Sole of the Foot

Introduction

• The sole is the bottom surface of the foot that comes in contact with the ground while standing or walking. It is also called the plantar region. The part of the sole beneath the heel bone (calcaneus) is known as the heel region.
• The sole and the palm have a similar basic structure. However, the sole is specially adapted for walking and bearing body weight. These adaptations include:
  • The great toe has become less mobile.
  • All the toes are shorter.
  • The first metatarsal and the bones of the great toe are larger and stronger.
  • The foot has developed arches that help in weight distribution and movement.

Skin of Sole

The skin of the sole has several special features:

1. It is thick because it has a well-developed stratum lucidum and stratum corneum.
2. It is hairless.
3. It is firmly attached to the underlying plantar aponeurosis, making it difficult to separate during dissection.
4. It has skin creases that improve grip while standing and walking.
5. It contains a large number of eccrine sweat glands, which help improve traction and regulate temperature.

Note: The sole does not contain apocrine sweat glands because these glands are associated with hair follicles, which are absent in the skin of the sole.

Superficial Fascia

• The superficial fascia of the sole contains subcutaneous fat arranged within a network of fibrous septa. These fibrous septa firmly attach the skin to the deep fascia (plantar aponeurosis). Thick fibrous bands, called skin ligaments (retinacula cutis), divide the fat into small compartments. The fat-filled compartments act as shock-absorbing cushions, especially under pressure-bearing areas such as the heel and the base of the great toe.

Cutaneous Nerves of Sole of Foot

The skin of the sole is supplied by the following nerves:

1. Medial calcaneal branches of the tibial nerve: Supply the skin over the heel and the back of the sole.
2. Cutaneous branches of the medial plantar nerve: Supply the front and medial part of the sole, including the medial 3½ toes.
3. Cutaneous branches of the lateral plantar nerve: Supply the front and lateral part of the sole, including the lateral 1½ toes.
4. Saphenous nerve: Supplies a small area along the middle of the medial border of the sole.
5. Sural nerve: Supplies a small area along the back of the lateral border of the sole.

• The sensory supply of the sole comes from the L4, L5, and S1 spinal nerve roots.

Deep Fascia

• The deep fascia of the sole is called the plantar fascia.
• It has a thick central part and thinner medial and lateral parts.

Modifications of deep (plantar) fascia

The plantar fascia has three important modifications:

1. Plantar aponeurosis
2. Deep transverse metatarsal ligaments
3. Fibrous flexor sheaths

Plantar aponeurosis

• The plantar aponeurosis is the thick, central part of the plantar fascia.
• Development: It is considered a separated part of the plantaris tendon that became detached as the heel enlarged during the evolution of bipedal walking.
• Shape: It is triangular.

Attachments

• The apex (proximal end) is attached to the medial tubercle of the calcaneus.
• The base (distal end) divides near the heads of the metatarsals into five bands, one for each toe.
• Small transverse fibers connect the adjacent bands at their point of division.

Each digital band divides into:

• A superficial slip, which blends with the skin.
• A deep slip, which splits into two parts. These surround the flexor tendons and finally join the fibrous flexor sheath and the deep transverse metatarsal ligament.

Superficial Transverse Metatarsal Ligaments

• Beyond the metatarsophalangeal joints, the digital bands of the plantar aponeurosis are connected by transverse fibers.
• These fibers form the superficial transverse metatarsal ligaments.

Intermuscular Septa

• The medial and lateral borders of the plantar aponeurosis give rise to vertical intermuscular septa.
• These septa divide the sole into three compartments:
  • Medial compartment
  • Intermediate compartment
  • Lateral compartment

Deep Transverse Metatarsal Ligaments

• There are four deep transverse metatarsal ligaments.
• They are short, flat fibrous bands that connect the plantar ligaments of adjacent metatarsophalangeal joints.
• They help maintain the alignment and stability of the forefoot.

Fibrous Flexor Sheaths

• The fibrous flexor sheaths are strong fibrous tunnels on the plantar surface of each toe.
• They provide a passage for the long flexor tendons.
• They keep the tendons close to the bones, prevent bowstringing during toe movement, and help distribute pressure across the sole during standing and walking.

Muscle of Sole

The sole of the foot contains 22 muscles, including 18 intrinsic muscles (located within the foot) and 4 extrinsic muscles (whose tendons come from muscles in the leg). These muscles are arranged in four layers, from superficial to deep.

