PROCEDURE

1. PRE-WORK (before lab) review the organization of the bony lower extremity from your atlas or on an articulated skeleton in the lab.
    (Netter515) (Netter516) (Netter517)

a) The most posterior (proximal) bones of the foot are the calcaneus and talus. These two bones are commonly classified as the 'hind-foot'

b) Directly anterior (distal) to the calcaneus and talus bones are the navicular and cuboid bones. The three cuneiform bones (medial, intermediate, and lateral) are just anterior to the cuboid bone. The navicular, cuboid and cuneiform bones are commonly classified as the 'mid-foot'.

c) The metatarsal bones (one for each digit) and the phalanges are collectively classified as the 'fore-foot'. There are two phalanges (proximal and distal) for the big toe and three phalanges (proximal, middle, and distal) for all the other toes.

Toes (and fingers) are numbered as well as having the common names. The big toe is number 1 and the little toe is number 5.

c) The bones of the foot are normally supported by ligaments that establish the arches of the foot (longitudinal and transverse arches).

Due to the arches, the primary weight bearing points of the foot are the calcaneus and the heads of the metatarsals.

d) Examine the calcaneus, which forms the heel of the foot.  On the medial side of the calcaneus is a bony shelf, the sustentaculum tali. This forms a groove for the tendon of flexor hallucis longus to 'hook' around and into the foot. Posterior on the calcaneus is the calcaneal tuberosity, the attachment point for the calcaneal (Achilles) tendon.

The calcaneus can exhibit abnormal deposition of bone during life that forms a 'heel spur', most commonly where the plantar aponeurosis or the calcaneal tendon insert on the tuberosity. Chronic irritation around the spur can cause inflammation and pain.

2. With the donor in prone position, place two stacked wood blocks under the ankles so that the plantar surface of the foot is elevated to improve access.
    (Figure548)

3. Palpate the ankle to identify the medial malleolus.
    (Figure514) (Netter520)

The medial malleolus is the most distal region of the tibia. The tibia is the primary weight bearing bone of the leg, with the fibula stabilizing the ankle joint.

4. Remove skin posterior to the medial malleolus working inferiorly down to the calcaneus tuberosity.
    (Figure514) (Netter520)

5. Return to the superficial posterior compartment muscles (gastrocnemius, plantaris, and soleus) and follow their shared calcaneal (Achilles) tendon to the insertion on the calcaneus tuberosity.
    (Figure514) (Netter520) (Photo5024)

6. Return to the deep posterior compartment muscles and follow their tendons distally to the medial malleolus, where they pass beneath the flexor retinaculum.
    (Figure514) (Netter520) (Photo5024)

The flexor retinaculum is a strong fibrous band, extending from the medial (tibial) malleolus to the margin of the calcaneus forming canals for passage of the tendons, posterior tibial artery, and tibial nerve.

7. Examine the relative position of each of the structures passing into the flexor retinaculum.
    (Figure514) (Netter520) (Photo5024)

Observe that, although the Tibialis posterior muscle is located in the middle in the leg, the tendon courses deep to the flexor digitorum longus tendon to take up the most anterior position as it passes into the flexor retinaculum.

The mnemonic Tom, Dick, and A Very Nervous Harry describes the anterior to posterior order of the tendinous and neurovascular structures as they pass around the medial malleolus: Tibialis posterior, flexor Digitorum longus, posterior tibial Artery and Vein, tibial Nerve, and flexor Hallucis longus.

All these structures pass deep to the flexor retinaculum (i.e. through the tarsal tunnel) into the sole of the foot.

The tendon of flexor hallucis longus lies in a groove inferior to a bony shelf, the sustentaculum tali. The flexor digitorum longus and tibialis posterior lie superior to the sustentaculum tali. You may be able to palpate the sustentaculum tali on your own foot and feel the flex of the tendons that course superior or inferior to the bone shelf when wiggling the respective toes.

PROCEDURE - Sole of the Foot

8. Starting at the calcaneus tubercle, make a shallow incision through the skin along the sole of the foot, then extend the ends of the incision laterally and medially.
    (Figure515) 

9. Remove the skin from the sole of the foot extending as far laterally and medially as the bony margin of the foot.
    (Figure515) 

On the lateral side of the foot this follows the side of the foot (i.e. from the side of the little toe to the side of the calcaneus). On the medial side, the bony margin arches such that the line to follow spans from the side of the big toe to the medial malleolus.

