Leg

The lower leg, or just “leg” in anatomical terms, is the part of the lower limb between the knee and the ankle joint. The bony structure is composed of the tibia and fibula bones, which articulate with each other at the proximal and distal tibiofibular joints. The muscles of the leg are grouped into the anterior, lateral, and posterior compartments by extensions of fascia and exert their action on the ankle, foot, and toes.

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Bones and Joints

Tibia

  • Medial, weight-bearing bone of the leg
  • Articulates with the femur and fibula proximally and the talus and fibula distally
  • Important landmarks:
    • Tibial condyles: 
      • Horizontal proximal surfaces that articulate with the femoral condyles
      • Separated by the lateral and medial intercondylar tubercles
    • Tibial tuberosity: 
      • Triangular, superior–anterior area where condyles merge
      • Site of attachment for the patellar tendon
    • Shaft: anterior, lateral, and posterior surfaces
    • Medial malleolus: 
      • Distal projection
      • Articulates with the talus as part of the ankle

Fibula

  • Thin, lateral bone of the leg
  • Articulates with the tibia proximally and distally and with the talus distally
  • Important landmarks:
    • Head: 
      • Serves as the site of attachment for ligaments of the knee
      • Articulates with the tibia
    • Neck:
      • Narrow
      • The common peroneal nerve wraps around it.
    • Shaft: medial, lateral, and posterior surfaces
    • Lateral malleolus: 
      • Distal projection
      • Articulates with the talus as part of the ankle
Leg Anterior and Posterior view

Anterior and posterior views of the tibia, fibula, and tibiofibular joints

Image by BioDigital, edited by Lecturio

Joints

  • Proximal tibiofibular joint: 
    • Arthrodial plane joint between the lateral tibial condyle and the head of the fibula
    • Stabilized by a sturdy capsule and multiple ligaments
  • Distal tibiofibular joint: 
    • Syndesmosis of the distal ends of the fibula and tibia
    • Stabilized by multiple ligaments and the interosseous membrane
  • Interosseous membrane: 
    • Fibrous tissue directed downward and laterally
    • Connects medial border of the fibula with lateral border of the tibia
    • Stabilizes the leg
    • Contains an opening on the proximal region that allows for the passage of the anterior tibial vessels to the anterior compartment of the leg
Posterior view of the arterial supply of the leg

Posterior view of the leg, featuring the opening on the interosseous membrane and the passage of the anterior tibial artery

Image by BioDigital, edited by Lecturio

Fascial Compartments

The leg is divided into 4 fascial compartments by the interosseous membrane and the anterior, posterior, and transverse intermuscular septa as follows:

  1. Anterior compartment:
    • Separated from the lateral compartment by the anterior septum
    • Separated from the deep posterior compartment by the interosseous membrane and the tibia
    • Contains 4 muscles:
      • Tibialis anterior
      • Extensor hallucis longus
      • Extensor digitorum longus
      • Peroneus tertius
  2. Lateral compartment:
    • Separated from the anterior compartment by the anterior septum
    • Separated from the superficial posterior compartment by the transverse septum
    • Contains 2 muscles:
      • Peroneal brevis
      • Peroneal longus
  3. Superficial posterior compartment:
    • Separated from the deep posterior compartment by the posterior septum
    • Separated from the lateral compartment by the transverse septum
    • Contains 3 muscles:
      • Soleus
      • Gastrocnemius
      • Plantaris
  4. Deep posterior compartment:
    • Separated from the anterior compartment by the interosseous membrane and the tibia
    • Separated from the superficial posterior compartment by the posterior septum
    • Contains 4 muscles:
      • Popliteus
      • Tibialis posterior
      • Flexor digitorum longus
      • Flexor hallucis longus
Cross section of the leg

Cross section of the leg, featuring the fascial compartments and the location of the major vessels

Image by Lecturio.

Anterior Compartment — Dorsiflexor Muscles

  • There are 4 muscles in the anterior compartment responsible for dorsiflexion or extension of the foot and/or toes:
    • Tibialis anterior: powerful dorsiflexor of the foot
    • Extensor digitorum longus: extends the 4 lateral toes and the foot
    • Extensor hallucis longus: extends the big toe (hallux) and the foot
    • Peroneus tertius: everts and dorsiflexes the foot
  • Common nerve supply: deep fibular nerve (ventral rami of L4–S2)
  • Common blood supply: anterior tibial artery, branch of the popliteal artery
Table: Anterior compartment — dorsiflexor muscles
MuscleOriginInsertionInnervationFunction
Tibialis anteriorLateral condyle, lateral surface of proximal tibia, and interosseous membrane (IO)Medial cuneiform and base of 1st metatarsalDeep fibular nerves (L4, L5, S1)
  • Dorsiflexion of ankle
  • Inversion of foot
Extensor digitorum longusLateral condyle of tibia, anterior surface of fibula, and IOMiddle and distal phalanges of digits 2–5
  • Extends digits 2–5
  • Dorsiflexes ankle
Extensor hallucis longusAnterior surface of midfibula and IODistal phalanx of hallux (digit 1)
  • Extends hallux
  • Dorsiflexes ankle
Fibularis tertiusAnterior surface of lower fibula and IOBase of 5th metatarsal
  • Dorsiflexes ankle
  • Supports eversion

