2.1 Musculoskeletal Anatomy of the Leg & Foot
Key Takeaways
- The foot has 26 bones (7 tarsals, 5 metatarsals, 14 phalanges) plus sesamoids, and its key joints are the ankle, subtalar, Chopart (talonavicular and calcaneocuboid), Lisfranc (tarsometatarsal), metatarsophalangeal, and interphalangeal joints.
- The leg has four compartments: anterior (dorsiflexors, deep peroneal nerve, anterior tibial artery), lateral (peroneus longus and brevis, superficial peroneal nerve), superficial posterior (gastrocnemius, soleus), and deep posterior (tibialis posterior, flexor digitorum longus, flexor hallucis longus, tibial nerve, posterior tibial and peroneal arteries).
- Tendon insertions determine deformity after partial foot surgery: tibialis anterior inserts on the medial cuneiform and first metatarsal base, peroneus brevis on the fifth metatarsal base, peroneus longus on the plantar first metatarsal base and medial cuneiform, and the Achilles on the calcaneal tuberosity.
- Plantar foot infections spread along compartments and tendon sheaths, so deep space infection can track proximally from the forefoot into the midfoot and leg.
- Compartment syndrome is a clinical diagnosis marked by pain out of proportion and pain with passive stretch; a delta pressure (diastolic pressure minus compartment pressure) of 30 mmHg or less supports urgent fasciotomy.
2.1 Musculoskeletal Anatomy of the Leg & Foot
Core Clinical Principle: Lower-extremity wounds sit on a mechanical structure. Knowing which bone lies under an ulcer, which tendon a debridement will expose, which compartment an infection has entered, and which tendon insertion an amputation will remove lets you predict probe-to-bone findings, deformity, and the spread of infection.
The CWSP outline lists musculoskeletal anatomy and physiology in the Wound Healing Environment domain. Most questions apply the anatomy to diabetic foot ulcers, osteomyelitis, partial foot amputations, and traumatic wounds of the leg.
Bones and Joints of the Foot
The foot contains 26 bones: 7 tarsals (calcaneus, talus, navicular, cuboid, and the medial, intermediate, and lateral cuneiforms), 5 metatarsals, and 14 phalanges, plus the two hallux sesamoids within the flexor hallucis brevis tendons.
| Region | Bones | Key Joints | Wound Relevance |
|---|---|---|---|
| Hindfoot | Talus, calcaneus | Ankle (talocrural), subtalar | Heel pressure injuries; calcaneal osteomyelitis; heel pad is thick, septated fat |
| Midfoot | Navicular, cuboid, three cuneiforms | Chopart joint (talonavicular + calcaneocuboid); Lisfranc joint (tarsometatarsal) | Charcot collapse produces rocker-bottom ulcers under the cuboid or navicular |
| Forefoot | Metatarsals, phalanges, sesamoids | Metatarsophalangeal (MTP), interphalangeal (IP) | Plantar metatarsal head and hallux ulcers; sesamoid osteomyelitis; claw toe tip ulcers |
Arches (medial longitudinal, lateral longitudinal, and transverse) spread load. The plantar fascia tightens as the toes dorsiflex during push-off (the windlass mechanism), raising the arch. In claw toe deformity from intrinsic muscle weakness, the proximal phalanges extend at the MTP joints and pull the plantar fat pad distally, leaving the metatarsal heads covered by thin skin, one reason forefoot ulcers form there.
Compartments of the Leg
| Compartment | Muscles | Nerve | Artery | Clinical Clue When Injured |
|---|---|---|---|---|
| Anterior | Tibialis anterior, extensor hallucis longus, extensor digitorum longus, peroneus tertius | Deep peroneal | Anterior tibial (continues as dorsalis pedis) | Foot drop; numbness in the first web space |
| Lateral | Peroneus longus, peroneus brevis | Superficial peroneal | Peroneal artery branches | Weak eversion; dorsal foot numbness |
| Superficial posterior | Gastrocnemius, soleus, plantaris | Tibial | Sural branches of popliteal | Weak plantarflexion; the Achilles tendon forms from gastrocnemius and soleus |
| Deep posterior | Tibialis posterior, flexor digitorum longus, flexor hallucis longus | Tibial | Posterior tibial and peroneal | Weak toe flexion and inversion; plantar numbness |
Tendon Insertions That Matter After Surgery
- Tibialis anterior: Medial cuneiform and base of the first metatarsal (dorsiflexion and inversion).
