1.2 Osseous, Ligamentous, and Vascular Anatomy of the Foot

Osseous Framework and Functional Tripartite Division of the Foot

The human foot comprises 26 major structural bones (28 including the two persistent sesamoid bones beneath the first metatarsal head), 33 complex synovial articulations, and over 100 stabilizing ligaments, tendons, and muscles. From a clinical and functional perspective, the foot is divided into three interconnected anatomical zones: the rearfoot (hindfoot), the midfoot, and the forefoot.

+-------------------------------------------------------------------------+
|                        SKELETAL DIVISIONS OF THE FOOT                   |
+-----------------------+-----------------------+-------------------------+
|   REARFOOT (HINDFOOT) |        MIDFOOT        |         FOREFOOT        |
+-----------------------+-----------------------+-------------------------+
| - Calcaneus (os calcis)| - Navicular           | - 5 Metatarsals (1 to 5)|
| - Talus (astragalus)  | - Cuboid              | - 14 Phalanges          |
|                       | - Medial Cuneiform    | - 2 First MTP Sesamoids |
|                       | - Intermediate Cunei. |   (Tibial and Fibular)  |
|                       | - Lateral Cuneiform   |                         |
+-----------------------+-----------------------+-------------------------+
|           |                       |                         |           |
|           +------- CHOPART -------+-------- LISFRANC -------+           |
|              (Midtarsal Joint)        (Tarsometatarsal Joint)           |
+-------------------------------------------------------------------------+

1. The Rearfoot (Hindfoot)

The rearfoot houses the two largest bones of the foot:

• Talus (Astragalus): The central weight-distributing hub connecting the leg to the foot. The talar dome articulates superiorly with the distal tibial plafond and medially/laterally with the malleoli at the talocrural (ankle) joint. Crucially, the talus has no direct muscular or tendinous attachments; over 60% of its surface is covered by articular cartilage, rendering its vascular supply (entering via the tarsal canal and sinus tarsi) precarious and vulnerable to avascular necrosis.
• Calcaneus (Os Calcis): The largest bone of the foot, forming the structural foundation of the heel and bearing initial ground contact during heel strike. It features the sustentaculum tali, a prominent medial horizontal shelf supporting the middle articular facet of the talus and serving as a pulley for the flexor hallucis longus tendon. Posteriorly, the calcaneal tuberosity receives the massive inserting fibers of the Achilles tendon (tendo calcaneus).
• Subtalar (Talocalcaneal) Joint: The articulation between the inferior talar facets and superior calcaneal facets. This joint operates as a triplanar joint that converts the transverse rotation of the lower extremity into frontal plane motion, executing pronation (a composite movement of eversion, abduction, and dorsiflexion) and supination (inversion, adduction, and plantarflexion).

2. The Midfoot

The midfoot comprises five irregular tarsal bones that construct the dynamic longitudinal and transverse arches of the foot:

• Navicular: Situated medially between the talar head and the three cuneiforms. Its medial prominence, the navicular tuberosity, provides the primary insertion for the powerful tibialis posterior tendon. On the plantar-medial aspect, the gap between the sustentaculum tali and the navicular is bridged by the plantar calcaneonavicular ligament (spring ligament), an elastic fibrocartilaginous strap that cradles the talar head and maintains the apex of the medial longitudinal arch.
• Cuboid: Situated laterally between the anterior calcaneus and the bases of the fourth and fifth metatarsals. Its plantar surface features an oblique groove for the gliding passage of the peroneus longus (fibularis longus) tendon.
• Three Cuneiforms (Medial/First, Intermediate/Second, Lateral/Third): Wedge-shaped bones interposed between the navicular proximally and the bases of the first, second, and third metatarsals distally. Together with the cuboid, their wedge-like geometry forms the transverse arch of the foot.

3. The Forefoot

The forefoot is the dynamic propulsion unit of the foot, comprising:

• Five Metatarsals: Numbered one through five from medial to lateral. Each metatarsal consists of a proximal base, a central shaft, and a distal convex head. The first metatarsal is the shortest, thickest, and bears substantial weight during terminal stance. Metatarsals 2 through 4 are slender and tightly constrained.
• Fourteen Phalanges: Two in the great toe (hallux: proximal and distal) and three in each of the four lesser digits (proximal, middle, and distal phalanges).
• Sesamoid Complex: Two small oval sesamoid bones—the tibial (medial) sesamoid and the fibular (lateral) sesamoid—embedded within the tendons of the flexor hallucis brevis beneath the plantar surface of the first metatarsal head. They function as anatomical pulleys to amplify the mechanical advantage of hallux plantarflexion and elevate the first metatarsal head off the floor.

