Ankle Joint: Anatomy, Ligaments, and Common Injuries

Overview

The ankle joint, or talocrural joint, is a hinge (ginglymus) synovial joint that allows dorsiflexion and plantarflexion of the foot. It is the most commonly injured joint in the body and one of the most important weight-bearing articulations. The joint is formed by a mortise and tenon arrangement — the bony socket (mortise) of the tibia and fibula gripping the trochlea (dome) of the talus.

Articular Surfaces

The Mortise

Formed by three bones:

• Medial malleolus (distal tibia): provides the medial buttress
• Articular surface of tibia (plafond): horizontal roof of the joint, bearing most weight
• Lateral malleolus (distal fibula): provides the lateral buttress; extends ~1 cm more distally than the medial malleolus

The Tenon (Talus)

• Superior articular surface (trochlea): wider anteriorly than posteriorly
• This anterior widening is clinically important: in dorsiflexion, the wider part enters the mortise → joint becomes more stable (more stable in dorsiflexion than in plantarflexion)
• Covered in hyaline cartilage on superior and both sides

Stability and Ligaments

Medial Ligament Complex (Deltoid Ligament)

• Strong, fan-shaped ligament from the medial malleolus
• Deep layer: anterior and posterior tibiotalar ligaments (strongest; resist eversion and lateral talar displacement)
• Superficial layer: tibionavicular, tibiocalcaneal, and anterior tibiotalar fibers
• Resistance to eversion makes isolated deltoid ligament tears uncommon (fractures of the fibula or avulsion fractures of medial malleolus more likely)

Lateral Ligament Complex (Three Bands)

• Anterior talofibular ligament (ATFL): most commonly injured ligament; limits anterior talar displacement; taut in plantarflexion; torn in inversion sprains
• Calcaneofibular ligament (CFL): limits inversion; taut in neutral and dorsiflexion
• Posterior talofibular ligament (PTFL): strongest lateral ligament; limits posterior talar displacement; rarely torn

Tibiofibular Syndesmosis

• The inferior tibiofibular joint is a fibrous joint (syndesmosis) held by:
  • Anterior and posterior inferior tibiofibular ligaments
  • Interosseous ligament and membrane
• Maintains the width of the ankle mortise; disruption → syndesmotic (high ankle) sprain; unstable ankle
• Cotton test / external rotation stress test for syndesmotic integrity

Capsule and Synovium

• Capsule: thin anteriorly and posteriorly; thickened medially (deltoid) and laterally (lateral ligaments)
• Extends anteriorly to include tibiotalar joint + part of talar neck (injection site: anterior to extensor tendons)

Blood Supply

Genicular anastomosis of the ankle:

• Anterior tibial artery → anterior medial and lateral malleolar arteries
• Posterior tibial artery → medial malleolar branches
• Fibular (peroneal) artery → lateral malleolar branches

Neurovascular Compartments at the Ankle

Six tendon/vessel/nerve structures pass around the ankle in osseofibrous tunnels:

Anterior: (lateral to medial)

1. Tibialis anterior
2. Extensor hallucis longus
3. Anterior tibial artery (becomes dorsalis pedis)
4. Deep peroneal nerve
5. Extensor digitorum longus
6. Fibularis (peroneus) tertius

Medial (tarsal tunnel): Tom, Dick, and Harry (Nervous)

• Tibialis posterior
• Digitorum longus flexor (flexor digitorum longus)
• Posterior tibial artery and vein
• Nerve (tibial nerve → medial and lateral plantar nerves)
• Hallucis longus flexor (flexor hallucis longus)

Lateral:

• Fibularis (peroneus) longus and brevis (in a single sheath)

Sub-talar (Talocalcaneal) Joint

• Permits inversion and eversion of the subtalar complex (not the ankle joint itself)
• Key for walking on uneven terrain
• The interosseous talocalcaneal ligament (in the sinus tarsi) is the primary stabilizer

Clinical Relevance

• Lateral ankle sprain (most common ligament injury in sports): inversion + plantarflexion; ATFL first, then CFL; >85% respond to conservative management (RICE, rehabilitation)
• Ottawa Ankle Rules: indication for X-ray — bone tenderness at malleolus with inability to weight-bear 4 steps
• Maisonneuve fracture: proximal fibula fracture with medial malleolus fracture/deltoid tear and syndesmotic rupture; "high ankle sprain" that requires open reduction
• Pilon fracture: axial loading fracture of tibial plafond; difficult to treat; post-traumatic arthritis common
• Tarsal tunnel syndrome: compression of the tibial nerve behind the medial malleolus; causes medial foot burning/numbness (analogous to carpal tunnel in the wrist)

Study Points

• Describe the bony mortise and explain why the ankle is more stable in dorsiflexion.
• Name the three lateral ankle ligaments and explain which is most commonly torn in ankle sprains.
• Describe the deltoid ligament and explain why isolated medial ankle sprains are uncommon.
• Apply the Ottawa Ankle Rules to clinical decision-making.
• Describe the tarsal tunnel and name the structures passing through it in order (Tom, Dick and Harry Nervous).