Accessory navicular bone or os tibiale externum, or os naviculare accessorium, is a large accessory ossicle [small bone] that can be present adjacent to the medial side of the navicular bone. The tibialis posterior tendon which normally inserts on the navicular attached to the ossicle when present.
An accessory navicular bone is present in 10-12% of the population and first appears in adolescence. It is common in females. Reported bilateral incidence is 50-90%.
Other names for accessory navicular bone are prehallux and navicular Secundum.
It is thought to have been first described by Bauhin in 1605.
It is an asymptomatic condition in most of the people but a small proportion may have the pain because of tendinosis due to traction between the ossicle and the navicular. This condition is called an accessory navicular syndrome.
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
Relevant Anatomy
The navicular is an intermediate tarsal bone on the medial side of the foot, which articulates proximally with the talus and distally with the cuneiforms.
In a few individuals, it also articulates laterally with the cuboid. The tibialis posterior tendon inserts into the navicular bone. It causes inversion of the foot and also secondary plantar flexor. It also supports the medial arch.
An abnormal insertion of the tendon into the accessory navicular bone results in a loss of suspension of the tibialis posterior tendon and can cause flat foot though the evidence for that is not clear.
Plantar calcaneonavicular ligament or spring ligament originates from sustentaculum tali and inserts on to navicular. It provides plantar support for talus.
Bifurcate ligament attaches the anterior process of the calcaneus to the navicular and cuboid bones. It provides lateral support.
Dorsal support is provided by the dorsal talonavicular ligament which connects the neck of the talus to the dorsal surface of the navicular bone.
Blood Supply to navicular bone is by dorsalis pedis artery on dorsal aspect and medial plantar artery on plantar aspect. Anastomosis between dorsalis pedis and medial plantar arteries supply the medial surface of the tuberosity.
Navicular bone normally has a single center of ossification. It ossifies at age 3 in girls and 5 in boys and fuses at 13 years of age.
An accessory navicular is a normal variant from which the tuberosity of the navicular develops from a secondary ossification center that fails to unite during childhood. The accessory navicular ossifies around 8 years of age.
Pathoananatomy
There is a plantar medial enlargement of the navicular bone which is due to the accessory navicular bone [Located posterior to the posteromedial tuberosity of the tarsal navicular bone.]
The presence of type I or II accessory navicular is also a cause of Posterior Tibial tendinopathy. This accessory bone could be separate or fused with the navicular appearing a completely ossified extension of the navicular.
Autosomal dominant inheritance has been reported.
Flat feet and posterior tibial tendon insufficiency have been reported as the association.
Classification of Accessory Navicular Bone
This classification was proposed by Geist in 1914 The Geist classification divides these into three types.
Type 1
It is also called as os tibiale externum. It accounts for 30% of accessory navicular bones
There is 2-3 mm sesamoid bone embedded within the distal portion of the posterior tibial tendon
There is no cartilaginous connection to the navicular tuberosity and may be separated from it by up to 5 mm.
Type 2
It is most common type. Accounts for 55% of all accessory navicular bones. The ossicle here is triangular or heart-shaped and typically measures around 12 mm.
It is connected to the navicular tuberosity by a 1-2 mm thick layer of either fibrocartilage or hyaline cartilage.
Eventually, the ossicle may fuse.
Type 3
Here, the ossicle is fused with navicular bone resulting in prominent navicular tuberosity called a cornuate navicular.
It is thought to represent a fused type 2.
It is occasionally symptomatic due to painful bunion formation over the bony protuberance
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.
Clinical Presentation
The condition is asymptomatic in most of the cases. The typical complaint in symptomatic patients is medial arch pain which worsens with the overuse.
The cause of pain is often repeated microfracture or inflammation of the posterior tibialis tendon insertion.
The examination may reveal swelling in the area. There may be tenderness of the medial foot [At the medial and plantar aspect of the navicular bone].
Flat foot or pes planus is often present.
Differential Diagnosis
Imaging
AP, lateral, external oblique x-rays are the recommended views. The accessory bone is best seen with an external oblique view.
A bony enlargement of navicular or accessory bone would be visible.
MRI may be used in rare cases to exclude a tumor, fracture of the medial tuberosity, or bone marrow edema.
Symptomatic accessory navicular bones may appear as a ‘hot spot’ on bone scan.
MRI may show bone marrow edema.
Treatment of Accessory Navicular Syndrome
Conservative Treatment
It is the first line of treatment. The treatment consists of activity restriction, shoe modification, and NSAIDs. Shoe modifications include arch supports or pads over the bony prominence
The UCBL orthosis may invert the heel during walking and decrease symptoms. A short period of cast immobilization is considered for pain refractory to activity modification and shoe modifications
Steroid injections locally may be used in some cases [often with immobilization].
Conservative treatment should be maintained for at least 4- 6 months before considering any surgical intervention.
Activity modification includes limiting or stopping any strenuous activities which may cause the accessory navicular bone to become symptomatic. Gait retraining and stability exercises may be required.
Well-padded shoe orthotics should be worn for arch support. This decreases direct pressure over the navicular.
Strength exercises for the peroneal muscles, posterior tibialis muscle, intrinsic foot muscles and the lateral rotators of the pelvis should be done.
Surgical Treatment
Most common surgery for this condition os removal of accessory navicular bone. Symptoms are relieved in 90% of cases.
In another variation of surgery, the bony prominence is cut and the posterior tibial tendon is split advanced further up the medial side of the foot
Partial weight bearing below knee cast is applied for 8 weeks after which full weight bearing is permitted.
Once the cast is removed, a strength and conditioning programme is highly recommended.
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.
The most common complication after surgery is persistent medial prominence and pain when the body of the navicular is not trimmed sufficiently.
