Accessory Navicular Syndrome: Understanding the Condition

The accessory navicular bone (ANB) is a common anatomical variant in the human foot. It is often asymptomatic, but in some cases, it can lead to significant foot pain and dysfunction. This bone is part of a group of accessory bones that can develop in the foot, and while most of these bones do not cause problems, the accessory navicular can be associated with certain pathologies that are important to identify for proper diagnosis and treatment.

What is the Accessory Navicular Bone?

The accessory navicular is an extra bone or piece of cartilage located on the medial (inner) side of the foot, near the arch, just behind the navicular bone. It is closely associated with the posterior tibial tendon (PTT), which plays a crucial role in maintaining the arch of the foot. The presence of this additional bone is typically detected through imaging techniques like X-rays or MRI, often when the patient presents with chronic foot pain or after an injury.

Prevalence and Types of Accessory Navicular Bone

Accessory navicular bones are relatively common, with studies showing a prevalence rate ranging from 2% to 25% in the general population. In a study conducted at a university hospital, ANB was found in 23.3% of the patients who attended foot and ankle clinics, with a higher prevalence in females (67.5%) compared to males (32.5%). The bone can be classified into three types, each with distinct characteristics and potential for causing symptoms:

1. Type I: This is a small, round or oval-shaped bone that is completely separate from the navicular bone. It is usually asymptomatic and often found incidentally on radiographs.
2. Type II: This type is connected to the navicular bone via a thin fibrocartilaginous plate. It is more prone to becoming symptomatic, especially when subjected to mechanical stress or injury. It can sometimes be misdiagnosed as a fracture of the navicular tuberosity.
3. Type III: This type is fused with the navicular bone by a bony bridge, creating a cornuate (horn-shaped) appearance. Type III ANB is typically the largest and most likely to cause symptoms due to its anatomical fusion and its potential for altered foot mechanics.

Symptoms and Causes of Pain

Although most cases of accessory navicular bone are asymptomatic, some individuals may experience pain, particularly with Type II and Type III variants. The pain is usually localized to the medial side of the foot, near the arch, and may be exacerbated by activity or pressure from footwear. Pain can occur when the bone is subjected to repetitive stress or trauma, such as in athletes or individuals who engage in high-impact activities. It is often associated with mechanical irritation of the posterior tibial tendon, which attaches to the navicular bone.

In children, the pressure from shoes rubbing against the accessory bone can lead to discomfort, while in adults, trauma, such as a twisting injury, can precipitate pain. The mechanical forces acting on the accessory bone, such as tension, shear, and compression from the posterior tibial tendon and the subtalar joint, can lead to chronic inflammation and even stress fractures of the synchondrosis, the cartilage that connects the accessory bone to the navicular .

Diagnosing Accessory Navicular Syndrome

Diagnosis typically involves a thorough physical examination and imaging studies. The pain is often reproducible with direct palpation over the medial aspect of the foot, particularly over the accessory navicular. In some cases, additional imaging, such as MRI or bone scintigraphy, may be used to further assess the involvement of the posterior tibial tendon and the degree of inflammation in the affected area. Radiographs are usually sufficient for identifying the bone and determining the type and size of the accessory navicular.

Treatment Options for Accessory Navicular Syndrome

Treatment for symptomatic accessory navicular syndrome typically starts with conservative management. This may include:

• Shoe Modifications: Wearing shoes with a wide toe box or using cushioned insoles to reduce pressure on the affected area.
• NSAIDs (Nonsteroidal Anti-Inflammatory Drugs): To manage pain and reduce inflammation.
• Orthotics: Custom orthotic devices may be used to provide additional support to the arch and alleviate pressure on the accessory navicular.
• Physical Therapy: Targeted exercises to strengthen the posterior tibial tendon and improve foot mechanics can be beneficial, particularly if the tendon is weak or dysfunctional.
• Casting or Bracing: In cases of acute pain or injury, immobilization with a short leg cast or a removable fracture boot may be recommended to allow the area to heal.

If conservative measures fail and symptoms persist, surgical intervention may be considered. The most common surgical procedure for treating symptomatic accessory navicular is excision of the ossicle. In some cases, the posterior tibial tendon may also need to be rerouted to relieve tension and restore normal foot function. The Kidner procedure, which involves excision of the accessory bone and transfer of the posterior tibial tendon, has been widely used with good results.

Conclusion

Accessory navicular syndrome is a condition that should be considered in patients who present with chronic medial foot pain, particularly when associated with a history of repetitive stress or trauma. With a prevalence rate of up to 23% in the general population, it is important for healthcare providers to recognize this condition and differentiate it from other foot pathologies, such as fractures or arthritis. While most cases are asymptomatic, patients who experience pain can benefit from conservative treatments, and when necessary, surgical procedures such as the Kidner procedure can provide significant relief and restore foot function.