Foot and Ankle Biomechanics

== Arches of the Foot ==
There are three arches formed by the tarsal and metatarsals creating an elastic shock-absorbing system.
=== Lateral longitudinal arch ===
* Formed by the calcaneus, cuboid, fourth metatarsal, and fifth metatarsals.
* Relatively flat and limited mobility
* Lower than the medial arch, may make contact with the ground and bear some weight
=== Medial longitudinal arch ===
* Runs across the calcaneus to the talus, navicular, cuneiforms, and first three metatarsals.
* More mobile and flexible than the lateral arch, plays a significant role in shock absorption, by transmitting the vertical load through deflection of the arch.
* After heel strike, initially the fat pad attenuates some of the force, then the medial arch rapidly elongates until toe contact with the ground. It then shortens at mid-support, and then slightly elongates and again rapidly shortens at toe-off.
* It doesn't make contact with the ground unless the individual has functional flat feet.
* Two models, both have validity
** Beam model: 
*** arch is curved beam made up of interconnecting joints. The stability is dependent on the joints and ligamentous interconnections.
*** Tensile forces on inferior surface of the beam, compressive forces on the superior surface
** Truss model: 
*** arch has a triangular structure with two struts connected at the base by a tie rod.
*** The struts are under compression and the tie rod is under tension.
* Windlass effect
** Plantar fascia is the tie rod in truss model
** Dorsiflexion of MTPJ puts traction on the plantar fascia and causes elevation of the medial arch through the windlass effect.
** Sesamoid bones within the fascia increase mechanical advantage and tension.
** During toe-off, the toes are dorsiflexed passively while the body passes over the foot. The plantar fascia tightens and shortens the distance between the MTP heads and heel, and thereby elevates the arch. This traction also helps to invert the calcaneus through the attachment on the medial plantar aspect of the calcaneus.
* Arch stability
** Keystone navicular bone
** Plantar fascia
** Long and short plantar ligaments
** Spring ligament
* Variations
** Pes cavus: high-arched, no contact with the ground, little or no inversion in stance, poor shock absorption.
** pes planus: flat-footted, hypermobile, weakened medial side, associated with excessive pronation.
=== Transverse arch ===
* Formed by wedging of the tarsals and the base of the metatarsals
* These bones act as beams for support
* Flattens with weightbearing, can support 3-4 times bodyweight.
* Flattening of the arch causes the forefoot to spread in the shoe
== Soft Tissues ==
* Dorsal skin loosely attached
* Plantar skin firmly attached by extensions of the plantar fascia
=== Heel pad ===
* Absorbs shock
* Consists of comma-shaped or U-shaped fat-filled columns arrayed vertically. 
* The septae are reinforced internally with elastic transverse and diagonal fibres to produce a spiiral honeycomb effect. 
* Septal degeneration and fat atrophy occur with age.
=== Plantar fascia ===
The plantar fascia originates on the medial tuberosity of the calcaneus and spans the transverse tarsal, tarsometatarsal, and metatarsophalangeal joints to insert on the metatarsophalangeal plantar plates and collateral ligaments as well as the sesamoids. Digital slips extend beyond the MTPJs. 
Dorsiflexion of the metatarsophalangeal joints places traction on the plantar fascia and causes elevation of the arch through a mechanism known as the “windlass effect”. During toe-off in the gait cycle, the toes are dorsiflexed passively as the body passes over the foot and the plantar fascia tightens and acts to shorten the distance between the metatarsal heads and the heel, thus elevating the arch. This supports both arches and protects the underlying neurovascular bundles.
It receives a wide range of tensions as it is flattened in dorsiflexion and increased in plantarflexion. Plantar fasciitis can develop in the setting of decreased ankle dorsiflexion (running on forefoot) and high body mass index.
The plantar skin is firmly attached to the underlying bones, joints, and tendon sheaths of the heel and forefoot by specialized extensions of the plantar fascia. This function of the plantar fascia is essential for traction between the floor and the foot’s weight-bearing skeletal structures to occur. The heel pad consists of comma-shaped or U-shaped fatfilled columns arrayed vertically. The septae are reinforced internally with elastic transverse and diagonal fibers to produce a spiral honeycomb effect.
== Movement ==
Abduction motion of the foot/ankle can be demonstrated by maintaining the foot flat on the floor in a seated position and sliding the surface of the foot laterally (without moving the tibia/fibula). Abduction/adduction motion of the foot/ankle primarily occurs at the subtalar and midtarsal joints and is quite limited. Second, the terms inversion and eversion represent motion in the coronal plane about an anterior/posterior axis (this motion is termed abduction/adduction elsewhere in the body). Inversion/eversion occurs primarily at the subtalar and midtarsal joints and can be demonstrated by moving the plantar surface of the foot to face medially (inversion) or laterally (eversion). Flexion/extension of the foot is termed dorsiflexion and plantarflexion respectively and occurs around a medial/lateral axis in the sagittal plane. This motion occurs primarily at the talocrural joint.
Major joints of the ankle/foot (talocrural, subtalar, midtarsal) have axes of motion that are oblique to the standard orthogonal axes. Supination and pronation are the appropriate terms to describe motion around the oblique axes of the foot that occur at the talocrural, subtalar, and midtarsal joints. Supination and pronation describe motion around a single axis.
Supination and pronation are therefore often termed triplanar; meaning that supination has components of plantarflexion, adduction, and inversion and that pronation has components of dorsiflexion, abduction, and eversion.
* Abduction and adduction
** occur around a vertical axis rather in the transverse plane than anterior-posterior axis. This motion is called internal and external rotation elsewhere in the body.
