Accessory Navicular

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Accessory Navicular (also known as Os Tibiale Externum or Prehallux) is an accessory ossicle of the foot located medial to the navicular bone. It is the second most common accessory bone of the foot (after the Os Trigonum).

Pathophysiology of Symptoms: While many are incidental findings, symptoms usually arise in adolescence due to:

1. Trauma: A twisted ankle or direct blow disrupts the fragile Type II synchondrosis.
2. Overuse: Repetitive traction from the Tibialis Posterior tendon (which inserts partly onto the ossicle).
3. Pressure: Direct footwear conflict against the prominence.

The "Weak Link": In Type II, the synchondrosis (cartilage bridge) is a weak point. Motion occurs here. Histology of painful excised fragments often shows chronic inflammation, micro-fractures, and reparative change consistent with a prominent chronic non-union.

Association with Flatfoot: There is a debated association with pes planus. The theory is that because the Tibialis Posterior inserts into the accessory bone (more proximal), it loses its lever arm to plantarflex the navicular and support the arch. This is functionally like a "lengthened" tendon. However, large studies suggest the incidence of flatfoot is the same in those with and without the ossicle.

Normal Navicular:

• Boat-shaped bone.
• Keystone of the medial longitudinal arch.
• Articulates with Talus (proximal) and Cuneiforms (distal).

Tibialis Posterior Tendon:

• Main Insertion: Navicular Tuberosity.
• Slip Insertions: Plantar cuneiforms, cuboid, bases of metatarsals 2-4.
• Function: Inversion and Plantarflexion. Primary dynamic stabilizer of the arch.

Pathoanatomy of Accessory Navicular:

• Location: Medial and plantar to the navicular tuberosity.
• Tendon Insertion: In Type II/III, the tendon often inserts broad-based onto the accessory bone rather than the native navicular.
• Nerve: The medial cutaneous nerve of the leg or branches of the Saphenous nerve can be compressed over the lump.

History:

• Pain: Medial midfoot pain. Intense "toothache" like pain or sharp pain with activity.
• Onset: Often insidious, or after a specific "sprain" (eversion injury) which disrupts the synchondrosis.
• Shoes: Pain with narrow shoes, skates, or ski boots.

Physical Exam:

• Inspection: Visible bony prominence on the medial side of the foot (proximal to the 1st ray).
• Palpation: Pinpoint tenderness over the prominence. This is the key sign.
• ROM: Resisted inversion reproduces pain (engages Tib Post). Resisted plantarflexion may also hurt.
• Heel Rise: Single leg heel rise test is usually normal (tendon is intact), unlike in adult acquired flatfoot.
• Pes Planus: Check for flexible flatfoot (often present).

X-rays (Weight Bearing):

• AP Foot: May miss the bone due to overlap.
• Lateral Foot: Shows the "double floor" or overlapping density.
• External Oblique (45 deg): The Money View. clearly profiles the medial side of the navicular and the accessory bone.

Advanced Imaging:

• MRI:
  • Indications: Diagnostic dilemma, or persistent pain with normal X-ray (Type I).
  • Findings: Bone marrow edema (BME) in the accessory bone and the adjacent navicular body is diagnostic of "symptomatic synchondrosis". If no edema, looking for other causes (stress fracture).
• Bone Scan (SPECT):
  • Hot spot at the navicular. (Historical, MRI preferred).
• CT:
  • Confirmation of fusion (Type III) vs tight Type II.

(Historical Note: The original Kidner procedure involved transposing the tendon to the plantar surface. Modern modification is just simple advancement).

Percutaneous Screw Fixation:

• Concept: Turn the painful Type II (synchondrosis) into a painless Type III (synostosis).
• Technique: Percutaneous screw across the fragment.
• Pros: Preserves tendon insertion, minimally invasive.
• Cons: Often fails to fuse (high non-union rate), screw irritation.
• Verdict: Rarely performed. Kidner is superior.

• Weeks 0-2: Backslab/Cast. Non-Weight Bearing. Elevation.
• Weeks 2-6:
  • Kidner: CAM boot, weight bearing as tolerated (if anchor is strong). Some surgeons prefer NWB for 4 weeks.
  • Start active inversion to stimulate tendon.
• Weeks 6-12:
  • Wean boot.
  • Arch support insole (essential to protect the repair).
  • Physiotherapy (eccentric loading).
• Return to Sport: 3-4 months.

