Orthopaedic - Accessory Navicular

Basics

An accessory navicular is an anatomical variant in which an additional ossicle is present along the medial border of the navicular bone. Accessory ossicles arise from secondary ossification centers that fail to fuse with the main bone. In many individuals, the accessory navicular is discovered incidentally on radiographs and remains completely asymptomatic.

Symptoms may develop after repetitive activity, overuse, direct trauma, or mechanical irritation. When the accessory navicular becomes painful, the condition is referred to as accessory navicular syndrome (ANS). Both children and adults may develop symptoms, although presentation is particularly common during adolescence.

Classification

Accessory navicular bones are generally classified into three major types according to their morphology and relationship with the posteromedial navicular tuberosity.

Type I, representing approximately 30% of cases, is a small 2–3 mm sesamoid ossicle located within the posterior tibialis tendon (PTT). It has no cartilaginous connection with the navicular tuberosity and is also known as the os tibiale externum.

Type II, which accounts for approximately 50% of cases and is the type most commonly associated with symptoms, is larger than type I. It originates from a secondary ossification center of the navicular and typically appears triangular or heart-shaped. It is connected to the main navicular by a cartilaginous or fibrocartilaginous synchondrosis. Type II may be further subdivided into types IIa and IIb according to its position.

Type III consists of an enlarged navicular tuberosity and is generally considered the result of fusion of a previous type II accessory navicular. It may have a prominent or pointed configuration.

Other terms used for an accessory navicular include os tibiale, os tibiale externum, naviculare secundum, and prehallux.

Pediatric Considerations

Accessory navicular syndrome frequently presents during adolescence or early adulthood. Patients may have associated pes planus, or flatfoot, together with pain around the medial longitudinal arch. However, the severity of the flatfoot deformity does not necessarily correlate with the degree of pain or symptoms caused by the accessory navicular.

Epidemiology

The reported incidence of an accessory navicular ranges from approximately 4% to 21%, making it one of the more common accessory ossicles of the foot. It is usually located adjacent to the medial aspect of the navicular bone and is commonly identified in adolescents.

Many cases are bilateral, with some studies reporting bilateral involvement in up to 90% of affected individuals. Despite its relatively frequent radiographic appearance, fewer than 1% of individuals with an accessory navicular develop clinically significant symptoms.

Prevalence

Symptoms are most frequently encountered in teenagers and young adults. The condition tends to occur more commonly and at an earlier age in females than in males.

Accessory navicular bones may also be identified incidentally in older adults. Rarely, older individuals may develop symptoms because of degenerative changes at the synchondrosis, posterior tibial tendon degeneration, or inflammation of an adjacent bursa.

Etiology and Pathophysiology

The accessory navicular itself represents a normal anatomical variation rather than a disease. Symptoms may arise when the medial bony prominence rubs against footwear or is exposed to repetitive mechanical stress.

Patients may experience diffuse pain along the medial or plantar aspect of the arch. The accessory ossicle can alter the normal insertion of the posterior tibialis tendon, reducing its mechanical efficiency and potentially contributing to dysfunction of the medial arch.

In patients with marked flatfoot deformity, lateral-sided pain may also occur because of impingement between the calcaneus and fibula. Acute trauma can injure the fibrocartilaginous synchondrosis joining a type II accessory navicular to the main navicular bone, producing pain and inflammation.

Associated Conditions

Accessory navicular syndrome may be associated with flatfoot deformity and, in some individuals, a secondary Achilles tendon contracture.

Diagnosis

Signs and Symptoms

Pain may begin after wearing poorly fitting or narrow shoes, participating in sports, increasing weight-bearing activities, or sustaining direct trauma to the foot.

Typical findings include tenderness and pain over the medial aspect of the foot at the accessory navicular. Patients may also experience pain or weakness while attempting to rise onto the toes, run, or jump. A visible or palpable prominence may be present over the medial navicular, sometimes accompanied by local swelling and erythema.

History

Patients usually describe pain centered over the medial portion of the navicular. Symptomatic accessory navicular is particularly recognized in young athletes.

Symptoms are commonly worsened by prolonged standing, walking, running, sporting activity, or wearing narrow shoes that press against the prominence. Pain generally decreases with rest and avoidance of the aggravating activity.

Physical Examination

Examination usually demonstrates localized tenderness over the medial pole of the navicular. Discomfort may increase when the foot is actively or passively abducted and adducted.

The patient’s footwear and shoe insole should also be assessed, as direct pressure from the shoe may contribute to symptoms.

Posterior tibialis tendon function should be evaluated by testing resisted plantarflexion and inversion. The ability to perform repeated single-leg heel rises is also useful when assessing PTT strength and function.

Ankle and subtalar joint motion should be examined, and the clinician should assess for associated Achilles tendon or gastrocnemius-soleus contracture.

Imaging

Initial evaluation should include weight-bearing anteroposterior, lateral, and oblique radiographs of the foot.

