Accessory Bones of the Foot: Types, Symptoms, Diagnosis, and Treatment Options

Accessory bones of the foot are extra small bones formed during development and are part of normal anatomical variation. About 5-40% of people have one type of accessory bone or another. Most cause no symptoms, but some can trigger pain or dysfunction because of external irritation or their anatomical position.

Common accessory bones of the foot include the accessory navicular (Accessory Navicular), the os trigonum (Os Trigonum), the os peroneum (Os Peroneum), the os vesalianum (Os Vesalianum), the os intermetatarseum (Os Intermetatarseum), the os supranaviculare (Os Supranaviculare), the os calcaneus secundarius (Os Calcaneus Secundarius), the os sustentaculi (Os Sustentaculi), the os subtibiale (Os Subtibiale), and the os subfibulare (Os Subfibulare).

The sections below describe in detail several accessory bones of the foot that are clinically important or common, covering their anatomical features, causes, age-related prevalence, clinical significance, symptoms, diagnosis, and treatment.

Accessory Navicular

The accessory navicular (Accessory Navicular) is an extra bone on the medial side of the navicular that is often broadly connected to the posterior tibial tendon. It arises when a secondary ossification center of the navicular fails to fuse with the main bone, leaving an independent bone fragment after ossification in adolescence.

Age-related prevalence and clinical significance

• About 4-21% of the population has this variant. It is more common in women, and roughly 50-90% of cases are bilaterally symmetric.

• Symptoms occur more often in adolescents (10-20 years), because the bones have not yet fully fused at this stage and activity levels are higher.

• Adults are usually asymptomatic, but chronic pain can occur if the posterior tibial tendon becomes inflamed.

The accessory navicular can be divided into three types based on size and degree of fusion: type I is a small round sesamoid bone 2-3 millimeters in diameter embedded within the posterior tibial tendon; type II is the most common, an approximately 1 cm triangular fragment connected to the navicular by fibrocartilage; type III is a large navicular tuberosity (also called a “cornuate navicular”), which may result from fusion of a type II fragment with the navicular.

The accessory navicular usually causes no discomfort and is often found incidentally on a foot X-ray. However, a minority of people (usually type II, with a larger accessory bone) develop pain along the medial arch, known as accessory navicular syndrome. The pain often worsens with prolonged walking, running, jumping, and similar activities, and a prominent bony lump that is tender to the touch may be felt over the area.

Because part of the posterior tibial tendon attaches to the accessory navicular, repeated traction stress can cause chronic tenosynovitis or overuse strain of that tendon. In severe cases, the presence of the accessory navicular may weaken the ability of the posterior tibial muscle to support the arch, which over the long term can lead to arch collapse (flatfoot deformity) or even a partial tear of the posterior tibial tendon.

For treatment, asymptomatic cases need no intervention; symptomatic cases can be managed conservatively with reduced activity, ice, insole support, manual therapy, and oral anti-inflammatory drugs. For patients with severe pain or impaired daily activity, excision of the accessory navicular (the Kidner procedure) may be considered, removing the accessory bone, trimming the medial navicular prominence, and reattaching the posterior tibial tendon.

Os Trigonum

The os trigonum (Os Trigonum) lies behind the talus and is a small accessory bone, usually located at the lateral tubercle of the posterior process of the talus. It arises when a secondary ossification center of the talus fails to fuse with the main bone.

Age-related prevalence and clinical significance

• About 7-10% of adults have an os trigonum, and bilateral cases account for about 2%.

• Adolescents (10-18 years) may have a higher prevalence because the bones are not yet fully developed.

• Professional athletes (such as ballet dancers and football players) are prone to posterior ankle impingement because of prolonged plantar flexion.

When the os trigonum is subjected to repeated compression or acute trauma, painful symptoms can arise, known as os trigonum syndrome (Os Trigonum Syndrome). This condition is common in people who repeatedly force the foot into extreme plantar flexion, such as classical ballet dancers (whose ankles are excessively plantar-flexed when standing on the toes) or football players. Excessive plantar flexion can lead to posterior ankle impingement, causing pain and swelling at the back of the ankle. In some patients, symptoms may stem from a single ankle sprain or a crush injury of the foot; in this acute-trauma setting, the os trigonum itself may fracture or separate from the posterior process of the talus (called a Shepherd fracture), which clinically resembles os trigonum syndrome.

Diagnosis includes clinical examination, X-ray, and MRI. For treatment, conservative measures include avoiding plantar flexion, using anti-inflammatory painkillers, cast immobilization, and physical therapy. If conservative treatment fails, the os trigonum can be removed by arthroscopy or a small-incision procedure.

Os Peroneum

The os peroneum (Os Peroneum) is a small sesamoid bone located within the peroneus longus tendon distal to the lateral malleolus, usually below and lateral to the cuboid. Many people are born with an ossified os peroneum, with a prevalence in the general population of about 4.7-30%; the commonly cited value is around 20%. Reports indicate that about 60% of os peroneum cases are bilaterally symmetric, and roughly one third is bipartite (in two parts). (A bipartite os peroneum appears on X-ray as two adjacent small fragments and is easily mistaken for a fracture fragment.) The formation of the os peroneum is related to development, but some studies suggest that certain cases are actually the result of the peroneus longus tendon avulsing a bone fragment in childhood that then healed chronically (so in some cases the os peroneum can be regarded as an old fracture remnant).

Age-related prevalence and clinical significance

• Its prevalence is about 4.7-30%, of which about 60% is bilaterally symmetric.

