Tarsal coalition is an abnormal connection between two bones in the midfoot or hindfoot. This condition is present from birth, resulting from a failure of the bones to separate completely during fetal development. Calcaneonavicular coalition (CNC) is the most frequently encountered type, accounting for over half of all tarsal coalition cases. Although the abnormal connection is congenital, symptoms typically do not appear until later childhood or early adolescence. The onset of pain and stiffness usually corresponds to the connecting tissue hardening over time.
Defining Calcaneonavicular Coalition
Calcaneonavicular coalition involves an abnormal union between the calcaneus (the heel bone) and the navicular bone (a tarsal bone located in front of the ankle joint). This connection is often referred to as a “bar” or “bridge.” The tissue forming this bridge varies in maturity, leading to three classifications:
- Fibrous (syndesmosis), consisting of dense connective tissue.
- Cartilaginous (synchondrosis), composed of flexible cartilage.
- Osseous (synostosis), where a solid bridge of bone has formed between the structures.
The fusion of the calcaneus and navicular bone directly affects the normal mechanics of the foot. Tarsal bones must move independently to allow for complex foot motion, especially the side-to-side movements of inversion (sole turning inward) and eversion (sole turning outward) at the subtalar joint. A rigid bar restricts this motion, essentially locking the bones together. This limitation prevents the foot from adapting to uneven surfaces. The limited motion places increased stress on other joints in the foot and ankle, often leading to pain and complications.
Clinical Presentation and Onset
Although the coalition is present from birth, symptoms usually begin between the ages of 8 and 14 years. This timeframe is when the initially fibrous or cartilaginous bridge begins to ossify, or turn into bone, becoming more rigid. This hardening process further limits subtalar joint motion, triggering the onset of pain and functional issues.
The primary complaint is often chronic or recurrent pain, typically localized to the anterolateral aspect of the foot, near the sinus tarsi. This discomfort is aggravated by physical activity and relieved by rest. Patients may also report frequent ankle sprains, a consequence of the foot’s limited ability to invert or evert for stabilization on uneven ground.
The restricted motion leads to a characteristic rigid flatfoot. The foot appears flat with a collapsed arch and the heel turned outward (valgus alignment). This deformity remains fixed even when the patient stands on their toes, unlike a flexible flatfoot. This condition has historically been referred to as “peroneal spastic flatfoot,” although true muscle spasm is not present. The term describes the shortening and potential pain of the peroneal muscles, which attempt to stabilize the foot against the mechanical limitations imposed by the coalition.
Diagnosis Using Medical Imaging
Diagnosis often begins with a physical examination of the foot and ankle. A clinician looks for a rigid flatfoot posture, tenderness over the coalition site, and a limitation in the side-to-side motion of the subtalar joint. Comparing the range of motion to the asymptomatic foot helps identify the restriction.
Standard X-rays are the initial and often definitive imaging tool. A specific 45-degree oblique radiographic view is usually necessary, as it best visualizes the abnormal bar connecting the calcaneus and navicular bones. Even if the bar is not fully ossified and visible, indirect signs such as the “anteater nose sign”—an elongation of the anterior process of the calcaneus—can suggest the presence of a coalition.
Computed tomography (CT) scans are frequently required, especially if initial X-rays are inconclusive or for surgical planning. CT provides superior detail of the bony anatomy, clearly defining the exact extent and composition of the coalition (bony, cartilaginous, or fibrous). Magnetic resonance imaging (MRI) is reserved for cases where a fibrous or cartilaginous coalition is suspected but not seen on X-ray, or when there is concern for associated soft tissue involvement, such as bone marrow edema indicating a stress reaction.
Management and Treatment Options
The initial approach to managing a symptomatic calcaneonavicular coalition is non-surgical, aiming to reduce pain and inflammation. Non-steroidal anti-inflammatory drugs (NSAIDs) manage pain and swelling. Rest and activity modification, particularly avoiding high-impact sports, are recommended to reduce stress on the foot.
Immobilization in a short leg cast or a removable boot for four to six weeks can calm acute symptoms by resting the painful joint. Custom-made foot orthotics (arch supports) are prescribed to support the arch and reduce strain on affected structures. These conservative measures may alleviate symptoms in many patients, especially those with milder forms of the condition.
Surgical intervention is considered when conservative treatments fail to provide lasting relief and the patient remains symptomatic. The most common procedure is a coalition resection, which involves removing the abnormal bridge of tissue to restore motion. During surgery, the surgeon creates a generous space between the calcaneus and navicular, typically removing at least a centimeter of tissue.
To prevent the coalition from regrowing, a piece of interposition material, such as fat or muscle, is placed into the gap created by the resection. Recovery usually involves a period of non-weight-bearing in a cast, followed by a gradual return to activity and physical therapy. If the coalition is very large, involves multiple joints, or is associated with significant degenerative arthritis, a joint fusion procedure, called arthrodesis, may be necessary to permanently stabilize the hindfoot and eliminate painful motion.