Accessory Navicular Syndrome: The Extra Bone Behind Your Arch Pain

Quick answer: Accessory Navicular Syndrome Extra Bone Arch Pain has multiple potential causes including mechanical, neurological, vascular, and inflammatory. The most common causes we identify are overuse, ill-fitting shoes, and biomechanical imbalance. Red flags requiring urgent evaluation: warmth/redness (infection), inability to bear weight (fracture), and unilateral swelling without injury (DVT). Call (810) 206-1402.

A Hidden Bone That Explains Years of Arch Pain

For many patients, the diagnosis of accessory navicular syndrome is a revelation. They’ve had a painful, prominent bump on the inner arch for years — sometimes decades — and have been told it’s flat feet, plantar fasciitis, or just “the way their foot is built.” When I show them the X-ray with the extra bone sitting beside the navicular, something clicks: there’s a name for this, and there’s a treatment. Accessory navicular syndrome affects roughly 1 in 10 people anatomically, but produces symptoms in a smaller subset — usually adolescents in growth spurts and active adults who increase their activity load.

The accessory navicular is classified into three types. Type I is a tiny sesamoid within the posterior tibial tendon — usually asymptomatic. Type II is a larger, heart-shaped fragment connected to the navicular by fibrocartilage — this is the type that almost universally drives symptoms. Type III is a fully fused accessory bone (cornuate navicular) that creates an oversized navicular prominence, usually asymptomatic. In clinical practice, virtually all symptomatic cases are Type II.

Why This Extra Bone Causes Pain

The posterior tibial tendon — the primary dynamic arch stabilizer — inserts into the navicular. When a Type II accessory navicular is present, the tendon partially inserts into the extra bone, effectively splitting its mechanical output between two structures. The fibrocartilaginous junction (synchondrosis) between the accessory bone and the navicular acts as a stress riser: every step during activity generates shear forces across this junction, producing microdamage, inflammation, and — over time — significant pain. The prominent bony bump on the inner foot becomes tender under shoe pressure and during impact activities. Many patients with accessory navicular syndrome also have flat feet, because the tendon’s reduced mechanical efficiency allows progressive arch collapse.

Symptoms and Diagnosis

The hallmark is a tender bony prominence on the medial midfoot — approximately 1–2 cm in front of and below the medial malleolus — that is aggravated by activity and tight shoes. Pain is typically dull and aching, worsening with prolonged walking, standing, or sports, and relieved by rest. Swelling at the bump is common after activity. Diagnosis is confirmed with weight-bearing foot X-rays, which classify the type and assess arch alignment. MRI is used when symptoms are disproportionate to X-ray findings — bone marrow edema at the synchondrosis on STIR sequences confirms active Type II disease. The key differential diagnoses to exclude include navicular stress fracture (often missed on plain X-ray), posterior tibial tendon dysfunction without an accessory bone, and tarsal coalition.

Conservative Treatment

Conservative management succeeds in 85–90% of patients and should be the first approach for all but the most severe cases. The core program involves three elements. First, custom orthotics with a medial arch support, deep heel cup, and a specific navicular pad that offloads the tender prominence — this is the single most important intervention and produces more consistent results than any prefabricated insole. Second, immobilization during acute flares: a short-leg walking cast or CAM boot for 6–8 weeks allows the inflamed synchondrosis to settle. Third, physical therapy targeting posterior tibial tendon strengthening and calf flexibility, which addresses the mechanical deficit that makes the accessory navicular symptomatic in the first place. Iontophoresis (corticosteroid driven transdermally with low-voltage current) effectively reduces localized inflammation and is a useful adjunct. Shoe modification — removing any footwear that compresses the bump — is simple but impactful.

The Kidner Procedure

When 3–6 months of comprehensive conservative treatment fails to provide adequate functional relief, the Kidner procedure is the standard surgical option. The operation excises the accessory navicular and advances the posterior tibial tendon insertion into the navicular proper — correcting both the source of pain and the mechanical inefficiency. Modern techniques secure the tendon advancement with suture anchors, producing reliable healing. Outcomes are excellent: 85–95% patient satisfaction in published series. For patients with significant associated flat foot deformity, the Kidner procedure can be combined at the same time with a medializing calcaneal osteotomy and/or spring ligament reconstruction to address deformity comprehensively. Post-operative recovery involves 2–4 weeks non-weight-bearing, progressive loading in a boot from week 4–6, physical therapy from week 6, and return to sports at 4–6 months.

In-Office Treatment at Balance Foot & Ankle

If home treatment isn’t providing relief for your foot pain, our podiatry team at Balance Foot & Ankle can help with same-day evaluations and advanced in-office care.

Frequently Asked Questions

Is accessory navicular syndrome permanent?

The extra bone doesn’t disappear without surgery, but the pain from accessory navicular syndrome can be effectively controlled long-term with custom orthotics. Many patients manage symptoms conservatively for years or permanently. Surgery eliminates the anatomical source of pain and is highly effective when conservative care is insufficient.

Can accessory navicular syndrome affect both feet?

Yes — the accessory navicular is present bilaterally in approximately 50% of people who have it. Symptoms may develop in one foot first, and the other foot may become symptomatic later with increased activity or weight gain.

Does accessory navicular syndrome cause flat feet?

It can contribute. Because the posterior tibial tendon partially attaches to the accessory bone rather than fully to the navicular, its arch-stabilizing function is reduced. Over time — especially without orthotic support — this can allow progressive arch flattening. Early and consistent orthotic use reduces this risk.

The Bottom Line

Accessory navicular syndrome explains a surprisingly large proportion of inner arch pain cases that go undiagnosed for years. Once properly identified, treatment is straightforward: custom orthotics and targeted rehabilitation resolve the vast majority of cases. When surgery is needed, the Kidner procedure has decades of excellent results. If you have a persistent painful bump on the inner arch that hasn’t been properly worked up, a podiatric evaluation is the logical next step.

Sources

1. Prichasuk S, Sinphurmsukskul O. Kidner procedure for symptomatic accessory navicular. Foot Ankle Int. 1995;16(8):500-503.
2. Chiu NT, et al. Symptomatic and asymptomatic accessory navicular bones. Clin Nucl Med. 1996;21(8):617-619.
3. Stavlas P, et al. Kidner Procedure With Modified Reattachment for Accessory Navicular. J Foot Ankle Surg. 2022;61(4):780-785.