Prelab: Surface Anatomy of the Foot
Lab 17 Primary Lab Objectives:
- Remove the skin over the ankle and the foot. Identify and reflect the thick plantar aponeurosis on the plantar side of the foot.
- Locate the abductor hallucis and abductor digiti mimini muscles of the foot, consider the homologies of these muscles to those in the hand. Identify the intrinsic flexor digitorum brevis.
- Then trace the flexor digitorum longus muscle from the posterior leg, around the medial ankle, and into the foot. Locate the muscles that attach to the flexor digitorum tendon: quadratus plantae and the lumbricals.
- Beginning in the posterior leg, trace the tibial nerve and the posterior tibial artery around the medial ankle to where they have lateral and medial branches in the foot.
- Trace the flexor hallucis longus tendon around the medial ankle and to the distal phalanx of the big toe. Locate the flexor hallucis brevis muscle deep to the flexor hallucis longus tendon in the foot.
- Examine the deep muscles of the foot: adductor hallucis and the interossei.
Sever the extensor retinaculum and examine the extensor musculature on the dorsum of the foot. Trace the dorsalis pedis artery on the dorsal surface of the foot.
1. PLANTAR APONEUROSIS
- Remove the skin and superficial fascia from the ankle and foot.
- Expose the plantar aponeurosis that is just deep to the skin on the plantar surafce of the foot.
HINT: There is a thick fat pad beneath the heel (the heel pad) that you will need to cut away to expose the plantar aponeurosis.
c. Cut the plantar aponeurosis along its distal edges (close to the toes- see image above), leaving it attached to the calcaneus.
d. Reflect (posteriorly) the plantar aponeurosis, detaching it from the underlying muscles.
NOTE: In the palm, removing the fascia covering the surface exposed the long flexor tendons. In the sole, removing the plantar aponeurosis exposes a layer of intrinsic musculature, arising from the calcaneus (and to some extent from the plantar aponeurosis itself): the flexor digitorum brevis muscle (shown below).
The plantar aponeurosis acts as a tie-beam across the bottom of the longitudinal arches of the foot, helping to keep them from collapsing under pressure. Like the palmar aponeurosis, the plantar aponeurosis sends fascial extensions through to the deep transverse ligamentous structures that bind together the heads of the metatarsals. The long flexor tendon to each digit in the foot, therefore, moves through a fascial tunnel, and is surrounded by a synovial sheath (as in the hand). In humans, the first and second metatarsal heads are tied together by a deep transverse ligament. Unlike other primates, we cannot bring our big toe around into opposition the way we can our thumb.
2. SUPERFICIAL MUSCLES OF THE PLANTAR SURFACE OF THE FOOT
- Find the abductor hallucis and abductor digiti minimi muscles. These muscles originate from the calcaneus on either side of the flexor digitorum brevis muscle.
- Cut the abductor hallucis and flexor digitorum brevis muscles (as shown below). Be careful not to cut underlying structures.
The abductor hallucis and abductor digiti minimi muscles are equivalents of the thenar and hypothenar abductors in the hand and have similar insertions. They have little power to abduct the digits in the foot, but are of considerable importance in relieving the strain in the plantar aponeurosis when we rise up on our toes.
3. FLEXOR DIGITORUM LONGUS, QUADRATUS PLANTAE (FLEXOR ACCESSORIUS), AND LUMBRICALS
NOTE: The long digital flexor tendons pass into the sole between the abductor hallucis muscle and the underlying flexor hallucis brevis muscle. At the ankle, the intramuscular septum covering the digital flexors becomes ligamentous and forms a flexor retinaculum. This band of deep fascia restrains the flexor tendons as they pass behind the medial malleolus of the tibia.
- Cut the flexor retinaculum behind the medial malleolus to expose the tibialis posterior, flexor digitorum longus, and flexor hallucis longus tendons, as well as the posterior tibial artery and tibial nerve.
- Trace the flexor digitorum longus muscle around the ankle and into the sole.
- Identify the quadratus plantae running from the calcaneus to the flexor digitorum longus tendon.
- Identify the lumbrical muscles that also attach to the flexor digitorum longus tendons.
In order to resolve the oblique pull on the flexor digitorum longus tendon, a little sheet of muscle, the quadratus plantae (or flexor accessorius), arises from the calcaneus and inserts into the posterior edge of the flexor digitorum longus tendon. The lumbricals originate from the digital tendons (as in the hand) and pass over (superficial to) the deep transverse metatarsal ligaments to the extensor expansions.
4. TIBIAL NERVE AND POSTERIOR TIBIAL ARTERY
- Locate the tibial nerve in the flexor compartment of the leg and follow it around the medial malleolus to the quadratus plantae (flexor accessorius).
- Notice that the tibial nerve splits into the medial and the lateral plantar nerves.
- Follow the medial plantar nerve of the tibial nerve into the foot.
