The accessory navicular bone (ANB) is a common congenital variation found in the foot of approximately 10% to 14% of the population. This extra bone tissue is present from birth, representing an anatomical difference rather than an acquired injury or disease. For most individuals, the presence of the ANB remains unknown and causes no issues throughout their lives. However, for a smaller group, this variation can lead to pain and inflammation, a condition known as Accessory Navicular Syndrome.
What Exactly Is the Accessory Navicular Bone?
The accessory navicular is an extra bone or piece of cartilage located on the medial side of the foot, positioned just above the arch. It sits next to the navicular bone, which is one of the tarsal bones in the midfoot that helps form the arch. The location of this extra bone places it directly at the point where the posterior tibialis tendon attaches to the foot.
The accessory navicular is categorized into three types based on its structure and connection to the main navicular bone. Type 1 is a small, round sesamoid bone embedded entirely within the posterior tibialis tendon, lacking any bony connection to the navicular. Type 2 is a larger, typically triangular piece connected to the main navicular bone by a layer of fibrocartilage or hyaline cartilage (a synchondrosis). Type 3, also called a cornuate navicular, represents a full fusion with the navicular, resulting in an enlarged, prominent tuberosity.
The Developmental Origin of the Accessory Navicular Bone
The accessory navicular bone is a congenital variation, occurring during development rather than due to later injury. The navicular bone develops from a primary center of ossification, turning from cartilage into bone around ages three to five. Separately, a secondary ossification center forms at the prominent tuberosity on the inside of the foot.
The accessory navicular bone is formed when this secondary ossification center fails to fuse with the main body of the navicular bone during childhood. This process usually begins around eight to twelve years of age, a period of rapid skeletal maturation. Instead of uniting into a single, seamless bone, the secondary center remains separate, connected either by a fibrous cord, cartilage, or remaining completely detached depending on the type.
This failure of fusion is the direct cause of the accessory navicular bone, resulting in an additional bony structure that may possess a genetic component. The bone itself does not change or grow larger in adulthood. This leaves a separate ossicle behind due to the body’s inability to complete the typical bone-forming process at this specific location.
Accessory Navicular Syndrome and Its Symptoms
The mere presence of the accessory navicular bone rarely causes pain; discomfort arises from Accessory Navicular Syndrome. This syndrome occurs when the bone or surrounding soft tissues become irritated or inflamed. The mechanisms of pain are often related to the accessory bone’s location at the insertion of the powerful posterior tibialis tendon.
In Type 2 accessory navicular, the most common symptomatic type, the constant pull of the posterior tibialis tendon across the cartilaginous connection (synchondrosis) can cause chronic inflammation. Trauma, repetitive stress from activities like running, or pressure from tight footwear can aggravate this connection. This chronic irritation of the fibrous tissue or the tendon is the primary reason the condition becomes painful, often manifesting during adolescence.
Common symptoms include a visible, tender bony prominence on the inside of the foot, just above the arch. Patients often experience pain and localized swelling in this area, particularly during or after weight-bearing activities. The discomfort can be aggravated by shoes that rub against the prominence, and the chronic strain on the posterior tibialis tendon can sometimes contribute to a flatfoot appearance.
Identifying and Managing the Condition
A medical professional can identify the accessory navicular bone during a physical examination by noting the prominent bony lump and localized tenderness on the medial midfoot. X-rays are routinely ordered to confirm the diagnosis, as they clearly visualize the extra bone and allow classification of its specific type. Further imaging, such as an MRI, may be used to assess the degree of inflammation in the synchondrosis or the posterior tibialis tendon.
Initial management for Accessory Navicular Syndrome is typically conservative and aims to reduce inflammation and pain. Non-surgical options include rest, applying ice to the area, and using nonsteroidal anti-inflammatory drugs (NSAIDs). Immobilization with a cast or walking boot may be necessary to allow the acute inflammation to subside fully.
Long-term non-surgical management often involves the use of custom or over-the-counter orthotics to provide arch support and reduce the strain on the posterior tibialis tendon. If conservative treatments fail to provide lasting relief after several months, surgical intervention may be considered. Surgery usually involves removing the accessory bone and, if necessary, reattaching the posterior tibialis tendon to the main navicular bone in a procedure known as a Kidner procedure.