Prelab: Surface Anatomy of the Foot
Lab 16 Primary Lab Objectives:
- Remove the skin over the ankle and the foot. Identify and reflect the thick plantar aponeurosis on the sole 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.
- Beginning in the leg, trace the tibial nerve and the posterior tibial artery. Then trace the tibialis posterior, flexor digitorum longus, and flexor hallucis longus muscles into the foot. Consider the functional aspects of their pathways around the ankle. Also, identify the flexor hallucis brevis muscle.
- In the foot identify the quadratus plantae (flexor accessorius) and the lumbrical muscles.
- Trace the lateral and medial plantar branches of the tibial nerve as well as the posterior tibial artery onto the plantar surface of the foot. Trace the dorsalis pedis artery on the dorsal surface of the foot.
- Locate the adductor hallucis and the interossei muscles. Examine the deep muscles of the foot.
- Follow the muscles of the leg onto the lateral and medial aspects of the foot. Consider the relationship between these muscles as they pass into the foot and their role in foot movement.
- Sever the extensor retinaculum and examine the extensor musculature of the foot.
- Identify the fibularis longus and brevis muscles, noting the relationship between insertions of the fibularis longus and tibialis posterior muscles.
- Appreciate the origin and course of the tibialis posterior muscle, noting the complexity of its insertion.
1. PLANTAR APONEUROSIS
- Remove the skin and superficial fascia from the ankle and foot.
- Expose the plantar aponeurosis on the sole of the foot.
HINT: There is a thick fat pad beneath the heel that you will need to cut away to expose the plantar aponeurosis.
- Cut the plantar aponeurosis along its distal edges (close to the toes - see image below), leaving it attached to the calcaneus.
- 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. ABDUCTOR MUSCLES
- Find the abductor hallucis and abductor digiti minimi muscles. These muscles originate from the calcaneus on either side of the flexor digitorum brevis muscle.
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. SUPERFICIAL MUSCLES OF THE SOLE
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.
- Sever the flexor retinaculum.
- Transect the abductor hallucis and flexor digitorum brevis muscles (as shown below) --BE CAREFUL TO NOT CUT UNDERLYING STRUCTURES!
- Reflect these structures and trace the tibial nerve and the posterior tibial artery.
- Trace the three tendons belonging to the tibialis posterior, flexor digitorum longus, and flexor hallucis longus muscles around the ankle and into the sole.
- Note that the flexor hallucis longus muscle runs in a compartment of its own, which grooves the tibia and talus.
- Examine these grooves on a skeleton.
- Identify the underlying flexor hallucis brevis muscle.
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.
4. QUADRATUS PLANTAE (FLEXOR ACCESSORIUS)
- Locate where the flexor hallucis longus and the flexor digitorum longus muscles cross in the foot.
- Identify and define the borders of the quadratus plantae and the lumbrical muscles.
HINT: The lumbricals are tiny muscles that take their origin from the tendon of the flexor digitorum longus muscle.
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 long flexor tendon. The lumbricals originate from the digital tendons (as in the hand) and pass over (plantar to) the deep transverse metatarsal ligaments to the extensor expansions.
5. TIBIAL NERVE AND POSTERIOR TIBIAL ARTERY
NOTE: The tibial nerve divides into the lateral and medial plantar branches (respectively equivalent to the ulnar and median nerves in the palm) as it passes under the flexor accessorius.
- Locate the tibial nerve in the flexor compartment of the leg and follow it to the quadratus plantae (flexor accessorius).
- Determine where it splits into the medial and the lateral plantar branches.
Like the median nerve, the medial plantar nerve sends motor branches to the muscles of the first digit, and superficial sensory branches to the first 3 1/2 digits. The median nerve innervates the flexor digitorum superficialis muscle in the forearm; likewise, the medial plantar nerve innervates the equivalent flexor digitorum brevis muscle in the sole of the foot.
- Follow the medial plantar branch of the tibial nerve into the foot.
- Trace offshoots of this nerve into the flexor hallucis brevis and flexor digitorum brevis 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.
- Follow the lateral plantar branch 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).
