= abnormal fibrous / cartilaginous / osseous fusion of two or more tarsal ossification bones

◊Clinically most important congenital problem of calcaneus

Prevalence: 1–2% of population

Cause: abnormal segmentation of primitive mesenchyme with lack of joint formation

Age: fibrous coalition at birth, ossification during 2^nd decade of life with onset of symptoms; M÷F = 1÷1

• asymptomatic: often first noted after antecedent trauma / weight gain / increase in athletic activity
• peroneal spastic / rigid pes planus (= flatfoot) in adjustment for calcaneus valgus (= heel valgus)
• hindfoot / tarsal pain or stiffness
• both feet affected in 20–50%
• osseous bars between bones of hindfoot / bones in close proximity with irregular surfaces

MR (of joint space):

• bone marrow contiguity (osseous coalition)
• fluid- / cartilage-intensity (cartilaginous coalition)
• intermediate- to low-signal intensity (fibrous coalition)
• reactive periarticular bone changes
• bone marrow edema along fused joint (STIR images)

Types:

1. CALCANEONAVICULAR COALITION (45%)
   Age: 8–12 years ← earlier ossification
   • rigid flat foot ± pain in 2^nd decade of life
   
   Radiographs:
   • narrowed calcaneonavicular joint with indistinct articular margins (bones that usually do not articulate)
   • widening / flattening of anteromedial calcaneus
   • “anteater's nose” = elongation of anterior dorsal calcaneus on lateral radiograph
   • hypoplastic talar head
   
   CT (axial scan):
   • broadening of medial aspect of anterodorsal calcaneus in apposition to navicular
   • narrowing of space between calcaneus and navicular + minimal marginal reactive sclerosis
   
   Dx: mostly diagnosed on 45° internal oblique films
2. TALOCALCANEAL COALITION (45%)
   Age: 12–16 years
   • painful peroneal spastic flat foot, relieved by rest
   
   Site: middle facet at level of sustentaculum tali (most frequently)
   Secondary radiographic signs ← alteration in hindfoot biomechanics:
   • prominent talar beak (66%) arising from dorsal aspect of head / neck of talus ← impaired subtalar joint motion
   • rounding of the lateral talar process
   • narrowing of posterior subtalar joint
   • lack of depiction of middle facets
   • asymmetric anterior talocalcaneal joint
   • “ball-in-socket” ankle mortise in severe cases
   • “C” sign = C-shaped outline of the medial talar dome + posteroinferior sustentaculum on lateral radiograph (from bone bridge between talar dome + sustentaculum)
   
   CT (coronal scan):
   • bony bar bridging the middle facet of subtalar joint
   • narrowed middle facet with reactive cystic + hypertrophic changes
   • downward or horizontal slope of sustentaculum, instead of upward
   
   Dx: requires cross-sectional imaging for diagnosis
3. TALONAVICULAR COALITION
4. CALCANEOCUBOID COALITION
5. CUBONAVICULAR COALITION

Rx: orthotics, casting, NSAID, steroid injections, physical therapy, resection, arthrodesis

DDx: acquired intertarsal ankylosis (infection, trauma, arthritis, surgery)