon repair can place the sural nerve at risk laterally during blind passage of sutures and needles. Two recent cadaver studies using a popular device for minimally invasive Achilles repair evaluated the incidence of sural nerve puncture during needle passage. One study showed zero nerve penetrations, one needle touching the nerve, and no nerve entrapment after jig removal.2 The other study showed 9 sural nerve penetrations in 4 of 10 specimens or 18% of the passes; however, removal of the device led to zero incidence of nerve entrapment.3

Percutaneous posterior-to-anterior fixation for posterior malleolar fractures can also place the sural nerve at risk, but a recent anatomic study found a safe zone for screw placement immediately lateral to the Achilles tendon and 1 cm proximal to the tip of the medial malleolus. This avoided injury to the sural nerve and short saphenous vein in all specimens.4 Similarly, anterior-to-posterior percutaneous fixation of posterior malleolar fractures can place the anterior neurovascular structures at risk. A 2021 anatomic study showed no damage to neurovascular structures when screws were placed medial to the tibialis anterior, but damage or close proximity to neurovascular structures when placed lateral to the tibialis anterior or extensor digitorum longus.5

Most TAA implants require an anterior approach to the ankle, involving anterior-to-posterior blind placement of pins, chisels, and saws to secure resection guides and resect the distal tibia. This places the posterior tibial tendon, posterior tibialis artery/vein, and tibial nerve at risk. An additional posteromedial incision just proximal to the tip of the medial malleolus, with placement of a blunt retractor between the posterior tibial tendon and the posterior cortex of the distal tibia, has been proposed as a way to prevent iatrogenic neurovascular injury during anterior approach TAA.6

Blood supply to the talus is known to be tenuous because of the bone’s high surface area covered with articular cartilage (Figure 2). This limited blood supply is thought to contribute to the frequent occurrences of osteochondral lesions of the talus and necrosis. A common procedure used to fill subchondral bone defects associated with osteochondral lesions of the talus involves injecting a highly porous synthetic calcium
phosphate bone graft substitute into the talus. However, two recent case series have shown a high rate of osteonecrosis of the talus after this procedure, which can lead to devastating articular collapse and arthritis of the ankle7,8 (Figure 3). It has been suggested that this technique may damage the extraosseous blood supply of the talus and the delicate intraosseous network, thereby preventing revascularization.6

Tendons and Ligaments

The ankle joint is stabilized by three ligamentous complexes that give it stability throughout the range of motion: laterally the anterior talofibular ligament, calcaneofibular ligament, and posterior talofibular ligament; the deltoid ligament complex medially; and the syndesmotic ligaments and interosseus membrane, which stabilize the distal tibiofibular joint.

One of the syndesmotic ligaments, the posterior inferior tibiofibular ligament, extends from the fibula to the posterior malleolus. Previous understanding was that a posterior malleolus fracture gave rise to syndesmotic instability because of insertion of the deep posterior inferior tibiofibular ligament on the posterior malleolus. However, a 2019 anatomic study showed that the superficial posterior inferior tibiofibular ligament has a very broad insertion on the posterior tibia that is larger than the average size of the posterior malleolar fragment. Therefore, the presence of syndesmotic instability in the setting of a posterior malleolar fracture requires that a ligamentous injury must also occur.9

The foot’s architecture and bony tripod is stabilized by several critical ligamentous structures, with plantar ligaments on the tension side being stronger and more robust than dorsal ligaments. The Lisfranc ligament is a short, thick, interosseous ligament that runs from the medial cuneiform to the base of the second metatarsal and is critical for stabilization of the midfoot and medial longitudinal arch. A recently described lateral Lisfranc ligament spans between the bases of the second through fifth metatarsals and blends with the long plantar ligament, creating a transverse suspensory metatarsal ligament.10 This ligamentous complex provides a connection of both the transverse and longitudinal arches of the foot, which may explain why lateral column instability can sometimes be overcome with medial column stabilization in complex tarsometatarsal joint injuries.

Ligaments provide static stability and structure to the foot and ankle, and multiple opposing tendon groups contribute to the dynamic stability of the foot and ankle and allow for the complex motion required for gait and activity. Any imbalance in these tendon forces can cause imbalance and deformity.


Plain Radiography

Plain radiography is the preferred initial study in the evaluation of a patient with foot and ankle complaints. Standard views include AP, oblique, and lateral weight-bearing views of the foot and AP, mortise, and lateral weight-bearing views of the ankle. It may be helpful to include both extremities in the AP views
for comparison. Weight-bearing views have become the standard of care (when tolerated by the patient) as they provide better dynamic evaluation of foot and ankle deformity, malalignment, arthritic collapse, instability, and impingement under physiologic loading.11

Ankle stress radiographs are used to determine stability of isolated lateral malleolar ankle fractures, syndesmotic injuries, and in the setting of chronic ankle ligamentous instability. Manual foot stress radiographs may be helpful in the evaluation of Lisfranc injuries and turf toe injuries.

Additional views can be helpful when evaluating certain pathologies. An axial Harris view helps define calcaneus fractures, a Broden view is used to view the posterior facet of the subtalar joint, and the Canale view gives the best image of the talar neck. An internal oblique AP foot view helps evaluate an accessory navicular, and a sesamoid view is useful for the evaluation of sesamoid pathology and their alignment relative to the cristae of the first metatarsal (Figure 4). The Saltzman view is used for evaluation of hindfoot alignment in relationship to the ankle and can be particularly useful for surgical planning of deformities. It is important to be familiar with these special views for clinical evaluation in the office and for intraoperative use.

There has been some interest in the intraoperative use of three-dimensional fluoroscopy, which involves a modified mobile C-arm unit, instead of standard two-dimensional fluoroscopy for complex anatomy or fracture patterns. However, a 2020 study suggests longer surgical time without any improvement in clinical outcomes or quality of calcaneal fracture reduction.12 This technology may provide new benefits as it evolves.

CT/Weight-Bearing CT

Standard multidetector CT scans are beneficial for three-dimensional evaluation in certain clinical settings including preoperative planning and understanding of deformity or intra-articular fractures, definition of arthritic changes, osteochondral fractures, coalitions, and evaluation of postoperative fusion or osteotomy/fracture healing. They are readily available and can be done quickly in most clinic and hospital settings but do require radiation.

The recent development of weight-bearing CT scans with the use of cone beam technology has expanded some of the indications for CT.11 There is a growing body of literature supporting the use of weight-bearing CT in specific clinical scenarios. They have demonstrated value for evaluation of progressive collapsing foot deformity with regard to subtalar and talonavicular alignment, subtalar or subfibular impingement, and forefoot position.13 Similarly, weight-bearing CT is useful in hallux valgus deformity to assess first ray pronation, sesamoid alignment, and midfoot instability. Weight-bearing CT can also be used in the setting of acute injuries to detect subtle instability, such as in Lisfranc or syndesmotic injuries that can be missed on plain radiographs14,15 (Figure 5). Additionally, it has been shown that time spent on image acquisition is lower for weight-bearing CT alone compared with radiographs with conventional CT scan, as is standard for many injuries.16 Many current software programs can create radiographs from CT images, which would allow for a single imaging modality with greater reproducibility. Furthermore, the radiation dose from a weight-bearing CT has been shown to be lower than a conventional CT scan.16 Accessibility to weight-bearing CT is a barrier for many surgeons at this time, but as indications evolve and expand this will likely become a more widely used imaging modality for many foot and ankle conditions.

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May 1, 2023 | Posted by in ORTHOPEDIC | Comments Off on Foot and Ankle Anatomy and Biomechanics

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