An accessory navicular bone is an extra piece of bone or cartilage located on the inner side of the foot, just above the arch. This congenital anatomical variation is present from birth and occurs in approximately 2% to 14% of the population. Although it is not part of the standard foot structure, it often remains unnoticed throughout a person’s life. When it causes discomfort, the resulting painful condition is called Accessory Navicular Syndrome.
Understanding the Anatomy and Types
The navicular bone is one of the seven tarsal bones that form the midfoot, positioned on the inside of the foot near the ankle. The accessory navicular bone, also known as the os naviculare or os tibiale externum, is located adjacent to the main navicular bone’s tuberosity, a bony bump on the inner foot. This accessory bone develops when the secondary center of ossification fails to completely fuse with the main navicular bone during childhood. The posterior tibial tendon, which supports the arch, attaches to the area where the accessory bone is found.
Accessory navicular bones are classified into three types based on their structure and connection to the main bone.
Type 1
The Type 1 accessory navicular is a small, round or oval bone, usually 2 to 3 millimeters, embedded entirely within the posterior tibial tendon. This type is generally asymptomatic and accounts for about a third of all cases.
Type 2
The most common variant is the Type 2 accessory navicular, a triangular ossicle connected to the navicular tuberosity by cartilage or fibrous tissue. This connection forms a pseudo-joint that can become irritated, making Type 2 the most frequently symptomatic classification.
Type 3
The Type 3 accessory navicular, often called a cornuate navicular, represents a complete fusion of the accessory bone to the navicular. This results in an enlarged, prominent bony bump that can still cause problems due to its size, though this type is less common.
Why Accessory Navicular Bone Pain Occurs
Accessory Navicular Syndrome occurs when the bone is subjected to chronic irritation or acute trauma. This often happens during adolescence as the foot bones mature and the accessory bone fully ossifies. Pain is frequently triggered by direct pressure from shoes, especially those with tight sides that rub against the bony prominence on the inner arch.
A primary cause of pain is the chronic strain placed on the posterior tibial tendon, which attaches directly to the accessory bone in Type 1 and Type 2 variants. Since this tendon maintains the arch of the foot, its function can be compromised when the accessory bone is present. Excessive activity, overuse, or a sprain can aggravate the tendon and the connection between the accessory bone and the navicular.
Many individuals who experience symptoms also have flat feet (pes planus deformity), which further stresses the posterior tibial tendon. A flat arch forces the tendon to work harder, leading to inflammation and irritation of the accessory bone and surrounding soft tissues. Symptoms include throbbing pain in the midfoot and arch, redness, and swelling over the visible bony prominence, often worse after activity.
Diagnosis and Identification
Diagnosis begins with a physical examination, checking for a visible bony prominence and evaluating the area for tenderness, redness, and swelling. Assessing the patient’s gait and foot structure, particularly checking for flatfoot, helps determine the condition’s impact. The doctor will press on the painful area to pinpoint the source of discomfort and evaluate the posterior tibial tendon’s function.
Standard X-rays are the initial and most effective imaging tool used to confirm the presence of an accessory navicular bone and identify its specific type. Specialized views, such as the external oblique view, provide a clearer image of the medial aspect of the foot where the extra bone resides. X-rays confirm the ossicle’s size and its relationship to the main navicular bone.
If the diagnosis remains unclear or soft tissue involvement is suspected, advanced imaging studies may be ordered. Magnetic Resonance Imaging (MRI) is useful for evaluating inflammation, as it can show bone marrow edema within the accessory bone, indicating a chronic stress injury. MRI is also valuable for assessing the posterior tibial tendon for signs of tendinosis, thickening, or a tear contributing to the pain.
Managing the Condition
Management of Accessory Navicular Syndrome prioritizes conservative, non-surgical approaches to reduce inflammation and relieve strain. Initial treatment involves rest, activity modification, and applying ice to the painful prominence to minimize swelling. Immobilization with a cast or removable walking boot may be necessary for a short period to allow inflamed tissues to recover.
Non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, manage pain and reduce inflammation. Supporting the foot arch is a foundational aspect of conservative care, often achieved using custom orthotic inserts. These devices reduce mechanical strain on the posterior tibial tendon, which alleviates irritation of the accessory bone.
If conservative treatments fail after several months, surgical intervention may be considered. The most common procedure is the Kidner procedure, which involves removing the accessory bone. During this operation, the surgeon detaches the posterior tibial tendon from the accessory bone and reattaches it to the main navicular bone, improving the tendon’s function in supporting the arch.
In some cases, simple excision of the accessory bone and remodeling of the remaining navicular tuberosity is performed, especially if the posterior tibial tendon is not significantly damaged. The decision to proceed with surgery depends on the severity of symptoms and the patient’s failure to respond to extended non-operative care. Surgical treatment is successful in relieving chronic pain, allowing patients to return to normal activities after recovery and rehabilitation.