Lateral Compartment Evertor Muscles

  • Also called the peroneal compartment of the leg
  • The muscles listed here are are primarily responsible for foot eversion; they also contribute to plantar flexion and dorsiflexion.
    • Fibularis, or peroneus, longus
    • Fibularis, or peroneus, brevis
  • Both muscular tendons pass posterior to the lateral malleolus to enter the foot deep in relation to the fibular (or peroneal) retinaculum.
  • Common nerve supply: primarily by the superficial fibular nerve
  • Common blood supply: perforating branches from the anterior tibial artery
Table: Lateral compartment — evertor muscles
MuscleOriginInsertionInnervationFunction
Fibularis longusHead and proximal ⅔ of lateral fibula1st metatarsal and medial cuneiformSuperficial fibular nerves (L5, S1)
Fibularis brevisMidportion of lateral fibulaBase of 5th metatarsalEverts foot; weak plantar flexion

Posterior Compartment Flexor Muscles

  • Muscles of the posterior compartment are primarily responsible for plantar flexion.
  • The flexor muscles are divided into superficial and deep layers by the transverse intermuscular septum.
    • Muscles of the superficial layer include:
      • Triceps surae: consists of the soleus and paired gastrocnemii
      • Plantaris
    • Muscles of the deep layer include:
      • Popliteus
      • Tibialis posterior
      • Flexor digitorum longus
      • Flexor hallucis longus
      • Muscular tendons (except for popliteus) pass posterior to the medial malleolus (tibialis posterior, flexor digitorum longus, and flexor hallucis longus, from medial to lateral).
  • Common nerve supply: tibial nerve
  • Common blood supply: posterior tibial artery and fibular vessels (deep layer)

Superficial layer of the posterior compartment of the leg

MuscleOriginInsertionInnervationFunction
Gastrocnemius
  • Lateral head: lateral condyle of femur
  • Medial head: medial condyle of femur
Posterior aspect of calcaneus via calcaneal tendonTibial nerves (S1, S2)
  • Plantar flexes ankle
  • Flexes knee
SoleusPosterior surface of fibula and soleal line of tibiaPlantar flexes ankle
PlantarisInferior lateral supracondylar ridge of femurWeak plantar flexion

Deep layer of the posterior compartment of the leg

MuscleOriginInsertionNerve supplyFunction
PopliteusLateral aspect of lateral condyle of femur and lateral meniscusPosterior tibia above soleal lineTibial nerves (L4, L5, S1, S2, S3)
  • Weak knee flexion
  • Unlocks knee by lateral rotation of femur on stable tibia
Flexor digitorum longusPosterior surface of tibia below soleal lineDistal phalanges of digits 2–5
  • Flexes digits 2–5
  • Plantar flexes ankle
Flexor hallucis longusLower ⅔ of posterior fibula and interosseous membraneDistal phalanx of hallux
  • Flexes hallux
  • Weak plantar flexion
Tibialis posteriorInterosseous membrane and posterior borders of tibia and fibulaTuberosity of the navicular bone, 1st–3rd cuneiform bones, and 2nd–4th metatarsal bones
  • Plantar flexion
  • Adducts lower ankle

Blood Supply

Arteries of the lower leg

A continuation of the popliteal artery supplies blood to the lower leg. The artery bifurcates at the lower margin of the popliteal fossa and sends branches to the anterior compartment, the anterior tibial artery, and the posterior and lateral compartments, forming the tibioperoneal trunk.

OriginTrajectoryBranchesSupply
Anterior tibial artery (accompanied by the deep peroneal nerve)Division of popliteal artery into anterior tibial artery and tibiofibular trunk
  • Begins in popliteal fossa
  • Enters anterior compartment through upper opening of interosseous membrane
  • Continues as the dorsalis pedis
  • Recurrent branches to knee joint anastomosis
  • Muscular branches
  • Medial and lateral malleolar branches
Anterior compartment of leg
Posterior tibial artery (accompanied by tibial nerve)Terminal branch of tibioperoneal trunk
  • Begins at lower border of popliteal fossa
  • Lies deep to soleus muscle
  • Passes under medial malleolus
  • Divides into medial and lateral plantar arteries
  • Circumflex branch to knee
  • Muscular branches
  • Medial malleolar and calcaneal branches
  • Posterior compartment of leg
  • Plantar aspect of foot
Peroneal arteryTerminal branch of tibioperoneal trunk
  • Begins at lower border of popliteal fossa
  • Travels along medial border of fibula
  • Divides into calcaneal branches
Muscular branchesLateral compartment and deep layer of posterior compartment of leg

Veins of the lower leg

There are 2 venous drainage networks in the leg: 