- Peroneus brevis: Tuberosity at the base of the fifth metatarsal (eversion). Resecting the fifth metatarsal base without reattaching it allows the foot to drift into varus.
- Peroneus longus: Plantar base of the first metatarsal and medial cuneiform (plantarflexes the first ray, everts).
- Tibialis posterior: Navicular tuberosity with slips to the other tarsals and metatarsal bases (inversion, arch support).
- Achilles tendon: Calcaneal tuberosity. After transmetatarsal, Lisfranc, and especially Chopart amputations, the loss of dorsiflexor insertions leaves the Achilles relatively unopposed, which is why tendon lengthening is often added.
Bone Blood Supply and Healing
Long bones receive most of their inner cortical supply from the nutrient artery and the outer part of the cortex from the periosteum. Stripping periosteum during trauma or debridement devitalizes cortical bone, which is why bare cortex does not support a skin graft and why sequestra form in chronic osteomyelitis. Bone heals through inflammation, soft and hard callus, and remodeling; diabetes, smoking, and poor perfusion slow each step.
Tendons are covered by paratenon or sit within synovial sheaths. Tendon with intact paratenon can granulate; tendon stripped of paratenon desiccates and often needs debridement or flap coverage.
How Deep Foot Infection Spreads
The plantar foot is divided by fascial septa into medial, central, and lateral compartments (with additional interosseous and adductor spaces). Infection that enters a plantar compartment is confined by tough fascia, raises pressure, and tracks proximally along tendon sheaths such as the flexor hallucis longus and flexor digitorum longus toward the tarsal tunnel and deep posterior leg. Signs include plantar swelling and loss of the arch contour, pain on dorsiflexing the toes (which stretches the flexor tendons), and dorsal swelling from drainage through the web spaces. Deep space abscesses need prompt surgical drainage.
Muscle Physiology and Compartment Syndrome
Skeletal muscle tolerates only a few hours of warm ischemia before irreversible necrosis. Compartment syndrome occurs when pressure within a closed fascial space rises until it blocks capillary perfusion, typically after fractures, crush injury, reperfusion after revascularization, burns, or tight casts.
- Early findings: Pain out of proportion to the injury and pain with passive stretch of the muscles in the compartment.
- Late findings: Paresthesia, paralysis, and pulselessness; distal pulses are often still present when muscle is already dying.
- Measurement: A delta pressure (diastolic blood pressure minus compartment pressure) of 30 mmHg or less supports the diagnosis.
- Treatment: Urgent fasciotomy; the leg is typically decompressed through two incisions to release all four compartments, and the fasciotomy wounds are closed later or grafted.
Clinical Traps
Trap 1: Missing a Deep Plantar Space Infection
A small plantar ulcer with dorsal foot swelling and pain on toe dorsiflexion may drain a deep compartment abscess. Imaging and surgical exploration, not a longer oral antibiotic course, are the next steps.
Trap 2: Relying on Pulses to Exclude Compartment Syndrome
Palpable pedal pulses do not rule out compartment syndrome, because compartment pressure blocks capillaries well before it blocks large arteries.
A 58-year-old man with diabetes has a chronic plantar-lateral ulcer over the base of the fifth metatarsal with osteomyelitis. The surgeon plans to resect the proximal fifth metatarsal, including the tuberosity. Which deformity is most likely if the tendon attached there is not reattached?
A 66-year-old woman with diabetes has a small ulcer under the first metatarsal head. Two days later she develops swelling across the dorsum of the foot, loss of the medial arch contour, and severe pain when the hallux and lesser toes are passively dorsiflexed. Which explanation best fits these findings?
During debridement of a traumatic pretibial wound, the surgeon removes a broad strip of periosteum, leaving white, dry cortical bone at the base. Why does this bare cortex fail to support a split-thickness skin graft?