- Trace the posterior tibial artery into the foot.
NOTE: The posterior tibial artery also divides into the lateral and medial plantar branches. The lateral plantar artery is equivalent to the ulnar artery and has corresponding deep and superficial branches. The medial plantar artery is not entirely equivalent to the radial artery, only to its superficial branch. The deep branch of the radial artery is represented in the foot by the artery of the extensor compartment (the anterior tibial artery).
- Locate its medial and lateral plantar branches accompanying the nerves of the same names.
- Trace the anterior tibial artery 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.
6. DEEP MUSCLES OF THE SOLE
- Transect the belly of the quadratus plantae muscle along with the tendon of the flexor digitorum longus muscle (as shown below).
- Reflect proximally (towards the heel) these two muscles and confirm that a few fibers of the flexor hallucis longus muscle go to the flexor digitorum longus tendon (a vestige of the primitive union of these two tendons).
- Cut this connection.
- Reflect the flexor digitorum longus muscle distally with the attached short muscles (lumbricals and quadratus plantae muscles)
- Expose and identify the deep intrinsic muscles of the foot: the interossei and the adductor hallucis muscles.
- Trace the tendon of the flexor hallucis longus muscle out to its insertion
- Cut the tendon proximal to the insertion, and reflect it proximally towards the heel. This exposes the deeper layer of the first digit muscles more completely.
7. FLEXOR HALLUCIS LONGUS AND BREVIS
The insertions of the flexor hallucis brevis muscle and the flexor brevis muscle of the fifth toe are equivalent to the hand muscles except that the flexor hallucis brevis muscle attaches to both sesamoids of the big toe. These sesamoids, rather than the head of the first metatarsal itself, bear the weight transmitted through the ball of the foot. They are needed to ensure that no weight falls on the tendon of the flexor hallucis longus muscle, which passes between the sesamoid bones.
- Trace the flexor hallucis brevis muscle to the sesamoids.
- Locate the tendon of the flexor hallucis longus muscle passing between the sesamoids.
- Reassemble the sole of the foot, and compare it with the palm of the hand.
8. EXTENSORS OF THE FOOT
The deep fascia on the dorsal surface of the foot is condensed to form the extensor retinacula. The extensor retinacula 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 retinacula and trace the tendons of the extensor digitorum longus and extensor hallucis longus muscles down to their respective insertions.
- Dissect out the extensor expansion on the second digit.
- Compare the second digit of the foot with the second digit of the hand.
- Transect and reflect distally the extensor digitorum longus muscle to see the underlying extensor digitorum brevis muscle.
- Underneath and distal to the extensor digitorum brevis muscle identify the dorsal interossei muscles.
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.
9. FIBULAR COMPARTMENT
- Open the fibular compartment by cutting the fibular retinaculum.
- Sever any remaining fibers of the extensor retinaculum.
- Trace the fibularis brevis muscle to its insertion.
- Examine the small fibularis tertius muscle, a small muscles found primarily in humans, and trace it to its insertion. Note that this muscles follows a different path from the other muscles that bear the name fibularis.
NOTE: The fibularis longus and tibialis anterior muscles (both elevators) form a sling or stirrup underneath the foot, which may help to support the arch when they contract.
- Expose the sole of the foot.
- Locate the insertions of the fibularis longus and tibialis anterior muscles.
10. TIBIALIS POSTERIOR MUSCLE
The tibialis posterior muscle was not examined previously because its insertion lies deep to all the other structures of the sole. Like the flexor hallucis longus muscle, the tibialis posterior muscle occupies a groove in the distal end of the tibia just behind the ankle. The tibialis posterior is a specialized flexor muscle whose tendon spreads out to insert into the tarsal bones (except the talus) and at least the middle three metatarsals. Some of its fibers pass into the flexor hallucis brevis muscle, so it has a secondary attachment to the big toe as well. It aids the deeper ligaments of the foot in sustaining the medial arch and in preventing the head of the talus from being pushed downward between the calcaneus and the navicular.
- Trace the tibialis posterior muscle behind the ankle around to its complex insertion.
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.