Ankle Joint, Ligaments & Tarsal Tunnel
Anatomy · Lower Limb · lean revision notes
Ankle Joint, Ligaments & Tarsal Tunnel
The ankle is a uniaxial synovial hinge (ginglymus) joint built like a mortise-and-tenon carpentry joint. For NEET PG, the high-yield integration runs: bony mortise → deltoid vs lateral ligament complex → which ligament sprains first → retinacula → tarsal tunnel contents in strict order. Anatomy here marries seamlessly with orthopaedics MCQs on ankle sprain and tarsal tunnel syndrome.
Bony anatomy — the mortise (ankle mortise / talocrural joint)
The talocrural joint is formed by a deep socket (mortise) that grips the body of the talus (tenon):
- Roof & medial wall: distal end of the tibia (inferior articular surface) + medial malleolus.
- Lateral wall: lateral malleolus of the fibula (extends more distal and posterior than the medial malleolus — explains why eversion/lateral stability is greater and inversion injuries are commoner).
- Tenon: the trochlea (body) of the talus, which is wider anteriorly than posteriorly.
High-yield: The trochlea of the talus is wider in front. In dorsiflexion the wide anterior part wedges into the mortise → joint is close-packed, most stable. In plantarflexion the narrow posterior part lies in the mortise → joint is loose, least stable → this is when most ankle sprains occur (e.g., stepping off a kerb, landing from a jump).
The talus has no muscular attachments and is largely covered by articular cartilage — clinically important because its blood supply is precarious (retrograde), making the talar neck fracture prone to avascular necrosis (Hawkins sign on radiograph = subchondral lucency, indicates intact blood supply, a good sign).
| Feature | Medial malleolus | Lateral malleolus |
|---|---|---|
| Bone | Tibia | Fibula |
| Level | Higher (proximal) | Lower (distal) by ~1 cm |
| Position | More anterior | More posterior |
| Ligament | Deltoid (strong) | Lateral complex (weaker) |
| Common injury | Eversion (deltoid) — rarer | Inversion (ATFL) — common |
Movements and the axis
The ankle permits only dorsiflexion and plantarflexion about an oblique axis passing roughly through the two malleoli. Inversion and eversion occur NOT at the ankle but at the subtalar (talocalcaneal) and transverse tarsal (Chopart: talonavicular + calcaneocuboid) joints.
Dorsiflexion → tibialis anterior, EHL, EDL, peroneus tertius (deep peroneal nerve). Plantarflexion → gastrocnemius, soleus, plantaris, tibialis posterior, FDL, FHL, peroneus longus/brevis (tibial nerve, except peronei = superficial peroneal).
High-yield: Plantarflexors are far more powerful than dorsiflexors (push-off in gait, standing on tiptoe). Foot drop (high-stepping gait) results from common peroneal nerve palsy affecting dorsiflexors and evertors.
Ligaments
Medial — Deltoid ligament
A strong, triangular ligament resisting eversion and pronation. Apex on medial malleolus; base fans out to talus, navicular, calcaneus (spring ligament) and sustentaculum tali. Four parts:
- Tibionavicular
- Tibiocalcaneal
- Anterior tibiotalar
- Posterior tibiotalar (deepest, strongest)
High-yield: Because the deltoid is so strong, a forced eversion injury tends to avulse the medial malleolus or rupture the syndesmosis rather than tear the ligament itself. Isolated deltoid tears are uncommon.
Lateral — Lateral collateral complex
Three separate, weaker bands resisting inversion (recall A-C-P, front-to-back):
| Ligament | Abbrev. | Position | Clinical note |
|---|---|---|---|
| Anterior talofibular | ATFL | Anterior | Most commonly sprained ligament of the body; torn first in inversion |
| Calcaneofibular | CFL | Middle | Torn second (more severe sprain) |
| Posterior talofibular | PTFL | Posterior | Strongest of the three; rarely torn |
High-yield (most-tested): ATFL is the most frequently injured ligament in the body. The classic mechanism is inversion + plantarflexion (foot is in the unstable position). Sequence of failure with increasing force: ATFL → CFL → PTFL.
Anterior drawer test stresses the ATFL; talar tilt (inversion stress) test stresses the CFL.
Distal tibiofibular syndesmosis ("high ankle")
The anterior & posterior tibiofibular ligaments + interosseous ligament bind the tibia and fibula above the joint. A "high ankle sprain" involves the syndesmosis (external rotation/dorsiflexion mechanism) and takes much longer to heal. Squeeze test and external rotation test are positive.
