Accessory Navicular (Os Tibiale Externum)

Epidemiology & Anatomy

• Present in 10–14% of the normal population (some sources cite up to 12%)
• Located along the proximal/medial aspect of the navicular tuberosity, within the direct insertion of the tibialis posterior tendon
• Also called os tibiale externum
• ~50% of individuals with an accessory navicular have flexible flatfoot

Classification (3 Types)

Accessory ossicles of the foot. — Miller's Review of Orthopaedics

Clinical Features

• Typically presents with medial arch/midfoot pain aggravated by activity
• Tenderness and prominence over the medial navicular
• The posterior tibial tendon typically inserts into the accessory bone rather than the native navicular, which can alter biomechanics and contribute to flatfoot deformity
• Symptoms may be triggered by direct shoe pressure (Types 2 and 3) or synchondrosis stress/micro-instability (Type 2)

Imaging

• Weight-bearing foot X-rays are the primary modality; an external oblique (supination oblique) view is most helpful for demonstrating the accessory bone
• Type 1 on CT scan: small separate ossicle; Type 2 shows the articulation clearly on oblique radiograph

(a) 3D CT reconstruction — Type 1 accessory navicular; (b) Oblique radiograph — Type 2 accessory navicular. — Imaging Anatomy Atlas Vol. 3

• MRI is useful when soft tissue assessment is needed (posterior tibial tendon integrity, synchondrosis edema, bone marrow edema)

Treatment

Conservative (First-line)

• Activity restriction and shoe modification (avoid pressure over the medial prominence)
• Orthosis — UCBL (University of California at Berkeley Laboratory) orthosis to control hindfoot valgus
• Short-leg cast immobilization for 4–6 weeks
• Most cases resolve spontaneously

Surgical (if conservative fails)

• Kidner procedure: excision of the accessory bone with repair and advancement of the posterior tibial tendon back to the native navicular
• In adolescents with flexible flatfoot, combined procedures (e.g., subtalar arthroereisis + Kidner procedure) have been studied — a 2025 systematic review and meta-analysis (PMID: 41137245) assessed efficacy of this combined approach

Key Differentials

• Navicular stress fracture — acute onset, different location; must not be confused on plain films
• Avulsion fracture of the navicular tuberosity
• Posterior tibial tendon dysfunction (may coexist)

• Miller's Review of Orthopaedics, 9th Ed., pp. 244, 566
• Imaging Anatomy Text and Atlas, Vol. 3, pp. 839, 875
• Rosen's Emergency Medicine, block 8