An accessory navicular is an extra piece of bone on the inner side of your foot, located just above the arch near a bone called the navicular. It’s a normal anatomical variation, not a disease, and most people who have one never know it’s there. But in some cases, this extra bone causes pain, swelling, and problems with how the foot functions.
Where It Is and Why It’s There
The navicular bone sits in the midfoot, roughly at the top of your arch on the inner side. It serves as an anchor point for the posterior tibial tendon, a thick cord of tissue that runs down from your calf, wraps behind your ankle, and attaches to several bones in the foot. This tendon is the main structure supporting your arch.
An accessory navicular develops during childhood when the foot bones are still forming. Instead of the navicular solidifying into a single piece, a separate fragment of bone develops alongside it. This fragment is often embedded within the posterior tibial tendon itself, which is why it can affect how the tendon works. The extra bone typically appears on both feet, though it may only cause problems on one side.
The Three Types
Not all accessory naviculars are the same. They’re classified into three types based on size, shape, and how they connect to the main navicular bone.
- Type I is a small, round or oval bone fragment that sits within the posterior tibial tendon but isn’t attached to the navicular. It’s almost always painless and is usually discovered by accident on an X-ray taken for something else.
- Type II is larger, typically 8 to 12 millimeters, and triangular or heart-shaped. It connects to the navicular through a thin strip of cartilage about 1 to 3 millimeters wide. This is the type most likely to cause symptoms because the cartilage connection can become irritated or partially torn with stress. It’s sometimes misdiagnosed as a fracture of the navicular.
- Type III is fully fused to the navicular, creating a single bone with a prominent bump (sometimes called a cornuate navicular). This enlarged shape can cause friction against shoes and may also affect tendon function.
What Makes It Start Hurting
Many people live their entire lives with an accessory navicular and never experience symptoms. When problems do develop, there’s usually a triggering event. A twist, stumble, or fall can stress the cartilage connection between the extra bone and the navicular (in Type II), causing inflammation and pain. Repetitive stress from running, dancing, or other high-impact activities can have the same effect over time.
Shoes also play a role. Because the accessory navicular creates a bony bump on the inner foot, tight or narrow shoes can press directly against it. This pressure irritates the soft tissue over the bone and can make the area chronically sore. The condition shows up most often in adolescence, when the bones are finishing their growth and young athletes are increasing their training loads.
The Connection to Flat Feet
An accessory navicular changes where the posterior tibial tendon attaches. In a Type II, for example, the tendon inserts partly onto the extra bone rather than entirely onto the navicular. This can reduce the tendon’s mechanical advantage, weakening its ability to hold up the arch. The result is that people with a symptomatic accessory navicular often have flat feet or flexible arches that collapse under weight.
Research shows this association is strongest with Type II and Type III accessory naviculars. Whether the flat foot is truly caused by the extra bone or simply tends to occur alongside it is still debated, but the two conditions frequently appear together, especially in children and adolescents. Flat feet, in turn, can cause their own chain of problems: ankle fatigue, shin pain, and altered gait.
How It’s Diagnosed
The most obvious sign is a visible or palpable bony bump on the inner side of the foot, just below and in front of the ankle bone. Pressing on this spot typically reproduces the pain. Swelling and redness over the area are common during flare-ups.
A standard set of foot X-rays will confirm the extra bone and show which type it is. The external oblique view, where the foot is angled outward, gives the clearest picture. If the X-ray looks normal but you’re still in pain, an MRI can reveal inflammation in the cartilage connection, stress reactions in the bone, or problems with the posterior tibial tendon itself.
Conservative Treatment
The first approach is almost always non-surgical, and for many people it works well. Treatment typically starts with switching to wider, more comfortable shoes that don’t press against the inner foot. Reducing or temporarily stopping activities that aggravate the pain, combined with over-the-counter anti-inflammatory medications, can bring the inflammation down.
Custom or over-the-counter arch-supporting insoles help by redistributing pressure away from the bump and supporting the arch, which takes strain off the posterior tibial tendon. For more stubborn cases, a walking boot or cast can enforce rest and prevent the repetitive micro-injuries that keep the area irritated. In a study of pediatric patients treated conservatively, about half received both immobilization and shoe inserts, while the rest were treated with one or the other. Those who achieved complete pain relief averaged about eight months of treatment.
Bracing, taping, and targeted physical therapy to strengthen the muscles that support the arch have also shown good results. A case report comparing two adolescent dancers found that the one treated non-surgically, with bracing, taping, and orthotics, returned to activities with pain relief comparable to the one who had surgery.
When Surgery Becomes an Option
If conservative treatment fails after several months of consistent effort, surgery may be recommended. The average length of non-surgical treatment before patients moved to surgery in one study was about 12 months. The most common procedure is called the Kidner procedure: the extra bone is removed and the posterior tibial tendon is reattached directly to the navicular in a position that better supports the arch. A simpler version involves just removing the extra bone without repositioning the tendon, though this may be less effective for people with flat feet or tendon dysfunction.
Recovery after surgery follows a structured timeline. You’ll typically spend the first six weeks in a short-leg cast and off your feet. During the initial three weeks, the foot is positioned in a slightly downward and inward angle to protect the tendon repair. For the next three weeks, the cast holds the foot in a neutral position, and gradual weight-bearing begins. After the cast comes off at six weeks, a supportive brace is worn for another six weeks. Full return to high-impact activities usually takes several months beyond that, depending on how the tendon heals and how quickly strength returns with physical therapy.
What to Watch For Over Time
A pain-free accessory navicular doesn’t need treatment. If yours was found incidentally on an X-ray and isn’t bothering you, there’s nothing to do about it. But if you notice a growing bump on the inner side of your foot, pain with activity, or your arch gradually flattening, those are signs the extra bone is starting to affect your foot mechanics. Catching these changes early, when shoe modifications and orthotics can still make a difference, gives you the best chance of avoiding surgery.