What Is an Accessory Navicular Bone?

Medically Reviewed by Jabeen Begum, MD on October 06, 2022
5 min read

The accessory navicular is an extra bone growth usually found on the inside part of the foot, near the navicular bone. It’s not part of a typical bone structure in humans, and therefore, not many people may have it. Since it’s an extra bone taking up space in the foot, it can sometimes be painful. 

We will look at some of the causes and symptoms of this condition and how it’s diagnosed and treated.

The accessory navicular bone is a surplus piece of cartilage or bone fragment. It usually forms in the inner part of the foot, right above the arch. It’s called the accessory navicular since it’s found near the navicular bone, which runs across the foot. This additional bone typically forms between the navicular bone and the posterior tibial tendon (one of the tendons that connects the calf muscles to the ankle).

Research indicates that roughly 2.5% of the population has this condition, but it goes unnoticed during early childhood. When a child approaches adolescence, though, the accessory navicular begins to calcify (harden). This causes a bump to form on the inner part of the foot, which makes it more noticeable.

Although the accessory navicular bone is a tiny part, its distinct location in the foot and its effect on a person’s gait makes it a significant impediment. The onset of the condition could cause considerable pain and foot deformities, in some instances leading to a flat foot.

The accessory navicular bone is congenital (found from birth). It usually occurs due to a variation in the development of the bone centers near the navicular bone. 

Research suggests that the condition could have a genetic basis. Additionally, some studies indicate that up to 50% of individuals with this condition have bilateral accessory naviculars (extra growth in both feet).

In many cases, the condition is incorrectly diagnosed when people report pain in their feet, and it is commonly confused with an ankle sprain. 

The posterior tibial tendon is a major tendon that connects the calf muscle to the navicular bone. Its primary function is to support foot and ankle movement. The tendon meets the foot in multiple points, with the most prominent point of insertion being the navicular bone. Some advanced incidences of this condition could damage the posterior tibial tendon.

Accessory navicular syndrome is grouped into three types depending on the growth’s size and location.

  • Type 1. This leads to the formation of a small bone-like structure (measuring around 2 to 3 millimeters) within the part of the posterior tibial tendon that meets the navicular bone. This occurs in roughly 30% of accessory navicular syndrome incidences.
  • Type 2. In this type, the extra growth occurs within the part of the navicular bone that meets the posterior tibial tendon. It usually measures around 12 millimeters in size and accounts for around 50% of all accessory navicular syndrome incidences.
  • Type 3. In this type of accessory navicular syndrome, the bone usually projects outward on the same side as the arch of the foot.

People with accessory navicular syndrome often report a flat foot. According to the American Academy of Orthopaedic Surgeons, a flat foot is a condition in which the foot’s arch lies flat on the inner side and the foot points outwards. This places strain on the posterior tibial tendon and leads to inflammation of the accessory navicular.

Accessory navicular symptoms usually arise during adolescence as bones mature and cartilage evolves into bones. Sometimes, though, symptoms don’t appear until adulthood. 

Some of the most common symptoms of this condition include:

  • A noticeable bony projection in the middle section of the foot (on the inner part, just above the arch)
  • Swelling accompanied by redness
  • Pain in the middle section of the foot and in the arch (typically occurring during or after completing any physical activity)

A foot and ankle surgeon will usually physically examine the affected part of the foot. This specialist may also ask about symptoms and evaluate the posterior tibial tendon to check if there are signs of tenderness in the area. The specialist will also check for possible misalignment in the foot and the ankle that could affect your gait. 

In some cases, you may be asked to undergo an x-ray or a magnetic resonance imaging (MRI) scan to confirm the diagnosis. An MRI detects possible inflammation in the navicular bone and the posterior tibial tendon. It also locates any tears in the posterior tibial tendon. 

A triple-phase bone scan is another highly sensitive test used to diagnose this condition.

Accessory navicular syndrome can be treated using surgical and nonsurgical methods. Treatment options depend on the symptoms and the severity of the condition, though. Nonsurgical treatment typically aims to relieve symptoms. Some of the nonsurgical treatments include:

  • Rest. Nonoperative treatments are the first course of action to treat this condition. This includes reducing or completely stopping movements and activities that you may otherwise engage in, such as sports, which will aid the healing process.
  • Medicines. Your doctor may prescribe non-steroidal oral anti-inflammatory medicines like ibuprofen to reduce the pain. Your doctor may also sometimes prescribe steroids if the situation calls for it.
  • Physical therapy. Your doctor may recommend physical therapy to strengthen the muscles around the affected region and reduce inflammation.
  • Insoles. Your doctor may recommend wearing customized insoles that reduce the pressure on the navicular bone. These insoles can be placed inside your shoes.
  • Cast. Sometimes, a plaster cast or a boot may help you walk without pain, aiding healing by lowering inflammation.
  • Ice treatment. This treatment involves placing an ice pack on the affected area to reduce swelling and pain.

Although nonsurgical treatments resolve many cases of accessory navicular syndrome, they can sometimes reappear. In such cases, nonsurgical treatments are repeated. Doctors will only explore surgical options if nonsurgical treatment methods prove unsuccessful in relieving symptoms. These surgeries may involve removing the extra bone or repairing the posterior tibial tendon to help it function correctly.

A foot and ankle surgeon usually performs the surgery.