Os tibiale externum, more commonly called the accessory navicular, is a developmental variation of the medial navicular region rather than a true “extra” bone in the ordinary sense. It is usually located on the medial or dorsomedial aspect of the navicular and may be incorporated within, adjacent to, or connected with the posterior tibial tendon.
In clinical practice, the os tibiale externum matters because it is both common and often misunderstood. Many people have it without symptoms, but in some patients it becomes a significant source of medial midfoot pain, shoe irritation, altered mechanics, and posterior tibial tendon stress.
The term itself is a historical one, and “accessory navicular” is now used more often in modern medical writing. Radiology and orthopaedic sources describe it as an accessory ossicle near the navicular tuberosity, while older terminology reflects its position relative to the tibia and navicular region.
Anatomy and classification
The os tibiale externum is found at the medial side of the navicular, close to the insertion of the posterior tibial tendon. In some cases it appears as a separate ossicle, while in others it is a cartilage-connected or fused variant of the navicular tuberosity region.
A widely used clinical classification divides accessory naviculars into three types. Type I is a small sesamoid-like bone within the posterior tibial tendon, Type II is a larger ossicle linked to the navicular by a synchondrosis, and Type III is a fused or prominent navicular tuberosity that can represent a chronic union or remodeled variant.
This classification is useful because symptoms tend to correlate most strongly with Type II lesions, especially when the synchondrosis becomes irritated by load, footwear pressure, or traction from the posterior tibial tendon.
Development and prevalence
Accessory navicular bones are congenital variants, meaning they arise during development rather than from trauma. They represent an additional ossification center or an anatomic variant in the medial navicular region.
Prevalence estimates vary across populations, but the condition is common enough that it is regularly encountered in foot and ankle practice. Most cases are incidental findings on radiographs and never require active treatment, which is why recognition is important mainly when symptoms are present.
Clinical significance
The os tibiale externum becomes clinically relevant when it causes pain or contributes to dysfunction of the medial arch. Patients may report tenderness over the bony prominence, swelling, redness, pain with activity, and discomfort in enclosed shoes that rub against the medial midfoot.
Pain often develops because the accessory bone increases local pressure and can irritate the posterior tibial tendon or the surrounding soft tissue. In some patients, this may also be associated with flatfoot posture or posterior tibial tendon overload, which can amplify symptoms and make the area persistently irritated.
A useful way to think about it is this: the bone itself may be harmless, but the interface between bone, tendon, and shoe wear can become a mechanical pain generator. That is why symptom severity does not always match the size of the ossicle alone.
Diagnosis
Diagnosis begins with history and examination, especially when a patient presents with medial midfoot pain and a palpable prominence over the navicular tuberosity. Clinicians often look for localized tenderness, swelling, pain with resisted inversion or single-leg heel rise, and signs of posterior tibial tendon irritation.
Plain radiographs are usually sufficient to identify the accessory navicular and classify its type. Imaging may show a separate ossicle adjacent to the navicular, a synchondrosis in Type II lesions, or a fused prominence in Type III lesions; more advanced imaging may be helpful if tendon pathology or occult pain generators are suspected.
Differential diagnosis can include navicular stress injury, posterior tibial tendinopathy, plantar fasciopathy, tarsal coalition, and local soft-tissue bursitis. In practice, the combination of exam findings and imaging usually clarifies the diagnosis.
Treatment options
Most patients are treated conservatively first. Initial management commonly includes activity modification, temporary immobilization when symptoms are acute, footwear changes to reduce pressure, orthoses to support the medial arch, and anti-inflammatory strategies where appropriate.
Orthotic therapy is often used to reduce strain on the posterior tibial tendon and unload the painful prominence. Padding, wider shoes, and arch-supportive devices may help by decreasing direct shoe contact and improving midfoot mechanics.
Physiotherapy may be helpful when there is associated tendon overload, calf tightness, or altered foot function. Stretching, strengthening, and graded return to activity are commonly incorporated into management plans, especially when symptoms are linked to sport or prolonged standing.
Surgery is considered when pain persists despite well-conducted conservative care. Procedures may include excision of the accessory bone, contouring of the navicular prominence, and in selected cases repair or reattachment of the posterior tibial tendon, including approaches such as the Kidner procedure.
Surgical outcomes and concerns
Surgical treatment can be effective for carefully selected symptomatic cases, but procedure choice depends on the size and type of the accessory bone, tendon involvement, and the patient’s biomechanics. Contemporary surgical series show that Type II lesions are most often treated operatively because they are more likely to be symptomatic.
Postoperative recovery typically involves immobilization and a staged return to weight bearing and activity. As with any foot surgery, outcomes depend on accurate diagnosis, adequate offloading, and restoration of tendon function rather than removal of the bone alone.
Complications can include persistent pain, scar sensitivity, delayed return to sport, or ongoing posterior tibial tendon dysfunction if the underlying mechanics are not addressed. For that reason, surgery is usually reserved for patients whose symptoms are functionally limiting and resistant to conservative measures.
The os tibiale externum is best understood as a common accessory navicular variant with important biomechanical implications in a minority of patients. It is usually asymptomatic, but when it becomes painful, the problem is often a combination of bony prominence, synchondrosis irritation, footwear pressure, and posterior tibial tendon overload.
For clinicians, the key is to distinguish an incidental anatomic variant from a true pain source. For patients, the message is reassuring: most cases do not require surgery, and many symptomatic cases improve with load management, orthotic support, and targeted conservative treatment.