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Anatomy and Biomechanics


KEY FACTS

  • The foot and ankle comprise a complex “machine” consisting of 26 bones and joints working together. The individual parts do not work in isolation.

  • The ankle and hindfoot are 1 part of this machine, allowing the foot to adapt to uneven terrain, while the rest of the body remains upright.

  • Ankle joint is the principal joint for plantar flexion/dorsiflexion. The hindfoot joints (subtalar, talonavicular, and, to a lesser extent, calcaneocuboid) provide a complex motion that can be simply thought of as inversion and eversion.

    • Ankle and hindfoot joints act as a universal joint, so the foot can be positioned on any irregular surface, while the leg remains vertical for bipedal weight bearing.

  • Loss of motion from either the ankle or hindfoot will lead to overload of the other, as the joints attempt to make up for the lost motion.

    • This is why patients with ankle fusions universally develop hindfoot arthritis on x-rays at late follow-up.

  • The midfoot bridges the universal joint of the hindfoot to the metatarsal heads. At the joints between the navicular, cuneiforms, and medial metatarsals, stability is much more important than flexibility.

    • Arthrodesis of these joints probably does not impair foot function at all.

  • Toes, especially the 1st, provide propulsion during gait. Metatarsophalangeal motion is important in this function. Interphalangeal motion is not essential for walking.

  • In the normal human foot, contraction of the Achilles pulls on the calcaneal tuberosity at a short distance from the ankle. As the ankle begins to rotate in response to the Achilles contraction, the force is transmitted across a rigid foot to the metatarsal heads, at a distance from the ankle (center of rotation). The end result is amplification of the Achilles force for propulsion.

  • Overall, the human foot has changed from a flexible primate appendage used to grasp tree branches into a rigid lever for bipedal gait.

Overall schematic of the human foot is shown. The ankle and hindfoot provide flexibility. The longitudinal arch and the midfoot are important for stability. The toes remain flexible to facilitate propulsion during gait.

In the normal human, the Achilles tendon acts at a short distance from the center of rotation of the ankle
. With a rigid arch, the short but strong Achilles contraction passes to the metatarsal heads resulting in a large displacement at the forefoot with strong propulsive forces
.

The monkey foot is well adapted for an arboreal lifestyle. The 1st ray (hallux) is mobile so that it can grasp around a tree branch.

The monkey or primate foot does not have a rigid arch to act as a lever so that Achilles forces act more on the midfoot than the forefoot. The displacement of Achilles contractions
are not amplified, so there is no great propulsion with bipedal gait
.

Evolution of Modern Foot and Comparative Anatomy

Evolution

  • Human evolution diverged from chimpanzees ~ 5 million years ago.

  • Modern apes do not have a rigid arch.

    • The lever arm for the Achilles tendon is much smaller.

    • The ape foot has less propulsive power than the human foot.

  • In fact, the ape foot is better developed for grasping.

    • The 1st metatarsal is quite mobile at its articulation with the medial cuneiform, so the 1st ray (hallux) can be used to grasp tree branches.

  • The foot of the modern chimpanzee or gorilla is a compromise between a weight-bearing organ and a grasping one.

    • The foot retains the mobile hallux.

  • The modern human foot has a tightly packed, immobile 1st ray.

    • The hallux is no longer able to abduct because of increased rigidity at the 1st metatarsocuneiform joint.

    • Adduction of the 1st metatarsal developed along with stability of the longitudinal arch.

  • Fossil footprints of a purely bipedal gait are visible from 3.7 million years ago.

    • At that time, human ancestors ( Australopithecines ) had a brain case very much like that of a chimpanzee.

  • One theory proposes that development of a modern, bipedal gait was the 1st step in human evolution.

    • By freeing up the hands from any weight-bearing or tree-climbing obligations, the hands could specifically evolve for fine motor skills and tool use.

    • Such refinement in use of the hands induced rapid expansion of the cerebral cortex.

  • The foot of early hominids (such as Homo habilis from 1-2 million years ago) probably looks very much like a modern human foot.

Longitudinal Arch

  • Compared with other animals, the human foot is well adapted for prolonged walking but perhaps not as good for climbing or running.

  • A key anatomical feature of the modern human foot is the longitudinal arch.

  • The arch provides some shock absorption while walking and gives room for nerves and vessels to pass to the forefoot without being crushed.

  • More importantly, the arch provides a long lever arm for the Achilles tendon to act on the forefoot.

    • With a stable arch, the joints between the calcaneus and metatarsals are rigid, so that Achilles tendon forces can pass from the calcaneal tuberosity to the metatarsal heads with rotation at the ankle.

  • The rigid lever facilitates propulsion during gait.

  • No other living primate can walk with the sustained bipedal gait of the modern human.

Anatomy of Foot

  • Some articulations are vital for normal function, while others are relatively unimportant for normal walking and running.

Ankle

  • The tibia and fibula together make a tight socket (mortise) for the talar dome.

  • The talar dome is wider anteriorly than posteriorly, so that dorsiflexion tightens up the fit of the talus in the mortise and also causes the fibula to move slightly laterally.

  • The joint surfaces are highly conforming so that weight-bearing forces can be spread out over a broad surface area, minimizing joint pressures.

  • Alteration in these conforming surfaces can dramatically decrease contact area and increase pressure, leading to arthritis.

    • This might occur with syndesmotic widening or intraarticular fracture.

  • Widening of the mortise by 1 mm increases peak contact pressures almost 50%.

  • Several studies have shown that persistent widening of the ankle mortise after injury leads to poorer outcome.

  • The cartilage of the joint is relatively thick.

  • About 1/6 of weight-bearing forces are borne by the fibula. The remainder passes through the tibia.

  • The distal tibial articular surface (plafond) may have as much as 3° of valgus.

  • The mortise is externally rotated 20-30° relative to the knee.

Ankle Ligaments

  • Stability for the ankle during standing is primarily through the conforming shape of the joint surfaces.

  • Collateral ligaments play a role while walking and running.

  • On the medial side, the superficial deltoid ligament has fibers that pass from the medial malleolus to the talus, navicular, and calcaneus.

    • The deep deltoid is most important for stability.

      • –

        It passes from deep inside the medial malleolus to the medial body of the talus.

    • A major source of blood supply to the talus enters the body through these medial ligaments.

  • The lateral collateral ligaments include the anterior talofibular ligament (ATFL), calcaneofibular ligament (CFL), and posterior talofibular ligament.

    • The ATFL provides protection against inversion while the ankle is plantar flexed.

    • The CFL is more important when the ankle is dorsiflexed.

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