An accessory navicular bone is an extra piece of bone that forms on the inner side of the foot, near the arch. It develops before birth as a normal variation in skeletal growth, present in roughly 4% to 21% of the population depending on ethnicity. Most people who have one never know it exists, but in some cases it becomes a source of chronic foot pain.
How the Extra Bone Forms
During fetal development, the bones of the foot form from separate centers of cartilage that gradually harden into bone and fuse together. The navicular bone, which sits near the top of the arch on the inner side of the foot, sometimes develops with an additional cartilage center that never fully merges with the main bone. This leftover piece hardens into its own small bone, the accessory navicular, and typically becomes visible on X-rays around ages 9 to 11.
The trait appears to be genetic, passed down through families in a pattern where carrying the gene doesn’t guarantee the bone will actually develop. This explains why it can seem to skip generations or show up unpredictably among siblings. There’s no significant difference in how often it occurs between males and females.
Three Types of Accessory Navicular
Not all accessory naviculars look or behave the same way. Orthopedic specialists classify them into three types based on how they relate to the main navicular bone:
- Type 1: A small, separate bone fragment embedded within the posterior tibial tendon. It floats freely and rarely causes problems.
- Type 2: A larger piece of bone connected to the navicular by a bridge of cartilage. This is the type most likely to become painful, because the cartilage connection can loosen and become irritated under stress.
- Type 3: The accessory bone has fully fused with the navicular, creating an enlarged, horn-shaped navicular. This can create a prominent bump but often remains painless.
Type 2 is the source of most clinical symptoms. The cartilage bridge between the accessory bone and the navicular acts as a weak point. Repetitive force, a sudden injury, or even normal growth during adolescence can cause micro-fractures and inflammation at that junction.
Why It Becomes Painful
Having the bone itself isn’t the problem. Pain develops when mechanical forces overwhelm the connection between the accessory bone and the surrounding structures. Several specific triggers can set this off.
The most direct cause is irritation at the cartilage bridge in Type 2 bones. Tension, shear, and compressive forces travel through the posterior tibial tendon to the junction between the accessory bone and the navicular. Over time, this can produce micro-fractures and chronic inflammation, particularly during childhood and early adulthood when activity levels are high and the skeleton is still maturing.
Shoe pressure is another common trigger. The accessory navicular creates a bony bump on the inner side of the foot that rubs against footwear, especially stiff or narrow shoes. This external friction causes local pain, redness, and swelling over the bump itself.
Overuse plays a significant role too. Prolonged standing, excessive walking, and repetitive athletic activities can all push the accessory bone past its tolerance. Being overweight adds to the load on the inner foot, increasing the strain on an already vulnerable area.
The Connection to Flat Feet
A symptomatic accessory navicular frequently appears alongside flat feet, and the relationship between the two isn’t coincidental. The posterior tibial tendon, one of the key structures responsible for maintaining the foot’s arch, attaches to the navicular bone. When an accessory navicular is present, a significant portion of that tendon inserts into the extra bone instead of the main navicular.
This misplaced attachment changes the angle at which the tendon pulls on the foot. In the 1930s, orthopedic surgeon David Kidner proposed that the accessory navicular alters the tendon’s line of force, weakening its ability to support the arch. The result is an imbalance that allows the foot to roll inward excessively, a motion called hyperpronation. Over time, this can functionally lengthen and weaken the tendon, compounding the problem. It’s a feedback loop: the accessory bone weakens the tendon’s mechanical advantage, which flattens the arch, which puts more strain on the accessory bone.
What Triggers Symptoms in Adolescents
Symptoms most commonly surface during the teenage years, which makes sense given the bone’s developmental timeline. The accessory navicular appears on imaging around ages 9 to 11, and the years that follow bring a combination of rapid growth, increased body weight, and higher athletic demands. All of these factors load stress onto a connection that may already be fragile.
A specific injury can also spark symptoms in someone who previously had no trouble. An ankle sprain, a direct blow to the inner foot, or a sudden increase in training volume can disrupt the cartilage bridge or inflame the tendon attachment. Once the area becomes irritated, even normal walking can perpetuate the cycle of pain and swelling.
How It’s Managed
Most people with a painful accessory navicular improve without surgery. The initial approach focuses on reducing the mechanical stress that’s driving the irritation. This typically means wearing shoes with a wider toe box and less pressure on the inner foot, using arch-supporting insoles or custom orthotics to redistribute force away from the navicular, and temporarily limiting activities that worsen the pain. A short period of immobilization in a walking boot or cast can calm down an acute flare.
When conservative measures fail after several months, surgery becomes an option. The most common procedure involves removing the accessory bone and reattaching the posterior tibial tendon directly to the main navicular in a stronger position. Recovery involves a gradual return to walking, then jogging, then full athletic activity. Research on patients who had this procedure performed on both feet simultaneously showed no difference in recovery timeline compared to those who had one side done at a time, suggesting the surgery is well tolerated even when both feet are involved.
For many people, simply understanding the anatomy and adjusting their footwear is enough to manage symptoms long-term. The accessory bone doesn’t grow or change after skeletal maturity, so once you find a combination of support and activity modification that works, the condition tends to stabilize.