Tarsal Coalition

Definition: A failure of segmentation between two or more tarsal bones, producing a fibrous (syndesmosis), cartilaginous (synchondrosis), or osseous (synostosis) bridge.

Epidemiology:

• Incidence: Less than 1% of population.
• Genetics: Autosomal Dominant with variable penetrance.
• Laterality: Bilateral in 50% of cases.
• Types: CN and TC make up greater than 90% of all coalitions.

Pathophysiology:

• Congenital problem, but asymptomatic in young children because the coalition is cartilaginous/fibrous and allows motion.
• Symptoms start when the coalition ossifies (turns to bone) and becomes rigid, restricting subtalar motion.
• Peroneal Spastic Flatfoot: Rigid valgus foot causes shortening/tightness of peroneal tendons (not true spasm).

Aetiology:

• Failure of differentiation and segmentation of primitive mesenchyme is the leading theory.
• Most cases are isolated and autosomal dominant. Coalitions also occur as part of syndromes (fibular hemimelia, Apert, Nievergelt-Pearlman, and other limb-deficiency / proximal focal femoral deficiency phenotypes) where multiple coalitions are common.

Why it becomes symptomatic with age:

• The bar starts as fibrous or cartilaginous tissue that permits some motion; the foot is therefore asymptomatic in early childhood.
• As the bridge ossifies (CN typically 8-12 yr, TC 12-16 yr) it stiffens, abolishing subtalar motion and transferring stress to neighbouring joints and ligaments.
• The rigid valgus hindfoot causes adaptive shortening of the peroneal tendons ("peroneal spastic flatfoot" — a misnomer, as it is contracture, not true spasticity).

Relevant anatomy:

• CN coalition bridges the anterior process of the calcaneus to the lateral/dorsal navicular; the extensor digitorum brevis (EDB) origin overlies it and is the workhorse interposition tissue.
• TC coalition almost always involves the middle facet (sustentaculum tali). The flexor hallucis longus runs beneath the sustentaculum, and the posterior tibial neurovascular bundle and FDL/tibialis posterior tendons lie just medial — all at risk during medial resection.

History:

• Pain: Vague hindfoot/midfoot pain, worse with activity.
• Stiffness: "My foot doesn't move right."
• Sprains: Recurrent ankle sprains (rigid hindfoot can't accommodate uneven ground).
• Deformity: Progressive flatfoot.

Physical Examination:

• Inspection: Rigid Flatfoot (Pes Planus). Valgus heel.
• Motion:
  • Restricted Subtalar Motion (Inversion/Eversion).
  • Locking: Foot may be locked in valgus.
• Tender:
  • Sinus Tarsi (CN).
  • Medial Malleolus/Sustentaculum (TC).
• Toe Raise Test: Heel remains in valgus (does not invert) → Indicates RIGID flatfoot.

The cardinal task is separating a rigid painful flatfoot (coalition) from the far more common flexible flatfoot and from other causes of a stiff adolescent hindfoot.

X-ray:

1. Lateral:
   • C-Sign: Continuous C-shaped line from talar dome to sustentaculum tali (TC coalition).
   • Talar Beak: Dorsal spur on talar head (sign of increased stress, not the coalition).
   • Anteater Nose: Elongated anterior process of calcaneus (CN coalition).
2. Oblique (Slanted): Best for CN coalition.
3. Harris Axial: Can show TC coalition (middle facet angulation).

CT Scan (Gold Standard):

• Confirms diagnosis.
• Defines location and extent.
• Assesses % of joint involvement (Critical for TC).
• Look for degenerative changes.

MRI:

• Useful for fibrous/cartilaginous coalitions (non-ossified) if CT negative but high suspicion.

Complications of Surgery

Complications of Non-Treatment

• Progressive Valgus deformity.
• Peroneal tendon contracture/shortening.
• Subtalar and TN joint arthritis.

Protocol:

• Immobilization: Initial splinting/casting to protect the repair/fracture.
• Rehabilitation: Gradual Range of Motion (ROM) and strengthening as healing progresses.
• Weight Bearing: Progression depends on stability of fixation and healing.

• CN Resection: Usually very successful. Pain relief and improved motion.
• TC Resection: success depends on size (less than 50%). If greater than 50%, consider primary fusion.
• Recurrence: Bone can regrow. Interposition material helps prevent this.

• Resection thresholds for TC: The classic 50% facet / 16-degree valgus cut-offs (Wilde) are widely taught, but Luhmann & Schoenecker reported good outcomes in some feet exceeding these limits and argued the CT criteria should guide counselling, not absolute operative choice. There is no level-I evidence defining a hard cut-off.
• Best interposition material: EDB, fat graft, bone wax and vascularised adipofascial flaps have all been used. Fat-graft and EDB give comparable results; any interposition appears better than none for reducing reossification, but no randomised comparison exists.
• Arthroscopic / endoscopic resection: Posterior arthroscopic TC resection (Knörr) and endoscopic CN resection are emerging, with good early series and complete resection without recurrence — but follow-up is short and the technique demanding.
• Concomitant deformity correction: Whether to add a calcaneal lengthening / medialising osteotomy for marked hindfoot valgus at the time of resection (rather than relying on resection alone) is debated.
• Fusion vs joint-preservation in large/arthritic TC: When resection is unsuitable, subtalar versus triple arthrodesis (and timing in the growing foot) remains individualised.

Global epidemiology

• Population prevalence is classically quoted as under 1%, but cadaveric and CT studies suggest the true figure may approach 1-13%, with many coalitions remaining asymptomatic. CN and TC together account for over 90% of cases; bilateral in roughly 50%.
• Slight male predominance; autosomal-dominant inheritance with variable penetrance.

Society guidance — practical points (no dedicated coalition guideline exists)

Registry note: There is no national registry specific to tarsal coalition; outcome data come from single-centre series and the Garg 2023 systematic review rather than arthroplasty-style registries.

High- vs limited-resource practice

• Where CT/MRI is freely available, the resect-vs-fuse decision is CT-driven. In limited-resource settings the diagnosis often rests on plain radiographs (oblique for CN, Harris axial and lateral C-sign for TC) plus clinical examination, and management leans on prolonged casting/orthoses with surgery reserved for refractory cases.
• Terminology: Historically "peroneal spastic flatfoot"; "rigid (painful) flatfoot" is preferred, as the peroneal tightness is contracture, not spasticity.