First Layer

• The first layer covers almost the entire sole.
• It contains:
  1. Flexor digitorum brevis
  2. Abductor hallucis
  3. Abductor digiti minimi

Second Layer

The second layer contains two intrinsic muscles and the tendons of two extrinsic muscles.

Intrinsic muscles:

1. Flexor digitorum accessorius (Quadratus plantae)
2. Four lumbricals

Extrinsic muscle tendons:

1. Flexor digitorum longus
2. Flexor hallucis longus

Note: The intrinsic muscles of this layer are closely associated with the tendon of the flexor digitorum longus.

Third Layer

• The muscles of the third layer are located mainly in the metatarsal region.
• They include:
  1. Flexor hallucis brevis
  2. Flexor digiti minimi brevis
  3. Adductor hallucis

Fourth Layer

• The fourth layer occupies the spaces between the metatarsal bones.
• It contains intrinsic muscles and the tendons of extrinsic muscles.

Intrinsic muscles:

1. Three plantar interossei
2. Four dorsal interossei

Extrinsic muscle tendons:

1. Tendon of tibialis posterior
2. Tendon of fibularis (peroneus) longus

Muscles of first layer of sole

Flexor Digitorum Brevis (FDB)

• Origin: It arises from the medial tubercle of the calcaneus and the plantar aponeurosis.
• Direction of fibers: The muscle fibers run forward and divide into four tendons, one for each of the lateral four toes. Each tendon splits into medial and lateral slips opposite the base of the proximal phalanx, allowing the tendon of flexor digitorum longus to pass through.
• Insertion: The two slips reunite and attach to the sides of the middle phalanges of the 2nd to 5th toes.
• Innervation: Medial plantar nerve.
• Actions: It flexes the metatarsophalangeal and proximal interphalangeal joints of the 2nd to 5th toes and helps maintain the longitudinal arch of the foot.

Abductor Hallucis

• Location: It lies along the medial border of the foot.
• Origin: It arises from the medial process of the calcaneus, flexor retinaculum, and plantar aponeurosis.
• Direction of fibers: The muscle fibers pass forward and form a tendon.
• Insertion: It is inserted into the medial side of the base of the proximal phalanx of the great toe.
• Innervation: Medial plantar nerve.
• Actions: It abducts the great toe and helps support the medial longitudinal arch of the foot.

Abductor Digiti Minimi

• Location: It lies along the lateral border of the foot.
• Origin: It arises from the medial and lateral tubercles of the calcaneus.
• Direction of fibers: The muscle fibers run forward and form a tendon.
• Insertion: It is inserted into the lateral side of the base of the proximal phalanx of the little toe.
• Innervation: Lateral plantar nerve.
• Actions: It abducts the little toe and helps support the lateral longitudinal arch of the foot.

Muscles of Second Layer of Sole

Flexor Digitorum Longus (FDL)

• Origin: It arises from the upper two-thirds of the medial part of the posterior surface of the tibia, below the soleal line.
• Direction of fibers: The muscle fibers descend and form a tendon that enters the sole and divides into four tendons for the 2nd to 5th toes.
• Insertion: Each tendon is inserted into the base of the distal phalanx of the 2nd to 5th toes.
• Innervation: Tibial nerve.
• Actions:
  • Flexes the 2nd to 5th toes.
  • Produces plantar flexion of the ankle.
  • Produces inversion of the foot.
  • Helps maintain the medial longitudinal arch of the foot.

Flexor Hallucis Longus (FHL)

• Origin: It arises from the posterior surface of the fibula.
• Direction of fibers: The muscle fibers descend to form a tendon that enters the sole and passes toward the great toe.
• Insertion: It is inserted into the plantar surface of the base of the distal phalanx of the great toe.
• Innervation: Tibial nerve.
• Actions:
  • Flexes the great toe.
  • Produces plantar flexion of the ankle.
  • Helps maintain the medial longitudinal arch of the foot.

Flexor Digitorum Accessorius (Quadratus Plantae)

• Location: It is an accessory muscle to the flexor digitorum longus.
• Origin:
  • Medial head: Medial surface of the calcaneus.
  • Lateral head: Calcaneus in front of the lateral tubercle.
• Direction of fibers:
  • The medial head is larger and fleshy.
  • The lateral head is smaller and tendinous.
  • Both heads join in front of the lateral tubercle at an acute angle.
• Insertion: It is inserted into the lateral side of the tendon of the flexor digitorum longus.
• Innervation: Lateral plantar nerve.
• Actions: It straightens the pull of the flexor digitorum longus, allowing efficient flexion of the 2nd to 5th toes.