• Children and adolescents less often develop symptoms, because the os peroneum and tendon adapt better.

• Adults and older people with chronic peroneus longus tendinitis may develop painful os peroneum syndrome.

If the peroneus longus tendon is acutely injured or chronically degenerated, pain can develop around the os peroneum, known as painful os peroneum syndrome (POPS). This syndrome includes a range of possible lesions: fracture of the os peroneum (the bone splitting into two fragments after acute trauma), dislocation/distal migration of the os peroneum (usually accompanied by a partial tear of the peroneus longus tendon, causing the os peroneum to sit abnormally), and tenosynovitis or rupture of the peroneus longus tendon. Patients usually present with pain in the lateral arch or the lateral sole of the foot, worse with activity, especially when pushing off with the sole of the foot or everting the ankle, and in severe cases they cannot walk normally.

If the peroneus longus tendon ruptures, longitudinal support of the arch may decrease, and vague pain may appear near the base of the first metatarsal, because impaired peroneus longus function affects force transmission through the sole. On examination, tenderness may be felt near the cuboid, with possible local swelling and bruising; if there is an os peroneum fracture with tendon rupture, a defect may be felt at the peroneus longus tendon on palpation.

Most os peroneum bones cause no symptoms, but in patients with lateral foot pain, painful os peroneum syndrome should be considered, which may be caused by an os peroneum fracture, a tendon tear, or inflammation. Diagnosis is by X-ray, MRI, or ultrasound. Conservative treatment includes reduced weight bearing, ice, anti-inflammatory drugs, and cast immobilization. If symptoms are severe and conservative treatment fails, surgery can remove the os peroneum and repair the peroneus longus tendon.

Os Vesalianum

The os vesalianum (Os Vesalianum) sits on the lateral side of the base of the fifth metatarsal and is relatively rare. It arises when a secondary ossification center of the fifth metatarsal tuberosity fails to fuse, located on the lateral side of the base of the fifth metatarsal and related to the attachment of the peroneus brevis tendon. Reports indicate a prevalence of only about 2%.

Age-related prevalence and clinical significance

• About 2% of the population has this variant.

• It is more common in adolescents (12-18 years), because metatarsal development is not yet complete at this stage.

• In adults, os vesalianum syndrome may be related to overuse or repeated sprains of the lateral foot.

Because it lies close to the tuberosity at the base of the fifth metatarsal, it is very easily mistaken on X-ray for an avulsion fracture at the fifth metatarsal base. The distinction is that the os vesalianum usually has smooth margins and forms a joint-like structure with the fifth metatarsal, whereas a fresh fracture fragment has sharp edges and comes with clinical signs of acute trauma. In adolescents, it must also be distinguished from the fifth metatarsal apophysis (the tuberosity apophysis): a normal apophysis is longitudinally elongated and appears between ages 11 and 15, whereas the os vesalianum is usually a small oval bone. The os vesalianum is usually asymptomatic and found incidentally. However, in rare cases (for example, during an ankle-foot sprain), the cartilaginous connection between the accessory bone and the main bone can be strained and cause lateral foot pain.

Most os vesalianum bones cause no symptoms, but in patients with lateral foot pain, this structure should be considered as a possible source of pain. Diagnosis can be made with X-ray and MRI, and treatment is mainly conservative, such as rest, insole adjustment, and anti-inflammatory drugs. If pain persists, surgical removal of the accessory bone can be considered.

Other Rare Accessory Bones

• Os intermetatarseum (Os Intermetatarseum)

  • Location: between the first and second metatarsals

  • Clinical features: uncommon; some patients have dorsal foot pain or a sense of nerve compression

• Os supranaviculare (Os Supranaviculare)

  • Location: above the navicular

  • Clinical features: rare; easily mistaken on X-ray for a dorsal navicular avulsion fracture, but usually asymptomatic

• Os calcaneus secundarius (Os Calcaneus Secundarius)

  • Location: anterior process of the calcaneus

  • Clinical features: may be related to the attachment of the bifurcate ligament and occasionally causes ankle-foot pain

• Os sustentaculi (Os Sustentaculi)

  • Location: medial side of the calcaneus (sustentaculum tali)

  • Clinical features: may be related to a subtalar coalition; some patients have deep pain on the medial side of the ankle

• Os subtibiale (Os Subtibiale)

  • Location: below the tip of the medial malleolus

  • Clinical features: must be distinguished from a medial malleolus avulsion fracture; rarely causes symptoms

• Os subfibulare (Os Subfibulare)

  • Location: below the tip of the lateral malleolus

  • Clinical features: may be a bone fragment left over from a childhood lateral ankle sprain; some patients have ankle instability

Diagnosis and Treatment

Asymptomatic cases: usually need no treatment and are only found on X-ray.

Mild symptoms:

• Physical therapy and foot muscle training (such as ultrasound, electrotherapy, stretching, and foot exercises)
• Insole adjustment (such as arch support and insoles)
• Oral anti-inflammatory drugs (NSAIDs)

Severe symptoms:

• Local immobilization (a cast or a walking boot)
• Local corticosteroid injection
• Surgical removal of the accessory bone (such as accessory navicular excision or os trigonum excision)

Understanding the types, causes, age-related prevalence, and treatment of accessory bones of the foot can help clinicians and patients arrive at an appropriate diagnosis and management plan, keeping the foot healthy and functioning normally.

References

• https://www.orthobullets.com/pediatrics/4070/accessory-navicular

Further reading

• Sports Injury Overview
• PRP Prolotherapy
• Rehabilitation Exercise Prescription
• Pain Management