Introduction
Definition: An abnormal connection between two or more tarsal bones, which can be osseous (synostosis), cartilaginous (synchondrosis), or fibrous (syndesmosis).
Epidemiology:
Incidence: Documented at ~1%, though cadaveric studies suggest it may be as high as 12.7% due to asymptomatic cases.
Location: The joints most commonly affected are talocalcaneal (TC) (specifically the middle facet) and calcaneonavicular (CN), accounting for approximately 90% of cases.
Bilateralism: Occurs in 50% to 60% of patients.
Etiology:
Congenital failure of differentiation and segmentation of primitive embryonic mesenchyme.
Genetics: Inherited in an autosomal dominant pattern with nearly full penetrance. Approximately 39% of first-degree relatives are affected.
Pathophysiology:
Restriction of subtalar motion leads to compensatory movement at adjacent joints, resulting in accelerated degeneration.
Adaptive shortening or reflex spasm of the peroneal tendons occurs in response to pain, traditionally termed "peroneal spastic flatfoot".
Clinical Presentation
Onset: Symptoms typically manifest as the coalition progressively ossifies, altering joint kinematics.
Calcaneonavicular: Ages 8–12 years.
Talocalcaneal: Ages 12–16 years.
Symptoms:
Vague foot fatigue and pain localized to the sinus tarsi (CN) or medial subtalar joint (TC).
Pain is exacerbated by activity or walking on uneven surfaces.
Recurrent ankle sprains: Limited subtalar motion transfers forces to the tibiotalar joint, increasing injury risk.
Physical Exam:
Rigid valgus hindfoot with forefoot abduction.
Decreased subtalar motion: Significant reduction or absence of passive inversion and eversion.
Toe-rise test: Failure of the hindfoot to shift into inversion (varus) when rising on toes.
Heel-tip/Reverse Coleman test: Heel valgus remains uncorrected when the medial border is raised, confirming rigidity.
Imaging
Radiography: Initial screening involves AP, lateral, and 45° oblique views.
CN Coalition: Best seen on 45° lateral oblique view.
"Anteater nose" sign: Elongated anterior process of the calcaneus on the lateral view.
TC Coalition: Best seen on Harris (ski-jump) view.
"C-sign": A continuous C-shaped arc formed by the talar dome and sustentaculum tali on the lateral view.
Talar Beaking: A traction spur on the dorsal talus caused by abnormal tarsal mobility.
Computed Tomography (CT): The gold standard for evaluating the extent of osseous coalitions and preoperative planning.
"Drunken waiter" sign: Sloping joint line due to a dysplastic sustentaculum tali (TC).
Magnetic Resonance Imaging (MRI): Highly sensitive for identifying fibrous or cartilaginous coalitions that CT might miss, often showing marrow edema at the coalition site.
Treatment
Non-operative:
Indications: First-line treatment for all symptomatic patients.
Modalities: Activity modification, UCBL orthoses, or 4–6 weeks of immobilization in a short-leg walking cast.
Operative (Resection):
Indications: Symptomatic patients who fail conservative management.
CN Coalition Resection:
Approach: Ollier (dorsolateral tarsal sinus) incision.
Technique: Excision of a 1.5–2.5 cm trapezoidal segment of the bar.
Interposition: Use of EDB muscle origin, fat graft, or bone wax to prevent recurrence.
TC Coalition Resection:
Approach: Medial approach distal to the medial malleolus.
Technique: Exposure via retraction of the FHL and FDL tendons; resection of the bar until normal joint surfaces are visible.
Interposition: Fat graft, split FHL tendon, or bone wax.
Operative (Arthrodesis):
Indications: Extensive coalitions (>50% joint involvement), presence of global degenerative changes, or failed previous resection.
Options: Isolated subtalar fusion or triple arthrodesis (if midfoot joints are also degenerate).
Outcome & Complications
Outcomes: Successful long-term results reported in 70–90% of patients following resection. TC resection outcomes correlate with the amount of postoperative subtalar motion achieved.
Complications:
Recurrence: Symptomatic re-ossification if the resection or interposition is inadequate.
Violation of the talonavicular capsule: During CN resection, this may cause navicular subluxation and subsequent degeneration.
Anesthetic risk: A rigid foot that becomes flexible under anesthesia often indicates a fibrous coalition with associated muscle spasm.