Also known as Prehallux, Os Tibiale Externum and Navicular Secundum.
An accessory navicular bone is a bone of the foot that develops abnormally causing a plantar medial enlargement of the navicular. The accessory navicular bone presents as a sesamoid in the posterior tibial tendon, in articulation with the navicular or as an enlargement of the navicular itself.
The Geist classification divides these into three types:
Type I: is a sesamoid bone in the posterior tibialis tendon. There is a small gap of approximately 3mm or less between the sesamoid and the navicular.
Type II: consists of an accessory bone, up to 1.2cm in diameter, where synchondrosis develops between it and the navicular.
Type III: is the fused accessory navicular resulting in a large cornuate shaped navicular.
Clinically Relevant Anatomy
The navicular is an intermediate tarsal bone on the medial side of the foot, which articulates proximally with the talus. Distally it articulates with the three cuneiform bones. In some individuals it also articulates laterally with the cuboid. The tibialis posterior tendon inserts into the navicular bone. Tibialis posterior is an inverter of the foot, assists in the plantar flexion of the foot at the ankle and also has a major role in supporting the medial arch of the foot. This can be compromised where there exists an abnormal insertion of the tendon into the accessory navicular bone  and result in a loss of suspension of the tibialis posterior tendon, possibly causing peroneal spastic pes planus or simple pes planus. However, the cause and effect relationship between the accessory navicular and pes planus is speculative as there is no clear proof of that relationship. 
The presence of a type I or II accessory navicular is also a cause of Posterior Tibial tendinopathy as the insertion of the Tibialus Posterior tendon onto the accessory navicular is more proximal (dashed line). Leverage of the malleolus on the Tibialus Posterior tendon is reduced increasing stress on the tendon. 
The calcaneal pitch angle is also reduced in patients with a symptomatic accessory navicular than in normal subjects.
The foot and ankle have numerous accessory ossification areas, with the most common being the accessory tarsal navicular bone which occurs in 4-14% of the population. 
- An accessory navicular bone is present in ~10% of the population
- It first appears in adolescence, with incidence of 4-21% in children.
- It is more common in females 
- Reported prevalence bilaterally is ~70% (range 50-90%)
People who have an accessory navicular are often unaware of the condition as it causes no symptoms. Some individuals, however, will develop accessory navicular syndrome, a painful condition where the bone and/or posterior tibial tendon become aggravated. This can result from any of the following:
- Trauma, such as a foot or ankle sprain
- Chronic irritation from shoes or other footwear rubbing against the accessory bone
- High levels of activity or overuse
- Typically seen in young females (10-20 years of age) complaining of mid food/arch pain which may be insidious or post trauma
- Difficulty getting comfortable footwear
- Prominent navicular
- Tenderness over the prominence
- Pain over the posterior tibialis tendon and reduced mobility in the Achilles tendon in chronic cases
- pes planus is often present
- Inflamed bursa
- X-Ray: An accessory navicular is often clear to see in standing AP and lateral views, but in some cases an oblique view is also required in order to fully diagnose the extent of the navicular abnormality. Bilateral investigations are often done as there is a high incidence of symmetrical abnormalities.
- When examining the lateral weight bearing X-ray, alignment of the talonavicular cuneiform and first metatarsal dorsal should be carefully examined as well. “Sag” at this joint indicates structural integrity of the area.
- In rare cases, an MRI or CT is indicated in order to exclude a tumor, fracture of the medial tuberosity, or bone marrow edema.
Patients with an accessory navicular may present with complex pain patterns requiring a thorough examination.  The examination should include key assessments:
- Differentiation of the navicular prominence from the talar head prominence in flat foot deformities by inverting and everting through the subtalar joint with a thumb over the bony prominence.
- Assessing for any loss of structural integrity of the longitudinal arch is important as this component of the deformity will not be corrected by surgical treatment where surgery is required. 
- Thorough gait examination.