The skin is often tightly bonded to the underlying fascia and will commonly require careful scalpel cuts to separate skin from the underlying tissue. When removing skin from the sole of the foot take care as the subcutaneous tissue over the lateral and medial plantar surfaces can be quite thin.

The subcutaneous tissue over the calcaneus is very thick and has a firm density. This is the calcaneal fat-pad which serves to cushion impacts when walking. Cutting deeply across the calcaneus is safe as there is only the calcaneal bone deep to the fat-pad.

10. Use scraping motions with a dulled scalpel blade to clean subcutaneous fat from the plantar aponeurosis.
    (Figure516) (Netter523) (Photo5015)

The plantar aponeurosis spans the longitudinal arch of the foot, attaching posteriorly to the calcaneus and anteriorly to proximal phalanges.

The plantar aponeurosis consists of bands of longitudinal connective tissue extending out to each of the digits. At the distal end of the plantar surface, transverse fascicles of connective tissue support and stabilize the longitudinal bands.

Inflammation of these fibrous bands is called plantar fasciitis. Pain from this condition is usually most prominent along the medial side of the calcaneus.

11. Using the plantar aponeurosis as a guide, extend the skin removal to the distal ends of the first and second toes.
    (Figure517) 

12. Work a probe under the plantar aponeurosis and using the probe as a guide transect the aponeurosis close to the calcaneus.
    (Figure517) 

13. Retract the plantar aponeurosis and with blunt dissection detach it from the underlying tissue as far distally as the base of the toes.
    (Figure517) 

14. Cut the plantar aponeurosis attachments to digits 1 (big toe) and 2 such that the plantar aponeurosis can be reflected distally and laterally out of the way.

Leave the plantar aponeurosis attached to digits 3, 4, and 5 (little toe).

Deep to the plantar aponeurosis are the layers of the sole of the foot. We will go through each of the layers in the following dissection steps.

It is worth noting that although these are layers separated by fascia, they are not neat or well defined. Instead, the layers follow the curvature of the foot and bulge as needed to cover the layer contents (i.e. do not expect perfect or even visible 'layers').

15. Clean any remnants of the plantar aponeurosis and examine the first layer muscles/tendons.
    (Figure518) (Netter524) (Photo5016)

Abductor hallucis muscle: This is the most medial muscle, attached to the medial side of the calcaneus. Follow the muscle fibers out to its distal attachment on the medial side of the proximal phalanx of the great toe.

Flexor digitorum brevis muscle: This middle, or intermediate, muscle attaches the mid-region of the calcaneus. Follow the muscle distal to the toes where an individual tendon extends into each of digits 2-5.  

Abductor digiti minimi muscle: This is the most lateral of the first layer muscles, attaching to the lateral side of the tuberosity of the calcaneus. Follow the muscle fibers out to its distal attachment to the base of the proximal phalanx of the fifth (small) toe.

16. Transect the flexor digitorum brevis muscle close to the calcaneus and reflect it distally.
    (Figure519)

17. Return to the flexor retinaculum and push a probe deep to the flexor retinaculum following the path of the posterior tibial artery and tibial nerve.
    (Figure520)

The passage under the retinaculum is called the tarsal tunnel. Any inflammation or compression of the tibial nerve as it passes through the tunnel can result in referred pain from the sole of the foot (i.e. tarsal tunnel syndrome).

19. Using the probe as a guide, cut through the flexor retinaculum and spread it open to follow the posterior tibial artery and tibial nerve.
    (Figure520)

20. Where the posterior tibial artery and tibial nerve pass deep to the abductor hallucis muscle, push a probe under the abductor hallucis muscle all the way to the other side of the muscle following the trajectory of the posterior tibial artery and tibial nerve.
    (Figure521)

This path is sometimes referred to as the porta pedis, the 'portal to the foot'.

Just prior to the passage of the posterior tibial artery into the foot is the location where posterior pulse can be felt (i.e. the spot just distal to the flexor retinaculum and just proximal to the passage under the adductor hallucis muscle).

