Summary

An Accessory Navicular (Os Tibiale Externum) is an extra bone located on the medial side of the foot, embedded within the Tibialis Posterior tendon where it inserts into the Navicular. It is present in 10-15% of the population. While often asymptomatic, it can become painful, particularly in adolescence (skeletal maturity) or following trauma. The pain arises from stress across the fibrous junction (synchondrosis) between the accessory bone and the main navicular body. The condition is strongly associated with Pes Planus (Flatfoot), as the abnormal insertion of the tendon reduces its leverage to support the arch. Treatment ranges from immobilization to the Kidner Procedure (surgical excision). [1,2,3]

Key Facts

• The "Second Ankle Bone": Patients often point to the bump and say "my inner ankle bone hurts," but they are pointing distally to the medial malleolus.
• The Type 2 Villain: Not all accessory bones are painful. Type 1 is a tiny pebble (painless). Type 3 is a fused horn (painless). Type 2 is a triangular bone connected by cartilage. Shearing forces across this cartilage cause the pain.
• The Flatfoot Link: Because the Tibialis Posterior tendon attaches to the accessory bone (which is more proximal than normal), the tendon is effectively "too short" and pulls less effectively, allowing the arch to collapse.

Clinical Pearls

"It's a Fracture!": A teenager twists their ankle. X-ray shows a bone fragment near the navicular. It is misdiagnosed as an avulsion fracture. In reality, it is a Type 2 accessory navicular that has been disrupted. Look for smooth, rounded edges (chronic) vs sharp jagged edges (acute).

"The Kidner Trap": Simply removing the bone (excisional biopsy) is a mistake. You must detach the tendon, remove the bone, and then re-advance and re-attach the tendon to the main navicular body to restore arch function. This is the Kidner procedure.

"Rubbing the Boot": Skiers and skaters hate this bone. The rigid boot presses directly on the medial prominence, causing adventitial bursitis.

                 MEDIAL FOOT BUMP
                        ↓
            X-RAY: ACCESSORY NAVICULAR
           ┌────────────┴────────────┐
       ASYMPTOMATIC              SYMPTOMATIC
     (Incidental finding)       (Painful Type 2)
           ↓                         ↓
       DATA ONLY                CONSERVATIVE
                               (Boot / Arch Support)
                                     ↓
                                  FAILED?
                                     ↓
                                  SURGERY
                             (Kidner Procedure)

Protocol

• Immobilization: 4-6 weeks in a CAM boot to allow the disrupted synchondrosis to heal/scar down. High success rate for acute flare-ups.
• Orthotics: Medial arch support and a "navicular sweet spot" (pocket) to prevent shoe rubbing.
• Injection: Steroid into the synchondrosis. Diagnostic and therapeutic.

The Kidner Procedure

• Indication: Persistent pain despite conservative care.
• Steps:
  1. Medial incision.
  2. Split the Tibialis Posterior tendon longitudinally.
  3. Shell out the accessory ossicle.
  4. Resect the medial prominence of the main Navicular (make it flush).
  5. Advancement: Pull the tendon tight and reattach it to the plantar/medial aspect of the navicular (using Suture Augmentation/Anchors). This restores the "windlass" tension.

Complex Reconstruction

• If the flatfoot is severe (Stage 2 PTTD), a Kidner alone is insufficient. A Medial Calcaneal Osteotomy and FDL Transfer may be needed in addition.

Simple Excision vs Kidner

• Kopp et al: Comparing simple excision (shelling out) vs Kidner (advancement). Found that Kidner had superior results in restoring arch height and function, likely due to tightening the lax PTT.

Fusion?

• Fusion of the ossicle: Some surgeons attempt to screw the accessory bone to the main bone (arthrodesis of the synchondrosis).
• Success: Variable. High rate of non-union. Excision is generally preferred.

The Bone

You have an extra bone in your foot, like a kneecap for your arch muscle. It is connected by a bridge of gristle (cartilage).

The Problem

You have strained the bridge. It is inflamed and painful, like a broken bone that won't heal.

The Fix

We remove the extra bone and the inflamed bridge. Then, we take the tendon that was attached to it and anchor it firmly to the main bone so your arch stays strong.

Q1: Describe the Geist Classification of Accessory Navicular. A:

• Type 1: Sesamoid in tendon.
• Type 2: Synchondrosis (Painful).
• Type 3: Fused (Cornuate).

Q2: What is the main deformity associated with a symptomatic accessory navicular? A: Pes Planovalgus (Flatfoot). The effective insertion of the Tibialis Posterior is proximalized, reducing its mechanical advantage in supporting the arch.

Q3: What view on X-ray best demonstrates the synchondrosis? A: External Oblique view. The standard AP view often overlaps the bones.

Q4: Which nerve supplies sensation to the medial aspect of the midfoot? A: The Saphenous Nerve. It runs intimately with the Great Saphenous Vein and is at risk during the medial approach for a Kidner procedure.

(End of Topic)