A type II accessory navicular typically has well-defined, smooth cortical margins and a triangular or heart-shaped appearance. It may measure approximately 9 × 12 mm and is commonly located about 1–2 mm from the medial and posterior surfaces of the native navicular. An internal oblique view may provide particularly good visualization of the ossicle.

The presence of smooth margins and a mature cortical outline helps distinguish an accessory navicular from an acute navicular avulsion fracture.

MRI

Magnetic resonance imaging is useful when conventional radiographs do not adequately explain the patient’s symptoms and has high sensitivity for detecting associated abnormalities.

In a type II accessory navicular, MRI may demonstrate the fibrocartilaginous or hyaline cartilage layer forming the synchondrosis between the accessory ossicle and the navicular tuberosity. Increased soft-tissue signal and edema may indicate sprain or disruption of this synchondrosis.

MRI may also demonstrate altered marrow signal or bone marrow edema related to chronic mechanical stress and, occasionally, osteonecrotic change. It is particularly helpful for identifying associated degeneration or tendinopathy of the posterior tibialis tendon.

Pathological Findings

In painful type II accessory navicular lesions, histological examination may demonstrate increased osteoblastic and osteoclastic activity within the tissue between the accessory ossicle and the main navicular.

Cartilage proliferation and increased vascular mesenchymal tissue may also be present, supporting the concept that repetitive mechanical stress at the synchondrosis contributes to symptoms.

Differential Diagnosis

Important differential diagnoses include an acute avulsion fracture of the navicular tuberosity, posterior tibial tendinitis or tendinopathy, and a navicular stress fracture.

Treatment

General Measures

Initial management is usually conservative. The patient should reduce or temporarily discontinue sports and other activities that reproduce symptoms. Rest and activity modification help decrease repeated mechanical stress across the accessory navicular.

Anti-inflammatory medication can be used for symptomatic pain relief. Footwear modification is also important, particularly the use of softer and wider shoes that minimize direct pressure over the medial bony prominence.

Patients with associated flatfoot may benefit from a medial arch support or orthotic. However, some individuals cannot tolerate an orthosis if it applies direct pressure to the accessory ossicle.

Persistent symptoms can be managed with immobilization in a below-knee walking cast or removable walking boot for approximately 3–6 weeks. Physical therapy may subsequently be introduced, with emphasis on strengthening, flexibility, posterior tibialis rehabilitation, and cryotherapy when appropriate.

Medication

Nonsteroidal anti-inflammatory drugs may be used for pain and inflammation. There is no clear evidence demonstrating that one particular NSAID is consistently superior to another for this condition.

Surgical Management

Most patients with a painful accessory navicular improve with nonoperative management, particularly children and adolescents.

In skeletally immature children, conservative treatment is generally preferred, especially before complete calcaneal apophyseal fusion, because symptoms may lessen with continued skeletal maturation.

Surgery may be considered when pain is progressive, recurrent, or persistent despite an adequate trial of conservative treatment.

One surgical option is excision of the accessory navicular combined with naviculoplasty, in which part of the prominent medial navicular is reshaped to restore a more normal contour. This may be useful because patients with an accessory navicular can also have a relatively broad medial native navicular.

Kidner Procedure

The traditional Kidner procedure involves excision of the accessory navicular together with repositioning of the posterior tibialis tendon to a more plantar location on the navicular.

Modern variations commonly involve removal of the accessory ossicle followed by direct reattachment or advancement of the posterior tibialis tendon to the navicular. Fixation may be achieved using suture anchors or sutures passed through drill holes.

This approach generally provides satisfactory pain relief and functional improvement, particularly in adolescents with persistent symptomatic accessory navicular syndrome.

Percutaneous Drilling

Percutaneous drilling of the synchondrosis can be considered in selected adolescent athletes. The aim is to stimulate bone union between the accessory navicular and the native navicular rather than removing the ossicle.

This technique may be suitable in skeletally immature patients in whom successful fusion of the synchondrosis is considered achievable.

Associated Flatfoot Reconstruction

Patients with severe flatfoot deformity and associated lateral impingement may require correction of the underlying deformity in addition to treatment of the accessory navicular.

Procedures may include a calcaneal osteotomy, a medial column osteotomy, or other reconstructive techniques intended to improve foot alignment. Correcting the deformity decreases abnormal mechanical stress on the posterior tibialis tendon insertion and may improve long-term function.

Follow-Up and Complications

Patients should be followed until pain has resolved and normal activity can be resumed without significant symptoms. After surgical treatment, rehabilitation should include restoration of ankle and foot motion followed by progressive strengthening of the posterior tibialis tendon and surrounding musculature.

Potential complications include persistent or incomplete pain relief, residual weakness of the posterior tibialis tendon, recurrent symptoms, and continuation of an underlying flatfoot deformity. Patients with significant structural deformity may therefore require additional corrective procedures to achieve a satisfactory functional outcome.