• Success Rate: The Kidner procedure has a high success rate (90% good/excellent).
• Time to Recovery: Can take 6-12 months for full resolution of swelling and scar tenderness.
• Arch Height: Does not typically restore the arch in patients with flatfeet (a common misconception). It just removes the pain. If the flatfoot is severe, a medializing calcaneal osteotomy may be needed concurrently.

• Epidemiology: Very common in Australian school-aged children, especially netballers and dancers.
• Terminology: Often called "Navicular tuberosity prominence" in radiology reports.
• Referral: Most managed by Podiatry/Physio first. Ortho referral only for "failed conservative management".
• Waitlists: Elective Category 3 (Non-urgent) in public system. Can wait 12+ months.

What are we actually cutting out? When a Type II ossicle is excised, the histology typically shows:

• Chondro-osseous junction: Irregular cartilage columns.
• Fibrosis: Scar tissue replacing hyaline cartilage.
• Micro-fractures: Evidence of acute-on-chronic trauma.
• Avascular Changes: Sometimes zones of necrosis. This supports the theory that the pain is from a "chronic stress fracture" of the synchondrosis.

The "Other" Medial Lump:

• A Talonavicular (TN) coalition is rare, but a middle facet Calcaneonavicular (CN) coalition can present with vague medial/midfoot pain.
• Key Differences:
  • Motion: Accessory Navicular has FULL subtalar motion (unless severe spasm). Coalition has REDUCED/RIGID subtalar motion.
  • Onset: Coalition often slightly older (12-16).
  • X-ray: "C-Sign" for CN coalition. "Anteater nose" for CN.
• Why it matters:
  • If you do a Kidner on a Coalition patient, it will fail. The pain is from the rigid hindfoot, not the accessory bone.
  • Always check subtalar ROM before booking a Kidner.

The "Pulley" Concept:

• The Tibialis Posterior tendon uses the Navicular tuberosity as a "pulley" or fulcrum to generate inversion torque.
• In Type II/III: The insertion is more proximal (on the accessory bone).
• Effect: This effectively shortens the lever arm. The muscle has to work harder to achieve the same inversion force.
• Result: This can lead to earlier muscle fatigue and potentially predispose to tendon degeneration (tendinopathy) or "insufficiency" (flatfoot).
• Correction: By advancing the tendon distally onto the main navicular body (Kidner), we restore the mechanical advantage.

The "Windlass" Mechanism:

• The Tib Post locks the transverse tarsal joints (TN and CC) allowing the foot to become a rigidity lever for push off.
• If the insertion is painful (synchondrosis), the patient avoids firing the muscle ("antalgic inhibition"). due to pain.
• This leaves the foot flexible and flat during push off, leading to further strain.

Patient: 14-year-old female ballet dancer. Complaint: Right medial foot pain, worse "en pointe". History: No trauma. Gradual onset over 3 months. Exam:

• Bilateral mild pes planus.
• Right foot: Prominent navicular. Redness over the bump.
• Tenderness: 10/10 pin-point over the ossicle.
• Heel Rise: Painful but complete.

Imaging:

• X-ray: External oblique shows Type II accessory navicular with irregular sclerosis at the synchondrosis (Sign of stress).
• MRI: Bone marrow edema in the ossicle and navicular body.

Management:

1. Phase 1 (Acute): Moon boot for 4 weeks. No dancing. Ice. NSAIDs.
2. Phase 2 (Rehab):
   • Pain resolved after boot.
   • Started progressive loading.
   • Custom orthotic with "navicular accommodation" (sweet spot cutout).
3. Outcome: returned to dance at 3 months. Flare ups managed with taping. Surgery avoided.

Phase 1: Protection & Healing (Weeks 0-2)

• Immobilization: Backslab or Moon boot. Strict Non-Weight Bearing (NWB).
• Goal: Allow the bone-tendon interface to heal without shear stress.
• Edema Control: Elevate foot above heart level.
• Wound Care: Keep dry. Sutures removed at 2 weeks.