- Physical therapy
- Medications. Oral nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, may be prescribed. In some cases, oral or injected steroid medications may be used in combination with immobilisation to reduce pain and inflammation. 
Surgical: Depending upon the severity of symptoms, non-operative or conservative treatment should be maintained for at least 4- 6 months before any surgical intervention.
There are 2 surgeries that can be performed depending upon the condition and symptoms
- A simple surgical excision to remove the accessory navicular along with its prominence. In this procedure, the incision is made dorsally to the prominence of the accessory navicular. Symptoms are relieved in 90% of cases.
- The 2nd procedure excising the boney prominance in conjunction with a reattachment of the posterior tibial tendon. . The posterior tibial tendon is split and is reattached further up the medial side of foot to provide increased support to the longitudinal arch.
After surgery the lower leg is put into a cast for 4 weeks, which is moulded into the shape of the arch, with the foot maintained in a plantigrade position. Partial weight bearing is indicated for 8 weeks after which full weight bearing is permitted.. Once the cast is removed, a strength and conditioning programme is highly recommended. 
Occasionally, a limited fusion of the cuneiform metatarsal or talonavicular joints is also recommended. The rationale and efficacy of this operation have been questioned however.
Arthrodesis may be a reasonable treatment option in selected cases of patients with symptomatic recalcitrant Type II accessory naviculars that are large enough to accept small fragment screws.
Physical Therapy Management
If the accessory navicular bone becomes problematic, physical therapy may be prescribed.
This includes use of therapeutic modalities to relieve pain, increase strength and stability in the foot. An accessory navicular bone is often linked to posterior tibial dysfunction and pes planus. In some cases orthotics may be indicated.
- Well padded shoe orthotics should be worn for arch support. This decreases direct pressure over the navicular.
- Strength and conditioning exercises for the peroneal and posterior tibialis muscles.
- Strengthening of the intrinsic foot muscles and the lateral rotators of the pelvis.
- Activity modification in the initial stages, such as limiting or stopping any strenuous activities which may cause the accessory navicular bone to become symptomatic.
- Gait re-training and stability exercises.
Some examples of functional posterior tibialis strengthening:
Clinical Bottom Line
A type I accessory navicular is rarely symptomatic. Where symptoms do appear, type I usually responds well to conservative management. Patients with Type II have an increased risk of injury and onset is usually insidious or as a a result of trauma. It fails to respond to conservative treatment when severe and fusion of the accessory navicular to the navicular may successfully relieve pain without disrupting the tibialis posterior tendon insertion. With Type III, excising the accessory navicular is indicated over excising the navicular prominance.
- ↑ 1.01.11.21.3Raymond T., Morrissy and Stuart L.Weinstein .Lovell, Winter’s Padiatric Orthopaedics. United States: Lippincot William’s Wilkin Publication, 2005.
- ↑ 2.02.1D.Richard, V.Wayne, M. Adam, Gray’s Anatomy for Students. Spain: Elsevier Publishers, 2005
- ↑Golano P., ‘The anatomy of the navicular and periarticular structures.’ Foot Ankle Clinics, 2004, March, vol. 9, p. 1-23.
- ↑ 4.04.1Kiter E., Erdan N., Karatosun V., Gunall I., ‘Tibialis posterior tendon abnormalities in feet with accessory navicular bone and flatfoot’. Acta orthopaedica Scandinavia, 1999, December, vol. 70, p. 618-621
- ↑ 5.05.15.2Kulkarni. G.S. Textbook of orthopedics and trauma.India: Jaypee Brother Publication, 1999
- ↑ 6.06.16.26.3Kidner FC. The prehallux (accessory scaphoid) in its relation to flat-foot. J Bone Joint Surg 1929 : II : 831
- ↑ 7.07.17.2Kidner FC. Pre-hallux in relation to flatfoot. JAMA 1933; 101: 1539-42.
