Accessory Navicular Syndrome What Is Accessory Navicular? The accessory navicular (os navicularum or os tibiale externum) is an extra bone or piece of cartilage located on the inner side of the foot just above the arch. It is incorporated within the posterior tibial tendon, which attaches in this area. An accessory navicular is congenital (present at birth). It is not part of normal bone structure and therefore is not present in most people.

Accessory Navicular •Most common accessory bone of the foot •Located at Medial plantar border of the navicular •Associated with Tibialis Posterior tendon •Accessory ossicles are derived from unfused ossification centers •Synonyms: Os tibiale; Os tibiale externum; Naviculare secundum

What Is Accessory Navicular Syndrome? People who have an accessory navicular often are unaware of the condition if it causes no problem. This extra bone develop a painful condition known as accessory navicular syndrome when the bone and/or posterior tibial tendon are aggravated •Trauma, as in a foot or ankle sprain •Chronic irritation from shoes or other footwear rubbing against the extra bone •Excessive activity or overuse

Incidence 21% incidence; 89% of cases are bilateral One of the most common accessory ossicles in the foot It is seen over the medial pole of the navicular bone, usually in adolescent patients It is most commonly symptomatic in the 2nd decade of life and causes medial foot <1% of patients become symptomatic. Usually affects teens and young adults More frequent in females Many people with accessory navicular syndrome also have flat feet (fallen arches). Having a flat foot puts more strain on the posterior tibial tendon, which can produce inflammation or irritation of the accessory navicular.

Classification I Small, 2 - 3 mm sesamoid bone in the PTT (os tibiale externum) ~30% II Larger ossicle than type I Secondary ossification center of the navicular bone ~50% III Enlarged navicular tuberosity often with pointed shape ~20%

Etiology • Variant of normal anatomy • Symptomatic from the bony prominence impinging against shoe wear. • Diffuse medial and plantar arch pain. • Destabilizing the insertion and diminishing the pull of the PTT. • Severe flatfoot deformity, lateral pain may occur secondary to impingement of the calcaneus against the fibula. • A traumatic injury to the fibrocartilaginous synchondrosis that attache the ossicle to the main navicular.

Signs & Symptoms of Accessory Navicular Syndrome Adolescence is a common time for the symptoms to first appear. This is a time when bones are maturing and cartilage is developing into bone. The symptoms do not occur until adulthood.

The signs and symptoms of accessory navicular syndrome include: • A visible bony prominence on the midfoot (the inner side of the foot, just above the arch) • Redness and swelling of the bony prominence • Vague pain or throbbing in the midfoot and arch, usually occurring during or after periods of activity

Diagnosis Skin irritation or swelling. Bony prominence to assess the area for discomfort. Foot structure, muscle strength, joint motion X-rays are usually ordered to confirm the diagnosis. MRI or other advanced imaging tests may be used to further evaluate the condition.

Differential Diagnosis • Navicular fracture may mimic an acute avulsion fracture of the tuberosity of the navicular. • Posterior tibial tendinitis • Stress fracture of navicular

Associated Conditions •Flatfoot deformity •Secondary Achilles tendon contracture symptoms •Bursa, redness, irritation, local tenderness •Often presents in adolescent patients or young adults, with flatfoot deformity and arch pain

Bone scan MRI •May show increased activity over an accessory navicular •May be needed if a navicular stress fracture is suspected in the differential diagnosis MRI •Useful when plain films are unremarkable •Often, a type-II accessory navicular is attached to the tuberosity by a fibrocartilage or hyaline cartilage layer, and MRI may show soft-tissue edema consistent with a synchondrosis sprain or tear. •Altered signal intensity and bone marrow edema, suggestive of chronic stress and/or osteonecrosis •Also helpful in showing PTT degeneration

Treatment • Most patients assymptomatic or are successfully managed conservatively ◦ Rest and avoid athletics or aggravating activities. ◦ Anti-inflammatory medication ◦ Shoe-wear modification ■use of a softer, wider shoe ■If flatfoot is present, a medial arch support may be useful, but often the patient may not tolerate it because of direct pressure on the ossicle. ◦ Below-the-knee walking cast or removable fracture boot may be used for 3-6 weeks for persistent symptoms. ◦ Physiotherapy - strengthening exercises

Nonsurgical Treatment Approaches The goal of nonsurgical treatment for accessory navicular syndrome is to relieve the symptoms. • Immobilization. Placing the foot in a cast or removable walking boot allows the affected area to rest and decreases the inflammation. • Ice. To reduce swelling, a bag of ice covered with a thin towel is applied to the affected area. Do not put ice directly on the skin. • Medications. Oral nonsteroidal anti-inflammatory drugs (NSAIDs), or injected steroid medications may be used in combination with immobilization to reduce pain and inflammation.

• Physical therapy • Orthotic devices Exercises and treatments to strengthen the muscles and decrease inflammation. Prevent recurrence of the symptoms • Orthotic devices Custom orthotic devices that fit into the shoe provide support for the arch and may play a role in preventing future symptoms.

When Is Surgery Needed? If nonsurgical treatment fails to relieve the symptoms of accessory navicular syndrome, surgery may be appropriate. Surgery may involve removing the accessory bone, reshaping the area and repairing the posterior tibial tendon to improve its function. This extra bone is not needed for normal foot function.

Surgery ■Options Kidner procedure, the accessory navicular is excised, and the PTT is rerouted into a more plantar position Excision of the ossicle and reattachment of the PTT insertion to the navicular, with suture anchors or sutures passed through drill holes ■ Osteotomy of the calcaneus and/or medial column of the foot to improve alignment and decrease mechanical stress of the PTT insertion.

Further Reference : "Boning up on navicular disease history". Equus. Primedia (302): 8. Retrieved 5 December 2013. "Equine Navicular Syndrome in the Fossil Record" (PDF). Hoofcare and Lameness. Gloucester, MA: FYI Pub (76): 34–40. ISSN 1076-4704. OCLC 708289198. Retrieved 5 December 2013.

Q & A Dr. 熊永萬 Dr. Bear