Retinacula around the ankle
Thickened deep fascia that hold tendons close to bone (prevent bowstringing) and act as fulcra/pulleys.
| Retinaculum | Location | Key contents |
|---|---|---|
| Superior extensor | Above ankle, anterior | TA, EHL, EDL, peroneus tertius |
| Inferior extensor | Y/frond-shaped, dorsum | Same extensor tendons; anterior tibial vessels, deep peroneal nerve |
| Flexor retinaculum | Medial — medial malleolus to calcaneus | Roof of the tarsal tunnel (see below) |
| Superior peroneal | Lateral, behind lateral malleolus | Peroneus longus & brevis |
| Inferior peroneal | Lateral, on calcaneus | Peroneus longus & brevis |
High-yield: The flexor retinaculum (laciniate ligament) converts the bony groove behind the medial malleolus into the tarsal tunnel. Compression here = tarsal tunnel syndrome (the "carpal tunnel of the ankle").
Tarsal tunnel — contents in order (THE classic MCQ)
The tarsal tunnel lies posteroinferior to the medial malleolus, roofed by the flexor retinaculum, floored by the talus/calcaneus/medial malleolus. Contents from anterior (medial malleolus) → posterior:
Tibialis posterior → Flexor digitorum longus → posterior tibial Artery & Vein → tibial Nerve → Flexor hallucis longus
Mnemonics for the order:
- "Tom, Dick, And Harry" → Tibialis posterior, flexor Digitorum longus, Artery (+ vein), Hallucis longus (FHL). The nerve sits between the vein and FHL → extended version: "Tom, Dick, And a Very Nervous Harry" (And = Artery, Very = Vein, Nervous = Nerve, Harry = FHL).
- Front-to-back the bundle: 2 tendons, then the neurovascular bundle, then 1 tendon (FHL).
High-yield: Order from medial malleolus backward = Tibialis posterior, FDL, posterior tibial artery, vein, tibial nerve, FHL. FHL is the most posterior structure; tibialis posterior is the most anterior.
The tibial nerve divides within or just distal to the tunnel into medial plantar, lateral plantar, and medial calcaneal branches.
Tarsal tunnel syndrome (orthopaedics integration)
Definition: Compressive (entrapment) neuropathy of the posterior tibial nerve / its branches as it passes deep to the flexor retinaculum in the tarsal tunnel — analogous to carpal tunnel syndrome.
Etiology
- Space-occupying: ganglion, lipoma, varicosities, neurilemmoma, accessory muscle, tenosynovitis.
- Bony: post-traumatic (calcaneal/malleolar fracture, talar coalition), osteophytes.
- Deformity: hindfoot valgus / pes planus (stretches/compresses the nerve).
- Systemic: diabetes mellitus, rheumatoid arthritis, hypothyroidism, pregnancy oedema.
- Idiopathic in a large proportion.
Clinical features
- Burning, tingling, numbness over the sole and toes (medial + lateral plantar distribution); often spares the heel if compression is distal to the medial calcaneal branch (which can arise proximally).
- Symptoms worse on standing/walking/at night, relieved by rest and elevation.
- Positive Tinel sign behind the medial malleolus (percussion → tingling radiating into sole) — the key bedside test.
- Dorsiflexion-eversion test reproduces symptoms.
- Late: weakness/wasting of intrinsic foot muscles, abductor hallucis.
High-yield: Tarsal tunnel syndrome affects sensation of the SOLE (plantar nerves), NOT the dorsum (deep/superficial peroneal). Heel sensation may be spared. Contrast with carpal tunnel = median nerve at wrist.
Diagnosis
Clinical (Tinel sign) → Nerve conduction studies/EMG (investigation of choice for confirmation, shows prolonged distal motor/sensory latency) → MRI to identify a space-occupying cause/mass.
Management
Stepwise approach:
- Conservative first: NSAIDs, activity modification, orthotics (medial arch support to correct pes planus/valgus), physiotherapy.
- Local corticosteroid injection into the tunnel.
- Surgical decompression (release of flexor retinaculum) — for failed conservative management or a definite space-occupying lesion (excise ganglion/lipoma). Drug/treatment of choice for a structural cause is surgery.
Ankle sprain — quick orthopaedics integration
- Inversion sprain (commonest) → ATFL ± CFL. Lateral ankle pain/swelling.
- Ottawa Ankle Rules decide need for X-ray: image if there is bony tenderness at the posterior edge/tip of either malleolus, tenderness at navicular or base of 5th metatarsal, OR inability to bear weight (4 steps) immediately and in ED.
- Grade I (stretch) / II (partial tear) / III (complete tear).
- Management: PRICE/RICE (Protection, Rest, Ice, Compression, Elevation) + early functional mobilisation/bracing; surgery rarely for chronic instability.
High-yield: Ottawa ankle rules have very high sensitivity for excluding fracture and reduce unnecessary radiographs — a recurrent MCQ. Maisonneuve fracture = proximal fibula fracture + syndesmotic/deltoid injury from external rotation; palpate the whole fibula.
