The human foot is a marvel of anatomical engineering, composed of 26 bones, 33 joints, and over 100 muscles, tendons, and ligaments. Among these bones lies the navicular, a boat-shaped bone in the midfoot that plays a crucial role in maintaining the arch and facilitating movement. However, in some individuals, an extra bone—known as the accessory navicular—appears adjacent to the navicular. Though often asymptomatic, this anatomical variation can sometimes lead to discomfort and functional issues, a condition known as Accessory Navicular Syndrome.
This essay explores the anatomy, types, causes, symptoms, diagnosis, and treatment of the accessory navicular, shedding light on a condition that affects a small but significant portion of the population.
Anatomy and Definition
The accessory navicular, also called os tibiale externum or os naviculare secundarium, is an extra bone or piece of cartilage located on the inner side of the foot, just above the arch. It is typically embedded within the posterior tibial tendon, which is responsible for supporting the arch and stabilizing the foot during movement.
This bone is congenital, meaning it is present from birth. It is not part of the standard skeletal structure and is found in approximately 2.5% to 10% of the population
. Most individuals with an accessory navicular are unaware of its presence unless it becomes symptomatic.
Types of Accessory Navicular
There are three distinct types of accessory navicular bones, each with unique anatomical characteristics and clinical implications:
| Type | Description | Prevalence | Clinical Notes |
|---|---|---|---|
| Type 1 | Small, round or oval sesamoid bone within the posterior tibial tendon; not connected to the navicular | ~30% | Usually asymptomatic |
| Type 2 | Heart-shaped or triangular bone (~12mm), connected to the navicular by cartilage | ~55% | Most commonly symptomatic |
| Type 3 | Fused to the navicular via a bony bridge, forming a horn-shaped structure | ~15% | May cause structural issues |
Type 2 and Type 3 are more likely to cause Accessory Navicular Syndrome, especially when subjected to trauma or repetitive stress.
Causes of Accessory Navicular Syndrome
While many people with an accessory navicular experience no symptoms, certain factors can trigger Accessory Navicular Syndrome, a painful condition resulting from irritation or inflammation of the bone and surrounding tissues.
Common Causes:
- Trauma: Sprains, falls, or direct impact to the foot can aggravate the accessory navicular.
- Overuse: Excessive physical activity, especially in athletes, can strain the posterior tibial tendon.
- Footwear: Poorly fitting shoes or those lacking arch support can rub against the bone.
- Flat Feet (Pes Planus): Individuals with flat feet place more stress on the posterior tibial tendon, increasing the risk of inflammation.
Signs and Symptoms
Symptoms of accessory navicular syndrome often emerge during adolescence, when the cartilage begins to ossify into bone. However, symptoms can also appear in adulthood, especially following injury or increased activity.
Typical Symptoms:
- Visible bump on the inner side of the foot, above the arch
- Redness and swelling around the bony prominence
- Pain or throbbing in the midfoot, especially during or after activity
- Tenderness when pressure is applied to the area
- Difficulty walking or wearing shoes due to discomfort
These symptoms can range from mild to severe and may interfere with daily activities or athletic performance.
Diagnosis
Diagnosing accessory navicular syndrome involves a combination of clinical examination and imaging studies.
Diagnostic Steps:
- Physical Exam: The physician assesses the foot for swelling, tenderness, and structural abnormalities.
- X-rays: Used to confirm the presence and type of accessory navicular.
- MRI or CT Scan: May be ordered to evaluate soft tissue involvement, especially the posterior tibial tendon.
A thorough assessment also includes evaluating foot alignment, muscle strength, and gait mechanics, as these factors influence treatment decisions.
Treatment Options
Treatment for accessory navicular syndrome aims to relieve pain, reduce inflammation, and restore function. Most cases respond well to conservative management, though surgery may be necessary in persistent or severe cases.
Conservative Treatments:
- Immobilization: Using a cast or walking boot to rest the foot and reduce inflammation.
- Ice Therapy: Applying ice packs to the affected area to control swelling.
- NSAIDs: Medications like ibuprofen to alleviate pain and inflammation.
- Physical Therapy: Strengthening exercises and stretches to support the arch and improve tendon function.
- Orthotics: Custom arch supports or medial heel wedges to reduce pressure on the accessory navicular.
Surgical Intervention:
Surgery is considered when conservative treatments fail or symptoms recur frequently. The most common procedure is Kidner’s Procedure, which involves:
- Removing the accessory navicular
- Reattaching the posterior tibial tendon to the navicular bone
Post-surgical recovery typically includes immobilization, followed by physical therapy to regain strength and mobility.
Impact on Lifestyle and Activity
For athletes and active individuals, accessory navicular syndrome can be particularly disruptive. Pain and instability may limit participation in sports, especially those involving running, jumping, or lateral movements. Early diagnosis and appropriate treatment are essential to prevent long-term complications and maintain an active lifestyle.
With proper care, most individuals can return to normal activities without significant limitations. However, ongoing use of orthotics and attention to footwear may be necessary to prevent recurrence.
Psychological and Social Considerations
Chronic foot pain can affect more than just physical health—it may also impact mental well-being, self-esteem, and social participation, especially in adolescents. The visibility of the bony prominence and limitations in footwear choices can lead to self-consciousness. Supportive care, education, and reassurance are important components of holistic treatment.
Genetic and Developmental Aspects
The exact cause of accessory navicular formation is unknown, but there is evidence of a genetic predisposition. It is often bilateral (present in both feet) and may be inherited. Understanding the developmental timeline—from cartilage in infancy to ossified bone in adolescence—helps explain why symptoms often emerge during growth spurts.