- ↑ 8.08.18.28.184.108.40.206.7A. Bernaerts, F.M. Vanhoenacker, S. Van de Perre, A.M. De Schepper, P.M. Parizel1 Accessory navicular bone: Not Such a normal variant. JBR–BTR, 2004, 87 (5) page 250-252
- ↑Prichausuk S, Sinphurmsukskul O: Kinder Procedure for symptomatic accessory navicular and its relation to pes planus, Foot Ankle 16:500,1995
- ↑ 10.010.1Shands AR Jr, Wentz IJ. Congenital anomalies, accessory bones and osteochondritis in the feet of 850 children. Surg.Clin.North Am 1953:97:1643-1666
- ↑Kulkarni. G.S. Textbook of orthopedics and trauma.India: Jaypee Brother Publication, 1999.</ref><ref name="p7">Kidner FC. The prehallux (accessory scaphoid) in its relation to flat-foot. J Bone Joint Surg 1929 : II : 831
- ↑ShaychiITA. Tibialis Posterior Basic Strengthening. Available from: http://www.youtube.com/watch?v=zmh1FisBeeM [last accessed 24/11/12]
- ↑ShaychiITA. Tibialis Posterior Basic Multiplanar Strengthening. Available from: http://www.youtube.com/watch?v=qv76eBxGQXI [last accessed 01/12/12]
- ↑ ShaychiITA. Tibialis Posterior Functional Strengthening: Full Body Weight Maintaining Supination. Available from: http://www.youtube.com/watch?v=1C_C5N9reB8[last accessed 01/12/12]|}
- ↑ ShaychiITA. Tibialis Posterior Functional Strengthening: Maintain Arch with Compass Squats. Available from: http://www.youtube.com/watch?v=TdHgyFZbOPk[last accessed 24/11/12]|}
An accessory navicular bone is an accessory bone of the foot that occasionally develops abnormally in front of the ankle towards the inside of the foot. This bone may be present in approximately 2-21% of the general population and is usually asymptomatic. When it is symptomatic, surgery may be necessary.
Surgery can be performed at any age because it does not alter any other bones.
Symptoms of an accessory navicular bone may include plantar fasciitis, bunions and heel spurs.
The Geist classification divides the accessory navicular bones into three types.
- Type 1: An os tibiale externum is a 2–3 mm sesamoid bone in the distal posterior tibialis tendon. Usually asymptomatic.
- Type 2: Triangular or heart-shaped ossicle measuring up to 12 mm, which represents a secondary ossification center connected to the navicular tuberosity by a 1–2 mm layer of fibrocartilage or hyaline cartilage. Portions of the posterior tibialis tendon sometimes insert onto the accessory ossicle, which can cause dysfunction, and therefore, symptoms.
- Type 3: A cornuate navicular bone represents an enlarged navicular tuberosity, which may represent a fused Type 2 accessory bone. Occasionally symptomatic due to bunion formation.
This section is empty.You can help by adding to it.(December 2017)
Aside from surgery, there are a few options for handling an accessory navicular bone that has become symptomatic. This includes immobilization, icing, medicating, physical therapy, and orthotic devices. Immobilizing involves placing the foot and ankle in a cast or removable walking boot. This alleviates stressors on the foot and can decrease inflammation. Icing will help reduce swelling and inflammation. Medication involves usage of nonsteroidal anti-inflammatory drugs, or steroids (taken orally or injected) to decrease inflammation. Physical therapy can be prescribed in order to strengthen the muscles and help decrease inflammation. Physical therapy can also help prevent the symptoms from returning. Orthotic devices (arch support devices that fit in a shoe) can help prevent future symptoms. Occasionally, the orthotic device will dig into the edge of the accessory navicular and cause discomfort. For this reason, the orthotic devices made for the patient should be carefully constructed.
From left to right: Type 1, 2 and 3
Lateral projection of type 2
Type 2 on one foot (dark arrow) and type 3 on the other (white arrow)