Eponyms, criteria & cut-offs to remember
| Eponym/criterion | Relevance |
|---|---|
| Pott's fracture | Bimalleolar ankle fracture |
| Cotton fracture | Trimalleolar (medial + lateral + posterior malleolus) |
| Maisonneuve fracture | Proximal fibula + ankle ligament injury |
| Tillaux fracture | Avulsion of anterolateral tibia (Salter-Harris III in adolescents) |
| Dupuytren fracture | Fibula fracture with diastasis of syndesmosis |
| Hawkins sign | Subchondral lucency in talus = viable, good prognosis |
| Ottawa ankle rules | Decision rule for ankle radiography |
| Lauge-Hansen / Weber A-B-C | Classification of ankle fractures (Weber by fibula fracture level relative to syndesmosis) |
Weber classification (by fibula fracture level):
- A — below syndesmosis (stable, usually managed conservatively)
- B — at level of syndesmosis (variable stability)
- C — above syndesmosis (unstable, syndesmosis disrupted → often needs fixation)
Key differentials
- Tarsal tunnel syndrome vs plantar fasciitis: Fasciitis = mechanical heel pain, worst with first steps in the morning, point tenderness at medial calcaneal tubercle, NO neuropathic tingling, Tinel negative.
- Tarsal tunnel vs S1/L5 radiculopathy: Radiculopathy follows a dermatome up the leg, has back pain/positive SLR, EMG localises proximally.
- Tarsal tunnel vs peripheral (diabetic) neuropathy: Bilateral, stocking distribution, no focal Tinel.
- Lateral ankle pain DDx: peroneal tendon subluxation/tear, sinus tarsi syndrome, osteochondral talar dome lesion, 5th metatarsal base fracture.
- Morton neuroma: interdigital (3rd web space) pain — different location, not the tunnel.
Complications
- Ankle sprain: chronic mechanical/functional instability, recurrent sprains, peroneal tendon injury, missed osteochondral lesion, complex regional pain syndrome.
- Talar neck fracture: avascular necrosis, post-traumatic osteoarthritis.
- Tarsal tunnel syndrome: permanent sensory loss, intrinsic muscle wasting, persistent pain if untreated.
- Syndesmotic injury: chronic instability, early ankle arthritis, heterotopic ossification.
Recently asked / exam angle
- Most commonly sprained ligament of the body → ATFL (anterior talofibular). Recurrent single-best-answer.
- Contents of tarsal tunnel in order → match-the-following / sequence MCQs; remember FHL is most posterior, tibialis posterior most anterior.
- Ankle is most stable in which position? → Dorsiflexion (talus wide anteriorly is locked in mortise).
- Sprain mechanism → inversion in plantarflexion.
- Nerve compressed in tarsal tunnel → tibial (posterior tibial) nerve; sensory loss over sole, heel sparing.
- Investigation of choice for tarsal tunnel syndrome → nerve conduction study/EMG; MRI for mass lesion.
- Strongest part of deltoid → posterior tibiotalar; strongest lateral ligament → PTFL.
- Eponyms: Pott's (bimalleolar), Cotton (trimalleolar), Maisonneuve (proximal fibula).
- Ottawa ankle rules indications for X-ray.
- Integration with NEET PG ortho: Weber classification, talar AVN, Hawkins sign.
Rapid revision
- Ankle = hinge (talocrural) joint; mortise = tibia + both malleoli; tenon = talus body.
- Talus trochlea is wider in front → dorsiflexion = most stable (close-packed); plantarflexion = least stable.
- Lateral malleolus (fibula) is lower and more posterior than medial.
- Deltoid ligament = strong medial, resists eversion; posterior tibiotalar part strongest.
- Lateral complex = ATFL, CFL, PTFL; ATFL torn first and most commonly sprained ligament in the body.
- Sprain mechanism = inversion + plantarflexion; failure order ATFL → CFL → PTFL.
- Flexor retinaculum roofs the tarsal tunnel behind the medial malleolus.
- Tarsal tunnel order: Tom, Dick, And (a Very Nervous) Harry = TibPost, FDL, post tibial artery + vein, tibial nerve, FHL (most posterior).
- Tarsal tunnel syndrome = tibial nerve entrapment → burning sole pain, positive Tinel behind medial malleolus, heel often spared.
- TTS investigation of choice = NCS/EMG; treatment ladder = orthotics → steroid injection → retinaculum release.
- Ottawa ankle rules decide need for radiograph; Weber C = above syndesmosis = unstable.
- Pott's = bimalleolar, Cotton = trimalleolar, Maisonneuve = proximal fibula; talar neck fracture → AVN (Hawkins sign = good).