Second Layer of Sole

Introduction

Introduction to the Second Layer of the Sole

The second layer of the sole lies deep to the first layer (abductor hallucis, flexor digitorum brevis, abductor digiti minimi) and superficial to the third layer (adductor hallucis, flexor hallucis brevis, flexor digiti minimi brevis). It is a tendinomuscular layer dominated by the tendons of the flexor hallucis longus (FHL) and flexor digitorum longus (FDL), along with two muscles: the quadratus plantae and the four lumbricals. The neurovascular structures of the sole—the medial and lateral plantar nerves and vessels—run in the plane between the first and second layers, making this layer surgically important. The tendons of FHL and FDL cross obliquely in the midfoot, forming the 'knot of Henry', a key landmark for tendon transfer and repair. The lumbricals, unique to the foot, originate from the FDL tendons and insert into the dorsal digital expansions, balancing the toe movements.

Study

1. Contents of the Second Layer

The second layer comprises two intrinsic muscles (quadratus plantae and lumbricals) and two extrinsic tendons (flexor hallucis longus and flexor digitorum longus). The FDL and FHL tendons enter the sole from the tarsal tunnel, run within this layer, and insert into the toes. The neurovascular structures (medial and lateral plantar nerves and arteries) travel between the first and second layers, giving off branches to these muscles. This layer is key for toe flexion and arch support.

2. Flexor Digitorum Longus Tendon

The tendon of the flexor digitorum longus (FDL) enters the sole through the tarsal tunnel, passing behind the medial malleolus. It crosses the flexor hallucis longus tendon obliquely at the midfoot (the 'knot of Henry'), receiving the insertion of the quadratus plantae muscle at this crossing. The FDL tendon then divides into four slips, which pass to the lateral four toes. Each slip enters the fibrous flexor sheath, passes through the bifurcated tendon of the flexor digitorum brevis (forming the chiasma tendinum), and inserts into the distal phalanx. The FDL flexes the distal interphalangeal joints of toes 2–5 and assists in plantar flexion and arch support.

3. Flexor Hallucis Longus Tendon

The flexor hallucis longus (FHL) tendon also enters the sole via the tarsal tunnel, passing between the medial and lateral tubercles of the posterior talus and under the sustentaculum tali. It crosses deep to the FDL tendon at the knot of Henry, continuing forward between the two heads of the flexor hallucis brevis, then between the sesamoid bones of the great toe to insert into the distal phalanx. The FHL flexes the interphalangeal joint of the great toe and aids plantar flexion and arch support. It is the strongest toe flexor.

4. Quadratus Plantae

Quadratus plantae (also called flexor accessorius) is a flat, rectangular muscle in the second layer. Origin: two heads—medial head from the medial surface of the calcaneus, lateral head from the lateral border of the plantar surface of the calcaneus. Insertion: the tendon of the flexor digitorum longus, at the point where it divides into its four slips. Action: corrects the oblique pull of the FDL (which comes from the medial side), realigning the force to flex the toes in the sagittal plane; also assists in flexing the lateral four toes. Innervation: lateral plantar nerve (S2, S3). This muscle is crucial for coordinated toe flexion, preventing the toes from deviating medially during flexion.

5. Lumbricals

There are four small, worm-like lumbrical muscles in the second layer. Origin: the tendons of the flexor digitorum longus; the first (most medial) lumbrical arises from the medial side of the tendon to the second toe, the second lumbrical from the adjacent sides of the tendons to the second and third toes, the third from the tendons to the third and fourth toes, and the fourth from the tendons to the fourth and fifth toes. Insertion: each lumbrical passes distally on the medial side of the corresponding toe, inserting into the dorsal digital expansion (extensor hood). Action: flex the metatarsophalangeal joints and extend the interphalangeal joints of the lateral four toes, akin to the lumbricals in the hand. Innervation: the first (medial) lumbrical is innervated by the medial plantar nerve; the second, third, and fourth lumbricals are innervated by the deep branch of the lateral plantar nerve. Blood supply: from the plantar arch and metatarsal arteries.

6. Neurovascular Plane

The medial and lateral plantar nerves and arteries run in the plane between the first and second layers, deep to the flexor digitorum brevis and superficial to the FDL and FHL tendons. The medial plantar nerve supplies the abductor hallucis and flexor digitorum brevis (first layer), and gives a branch to the first lumbrical (second layer). The lateral plantar nerve supplies the quadratus plantae, the lateral three lumbricals, and all muscles of the third and fourth layers, via its superficial and deep branches. The lateral plantar artery forms the deep plantar arch with the deep plantar branch of the dorsalis pedis artery. This neurovascular plane is important for surgical approaches to the midfoot and for understanding compartment syndromes of the foot.

7. Clinical Relevance

Knot of Henry: The crossing point of FHL and FDL is a surgical landmark for tendon reconstruction. The FDL tendon can be harvested for flexor tendon grafts, and the FHL tendon can be transferred to augment the Achilles tendon. Injury to the knot may occur in midfoot fractures or dislocations.

Lumbrical dysfunction: In diabetic neuropathy or Charcot-Marie-Tooth disease, weakness of the lumbricals leads to an imbalance between the long extensors (unopposed) and the loss of MTP flexion, resulting in claw toe deformity (hyperextension at MTP, flexion at IP). This is surgically corrected by tendon transfers.

Quadratus plantae role: Without the quadratus plantae, the FDL pulls the toes medially during flexion, causing them to deviate. In patients with neurological disorders, loss of quadratus plantae function can contribute to toe deformities.

Midfoot crush injuries: Lisfranc and midtarsal joint injuries can damage the FDL and FHL tendons and the neurovascular bundle. Recognizing the anatomical layers helps in surgical exploration and repair.

Tarsal tunnel syndrome: While primarily affecting the tibial nerve, its branches (medial and lateral plantar nerves) run in the second layer plane and can be compressed by ganglia, varicosities, or tenosynovitis of the FHL or FDL tendons.

Summary

Summary of the Second Layer of the Sole

The second layer contains the tendons of FHL and FDL, the quadratus plantae muscle, and the four lumbricals. The FDL tendon crosses the FHL at the knot of Henry, receiving quadratus plantae's insertion to correct the line of pull. The lumbricals arise from the FDL tendons and insert into the extensor expansions, flexing the MTP and extending the IP joints. The medial plantar nerve supplies the first lumbrical, and the lateral plantar nerve supplies the quadratus plantae and lateral three lumbricals. The neurovascular bundle runs between the first and second layers. This layer is critical for balanced toe function, arch support, and clinical conditions such as claw toe and tarsal tunnel syndrome.