Os Tibiale Externum
- Three Types: Type I (Sesamoid), Type II (Synchondrosis), Type III (Fused/Cornuate).
- Type II is the trouble-maker (painful synchondrosis).
- Tibialis Posterior: Inserts into the accessory bone, effectively 'losing' its mechanical advantage to support the arch.
- Association: Often associated with Pes Planus (Flatfoot).
- Kidner Procedure: The standard surgical treatment.
- “Look for the 'double medial malleolus' appearance (prominent navicular)
- “Pain is directly over the bony prominence, NOT the tendon itself
- “Resisted inversion triggers pain (pulls on the ossicle)
- “Check for associated flatfoot
Clinical Imaging
Imaging Atlas
Navicular Stress Fracture is the killer differential. Common in high-level sprinters and gymnasts. CT scan is often required as X-rays can be subtle.
Accessory Navicular: Pain is medial and prominent (over the bump). Stress Fracture: Pain is dorsal and deep (vaguer midfoot pain).
| Feature | Accessory Navicular (Type II) | Tib Post Tendinopathy |
|---|---|---|
| Adolescent (10-18y) | Adult (greater than 40y) | |
| Focal on bony prominence | Along the tendon course | |
| Bony lump (Hard) | Soft tissue swelling (Boggy) | |
| Associated (Secondary) | Acquired (Progressive) | |
| Ossicle visible | Normal (or arthritis) |
SSSGeist Classification
Hook:The 3 S's of the Accessory Navicular.
SSSKidner Procedure Steps
Hook:Kidner's 3 S's.
PPPIndications for Surgery
Hook:The 3 P's of Surgery.
Overview/Epidemiology
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:
- Trauma: A twisted ankle or direct blow disrupts the fragile Type II synchondrosis.
- Overuse: Repetitive traction from the Tibialis Posterior tendon (which inserts partly onto the ossicle).
- 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.
Anatomy
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.
Classification Systems
Geist Classification (1914)
- Type I (30-40%):
- True sesamoid bone within the Tibialis Posterior tendon.
- Small (2-3mm), round.
- Separated from navicular by greater than 3mm.
- Clinical: Usually asymptomatic.
- Type II (50-60%):
- Bi-partite navicular.
- Triangular or heart-shaped.
- Connected by fibrocartilage/hyaline cartilage (Synchondrosis).
- Distance less than 2mm.
- Clinical: Most common cause of pain.
- Type III (10-20%):
- Cornuate Navicular.
- Fusion of the ossicle (Synostosis).
- Prominent "beak".
- Clinical: Pain from shoe rub (bunion-like).
Clinical Assessment
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).
Investigations
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.
Management Algorithm
Conservative Management (First Line)
Duration: Minimum 3-6 months. Components:
- Activity Modification: Avoid aggravating sports (jumping, ballet) temporarily.
- Shoewear: Wide toe box to reduce compression on the medial bump.
- Orthotics:
- Medial Arch Support: Offloads the Tibialis Posterior tendon, reducing traction on the synchondrosis.
- Heel Cup: Stabilizes the hindfoot.
- Immobilization:
- Short leg walking cast or CAM boot for 4 weeks.
- Indications: Acute flare-up or "fracture" of the synchondrosis.
Surgical Technique: The Kidner Procedure
The Kidner Procedure (Excision & Advancement)
1. Approach:
- Curvilinear incision 3-4cm centered over the medial prominence.
- Avoid the saphenous nerve and vein (dorsal/anterior).
2. Deep Dissection:
- Incise the Tibialis Posterior tendon sheath.
- You will see the tendon broadening to insert onto the accessory bone.
- Pearl: Make a "T" or "I" shaped incision in the periosteum/tendon to preserve flaps for repair.
3. Excision:
- Shell out the ossicle (Type I/II).
- Identify the synchondrosis line.
- Use an osteotome or saw to remove the prominence flush with the cuneiform.
- Check: Palpate for any remaining spur. It must be smooth.
4. Repair:
- The tendon insertion is now loose.