---

Critical Articular Complexes: Chopart and Lisfranc Joints

Two transverse joint lines bridge the divisions of the foot and are of prime diagnostic significance in diabetic limb salvage, especially regarding Charcot neuroarthropathy.

Chopart Joint Complex (Midtarsal or Transverse Tarsal Joint)

The Chopart joint separates the rearfoot from the midfoot and consists of two separate, anatomically distinct articulations working synchronously:

1. The Talonavicular Joint (medially)
2. The Calcaneocuboid Joint (laterally)

During locomotion, the function of the Chopart joint is governed by the rotational position of the subtalar joint:

• Subtalar Pronation (Flexible Shock Absorber): The axes of the talonavicular and calcaneocuboid joints become parallel to one another. In this parallel orientation, the midtarsal joint unlocks, allowing unrestricted midfoot motion, arch flattening, and compliant dissipation of ground reaction forces during the contact phase of gait.
• Subtalar Supination (Rigid Propulsive Lever): As the foot transitions through midstance, the subtalar joint inverts into supination. The axes of the two joints diverge and cross, locking the Chopart complex. This transforms the midfoot into a rigid, non-yielding beam capable of transmitting powerful triceps surae forces forward for push-off.

Lisfranc Joint Complex (Tarsometatarsal Articulation)

The Lisfranc joint marks the structural boundary between the midfoot and forefoot. It comprises nine separate planar articulations between the three cuneiforms and the cuboid proximally, and the bases of the five metatarsals distally.

           MEDIAL                  CENTRAL                  LATERAL
     [ Medial Cuneiform ]   [ Intermed. ]  [ Lateral ]     [ Cuboid ]
     [     (C1)         ]   [ Cunei.(C2)]  [Cunei.(C3)]    [        ]
           \      |                |             |          /      \
            \     |                |             |         /        \
      LISFRANC    | (No transverse |             |        /          \
      LIGAMENT    | intermetatarsal|             |       /            \
        (stout,   | ligament       |             |      /              \
        oblique)  | between M1-M2) |             |     /                \
            \     |                |             |    /                  \
             v    v                v             v   v                    v
          [ 1st Base ]        [ 2nd Base ]   [ 3rd ] [ 4th Base ]    [ 5th Base ]
          [ Metatarsal]       [ Metatarsal]  [ Base] [Metatarsal]    [Metatarsal]
                                (Recessed
                                 Mortise
                                "Keystone")

The architectural stability of the Lisfranc complex depends on two features:

1. The Roman Arch and the Second Metatarsal Keystone: The intermediate cuneiform is anatomically shorter than the medial and lateral cuneiforms, creating a deep bony mortise or "socket." The base of the second metatarsal is recessed proximally into this mortise, acting as the structural keystone of the arch. This interlocking osseous configuration prevents coronal plane displacement of the forefoot.
2. The Lisfranc Ligament Complex: Transverse intermetatarsal ligaments firmly bind the bases of metatarsals two through five together. However, there is NO transverse ligament connecting the base of the first metatarsal to the base of the second metatarsal. Instead, the entire mechanical integrity of the medial and central columns relies upon the Lisfranc ligament—an exceptionally stout, oblique, interosseous ligament passing from the lateral-plantar aspect of the medial cuneiform to the medial-plantar base of the second metatarsal.

In diabetic Charcot neuroarthropathy, repetitive microtrauma to a sensory-denervated foot causes attenuation or rupture of the Lisfranc ligament. The second metatarsal keystone dislocates dorsolaterally from its mortise, precipitating a structural collapse of the tarsometatarsal complex. This yields the classic rocker-bottom foot deformity, where the midfoot sags to the ground, creating a high-pressure osseous prominence along the midplantar surface that ulcerates readily.

---

Biomechanics of the Windlass Mechanism

The windlass mechanism, originally described by J.H. Hicks in 1954, is a fundamental biomechanical model explaining how the foot dynamically transitions from a compliant shock-absorber at heel strike into a rigid, non-deformable propulsion lever during terminal stance.