Lumbricals

• Number: There are four lumbrical muscles, numbered from medial to lateral.
• Origin: They arise from the tendons of the flexor digitorum longus.
• Direction of fibers:
  • The first lumbrical is unipennate and arises from the medial side of the first FDL tendon.
  • The second, third, and fourth lumbricals are bipennate and arise from adjacent FDL tendons.
• Insertion: They are inserted into the medial side of the base of the proximal phalanx and the extensor expansion of the 2nd to 5th toes.
• Innervation:
  • First lumbrical: Medial plantar nerve.
  • Second to fourth lumbricals: Lateral plantar nerve.
• Actions: They flex the metatarsophalangeal joints and extend the interphalangeal joints of the 2nd to 5th toes.

Muscles of Third Layer of Sole

Flexor Hallucis Brevis (FHB)

• Origin: It arises by a Y-shaped tendon with two heads:
  • Lateral head: From the plantar surface of the cuboid and lateral cuneiform.
  • Medial head: As a continuation of the tendon of tibialis posterior and from the medial cuneiform.
• Direction of fibers: The two heads unite and pass forward toward the great toe, where they divide into two separate tendons.
• Insertion: The tendons are inserted on both sides of the base of the proximal phalanx of the great toe.
• Innervation: Medial plantar nerve.
• Actions: It flexes the proximal phalanx of the great toe and helps support the medial longitudinal arch.

Flexor Digiti Minimi Brevis (FDMB)

• Origin: It arises from the base of the fifth metatarsal.
• Direction of fibers: The muscle passes forward across the plantar surface of the fifth metatarsal and forms a tendon.
• Insertion: It is inserted into the lateral side of the base of the proximal phalanx of the little toe.
• Innervation: Superficial branch of the lateral plantar nerve.
• Actions: It flexes the proximal phalanx of the little toe.

Adductor Hallucis

• Origin: It has two heads:
  • Oblique head: Arises from the bases of the 2nd to 4th metatarsals.
  • Transverse head: Arises from the deep transverse metatarsal ligaments and the plantar metatarsophalangeal ligaments of the 3rd, 4th, and 5th toes.
• Direction of fibers: The fibers of both heads pass toward the base of the proximal phalanx of the great toe.
• Insertion: It is inserted into the lateral side of the base of the proximal phalanx of the great toe.
• Innervation: Deep branch of the lateral plantar nerve.
• Actions:
  • Adducts the great toe.
  • Maintains the transverse arch of the foot.

Muscles of Fourth Layer of Sole

Plantar Interossei

• Number: There are three plantar interossei, arranged from medial to lateral.
• Origin: They arise from the bases and medial sides of the 3rd, 4th, and 5th metatarsals.
• Direction of fibers: Their tendons pass forward along the medial sides of the 3rd, 4th, and 5th toes.
• Insertion: They are inserted into the medial sides of the bases of the proximal phalanges and the dorsal digital expansions of the 3rd to 5th toes.
• Innervation: Lateral plantar nerve.
• Actions:
  • Adduct the 3rd, 4th, and 5th toes toward the second toe.
  • Flex the metatarsophalangeal joints.
  • Extend the interphalangeal joints.

Remember: PAD = Plantar interossei ADduct the toes.

Dorsal Interossei

• Number: There are four dorsal interossei, arranged from medial to lateral.
• Origin: They arise from the adjacent sides of the metatarsal bones.
• Direction of fibers: The muscles occupy the spaces between the metatarsals. Their fibers converge to form thin tendons that pass forward.
• Insertion:
  • First dorsal interosseous: Medial side of the base of the proximal phalanx of the second toe.
  • Second to fourth dorsal interossei: Lateral sides of the bases of the proximal phalanges of the 2nd, 3rd, and 4th toes.
• Innervation: Lateral plantar nerve.
• Actions:
  • Abduct the 2nd, 3rd, and 4th toes away from the second toe.
  • Flex the metatarsophalangeal joints.
  • Extend the interphalangeal joints.

Remember: DAB = Dorsal interossei ABduct the toes.

