Q5 LONG ANSWER 10M ★ High-yield

Describe the arches of the foot and the factors maintaining them. Add a note on the muscles of the sole and weight transmission.

Anatomy · 1st Professional — Pre-Clinical

The Arches of the Foot

The bones of the foot are arranged in arches, which distribute weight, act as springs, and adapt the foot to uneven ground.

Arch	Bones	Features
Medial longitudinal	Calcaneus, talus, navicular, three cuneiforms, first three metatarsals	Higher, more mobile, more resilient; the chief shock absorber. Talus is the keystone
Lateral longitudinal	Calcaneus, cuboid, fourth and fifth metatarsals	Lower, flatter, more rigid; transmits weight to the ground. Cuboid is the keystone
Transverse	Cuneiforms, cuboid, bases of the metatarsals	Complete only when the two feet are together; intermediate cuneiform is the keystone

Factors Maintaining the Arches

Factor	Structures
Shape of the bones	Wedge-shaped bones fit like a Roman arch
Ligaments (passive)	Spring (plantar calcaneonavicular) ligament — the most important; long and short plantar ligaments; plantar aponeurosis
Muscles (active)	Tibialis anterior and posterior, peroneus longus, flexor hallucis longus, flexor digitorum longus, and the intrinsic muscles
Tie beams	Plantar aponeurosis and the small muscles, acting like the tie of a bowstring

Ligaments are the chief passive support; muscles are the chief active support
Tibialis posterior is the most important dynamic supporter of the medial arch — its dysfunction is a common cause of acquired flat foot in adults
Peroneus longus crosses the sole obliquely and supports the transverse arch, acting like a stirrup with tibialis anterior

Weight Transmission

Muscles of the Sole

Layer	Muscles	Note
First	Abductor hallucis, flexor digitorum brevis, abductor digiti minimi	Most superficial
Second	Flexor digitorum accessorius (quadratus plantae), lumbricals; tendons of flexor digitorum longus and flexor hallucis longus	Quadratus plantae corrects the oblique pull of flexor digitorum longus
Third	Flexor hallucis brevis, adductor hallucis, flexor digiti minimi brevis	—
Fourth	Interossei (3 plantar, 4 dorsal); tendons of peroneus longus and tibialis posterior	Deepest

Nerve supply — the medial plantar nerve supplies abductor hallucis, flexor digitorum brevis, flexor hallucis brevis and the first lumbrical; the lateral plantar nerve supplies all the rest
The medial plantar nerve is the equivalent of the median nerve; the lateral plantar of the ulnar
In the foot the interossei abduct and adduct about the second toe, not the third

Plantar Aponeurosis

A thick triangular sheet from the medial tubercle of the calcaneus, fanning out into five slips to the toes
Functions — protects the sole, supports the longitudinal arches, and acts as a tie beam
Windlass mechanism — dorsiflexion of the toes at toe-off tightens the aponeurosis, raising the arch and making the foot a rigid lever

Arterial Supply of the Sole

Posterior tibial artery divides beneath the flexor retinaculum into medial and lateral plantar arteries
The lateral plantar artery forms the plantar arch, completed by the dorsalis pedis through the first interosseous space
Dorsalis pedis pulse is felt lateral to the tendon of extensor hallucis longus; posterior tibial pulse midway between the medial malleolus and the heel

Joints of Inversion and Eversion

Joint	Articulation	Movement
Subtalar (talocalcanean)	Talus with calcaneus, posteriorly	The chief site of inversion and eversion
Talocalcaneonavicular	Head of talus with calcaneus, navicular and the spring ligament	Inversion and eversion
Transverse tarsal (midtarsal)	Talonavicular + calcaneocuboid	Supplements inversion and eversion; the line of Chopart amputation
Tarsometatarsal	Cuneiforms and cuboid with metatarsals	Gliding; the line of Lisfranc amputation

The axis of inversion and eversion is oblique, so inversion is accompanied by adduction and plantar flexion (together called supination), and eversion by abduction and dorsiflexion (pronation)
The spring ligament supports the head of the talus; its stretching is a major factor in acquired flat foot

Applied Aspects

Flat foot (pes planus) — loss of the medial arch; may be flexible (normal in children under 3, arch reappears on tiptoe) or rigid; caused in adults by tibialis posterior dysfunction
Pes cavus — an abnormally high arch, often with an underlying neurological cause such as Friedreich ataxia or Charcot–Marie–Tooth disease
Plantar fasciitis — pain at the medial calcaneal tubercle, worst on the first steps in the morning; a calcaneal spur may be seen but is not the cause
Club foot (congenital talipes equinovarus) — the foot is plantar flexed, inverted and adducted; treated early by the Ponseti method of serial casting
Diabetic foot — neuropathy removes protective sensation and ulcers form at the pressure points of the tripod, particularly under the head of the first metatarsal
Peripheral pulses must be examined in every diabetic and every patient with leg pain — the dorsalis pedis is congenitally absent in about 10% of people, so its absence alone is not diagnostic
March (stress) fracture of the second or third metatarsal follows unaccustomed walking; the radiograph is normal at first and callus appears later
Hallux valgus — lateral deviation of the great toe with a bunion over the prominent first metatarsal head, aggravated by narrow footwear
Ingrowing toenail and interdigital infection are common and become serious in the neuropathic or ischaemic foot
Sole skin is thick and firmly bound to the aponeurosis, so infection there is intensely painful and swelling appears on the dorsum instead
Footwear advice is a genuine clinical intervention in diabetes — well-fitting shoes and daily inspection of the feet prevent more amputations than any drug
The arches are not present at birth; the infant foot is normally flat, and the medial arch develops as the child begins to walk

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