Fascial compartments and their boundaries
The basis of the topography of the sole is the plantar aponeurosis (aponeurosis plantaris), which forms the inferior wall of the entire plantar space. Two dense connective tissue structures—intermuscular septa (septa intermuscularia)—extend from the aponeurosis deep into the tissues:
- Medial intermuscular septum: firmly attaches to the calcaneus (calcaneus), navicular (os naviculare), medial cuneiform (os cuneiforme mediale), and first metatarsal bones.
- Lateral intermuscular septum: fixes to the long plantar ligament (lig. plantare longum) and the fifth metatarsal bone.
These septa serve as lateral walls, dividing the sole into three isolated compartments: medial, central, and lateral. The superior (deep) wall of these compartments in the tarsal region is formed by the bones and the long plantar ligament, and in the metatarsal zone by the deep fascia covering the plantar interosseous muscles.
Topography of the central compartment and the plantar canal
The central fascial compartment is the most extensive and contains several muscle layers. Directly above the plantar aponeurosis (in the tarsal part) lies the first muscle layer—the flexor digitorum brevis (m. flexor digitorum brevis), which originates from the calcaneus and has its own fascial sheath.
The second (middle) muscle layer includes two key structures:
- Tendon of the flexor digitorum longus (m. flexor digitorum longus), lying deeper (more superiorly) than the flexor digitorum brevis.
- Quadratus plantae muscle (m. quadratus plantae), which attaches to the tendon of the flexor digitorum longus on its lateral side.
The plantar canal—a direct continuation of the calcaneal canal—runs through this same zone. Inferiorly, it is bounded by the fascia of the flexor digitorum brevis, and superiorly by the oblique head of the adductor hallucis muscle (m. adductor hallucis). The second muscle layer and neurovascular bundles lie within the canal.
Fascial spaces and spread of pus
In the metatarsal section of the central compartment of the sole, two important fascial spaces are distinguished:
- Superficial: localized between the tendons of the flexor digitorum brevis and flexor digitorum longus.
- Deep: located between the tendons of the flexor digitorum longus (along with the lumbrical muscles) and the adductor hallucis muscle.
The central compartment has numerous clinically significant communications through which purulent processes can spread. Pathways for outflow are possible in several directions:
- To the dorsum of the foot: along the course of the anastomosis between the lateral plantar artery and the dorsalis pedis artery.
- Into the interdigital spaces: along the lumbrical muscles (mm. lumbricales).
- Into the subcutaneous tissue of the sole: through the commissural openings in the plantar aponeurosis along the path of the vessels.
- Into the medial compartment and the leg: pus perforates the medial septum along the tendon of the flexor hallucis longus and enters the deep space of the posterior compartment of the leg via the retromalleolar canal.
- Into the lateral compartment: the infection breaches the lateral septum along the tendon of the flexor digiti minimi brevis and the lateral vessels.
Blood supply and innervation of deep layers
The neurovascular bundles of the sole are divided into medial and lateral groups.
Medial group: The medial plantar vessels and nerve (a., v., n. plantares mediales) run within the thickness of the medial intermuscular septum. The medial artery gives off a superficial branch to the first digit and a deep branch that supplies the muscles of the great toe and anastomoses with the first metatarsal artery. The medial nerve innervates the muscles of the first digit, the flexor digitorum brevis, the two medial lumbrical muscles, as well as the skin of the first, second, third, and the medial side of the fourth digits.
Lateral group: The lateral plantar artery (a. plantaris lateralis) is larger than the medial one. Emerging from the calcaneal canal, it passes between the flexor digitorum brevis and the quadratus plantae muscle. At the base of the metatarsal bones, the artery forms the plantar arch (arcus plantae), which lies between the plantar interosseous fascia and the oblique head of m. adductor hallucis. The arch is completed by the deep plantar artery (a branch of the dorsalis pedis artery). Metatarsal arteries, perforating branches (to the dorsum of the foot), and common digital arteries arise from the arch.