- Use a Suture Anchor (SwiveLock or Corkscrew) placed in the plantar-medial aspect of the navicular body.
- Advance the tendon and suture it down under tension with the foot in inversion.
5. Closure:
- Repair the tendon sheath.
- Skin closure.
(Historical Note: The original Kidner procedure involved transposing the tendon to the plantar surface. Modern modification is just simple advancement).
Alternative: Fusion
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.
Complications
- Rate
- 5-10%
- Prevention/Management
- Usually due to inadequate resection (leaving a stump) or neuroma.
- Rate
- Common
- Prevention/Management
- Injury to medial cutaneous nerve. Bury nerve end if cut.
- Rate
- Rare
- Prevention/Management
- Failure to reattach tendon securely.
- Rate
- Common
- Prevention/Management
- Desensitization massage.
- Rate
- Very Rare
- Prevention/Management
- Over-aggressive trimming of tendon.
Postoperative Care
- 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.
Outcomes/Prognosis
- 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.
Evidence Base
- Original description of excision of the accessory navicular with re-routing of the tibialis posterior tendon
- Proposed the (now disputed) concept that the ossicle disadvantages the tendon and contributes to flatfoot
- Modern practice has simplified this to excision plus tendon advancement/repair
- Systematic review comparing simple excision with the Kidner procedure for symptomatic accessory navicular
- Both procedures were efficacious with low complication profiles and high patient satisfaction; minor complications were scar and wound irritation
- Recurrent pain after Kidner was concentrated in feet with planovalgus / hindfoot valgus, sometimes requiring revision
- 79 patients with accessory navicular syndrome; only 6.9% of athletes improved with conservative care vs 34% of non-athletes (p less than 0.001)
- Bone marrow oedema in both navicular and accessory bone was seen in 100% of athletes vs 80% of non-athletes
- Trauma history and inter-ossicle movement were more frequent in athletes
- Edema-like bone marrow pattern was present in all symptomatic navicular tuberosities and absent in asymptomatic ones
- Oedema intensity diminished as symptoms resolved, suggesting it tracks the active pain generator
- The pattern occurred even in patients without an accessory bone, implying an osteitis-like mechanism
- 16 patients (20 feet) with symptomatic Type II accessory navicular; Kidner vs local arthrodesis
- Good-to-excellent outcomes were comparable: 80% (Kidner) vs 83% (arthrodesis), p=0.696
- Calcaneal pitch improved in both groups with no significant difference in radiographic indices
- 21 patients revised for recurrent pain after Kidner; 20/21 (95%) had valgus heel alignment
- Contralateral feet showed similar deformity, indicating a pre-existing alignment problem rather than surgical error
- Adding a medial displacement calcaneal osteotomy improved radiographic and clinical outcomes
- 22 feet in 15 adolescents treated with suture-anchor osteosynthesis instead of excision
- Radiographic union in only 64% by 8 weeks, yet clinical scores rose from 54 to 96
- Bony union was modest but symptom relief was reliable in the short to mid term
Viva Scenarios
Practise clinical reasoning and management decisions out loud
“14-year-old female dancer. Medial foot pain. Visible lump. Failed physio.”
This is likely a symptomatic Type II Accessory Navicular. Dance (en Pointe) puts massive stress on the Tib Post. The differential is a stress fracture. I would order an external oblique X-ray. If confirmed, and she has failed conservative care (activity mod, orthotics), I would offer a Kidner Procedure.
“Tennis player. Lateral lunge. Felt a 'pop' medial foot. Now painful lump.”
This sounds like an acute disruption of a Type II synchondrosis. It's essentially a fracture through the cartilage bridge. I would treat this like a fracture: **Cast immobilization** for 4-6 weeks to allow it to heal (fibrous union). If pain persists after healing, then excision.
“30-year-old. Had surgery as a child. Now pain has returned in same spot.”
Recurrent pain could be due to: 1. **Regrowth** (incomplete excision), 2. **Neuroma** (medial cutaneous nerve), 3. **Hardware irritation** (if anchor used), or 4. **Tib Post degeneration**. I would examine for a tinel's sign (neuroma) and get an MRI to assess the tendon and bone remnant."