** Primarily occurs at the subtalar and midtarsal joints and is limited
* Inversion and eversion
** Occur in the coronal plane about an anterior/posterior axis. This motion is called abduction/adduction elsewhere in the body.
** Primarily occurs at the subtalar and midtarsal joints.
* Dorsiflexion and plantarflexion
** Occurs around a medial/lateral axis in the sagittal plane
** Primarily occurs at the talocrural joint.
* Pronation and supination
** Occurs around the oblique axes of the foot, around a single axis, as a component of motion in three planes. Often called triplanar, as the same three components of motion always occur.
** Pronation: components of dorsiflexion, abduction, and eversion.
** Supination: components of plantarflexion, adduction, and inversion
** Occurs at the talocrural, subtalar, and midtarsal joints.
* Varus and valgus
** Coronal (frontal) plane angulation towards or away from midline 
** Occurs at the hindfoot and forefoot
== Load Distribution ==
* The ankle has a large load-bearing surface area of 11 to 13 cm2
* Lower stresses than in the knee or hip
* Most load is transmitted through he tibial plafond to the talar dome
* Remainder transmitted to the medial and lateral talar facets
* Inversion -> increases medial talar facet load
* Eversion -> increases medial talar facet load
* Ankle dorsiflexion -> greatest talar contact and lowest pressureBasic Biomechanics of the Musculoskeletal System - Nordin 4th edition 2012
== Kinetics of the Foot ==
* Peak vertical forces 120% body weight during walking, and 275% during running
* Medial column (talus, navicular, cuneiforms, 1-3 metatarsal) -> bears most of load
* Lateral column (calcaneocuboid joint, 4-5 metatarsals) -> transmits lesser load
* Standing: greater pressures on the heel. Forefoot peak pressure is under the second metatarsal head.
* Barefoot walking: centre of pressure initially central heel, then moves rapidly across the midfoot to the forefoot where the velocity decreases. 
* Shoe wearing: reduced peak heel pressure. Forefoot load shifts medially, maximum pressure is under the 1st and 2nd metatarsal heads. Pressure under the toes increase.
* Running: two types of runners, rearfoot strikers and midfoot strikers. Rearfoot strikers: initial ground contact with posterior third of shoe. Midfoot strikers: middle third of shoe. Both groups, first contact is along the lateral border of the foot. Centre of pressure is in the distal most 20 to 40% of the shoe, indicating that most time is spent on the forefoot
== Effects of Shoes on Biomechanics ==
* Narrow toebox compresses the forefoot contributing to hallux valgus, hammer toes, and bunionettes.
* Elevated heels increase forefoot pressure, cause pain under the metatarsal heads, contribute to intermetatarsal neuroma formation, and lead to Achilles contracture with limited ankle dorsiflexion and altered gait
* Athletic shoes: questionable whether it can control foot motion
* Barefoot running: a big topic in itself. Increase in stress fractures in certain situations.
* Rocker bottom shoes: decrease plantar pressure especially over the forefoot, but unproven effects on the proximal joints.
== Ligaments ==
The ankle joint is extremely stable primarily owing to bony congruency and ligamentous support. Bony congruency afforded by the two malleoli and tibial plafond form a “mortise” joint with the dome of the talus. The talus is shaped like a truncated cone, or frustum, with the apex directed medially. The talus is 4.2 mm wider anteriorly than posteriorly. The anterior increase in dimension is important functionally because during dorsiflexion the anterior portion of the talus is compressed between the tibia and fibula (spreading the mortise slightly) and the ankle joint becomes “close packed” in a position of maximal stability.
The lateral ankle ligaments responsible for resistance to inversion and internal rotation are the anterior talofibular ligament, the calcaneofibular ligament, and the posterior talofibular ligament. The superficial and deep deltoid ligaments are responsible for resistance to eversion and external rotation stress. The ligaments responsible for maintaining stability between the distal fibula and tibia are the
syndesmotic ligaments. The syndesmotic ligaments consist of the anterior tibiofibular ligament, the posterior tibiofibular ligament, the transverse tibiofibular ligament, and the interosseous ligament. 
normal motion of 10° to 20° dorsiflexion and 40° to 55° plantarflexion.  Joints of the midfoot contribute 10% to 41% of clinical plantarflexion from neutral to 30° plantarflexion. Therefore, what appears to be clinical ankle plantarflexion is actually occurring distal to the ankle itself. This midfoot motion explains the apparent ability of the foot to dorsiflex and plantarflex following ankle fusion.
== Great Toe ==
The great toe provides stability to the medial aspect of the foot through the windlass mechanism of the plantar aponeurosis. As the body passes over the foot in toe-off, the metatarsal head is pressed into the floor through the stabilizing action of the fibularis longus. This has been confirmed by force plate analysis in the late stance phase, which shows that pressure under the first metatarsal head increases in this phase of gait. Wearing narrow, high-heeled shoes can predispose the individual to mechanical entrapment of the interdigital nerves (more commonly the third) against the transverse intermetatarsal ligaments by compressing the metatarsal heads and creating a painful neuroma.
Excessive stress on the first metatarsophalangeal joint may result in inflammation or stress fractures and resulting pain on the sesamoid bones that are embedded within the flexor hallucis brevis tendon.
== Reading ==
*open access article general overview
*old but open access article on the first rayWard M Glasoe, H John Yack, Charles L Saltzman, Anatomy and Biomechanics of the First Ray, Physical Therapy, Volume 79, Issue 9, 1 September 1999, Pages 854–859, DOI
==References==
Category:Biomechanics of the Lower Limb
Category:Foot and Ankle Anatomy