Anatomical Components of the Pulley

• The Plantar Aponeurosis (Plantar Fascia): A dense, inelastic band of longitudinally oriented Type I collagen fibers originating from the medial calcaneal tuberosity. As it courses anteriorly along the plantar vault, it divides into five digital slips near the metatarsal heads.
• The Digital Slips and Plantar Plates: Each slip inserts into the fibrocartilaginous plantar plate of the metatarsophalangeal (MTP) joint, the flexor fibrous sheaths, and indirectly into the bases of the proximal phalanges.
• The Metatarsal Heads as the Drum (Spindle): The convex, cylindrical articular surface of each metatarsal head functions as the drum or spool of a mechanical windlass.

The Kinetic Sequence of Windlass Activation

1. Heel-Off and Forefoot Loading: During the late midstance and terminal stance phases of gait, body momentum carries the center of mass forward over the metatarsal heads, lifting the heel off the ground.
2. Hallux Dorsiflexion: As the body moves forward, the first MTP joint undergoes passive dorsiflexion, typically reaching 50° to 65° of extension relative to the ground.
3. Tensioning the Plantar Fascia: Because the plantar fascia inserts distal to the center of rotation of the MTP joint, hallux dorsiflexion pulls the plantar fascia tightly around the metatarsal head, winding the cable around the drum.
4. Arch Elevation and Arch Shortening: This winding action draws the calcaneus toward the metatarsal heads, shortening the effective length of the foot and pulling the apex of the medial longitudinal arch upward.
5. Subtalar Supination and Forefoot Stabilization: The tensile pull through the plantar fascia exerts an inverting torque on the calcaneus, driving the subtalar joint into supination and locking the midtarsal (Chopart) joint. The entire tarsus becomes rigid, stabilizing the transverse and longitudinal arches and providing a stiff lever that enables the gastrocnemius-soleus complex to propel the body forward without energetic dissipation.

        PASSIVE HALLUX DORSIFLEXION (50° - 65°)
                   ^   /
                    \ /
                     X  <-- Metatarsophalangeal Joint
                    / \
                   /   \ (Plantar fascia winds around metatarsal drum)
                  /     \=========================================+
                 /                                                 |
  [ Proximal ]  /                                                  |
  [ Phalanx  ] v                                                   |
                                                                   v
      PLANTAR FASCIA TIGHTENS & SHORTENS                           |
      <--------------------------------------------------------+   |
      |                                                        |   |
      v                                                        v   v
  [ Elevation of Medial Longitudinal Arch ]             [ Inversion / Locking of ]
  [ Conversion to Rigid Propulsive Lever  ]             [ Subtalar & Chopart Jts ]

Diabetic Disruption: The Stiff Windlass and Equinus

In patients with diabetes, glycation of periarticular collagen causes non-enzymatic stiffening of tendons, ligaments, and joint capsules (cheiroarthropathy). This stiffness frequently limits first MTP joint dorsiflexion (hallux rigidus or functional hallux limitus). Concurrently, contracture of the gastrocnemius-soleus complex produces an equinus deformity (inability to dorsiflex the ankle past 0° with the knee extended).

When hallux dorsiflexion is restricted, the windlass mechanism cannot engage normally. Instead of an efficient propulsion stroke, the foot cannot roll over the hallux. The patient compensates by twisting the foot during push-off (abductory twist) or prematurely overloading the second and third metatarsal heads. This failure directly multiplies focal shear stress under the central metatarsal heads and the plantar hallux, precipitating neuropathic ulceration.

---

Vascular Anatomy and the Angiosome Concept

Tissue viability and wound healing depend on adequate arterial inflow. The arterial supply to the foot originates from the popliteal artery, which bifurcates into the anterior tibial artery and the tibioperoneal trunk (which subsequently splits into the posterior tibial artery and peroneal [fibular] artery).