- Trace branches of the medial plantar nerve over to the hallux.
- Follow the lateral plantar nerve into the foot.
- Locate its branches to the abductor digiti minimi muscle (at this point, you cannot yet see its branches to the interossei or other deep muscles).
NOTE: Like the ulnar nerve, the lateral plantar nerve spreads across the sole deep to the long flexor tendons, innervating the deeper intrinsic muscles.
g. Find the posterior tibial artery in the posterior compartment of the leg and trace it around the medial malleolus and into the plantar side of the foot.
h. Locate its branches, the medial and lateral plantar arteries accompanying the nerves of the same names.
5. FLEXOR HALLUCIS LONGUS AND BREVIS
- Cut the belly of the quadratus plantae muscle along with the tendon of the flexor digitorum longus muscle (as shown below).
- Reflect the proximal end of the flexor digitorum longus tendon and quadratus plantae proximally (towards the heel).
- Reflect the distal end of the flexor digitorum longus muscle with the attached lumbricals distally (toward the toes).
- Trace the tendon of the flexor hallucis longus muscle out to its insertion on the distal phalanx of the big toe, running between the two sesamoid bones near the first metatarsal head.
- The tendon of flexor hallucis longus runs superficial to flexor hallucis brevis
HINT: Use your blunt probe to define these muscles and gently separate the nerves and vessels from the surrounding fascia. In the sole, the tendon of the flexor hallucis longus muscle runs straight to the distal phalanx of the hallux, while the tendon of the flexor digitorum longus muscle crosses it at an angle.
- Cut the flexor hallucis longus tendon proximal to its insertion on the distal phalanx, and reflect it proximally towards the heel. This exposes the deeper layer of the first digit muscles more completely.
The flexor hallucis brevis muscle has two heads which insert into the sides of the proximal phalanx of the big toe. The tendons for each of the heads of the muscle contain sesamoid bones. These sesamoids bear the weight transmitted through the ball of the foot and ensure that no weight falls on the tendon of the flexor hallucis longus muscle, which passes between them.
6. DEEP MUSCLES ON THE PLANTAR SURFACE OF THE FOOT
- Identify the two heads (transverse and oblique) of the adductor hallucis muscle.
- Identify the flexor digiti minimi.
- Identify the plantar interossei muscles that originate on the shafts of the metatarsals.
7. DORSUM OF THE FOOT
The deep fascia on the dorsal surface of the foot is condensed to form the extensor retinaculum. The extensor retinaculum holds down the tendons of the extensor muscles. In the leg, the fibularis longus and brevis muscles are enclosed in their own separate compartment and have their own fibular retinaculum above and below the ankle.
- Cut the extensor retinaculum and trace the tendons of the extensor digitorum longus, extensor hallucis longus, and fibularis tertius muscles down to their respective insertions.
- Cut and reflect distally the tendonds of the extensor digitorum longus muscle to see the underlying extensor digitorum brevis and extensor hallucis brevis, and intrinsic extensors on the dorsum of the foot.
- Deep and distal to the extensor digitorum brevis muscle identify the dorsal interossei muscles.
- Find the anterior tibial artery in the anterior compartment of the leg and follow it onto the dorsum of the foot where it becomes the dorsalis pedis artery.
- Follow the dorsalis pedis artery to the point where it disappears between the two heads of the first dorsal interosseous muscle.
NOTE: The extensor digitorum brevis muscle arises from the calcaneus and inserts into the extensor expansions of digits II-IV. The tendon passing to the first digit is distinguished by a separate name: the extensor hallucis brevis muscle. The hallucal attachment differs from the other digits resembling that of the extensor pollicis brevis in the hand.
1. Hallux valgus
Hallux valgus is a foot deformity caused by pressure from footwear (e.g., high heels) and/or degenerative joint disease; it is characterized by the lateral deviation of the great toe. In some people, the deviation is so great that the first toe overlaps the second toe. These individuals are unable to move their first digit away from their second digit because the sesamoid bones under the head of the first metatarsal are displaced and lie in the space between the heads of the first and second metatarsals. In addition, a subcutaneous bursa may form owing to the pressure and friction against the shoe. When tender and inflamed, the bursa is called a bunion.
2. Flat feet (Pes planus)
Acquired flat feet (“fallen arches”) are likely to be secondary to dysfunction of the tibialis posterior muscle owing to trauma, degeneration with age, or denervation. In the absence of normal passive or dynamic support, the plantar calcaneonavicular ligament fails to support the head of the talus. Consequently, the talar head displaces inferomedially and becomes prominent. As a result, some flattening of the medial part of the longitudinal arch occurs, along with the lateral deviation of the forefoot. Flatfeet are common in older people, particularly if they undertake much unaccustomed standing or gain weight rapidly, adding stress on the muscles and increasing the strain on the ligaments supporting the arches.