Accessory Navicular Bone (Os Naviculare)

Summary

An accessory navicular bone is a common finding on many foot x-rays. Most accessory naviculars are asymptomatic. However, in some patients the prominent bone on the inside of foot will create discomfort leading to difficulty with shoe fitting. Alternatively, the fibrous union between the navicular bone and the “extra” accessory part may become irritated and create discomfort. Physical examination combined with plain x-rays of the foot will be diagnostic. Treatment is usually non-operative with a change in shoe wear, and activity modification. However, it is not uncommon for patients to have ongoing symptoms and in these individuals patients may consider surgery to remove the prominent accessory navicular and reattach the posterior tibial tendon if necessary.

Figure 1:  Normal Navicular compared to “Accessory” Navicular

Accessory Navicular Bone

Clinical Presentation

Many people have accessory (“extra”) naviculars (figure 1) – a prominent extra bone extending form the navicular bone. Most accessory naviculars are completely asymptomatic. However, some individuals will develop pain on the inside of their midfoot. Pain may occur from pressure of shoeware against the prominence irritating the bone itself or  the fibrous junction where the accessory bone meets the regular navicular; or for both of these reasons. Alternatively, the fibrous junction or interface may become painful as a result of tension applied by the posterior tibial tendon through it’s connection or insertion at that site.  Often individuals will be asymptomatic for years until a new pair of shoes or a change in their activity level precipitates symptoms. The accessory navicular itself typically develops during adolescence when the two areas of the navicular bone fail to fuse together.

On examining a foot with a symptomatic accessory navicular there will often be a bony prominence on the inside of the foot, just below and in front of the inside ankle bone (medial malleolus). The size of this prominence will vary from small to quite large. Pressing over this area will often produce discomfort. There may be an associated flat foot deformity as this is common (but not universal!) in patients with an accessory navicular. Stressing the posterior tibial tendon by raising the heel up and down on one foot or by forcing the foot to the inside against resistance may aggravate the symptoms as these maneuvers stress the posterior tibial tendon which attaches (inserts) on the inside of the accessory navicular bone. Patients may walk with a slight limp due to pain.

Imaging Studies

An accessory navicular can be seen on plain X-rays. These x-rays will allow the size of the accessory navicular to be determined. Occasionally if the status of the posterior tibial tendon needs to be assessed or other problems are suspected (ex. Navicular stress fracture) it may be necessary to perform an MRI.  Although this is not considered routine, MRI may be helpful in identifying the degree of irritation by demonstrating fluid or edema that may accumulate in the bone as a result of the irritation.

Treatment

Non-Operative Treatment

Many individuals with symptomatic accessory naviculars can be managed successfully without surgery. Standard non-surgical treatment includes:

Appropriate Comfortable Shoes

Shoes that are soft around the inside of the ankle can allow for any excess prominence of bone. This can be achieved by purchasing shoes with plenty of padding and space in this area or by having a shoemaker create extra space in this area. For example many patients will get their ski boots expanded in the area around the prominence to minimize irritation. In addition a shoe with a stiff sole will help disperse force away from the arch of the foot during walking and thereby minimize the force on the posterior tibial tendon.  An off the shelf arch support may help decrease the stress applied by the posterior tibial tendon.  If necessary, ankle brace applied to provide more substantial support to the arch may be successful where a simple arch support fails.

Application of local padding

Often applying local foam or gel padding such as can be purchased at many pharmacies can help improve symptoms.

Activity Modification

If symptoms have been aggravated by an increase in activity level then backing off on activities in the short term can be helpful. The foot is subject to a lot of repetitive loading during walking and minimizing this force will often help symptoms to settle. After symptoms have settled the activity level can then be gradually increased.

Weight Loss

Excess weight will increase the force on the posterior tibial tendon as it inserts into the accessory navicular and will tend to precipitate or aggravate symptoms. If a patient with a symptomatic accessory navicular is

overweight then losing weight can be very helpful. Even losing 5-10lbs will decrease by 15-30 lbs the amount of force going through the foot with each step. This is because the foot acts like a lever serving to magnify the force absorbed by the foot with each step.

Operative Treatment

If a patient truly fails non-operative treatment then surgical intervention may be warranted. The standard operative treatment of an accessory navicular is a Kidner procedure.

Modified Kidner Procedure (Resection of the Prominent Accessory Navicular)

The Kidner procedure involves resecting the prominent accessory navicular and ensuring that the posterior tibial tendon is still attached to the bone. Often the prominent bone can simply be shelled out from its position relative to the posterior tibial tendon leaving the tendon intact. However, often the tendon is loose and floppy once the extra bone has been remove and therefore requires suturing or tother means of attaching it into the remaining navicular bone.

Kidner Procedure

In the original Kidner procedure the entire posterior tibial tendon was released from the navicular and then rerouted through a drill hole placed through the navicular, As a means of treating isolated an accessory navicular the original Kidner procedure is rarely done anymore. Instead, a modification of the Kidner procedure has become more commonplace.  The modified Kidner procedure consists of carefully removing the accessory and anchoring the posterior tibial tendon to the surface of the navicular where the accessory was removed.  The repair may be done by passing suture through the tendon and then through drill holes in the navicular or by using a suture anchor.

Recovery

In most instances a patient’s recovery will be as follows:

0-6 weeks: Immobilization (in case or cast boot) non-weight-bearing or touch weight-bearing

6-10 weeks: Increasing activity in a cast boot. Physical therapy to work on strength and balance

Full recovery after 9-2 months

In some patients (where the posterior tibial tendon is still intact and functioning) the treating surgeon may allow weight-bearing as tolerated in a cast boot immediately after surgery.

Edited by Michael Castro, DO  on September 26th, 2013