The Accessory Navicular Bone: Anatomy, Implications, and Management

The accessory navicular, also known as os naviculare accessorium, is a congenital anatomical variant characterized by an extra bone or cartilage located on the medial side of the foot, adjacent to the navicular bone. While often asymptomatic, this condition can cause significant discomfort and functional limitations in some individuals, particularly those engaged in high-impact activities like running or sports. This essay explores the anatomy, prevalence, clinical significance, diagnosis, and management of the accessory navicular, shedding light on its implications for foot health and mobility.

Anatomy of the Accessory Navicular

The navicular bone is a critical component of the foot’s medial longitudinal arch, situated between the talus and the cuneiform bones in the midfoot. It plays a key role in weight-bearing and facilitating smooth foot motion during gait. The accessory navicular is an additional ossicle or cartilaginous structure that develops on the medial aspect of the navicular, near the insertion of the posterior tibial tendon. This tendon is vital for maintaining the foot’s arch and stabilizing the ankle during movement.

There are three main types of accessory navicular, classified based on their morphology and relationship to the main navicular bone:

1. Type I: A small, sesamoid-like ossicle embedded within the posterior tibial tendon, typically 2-3 mm in size. It is usually asymptomatic and not directly attached to the navicular.

2. Type II: A larger, triangular or heart-shaped ossicle connected to the navicular by a cartilaginous bridge (synchondrosis). This type is the most common and often associated with symptomatic cases due to stress at the synchondrosis.

3. Type III: A fused accessory navicular, also known as a cornuate navicular, where the extra bone is fully integrated with the main navicular, forming a prominent bony projection.

The accessory navicular arises during embryonic development due to incomplete fusion of ossification centers. While the exact cause is not fully understood, genetic factors are believed to play a role, as the condition often runs in families. It is typically bilateral, affecting both feet in approximately 50-90% of cases.

Prevalence and Demographics

The accessory navicular is relatively common, with an estimated prevalence of 4-21% in the general population, though some studies suggest it may be higher due to asymptomatic cases going undetected. It is more frequently observed in females than males, possibly due to differences in foot anatomy or hormonal influences on connective tissue. The condition is often identified in adolescence or early adulthood, as physical activity increases and symptoms, if present, become more apparent. Athletes, particularly those involved in running, dancing, or sports requiring repetitive foot motion, are more likely to experience symptoms due to the increased stress on the posterior tibial tendon and surrounding structures.

Clinical Significance and Symptoms

While many individuals with an accessory navicular remain asymptomatic, others experience pain and functional limitations, particularly in Type II cases. The condition can lead to a range of symptoms, including:

• Medial Foot Pain: Discomfort or tenderness on the inner side of the foot, near the arch, often exacerbated by weight-bearing activities or tight footwear.

• Swelling and Inflammation: Irritation of the posterior tibial tendon or surrounding soft tissues, sometimes accompanied by redness or warmth.

• Flatfoot Deformity: The accessory navicular can weaken the posterior tibial tendon, contributing to a collapse of the medial arch and the development of flatfoot (pes planus).

• Limited Mobility: Stiffness or reduced range of motion in the midfoot, particularly during activities requiring plantarflexion or inversion.

Symptomatic cases are often linked to biomechanical stress at the synchondrosis (in Type II) or irritation of the posterior tibial tendon. Repetitive microtrauma from activities like running can exacerbate these issues, leading to conditions such as posterior tibial tendon dysfunction (PTTD) or tendonitis. In severe cases, the accessory navicular may contribute to chronic pain or even stress fractures in the navicular bone.

Diagnosis

Diagnosing an accessory navicular typically begins with a thorough clinical evaluation. A healthcare provider will assess the patient’s medical history, symptoms, and physical activity level. Physical examination may reveal tenderness over the medial midfoot, a visible bony prominence, or signs of flatfoot. The clinician may also evaluate gait and foot alignment to assess biomechanical abnormalities.

