A tarsal coalition is an abnormal connection between two or more of the tarsal bones in the rearfoot and midfoot. This connection may be composed of bone, cartilage, or fibrous tissue, and it restricts the normal independent motion of the involved joints. Although present from birth, a coalition typically becomes symptomatic only when the abnormal bridge ossifies or stiffens during late childhood or adolescence. The result is a foot that loses its natural adaptability, often leading to pain, stiffness, recurrent sprains, and a characteristic flatfoot deformity.
Definition and Types
A tarsal coalition is best understood as a failure of segmentation during embryological development. Instead of separating into distinct bones, certain tarsal elements remain partially fused. The two most common coalitions are:
- Calcaneonavicular coalition — a connection between the calcaneus and navicular, usually fibrocartilaginous.
- Talocalcaneal coalition — a connection between the talus and calcaneus, often bony and involving the middle facet of the subtalar joint.
Less common variants include talonavicular, calcaneocuboid, and naviculocuneiform coalitions.
Each type affects foot mechanics differently, but all share the same fundamental problem: loss of subtalar motion, especially inversion and eversion.
Embryology and Pathophysiology
During fetal development, the tarsal bones form from a cartilaginous template that should segment into separate units. When segmentation is incomplete, a coalition forms. Early in life, this bridge is often soft and flexible, so children remain asymptomatic. However, between ages 8–16, depending on the coalition type, the tissue begins to ossify.
As the coalition stiffens, the subtalar joint loses mobility. The subtalar joint is crucial for:
- Shock absorption
- Terrain adaptation
- Inversion/eversion
- Smooth gait transitions
When this joint becomes rigid, the foot compensates elsewhere, often in ways that overload soft tissues and adjacent joints.
Biomechanical Consequences
The hallmark biomechanical effect of a tarsal coalition is restricted subtalar motion. This leads to:
- Rigid flatfoot — the arch collapses because the subtalar joint cannot invert.
- Peroneal muscle spasm — the peroneals contract reflexively to stabilise the stiff rearfoot.
- Increased stress on the ankle — leading to recurrent sprains.
- Altered gait — reduced shock absorption increases proximal loading at the knee and hip.
A coalition essentially forces the foot into a single, locked position. This rigidity is why many adolescents with the condition describe their foot as “always stiff” or “never bending properly.”
Epidemiology and Clinical Presentation
Tarsal coalitions occur in approximately 1–2% of the population, though many remain undiagnosed. Symptoms typically emerge during the ossification window:
- Calcaneonavicular: ages 8–12
- Talocalcaneal: ages 12–16
Common clinical features include:
- Medial or lateral foot pain
- Rigid flatfoot deformity
- Limited subtalar motion
- Recurrent ankle sprains
- Peroneal spasm
- Difficulty walking on uneven ground
Parents often report that their child “used to have flexible flat feet, but now the foot seems stiff.”
Diagnosis
Diagnosis begins with a detailed clinical examination, focusing on subtalar mobility and gait. Key clinical signs include:
- Absent or reduced inversion/eversion
- Rigid flatfoot that does not correct on tiptoe
- Tenderness over the coalition site
Imaging is essential:
- X?ray — may show the “anteater sign” (calcaneonavicular) or “C?sign” (talocalcaneal).
- CT scan — best for defining bony coalitions.
- MRI — ideal for detecting fibrous or cartilaginous coalitions.
Management Strategies
Treatment depends on symptoms, coalition type, and degree of deformity. Management falls into two broad categories:
1. Conservative Treatment
- Activity modification
- Orthotic devices to reduce rearfoot motion
- Immobilisation in a cast or boot for 4–6 weeks
- NSAIDs for pain and inflammation
- Physiotherapy to address peroneal spasm and improve gait
Conservative care is often effective, especially for calcaneonavicular coalitions.
2. Surgical Treatment
Surgery is considered when conservative measures fail. Options include:
- Resection of the coalition — often with interposition of fat or muscle to prevent recurrence.
- Subtalar fusion — used when the coalition is large or degenerative changes are present.
- Corrective osteotomies — for significant deformity.
Surgical outcomes are generally excellent, particularly in younger patients with isolated coalitions.
Long?Term Outlook
Most individuals with a tarsal coalition do very well with appropriate treatment. Early diagnosis is key, as prolonged rigidity can lead to secondary degenerative changes. With timely intervention, children and adolescents often return to full activity, including sport.
Adults with longstanding coalitions may require more complex management due to compensatory changes, but even in these cases, targeted treatment can significantly improve function and comfort.
Why Tarsal Coalition Matters Clinically
A tarsal coalition is more than a structural anomaly; it is a condition that reshapes the entire kinetic chain. Its impact on gait, muscle function, and joint loading makes it a critical diagnosis for clinicians working with children, adolescents, and adults with chronic foot pain.
Understanding the coalition’s biomechanical implications helps guide treatment decisions and prevents misdiagnosis—particularly in cases labelled as “recurrent sprains” or “unexplained flatfoot.”