This is also the region where tarsal tunnel syndrome occurs with injury or a mass effect compressing the tibial nerve.

21. Examine the location where the probe emerges for branches of the posterior tibial artery and tibial nerve in the sole of the foot, where they rename to plantar arteries and nerves.
    (Figure522) (Netter524) (Netter525) (Photo5018)

As the tibial nerve and posterior tibial artery pass onto the sole of the foot they divide into a medial plantar and a lateral plantar artery/nerve providing innervation to the foot/toes (Photo5017)

22. The second 'layer' of the foot contains two long tendons and two short muscles that we will examine next.

23. Return to the flexor digitorum longus muscle and follow its tendon through the flexor retinaculum onto the sole of the foot as far as the metatarsophalangeal joints.
    (Figure522) (Netter525) (Photo5019)

The tendon of the flexor digitorum longus muscle splits and enters toes 2-5, inserting into the distal phalanges.

24. Return to the flexor hallucis longus muscle in the posterior compartment of the leg and follow its tendon distally through the flexor retinaculum onto the sole of the foot as far as the metatarsophalangeal joint of the big toe.
    (Figure522) (Netter525) (Photo5019)

The tendon of flexor hallucis longus runs deep to the flexor digitorum longus tendon, i.e. they 'cross over' each other on the medial side of the sole of the foot.

25. Examine along the flexor digitorum longus tendon proximal to where the tendon separates into slips and identify fibers of the quadratus plantae muscle.
    (Figure522) (Netter525) (Photo5019)

The quadratus plantae muscle arises from the calcaneus and inserts into the tendon of flexor digitorum longus. As the name implies, the quadratus plantae muscle is roughly quadrangular in shape.

The quadratus plantae muscle is directly deep to the flexor digitorum brevis muscle.

26. The third layer of the foot consists of three muscles, but we will only examine the most important of these the flexor hallucis brevis.

The adductor hallucis muscle is deep and dissecting to that muscle would unduly disrupt our superficial structures. Since this muscle is of limited role in humans as we do not have prehensile toes, we will not dissect this muscle.

The flexor digiti minimi also has limited role as flexion of the little toe is a minor movement. Thus, we will also not dissect this muscle.

27. Identify the tendon of the flexor hallucis longus coursing on the plantar surface of the foot to the big toe (digit 1).
    (Figure522) (Netter525) (Photo5021)

27. Dissect into the tissue on each side of the tendon of flexor hallucis longus to identify muscle fibers of the lateral and medial heads of flexor hallucis brevis.
    (Figure522) (Netter525) (Photo5021)

Flexor hallucis brevis divides into medial and lateral heads sitting on each side of the flexor hallucis longus tendon proximal to the tendon entering the big toe (i.e. the long tendon 'splits' the brevis musculature to each side of the long tendon).

The flexor hallucis brevis muscle along with the flexor hallucis longus muscle are important for the final phase of walking. Flexion of the big toe during the final phase of gait generates a 'push off' lever action of the big toe while walking/running.

28. The fourth layer of muscles is formed by primarily by the  interossei muscles.
    (Figure523) (Figure 524) (Netter526) (Netter527)

We will not dissect these. 

There are three plantar interossei and four dorsal interossei. These are involved in adduction and abduction of the toes respectively, though functionally this movement is limited relevance in toes compared to fingers.

CHECKLIST

Skeletal Structures

Foot
    Talus
    Calcaneus
        Sustentaculum tali
        Tuberosity
    Navicular
    Cuboid
    3 cuneiforms
    Metatarsal bones
    Phalanges (proximal, middle, distal)

Soft Structures

Flexor retinaculum

Posterior tibial artery
    Plantar artery

Tibial nerve
    Plantar nerve

Tendon of tibialis posterior

Tendon of flexor digitorum longus

Tendon of flexor hallucis longus

Plantar aponeurosis

First layer
    Abductor hallucis muscle and tendon
    Flexor digitorum brevis muscle and tendon
    Abductor digiti minimi muscle and tendon

Second layer
    Tendons of flexor digitorum longus
    Tendons of flexor hallucis longus
    Quadratus plantae muscle

Third layer
    Flexor hallucis brevis muscle