Imaging plays a crucial role in confirming the diagnosis and classifying the type of accessory navicular. Common imaging modalities include:

• X-rays: Standard radiographs, particularly weight-bearing views, can identify the accessory navicular and its relationship to the main navicular bone. X-rays also help assess for associated conditions like flatfoot or navicular stress fractures.

• Magnetic Resonance Imaging (MRI): MRI is used to evaluate soft tissue structures, such as the posterior tibial tendon, for signs of inflammation, tearing, or degeneration. It can also detect bone marrow edema, indicating stress at the synchondrosis.

• Computed Tomography (CT): CT scans provide detailed images of the bone structure, useful for planning surgical interventions in complex cases.

Differential diagnoses include other causes of medial foot pain, such as navicular stress fractures, tarsal coalition, or tendonitis unrelated to the accessory navicular. Accurate diagnosis is essential to guide appropriate treatment.

Management and Treatment

The management of an accessory navicular depends on the severity of symptoms and the patient’s activity level. Treatment strategies are broadly categorized into conservative (non-surgical) and surgical approaches.

Conservative Management

For asymptomatic or mildly symptomatic cases, conservative treatments are typically effective. These include:

• Rest and Activity Modification: Reducing high-impact activities, such as running or jumping, can alleviate stress on the posterior tibial tendon and allow inflammation to subside.

• Orthotics and Supportive Footwear: Custom or over-the-counter arch supports can stabilize the foot, reduce pronation, and offload pressure from the accessory navicular. Shoes with a wide toe box and good arch support are recommended to minimize irritation.

• Physical Therapy: Exercises to strengthen the posterior tibial tendon and surrounding muscles can improve foot stability and reduce symptoms. Stretching routines may also enhance flexibility and alleviate tightness.

• Anti-inflammatory Measures: Nonsteroidal anti-inflammatory drugs (NSAIDs), ice therapy, or corticosteroid injections may be used to manage pain and inflammation in acute cases.

• Immobilization: In cases of significant pain or tendonitis, a short period of immobilization with a walking boot or cast may be prescribed to rest the foot and promote healing.

Surgical Management

Surgical intervention is considered when conservative treatments fail, typically after 3-6 months of persistent symptoms. The most common procedure is the Kidner procedure, which involves removing the accessory navicular and reattaching the posterior tibial tendon to the main navicular bone. In some cases, additional procedures, such as medial arch reconstruction or tendon repair, may be performed to address associated flatfoot or PTTD.

Surgical outcomes are generally favorable, with most patients experiencing significant pain relief and improved function. However, recovery can take 3-6 months, requiring immobilization, physical therapy, and gradual return to activity. Complications, such as infection or tendon weakness, are rare but possible.

Implications for Athletes and Active Individuals

For runners and athletes, the accessory navicular can pose unique challenges. The repetitive stress of running exacerbates symptoms, particularly in those with Type II accessory naviculars, where the synchondrosis is prone to microtrauma. Proper footwear, including shoes with advanced cushioning and stability features, is critical for managing symptoms. Brands like Brooks and Asics offer models with enhanced arch support and rocker soles that can benefit runners with this condition. Additionally, athletes may need to adjust training regimens, incorporating cross-training or low-impact activities to reduce stress on the foot.

Future Directions and Research

Ongoing research into the accessory navicular focuses on improving diagnostic accuracy and treatment outcomes. Advances in imaging, such as 3D ultrasound or high-resolution MRI, may enhance the ability to assess soft tissue and bone interactions. Additionally, biomechanical studies are exploring how orthotics and footwear can be optimized for individuals with accessory naviculars. Gene therapy and tissue engineering hold potential for addressing congenital anomalies like the accessory navicular, though these are still in early stages.

Craig Payne is a University lecturer, runner, cynic, researcher, skeptic, forum admin, woo basher, clinician, rabble-rouser, blogger and a dad.