Topography of the toes
The layers of the dorsal surface of the toes have a specific structure. The skin here is thin and mobile, and the subcutaneous tissue is poorly developed. The dorsal digital vessels and nerves run closer to the lateral surfaces of the digits. Anastomoses between the dorsal digital arteries are located at the base of the nails.
The fascia of the dorsum of the toes is thinned and firmly fused with the tendinous apparatus. The tendons of the extensor digitorum longus (II–V) attach to the bases of the middle (middle portions) and distal (lateral portions) phalanges. At the level of the proximal phalanges, the tendons are securely fixed by circular and cruciate ligaments.
Briefly: answers to topical questions
Which muscles form the third and fourth muscle layers of the sole of the foot?
The third and fourth layers of the sole of the foot are represented by deep muscles and tendons.
- Oblique head of the adductor hallucis muscle (m. adductor hallucis, caput obliquum) — forms the third muscle layer.
- Interosseous muscles (mm. interossei) — are located at the very bottom, forming the deep fourth layer of the central muscle group of the sole.
Also in the fourth layer, posterior to the oblique head of the adductor hallucis muscle, runs the tendon of the fibularis longus muscle (tendo m. fibularis longi).
Which nerves provide motor and sensory innervation to the sole of the foot?
Motor and sensory innervation of the sole of the foot is provided by branches of the tibial nerve and the saphenous nerve.
- Medial plantar nerve (n. plantaris medialis) — innervates the muscles of the first digit, the flexor digitorum brevis, the two medial lumbrical muscles; the skin of digits I–III and the medial half of digit IV, the medial margin, and the center of the sole.
- Lateral plantar nerve (n. plantaris lateralis) — innervates the muscles of the fifth digit, the quadratus plantae muscle, the adductor hallucis muscle, the interosseous muscles, and the two lateral lumbrical muscles; the skin of digit V and the lateral half of digit IV, and the lateral margin of the foot.
- Tibial nerve (n. tibialis) — innervates the skin of the calcaneal region.
- Saphenous nerve (n. saphenus) — innervates a narrow strip of skin on the medial surface.
What forms the walls of the calcaneal canal (canalis calcaneus) and what structures pass through it?
The walls of the calcaneal canal are formed by bone and muscle structures, between which elements of the neurovascular bundle and a tendon pass.
- Outer wall — calcaneus (calcaneus).
- Inner wall — abductor hallucis muscle (m. abductor hallucis).
The following anatomical structures pass through the calcaneal canal:
- Lateral plantar neurovascular bundle — runs from the retromalleolar canal into the central compartment of the sole.
- Tendon of the extensor digitorum longus — mentioned in sources as contents of the canal.
The pathways of pus spread from the central compartment can be remembered via guiding structures: vessels (to the dorsum and subcutaneously), lumbrical tendons (into the interdigital spaces), and the flexor hallucis longus tendon (into the medial compartment and leg).
Class and exam questions
Which muscles form the second layer in the tarsal region of the sole?
The second (middle) layer includes the tendon of the flexor digitorum longus and the quadratus plantae muscle (m. quadratus plantae), which attaches to this tendon.
How can pus from the middle fascial compartment of the sole spread to the leg?
The process spreads along the course of the flexor hallucis longus tendon and the neurovascular bundle, passing through the medial intermuscular septum and further through the retromalleolar canal into the deep space of the posterior compartment of the leg.
What forms and where is the deep plantar arterial arch located?
The arch is formed by the lateral plantar artery and the deep plantar artery. It is located between the plantar interosseous fascia and the oblique head of the adductor hallucis muscle.
Where does the superficial fascial space of the middle compartment form?
The superficial fascial space is localized between the tendons of the flexor digitorum brevis and flexor digitorum longus.
What else to study on this topic
- Topography of synovial sheaths and bursae of the foot
- Boundaries and contents of the malleolar and calcaneal canals
- Topography of the transverse section of the foot at the level of the metatarsal bones
- Ligamentous apparatus and tendon retinacula
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