Tibialis Posterior

• Origin:
  • Upper two-thirds of the lateral part of the posterior surface of the tibia below the soleal line.
  • Posterior surface of the fibula in front of the medial crest.
  • Interosseous membrane.
• Direction of fibers: The fibers converge into a strong central tendon, which passes behind the medial malleolus through a groove before entering the sole.
• Insertion: It is inserted into the plantar surfaces of all tarsal bones except the talus and the bases of the 2nd to 4th metatarsals.
• Innervation: Tibial nerve.
• Actions:
  • Inverts the foot.
  • Plantar flexes the ankle.
  • Maintains the medial longitudinal arch of the foot.

Fibularis (Peroneus) Longus

• Origin:
  • Head of the fibula.
  • Upper one-third and posterior half of the middle one-third of the shaft of the fibula.
• Direction of fibers: The fibers form a long tendon that passes behind the lateral malleolus, beneath the fibular retinacula, and then crosses the sole through a groove on the cuboid bone to reach the medial side of the foot.
• Insertion: It is inserted into the lateral side of the base of the first metatarsal and the medial cuneiform.
• Innervation: Superficial fibular (peroneal) nerve.
• Actions:
  • Everts the foot.
  • Helps maintain the longitudinal arches of the foot.
  • Helps maintain the transverse arch of the foot.

Nerves and Vessels of Sole

The sole of the foot receives its blood supply from branches of the posterior tibial artery and its nerve supply from branches of the tibial nerve.

Arteries

The arterial supply of the sole includes:

1. Medial plantar artery
2. Lateral plantar artery
3. Branches of the posterior tibial artery

Nerves

The nerve supply of the sole is provided by:

1. Medial plantar nerve
2. Lateral plantar nerve

Medial Plantar Nerve

• The medial plantar nerve is the larger terminal branch of the tibial nerve. It supplies four intrinsic muscles of the foot.
• It is the preaxial nerve of the foot and is comparable to the median nerve of the hand.

Beginning

• The medial plantar nerve begins as the larger terminal branch of the tibial nerve deep to the flexor retinaculum.
• Root value: L4, L5, and S1 ventral primary rami.

Course and Relations

• The nerve passes forward deep to the abductor hallucis muscle.
• It then runs between the abductor hallucis and the flexor digitorum brevis.
• Throughout its course, it is accompanied by the medial plantar artery, which lies on its medial side.

Branches and Distribution

Muscular Branches

The medial plantar nerve supplies the following muscles:

1. Abductor hallucis
2. Flexor digitorum brevis
3. Flexor hallucis brevis
4. First lumbrical

Cutaneous Branches

The cutaneous branches supply the medial part of the sole and the medial 3½ toes through four digital nerves:

1. First digital nerve: Supplies the medial side of the great toe.
2. Second digital nerve: Supplies the adjacent sides of the 1st and 2nd toes.
3. Third digital nerve: Supplies the adjacent sides of the 2nd and 3rd toes.
4. Fourth digital nerve: Supplies the adjacent sides of the 3rd and 4th toes.

• Each digital nerve also supplies the skin over the dorsum of the terminal phalanx of its corresponding toe.

Articular Branches

• These branches supply the tarsal and metatarsal joints.

Lateral Plantar Nerve

• The lateral plantar nerve is the smaller terminal branch of the tibial nerve. It supplies most of the intrinsic muscles of the foot.
• It is the postaxial nerve of the foot and is comparable to the ulnar nerve of the hand.

Beginning

• The lateral plantar nerve begins as the smaller terminal branch of the tibial nerve, deep to the abductor hallucis muscle.
• Root value: S1 and S2 ventral primary rami.

Course and Relations

• The nerve passes forward and laterally between the flexor digitorum brevis and flexor digitorum accessorius (quadratus plantae).
• It then runs between the first and second layers of the sole.
• Near the head of the fifth metatarsal, it divides into:
  • Superficial branch
  • Deep branch

Branches and Distribution

Branches from the Main Trunk

Muscular branches supply:

1. Abductor digiti minimi
2. Flexor digitorum accessorius (Quadratus plantae)

Cutaneous branches supply:

• The lateral part of the sole.

Superficial Branch

The superficial branch divides into lateral and medial branches.