Phase 2: Protected Weight Bearing (Weeks 2-6)

• Immobilization: Transition to CAM Walker (Moon Boot).
• Weight Bearing:
  • Week 2-4: Touch Down Weight Bearing (Crutches).
  • Week 4-6: Partial to Full Weight Bearing in boot.
• Exercises:
  • Active Plantarflexion/Dorsiflexion (limit inversion).
  • Toe yoga (intrinsic strengthening).
  • Static Quads/Glutes.

Phase 3: Weaning & Strengthening (Weeks 6-12)

• Footwear: Supportive sneaker with Medial Arch Support (Orthotic). This is critical to protect the repair during transition.
• ROM: Full active inversion/eversion.
• Strength:
  • Theraband Inversion (Yellow to Red to Green).
  • Double leg heel rises to Single leg heel rises.
  • Proprioception (Balance board). (The Tib Post is a key proprioceptor).

Phase 4: Return to Sport (Month 3-6)

• Criteria:
  • Pain-free single leg heel rise (3x10 reps).
  • Full ROM.
  • Able to run/hop without pain.
• Progression: Walk to Jog to Run to Cut to Jump.
• Dancers: Need specific "en pointe" training. Often takes 6 months for full return.
• Contact Sports: 4 months usually safe.

The Great Debate:

• Historically, some surgeons argued that fusing the Type II accessory navicular to the main body (turning it into a Type III) was superior because it preserved the "full" insertion of the tendon.
• The Problem:
  • The synchondrosis has poor blood supply.
  • Non-union rates for percutaneous screw fixation are high (up to 50% in some series).
  • The screw head can become symptomatic.
• Current Consensus:
  • Simple Excision (Kidner) is the gold standard.
  • It is reliable, technically easier, and has equivalent or better outcomes.
  • Fusion is reserved for very large ossicles (greater than 1.5cm) where excision would remove greater than 30% of the articular surface (risk of instability), but even then, many experts prefer excision with robust repair.

Does Excision Cause Flatfoot?

• Myth: Removing the bone "detaches" the arch support.
• Fact: The primary support of the arch is the spring ligament complex and the interosseous ligaments, not just the Tib Post insertion.
• Evidence: 10-year follow-up studies show NO radiographic progression of flatfoot deformity after Kidner procedure compared to non-operated controls.
• Caveat: If the patient already has a flexible flatfoot, the surgery won't fix it. It just fixes the pain.

Arthritis Risk:

• Does the altered mechanics cause naviculocuneiform arthritis?
• Data: Long term studies show minimal risk of OA.
• Conclusion: It is a safe, benign procedure.

Q: Did I cause this by buying tight shoes? A: No. This is a congenital (born with it) extra bone. The shoes might have irritated it and brought it to attention, but they didn't create the bone.

Q: Does removing the bone make the foot weak? A: No. The accessory bone is actually "stealing" the power of the Tibialis Posterior tendon. By removing it and re-attaching the tendon to the main bone, we often improve the mechanical efficiency of the muscle.

Q: Will the surgery fix the flat foot? A: Generally, no. A flat foot is a complex 3D deformity. The Kidner procedure fixes the pain from the lump. The arch height usually remains the same, which is fine as long as it's painless.

Q: Can it come back? A: True regrowth is rare if the excision is adequate. However, sometimes a small "stump" of bone is left behind which can be bothersome. This is why we are aggressive with shaving it flat.

Q: How long until my child can dance again? A: Usually 3-4 months for basic dance. Pointe work typically requires 6 months of strengthening to ensure the tendon can handle the load.

Incision Placement:

• Don't place the incision directly over the prominence. Place it slightly dorsal.
• Why? Because the shoe line often hits the prominence directly. A scar there is miserable.

Suture Anchor vs Drill Holes:

• Drill Holes: The classic technique. Drill two holes in the navicular, pass suture through. Inexpensive. Strong. Harder to do.
• Suture Anchors: Modern technique. Fast. Reproducible. Strong pull-out strength. Expensive.
• Verdict: Most surgeons now use anchors for ease of use.

The "Native" Navicular:

• Often, after removing the accessory bone, the medial aspect of the native navicular is still "beaked" or prominent.
• Rule of Thumb: Palpate the Cuneiform-Navicular joint. Your resection line should be flush with the cuneiform. If you leave a step, it will rub.