MCQ Practice Points
Q: Which type of accessory navicular is most commonly symptomatic? A: Type II. The movement at the synchondrosis causes pain. Type I is too small, Type III is fused.
Q: What is the primary purpose of the Kidner procedure? A: Pain relief. It is NOT primarily an arch-reconstruction procedure, although tendon advancement is performed.
Q: Which X-ray view best visualizes the accessory navicular? A: External Oblique (45 degree). It projects the ossicle away from the other tarsal bones.
Q: What sensory nerve is at risk during the medial approach to the navicular? A: Medial Cutaneous Nerve (branch of Saphenous).
Q: Which tendon inserts onto the accessory navicular? A: Tibialis Posterior. In Type II/III, the insertion is often broad-based and inserts onto the ossicle, losing its mechanical advantage.
Guidelines, Registries & Global Practice
Global epidemiology
- Accessory navicular is present in roughly 10-14% of the general population and is bilateral in 50-90% of cases, with a female predominance.
- The great majority are asymptomatic incidental findings; symptomatic accessory navicular syndrome clusters in adolescents and young adults, especially dancers, gymnasts and pivoting athletes.
Society / guideline positions (side by side)
- Position on management
- Conservative care first (rest, orthoses, immobilisation); surgery (excision or modified Kidner) for refractory pain. No dedicated standalone guideline.
- Position on management
- Same conservative-first pathway; foot and ankle surgeon referral after failed non-operative care; address coexisting planovalgus.
- Position on management
- Frames acute synchondrosis disruption as a stress-injury/non-union; immobilise acutely, excise if persistently symptomatic.
- Position on management
- Endorses excision or Kidner with equivalent results; recommends correcting hindfoot valgus to reduce recurrence.
Registry note
- No arthroplasty or implant registry tracks this condition (it is non-prosthetic). Evidence is limited to retrospective series and one systematic review; there are no randomised trials comparing excision with Kidner.
Resource-setting variation
- High-resource settings use weight-bearing radiographs plus MRI to confirm a symptomatic synchondrosis and suture anchors for tendon reattachment.
- Limited-resource settings rely on clinical examination and plain external-oblique radiographs, with transosseous suture (drill-hole) tendon repair as a low-cost, equally effective alternative to anchors.
- First-contact management is frequently led by physiotherapy/podiatry worldwide, with orthopaedic referral reserved for failed conservative care.
Deep Dive: Histopathology
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.
Differential Diagnosis: Tarsal Coalition
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.
Deep Dive: Biomechanics
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.
Case Study: The Dancer
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:
- Phase 1 (Acute): Moon boot for 4 weeks. No dancing. Ice. NSAIDs.
- Phase 2 (Rehab):
- Pain resolved after boot.
- Started progressive loading.
- Custom orthotic with "navicular accommodation" (sweet spot cutout).
- Outcome: returned to dance at 3 months. Flare ups managed with taping. Surgery avoided.
Detailed Rehabilitation Protocol
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.
Clinical Controversies: Fusion vs Excision
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.
Long Term Outcomes
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.
Self-Assessment Quiz
The primary cause of pain in a Type II Accessory Navicular is:
Disruption or micro-motion at the synchondrosis (cartilage bridge).
Which nerve is most at risk during the surgical approach?
The medial cutaneous nerve (branch of Saphenous). It runs dorsal to the navicular.
What is the approximate prevalence of an accessory navicular in the general population?
10-14%. It is very common, but usually asymptomatic.
A patient fails Kidner procedure and still has pain. What are 3 causes?
- Inadequate resection (Regrowth/Stump)
- Neuroma formation
- Missed Tarsal Coalition
True or False: The Kidner procedure restores the medial longitudinal arch height.
False. It relieves pain but does not significantly change radiographic arch parameters.
Parent's Guide: Frequently Asked Questions
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.
Deep Dive: Advanced Surgical Pearls
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.