The accessory navicular, also known as os naviculare or os tibiale externum, represents one of the most clinically significant accessory bones of the foot. This anatomical variant occurs as an additional ossification center that develops in relation to the navicular bone, located on the medial aspect of the foot. While often asymptomatic, the accessory navicular can become a source of considerable pain and functional limitation, particularly in adolescents and young adults engaged in athletic activities. Understanding its embryological development, anatomical variations, clinical presentation, and management options is crucial for healthcare providers who encounter patients with medial foot pain.
Embryological Development and Anatomy
The accessory navicular develops from a separate ossification center that appears during fetal development, typically around the ninth week of gestation. Under normal circumstances, this accessory center should fuse with the main navicular bone during skeletal maturation. However, in approximately 4-14% of the population, this fusion fails to occur completely, resulting in the persistence of the accessory navicular as a distinct osseous structure.
The accessory navicular is positioned on the posteromedial aspect of the navicular bone, where the posterior tibial tendon inserts. This anatomical relationship is crucial to understanding the pathophysiology of associated symptoms. The accessory bone varies in size and can range from a small ossicle to a substantial bony prominence that significantly alters the medial foot contour. Its presence creates an enlarged insertion site for the posterior tibial tendon, which plays a vital role in maintaining the medial longitudinal arch of the foot.
Classification System
Geist and Cooperman developed a widely accepted classification system that categorizes accessory naviculars into three distinct types based on their radiographic appearance and anatomical characteristics:
Type I represents a small, oval-shaped ossicle located within the distal aspect of the posterior tibial tendon. This variant is typically asymptomatic and measures 2-3 millimeters in diameter. The ossicle maintains a fibrous connection to the main navicular bone and rarely causes clinical problems.
Type II is the most common and clinically significant variant, occurring in approximately 50-60% of cases with accessory naviculars. This type appears as a larger, triangular or heart-shaped bone that measures 9-12 millimeters in diameter. It maintains a synchondrotic connection with the navicular bone through fibrocartilage. The Type II accessory navicular creates a prominent medial foot contour and is most likely to become symptomatic.
Type III, also known as a prominent navicular tuberosity or cornuate navicular, results from the fusion of a large accessory navicular with the main navicular bone. This creates an enlarged navicular tuberosity that extends prominently on the medial aspect of the foot. While technically representing a healed Type II variant, it can still cause symptoms due to its size and prominence.
Clinical Presentation and Pathophysiology
The accessory navicular becomes symptomatic through several mechanisms. The most common presentation involves localized pain and tenderness over the prominent medial aspect of the foot, particularly at the navicular tuberosity. Patients often report a dull, aching pain that worsens with activity and improves with rest. The pain typically develops during adolescence when increased activity levels and athletic participation place greater demands on the posterior tibial tendon complex.
The pathophysiology of symptomatic accessory navicular syndrome involves mechanical and biomechanical factors. The abnormal bony prominence creates friction against footwear, leading to localized inflammation and pain. Additionally, the altered insertion of the posterior tibial tendon can result in inefficient force transmission during push-off and weight-bearing activities. This mechanical disadvantage can contribute to posterior tibial tendon dysfunction and progressive flatfoot deformity in some cases.
Athletes, particularly those involved in running, jumping, and cutting sports, are at higher risk for developing symptoms. The repetitive stress placed on the posterior tibial tendon during these activities can lead to inflammation at the tendon-bone interface, resulting in what is essentially a chronic insertion tendinopathy.
Diagnostic Evaluation
The diagnosis of accessory navicular syndrome relies on a combination of clinical examination and imaging studies. Physical examination reveals tenderness and often visible prominence over the medial navicular region. Patients may demonstrate a positive “too many toes” sign when viewed from behind, indicating possible flatfoot deformity. Single heel rise testing can assess posterior tibial tendon function and may reveal weakness or inability to perform the maneuver.
Radiographic evaluation begins with standard weight-bearing foot radiographs, including anteroposterior, lateral, and oblique views. The accessory navicular is best visualized on the oblique and anteroposterior projections, where it appears as a distinct ossification center with smooth, rounded edges distinct from fracture fragments. The size, shape, and relationship to the main navicular bone can be assessed to determine the classification type.
Advanced imaging may be necessary in complex cases or when surgical intervention is being considered. Magnetic resonance imaging (MRI) can evaluate the posterior tibial tendon for signs of tendinopathy, partial tears, or dysfunction. It can also assess the fibrocartilaginous connection between the accessory bone and navicular, as well as identify associated soft tissue inflammation or marrow edema.
Treatment Approaches
Management of accessory navicular syndrome follows a stepwise approach, beginning with conservative measures and progressing to surgical intervention when necessary. The treatment strategy depends on symptom severity, functional limitations, and patient goals.
Conservative Management forms the foundation of initial treatment and is successful in resolving symptoms in approximately 90% of cases. Rest and activity modification help reduce inflammatory symptoms and allow acute pain to subside. Nonsteroidal anti-inflammatory drugs can provide symptomatic relief and reduce local inflammation.
Orthotic devices play a crucial role in conservative management by supporting the medial longitudinal arch and reducing stress on the posterior tibial tendon complex. Custom-molded arch supports or over-the-counter orthotic devices with good arch support can effectively redistribute forces and improve biomechanical function.
Physical therapy focuses on strengthening the posterior tibial muscle and other intrinsic foot muscles while improving flexibility and biomechanical function. Eccentric strengthening exercises have shown particular benefit in addressing tendon-related symptoms.
Surgical Management is reserved for patients who fail to respond to adequate conservative treatment after 3-6 months. Several surgical procedures have been described, with the choice depending on the accessory navicular type and associated pathology.
Simple excision of the accessory navicular with repair of the posterior tibial tendon insertion is the most commonly performed procedure for Type II variants. This approach removes the prominent bone while maintaining tendon function through secure reattachment to the remaining navicular bone.
The Kidner procedure involves excision of the accessory navicular combined with advancement and reattachment of the posterior tibial tendon to a more plantar position on the navicular. This technique is particularly useful when associated flatfoot deformity is present.
For Type III variants with large, prominent navicular tuberosities, partial excision with smoothing of the bony prominence may be sufficient to relieve symptoms while preserving tendon attachment.
Prognosis and Outcomes
The prognosis for accessory navicular syndrome is generally favorable with appropriate treatment. Conservative management successfully resolves symptoms in the majority of cases, particularly when implemented early in the course of symptoms. Patients who undergo surgical treatment typically experience good to excellent outcomes, with high rates of symptom relief and return to athletic activities.
Long-term outcomes depend on several factors, including the presence of associated flatfoot deformity, posterior tibial tendon dysfunction, and patient compliance with post-treatment recommendations. Regular follow-up and continued use of supportive orthotic devices may be necessary to prevent symptom recurrence.
The accessory navicular represents a common anatomical variant that can become a significant source of medial foot pain and dysfunction. Recognition of its clinical presentation, appropriate diagnostic evaluation, and implementation of evidence-based treatment strategies are essential for optimal patient outcomes. While conservative management remains the first-line approach with high success rates, surgical intervention provides an effective option for refractory cases. Understanding the complex relationship between the accessory navicular, posterior tibial tendon function, and foot biomechanics allows healthcare providers to develop comprehensive treatment plans that address both symptoms and underlying mechanical factors contributing to this condition.