Primary Arterial Pathways

1. Anterior Tibial Artery / Dorsalis Pedis: The anterior tibial artery descends through the anterior compartment of the leg, crosses the anterior ankle joint line beneath the extensor retinaculum, and becomes the dorsalis pedis artery. The dorsalis pedis courses over the navicular and cuneiforms, giving off the medial and lateral tarsal arteries and the arcuate artery. It terminates by dividing into the first dorsal metatarsal artery and the deep plantar artery. The deep plantar artery plunges vertically between the first and second metatarsal bases to join the lateral plantar artery in completing the deep plantar arterial arch.
2. Posterior Tibial Artery: The posterior tibial artery descends through the deep posterior compartment of the leg, passes behind the medial malleolus within the tarsal tunnel (covered by the flexor retinaculum), and enters the plantar foot. Here it bifurcates into:
   • Medial Plantar Artery: Courses along the medial border of the foot, supplying the abductor hallucis muscle, medial instep, and sending digital branches to the medial hallux.
   • Lateral Plantar Artery: Courses obliquely across the midfoot between the flexor digitorum brevis and quadratus plantae muscles toward the base of the fifth metatarsal. It then turns medially to form the deep plantar arch, which anastomoses with the deep plantar branch of the dorsalis pedis artery. The deep plantar arch issues plantar metatarsal arteries that bifurcate into digital arteries.
3. Peroneal (Fibular) Artery: Descends through the lateral compartment, supplying the fibular musculature. Proximal to the ankle, it issues an anterior perforating branch (which pierces the interosseous membrane to supply the anterolateral ankle) and terminates in lateral calcaneal branches to the lateral heel.

The Six Angiosomes of the Foot and Ankle

In 1987, Ian Taylor and colleagues introduced the angiosome concept, defining an angiosome as a continuous three-dimensional anatomical block of tissue (skin, subcutaneous tissue, fascia, muscle, and bone) supplied by a specific, dedicated source artery. In the foot and ankle, six distinct angiosomes are mapped to three main parent vessels:

+-------------------------------------------------------------------------+
|                   THE SIX ANGIOSOMES OF THE FOOT AND ANKLE              |
+------------------------------------+------------------------------------+
| SOURCE ARTERY                      | ANGIOSOME TERRITORY                |
+------------------------------------+------------------------------------+
| 1. Posterior Tibial Artery:        | Medial and plantar heel            |
|    - Calcaneal Branch              | (calcaneal weight-bearing pad)     |
|                                    |                                    |
| 2. Posterior Tibial Artery:        | Medial instep and medial arch;     |
|    - Medial Plantar Artery         | medial plantar midfoot             |
|                                    |                                    |
| 3. Posterior Tibial Artery:        | Lateral plantar sole, central      |
|    - Lateral Plantar Artery        | plantar forefoot, and plantar toes |
|                                    |                                    |
| 4. Anterior Tibial Artery:         | Entire dorsum of foot and          |
|    - Dorsalis Pedis Artery         | dorsal aspects of all five digits  |
|                                    |                                    |
| 5. Peroneal Artery:                | Lateral and posterior heel         |
|    - Calcaneal Branch              | (lateral calcaneal margin)         |
|                                    |                                    |
| 6. Peroneal Artery:                | Anterolateral ankle and            |
|    - Anterior Perforating Branch   | lateral dorsum of the midfoot      |
+------------------------------------+------------------------------------+

PLANTAR VIEW OF FOOT ANGIOSOMES:
+-------------------------------------------------------------+
|                        PLANTAR TOES                         |
|         (Supplied by Lateral Plantar Artery branches)       |
+-------------------------------------------------------------+
|    MEDIAL INSTEP & ARCH     |     CENTRAL & LATERAL SOLE    |
|  (Medial Plantar Artery)    |   (Lateral Plantar Artery)    |
+-----------------------------+-------------------------------+
|                     PLANTAR HEEL PAD                        |
|           (Posterior Tibial - Calcaneal Branch)             |
+-------------------------------------------------------------+

Clinical and Surgical Implications: Direct vs. Indirect Revascularization

In non-diabetic atherosclerotic peripheral artery disease (PAD), collateral vessels are often capable of compensating across angiosome borders via "choke vessels" or arterio-arterial connections. However, in patients with diabetes, severe microangiopathy and medial arterial calcification (Mönckeberg sclerosis) frequently paralyze collateral compensation.

Current PAD guidance prioritizes restoration of in-line flow to at least one foot artery. During an endovascular procedure, targeting the artery that supplies the ulcer may be considered when technically feasible and practical, but evidence is very uncertain and collateral pathways vary. Angiosome anatomy therefore informs planning without creating an absolute rule that a particular artery must always be treated.