Lateral branch supplies:

• Flexor digiti minimi brevis
• Third plantar interosseous
• Fourth dorsal interosseous
• Skin on the lateral side of the little toe

Medial branch supplies:

• Skin of the fourth interdigital cleft
• A communicating branch to the medial plantar nerve

Deep Branch

The deep branch supplies the following nine muscles:

1. Adductor hallucis
2. Second, third, and fourth lumbricals
3. First and second plantar interossei
4. First, second, and third dorsal interossei

Medial Plantar Artery

• The medial plantar artery is the smaller terminal branch of the posterior tibial artery.
• It accompanies the medial plantar nerve and supplies the medial side of the sole.

Beginning

• The medial plantar artery arises as the smaller terminal branch of the posterior tibial artery, deep to the flexor retinaculum.

Course

• It runs forward along the medial border of the sole.
• Throughout its course, it lies on the medial side of the medial plantar nerve.

Termination

• It ends by dividing into small and large terminal branches.

Branches and Distribution

The medial plantar artery gives off:

1. Muscular branches to the muscles of the medial part of the sole.
2. Cutaneous branches to the skin of the medial side of the sole.
3. Digital branches to the toes.

Note: The digital branches of the medial plantar artery anastomose with the first three plantar metatarsal arteries, providing collateral blood supply to the toes.

Lateral Plantar Artery

• The lateral plantar artery is the larger terminal branch of the posterior tibial artery.
• It accompanies the lateral plantar nerve and supplies most of the sole of the foot.

Beginning

• The lateral plantar artery arises as the larger terminal branch of the posterior tibial artery, deep to the flexor retinaculum.

Course

• The artery passes forward and laterally toward the base of the fifth metatarsal, lying on the lateral side of the lateral plantar nerve.
• At the base of the fifth metatarsal, it turns medially and continues across the sole as the plantar arch.
• The plantar arch runs toward the interval between the bases of the first and second metatarsals, where it joins the deep plantar branch of the dorsalis pedis artery, completing the arterial arch of the sole.
• During its course, the artery passes:
  • Between the calcaneus and adductor hallucis.
  • Between the flexor digitorum brevis and flexor digitorum accessorius (quadratus plantae).
  • Then between the flexor digitorum brevis and abductor digiti minimi.

Termination

• At the base of the fifth metatarsal, the artery turns medially and continues as the plantar arch.

Branches

The lateral plantar artery gives off the following branches:

1. Muscular branches to the muscles of the sole.
2. Cutaneous (superficial) branches to the skin of the sole.
3. Anastomotic branches, which pass around the lateral border of the foot and join branches of the arteries on the dorsum of the foot.
4. Calcaneal branch (occasionally present), which supplies the skin over the heel.
5. Terminal branch, which forms the plantar arch.

Plantar Arch

• The plantar arch is the main arterial arch of the sole. It is located between the third and fourth layers of the sole.

Beginning

• The plantar arch begins as the continuation of the lateral plantar artery at the base of the fifth metatarsal.
• It is completed by the deep plantar branch of the dorsalis pedis artery at the first intermetatarsal space.

Course

• The plantar arch runs from the base of the fifth metatarsal to the proximal part of the first intermetatarsal space.
• Its convexity is directed toward the toes.

Relations

• The plantar arch lies between the third and fourth layers of the sole.
• It is accompanied by the deep branch of the lateral plantar nerve, which lies in the concavity of the arch.
• It is also accompanied by venae comitantes (paired accompanying veins).

Branches

The plantar arch gives rise to:

1. Four plantar metatarsal arteries
2. Three proximal perforating arteries

Plantar Metatarsal Arteries

• The four plantar metatarsal arteries run forward in the corresponding intermetatarsal spaces.
• Each artery divides into two plantar digital arteries, which supply the adjacent sides of two toes.
• The first plantar metatarsal artery also gives a branch to the medial side of the great toe.
• The lateral side of the little toe is supplied by a branch of the lateral plantar artery.

Perforating Arteries

• The three proximal perforating arteries pass through the second, third, and fourth intermetatarsal spaces to join the dorsal metatarsal arteries (branches of the arcuate artery).
• The distal perforating arteries arise from the plantar metatarsal arteries and anastomose with the corresponding dorsal metatarsal arteries.

Functions

• The plantar arch provides the main blood supply to the sole and toes.
• Its numerous anastomoses with the dorsal arteries ensure a rich collateral circulation to the foot.

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