The accessory navicular is an extra bone or piece of cartilage located on the inner (medial) side of the foot, just above the arch. It is not a part of the normal bone structure and represents a common anatomical variant involving the navicular bone.
Anatomical Overview
The navicular bone is a key structure in the midfoot, supporting the arch and serving as a critical attachment for the posterior tibialis tendon. An accessory navicular develops from a secondary ossification center of the navicular tuberosity that fails to unite with the main body of the navicular during early development. The tibialis posterior tendon inserts into the navicular tuberosity, and the presence of this extra bone can affect the tendon’s function, sometimes leading to arch malalignment or associated foot conditions.
Epidemiology and Genetic Factors
An accessory navicular is found in approximately 4–21% of the population, with a notably higher prevalence of up to 45% reported in some Asian populations. It is more commonly diagnosed in females and is often present bilaterally, with studies reporting a bilateral prevalence between 50–90%. Genetics play a role, and the feature is often associated with a flat (pes planus) foot posture, further complicating the biomechanical picture of affected individuals.
Classification of Accessory Navicular
The most widely accepted classification system is the Geist classification (1914), which organizes accessory navicular bones into three types according to morphology, position, and connection with the navicular:
| Type | Description | Location & Connection | Prevalence | Symptoms |
|---|---|---|---|---|
| I | Small, round sesamoid bone; os tibiale externum | Within posterior tibial tendon, no cartilaginous connection to navicular | ~30% | Usually asymptomatic |
| II | Large, triangular or heart-shaped bone | Connected by fibrocartilage or hyaline cartilage to navicular tuberosity | ~50-60% | Most likely to be symptomatic |
| III | Enlarged navicular tuberosity (cornuate navicular) | Osseous fusion; prominent bony enlargement | ~10-20% | Occasionally symptomatic |
Type II is the most common and most likely to produce symptoms, often precipitated by trauma, overuse, or footwear irritation that exacerbates local swelling or pain.
Clinical Presentation
Most individuals with an accessory navicular are asymptomatic, discovering the condition incidentally via imaging. However, some develop accessory navicular syndrome, particularly during adolescence or after an acute injury or repetitive stress:
- Medial foot pain: especially aggravated by walking, running, weight-bearing activities, or stairs.
- Swelling or palpable bony prominence: on the medial side of the foot, just below the ankle.
- Shoe irritation: increased friction from footwear over the protuberance can induce inflammation.
- Difficulty with physical activity: pain may cause limp or limited activity.
- Association with flat feet: the extra bone can compromise posterior tibial tendon function and arch integrity, leading to pes planus and increased strain.
Diagnosis
Diagnosis of accessory navicular relies on a thorough history, physical examination, and imaging:
- X-rays (Weightbearing): reveal the size, location, and type of accessory navicular present.
- MRI: used to assess inflammation, particularly in symptomatic Type II cases.
- Additional imaging: may be required for surgical planning or to exclude other foot pathologies.
Treatment Modalities
Non-Surgical Management
Non-surgical treatment is the first-line approach for symptomatic cases. Options include:
- Activity modification: reduce activities that exacerbate symptoms.
- Immobilization: moon-boot or cast may help with acute pain.
- Pain relief: NSAIDs, ice, corticosteroid injections.
- Orthotics: arch supports, shoe modifications, and padding to limit friction.
Surgical Management
When non-surgical measures fail or symptoms persist, surgery may be needed. The principal surgical options include:
- Excision: removal of the accessory navicular bone (modified Kidner procedure), especially effective for smaller Type I and unstable Type II bones.
- Fixation: removal of the fibrous synchondrosis and fixation of the accessory navicular to the main body of the navicular with a screw, especially in larger or persistent symptomatic Type II cases.
- Additional procedures: such as correction of associated flatfoot deformity, may be performed when indicated.
Prognosis and Complications
Most individuals with an accessory navicular do not experience long-term complications and remain asymptomatic. For those requiring intervention, proper treatment—whether conservative or surgical—frequently leads to resolution of symptoms and restoration of normal activity levels. Surgical complications are infrequent but may include persistent pain, nerve irritation, or recurrence in rare cases.
The accessory navicular bone is a common, yet often overlooked, anatomical variant in the foot. While usually asymptomatic, it can cause significant discomfort and functional impairment in some cases, particularly adolescents and those with flat feet. Clinical management is guided by symptom severity and the specific anatomical subtype, with most individuals responding well to conservative therapy and only a minority requiring surgery for persistent symptoms. Awareness and early intervention when accessory navicular syndrome is suspected are crucial for optimal outcomes.