  • The deep venous branches unite to form the popliteal vein.
  • The superficial or saphenous veins are located in the subcutaneous tissue.
OriginTrajectoryTributariesDrainage
Great saphenous vein (accompanied by saphenous nerve)Continuation of medial end of dorsal venous arch of the foot
  • At ankle, passes anterior to medial malleolus
  • Ascends leg medially
  • Passes behind medial condyles of femur and tibia
  • Drains into saphenofemoral junction
  • Short saphenous vein
  • Superficial external pudendal vein
  • Superficial epigastric veins
  • Superficial circumflex iliac vein
  • Lateral femoral cutaneous vein
Anteromedial aspect of leg and plantar aspect of foot
Small saphenous vein (accompanied by sural nerve)Continuation of lateral end of dorsal venous arch of foot
  • At ankle, passes posterior to lateral malleolus
  • Ascends in midline of posterior leg
  • Drains into saphenopopliteal junction (variable)
Posterior aspect of leg
Medial and Posterior view of the leg featuring the venous drainage of the leg

Medial and posterior views of the leg, featuring the superficial venous drainage

Image by BioDigital, edited by Lecturio

Deep veins of the leg:

  • Located deep to the deep fascia of the leg
  • Accompany the arteries of the leg of the same name:
    • Anterior tibial veins
    • Posterior tibial veins
    • Peroneal veins
  • Converge at the inferior border of popliteus muscle to form the popliteal vein
Deep veins of the leg

Posterior view of the leg, featuring the deep venous drainage

Image by BioDigital, edited by Lecturio

Innervation

The sensory and motor innervation to the lower limb is supplied by the lumbosacral plexus (L1–S4).

  • Sciatic nerve: biggest and longest nerve from the lumbosacral plexus; gives off many branches and divides into the tibial nerve and the common peroneal nerve
    • Tibial nerve: provides motor function to the posterior compartment in the leg and multiple sensory branches to the entire leg (sural, medial calcaneal, and medial and lateral plantar nerves); passes inferior and posterior to the medial malleolus in the ankle, through the tarsal tunnel.
      • Sural nerve: sensory supply to the skin over the posterolateral aspect of the distal third of the leg and lateral border of the foot
    • Common peroneal nerve (common fibular nerve): provides motor function to the short head of the biceps femoris, then bifurcates into two branches:
      • Deep peroneal (fibular) nerve: terminal branch of the common peroneal nerve, provides motor function to the anterior compartment and sensory supply to the 1st interdigital space of the foot
      • Superficial peroneal (fibular) nerve: terminal branch of the common peroneal nerve, provides motor function to the lateral compartment and sensory supply to the lower anterior aspect of the leg and dorsum of the foot
  • Saphenous nerve: branch of the femoral nerve, arises from the femoral triangle, pure sensory function to the skin over the medial half of the leg

Clinical Relevance

The following are common conditions associated with the lower leg:

  • Tibial fracture: most common type of long bone fracture, usually caused by direct trauma: Tibial fractures may occur at any point of the tibia and often are associated with a concomitant fracture of the fibula. There is a high risk of open fracture, neurovascular injury, compartment syndrome, poor healing of the fracture, and wound infection.
  • Peripheral artery disease: condition due to atherosclerosis. Peripheral artery disease presents initially as intermittent claudication and may progress to ischemic pain at rest and very commonly presents in the calves. The underlying atherosclerosis produces arterial stenosis and/or occlusion of the arterial vessels in the extremities, causing poor tissue perfusion. Clinically, as the disease progresses, hypoperfusion causes the affected extremity to become pale, cold, and paresthetic.
  • Deep vein thrombosis: Phlebothrombosis occurs when a blood clot forms in a vein in the absence of an inflammatory reaction. The most common sites of venous thrombosis are the deep veins of the leg. The most dangerous complication is when part of the clot travels to the lungs, causing a pulmonary embolism. The thrombus can be visualized on ultrasonography. Anticoagulation is the main form of treatment.
  • Chronic venous insufficiency: When venous blood is unable to follow the normal antegrade path of return to the heart, it refluxes distally into the leg, causing venous congestion. This insufficiency presents as lower extremity edema, trophic skin changes, and discomfort.
  • Peripheral nerve injuries: A common peroneal (or fibular) nerve injury can present as foot drop (weakness with dorsiflexion) and numbness of the lateral leg/dorsum of the foot. Injury to the common peroneal nerve can occur at the proximal fibula secondary to a fracture of the fibula or pressure from a cast.
  • Tarsal tunnel syndrome: a compressive neuropathy caused by compression of the tibial nerve under the flexor retinaculum at the posterior–medial aspect of the ankle. Tarsal tunnel syndrome is often secondary to trauma, rheumatoid arthritis, postsurgical changes, ganglionic cysts, and wearing tight shoes and presents as pain and paresthesia in the medial–plantar surface of the foot, particularly while walking and standing.

References

  1. Drake, R. L., Vogl, A. W., Mitchell, A. W. M. (2014). Gray’s Anatomy for Students (3rd ed.). Philadelphia, PA: Churchill Livingstone.
  2. Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Clinically Oriented Anatomy (7th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.
  3. Netter, F. (2019). Atlas of Human Anatomy (7th ed.). Philadelphia, PA: Saunders.

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