Accessory Bones at the Foot and Ankle - An Overview

Introduction

Accessory bones are normal or anatomical variants of the skeletal system. These are primarily seen in the foot and ankle regions. They mainly occur due to failure to unify the secondary ossification centers close to the central bony mass. These may be present separately or close to the main bone mass. These ossicles can be unilateral or bilateral; in most cases, they are detected accidentally.

The skeletal foot variants are classified as sesamoid bones, accessory bones, coalitions, and bipartitions. Accessory bones are also referred to as inconstant or supernumerary bones. These are anatomical skeletal variants, and almost 40 have been described so far. The prevalence of the condition varies from 21 to 49 percent. In most cases, these are asymptomatic but may cause symptoms in some rare cases.

What Is the Etiology?

The foot is a structure that was subject to drastic changes during evolution. Studies suggest that accessory bones may be rudimentary remnants of supernumerary tarsal bones found in amphibians. Another theory suggests that accessory bones result from the failure to unify ossification centers during prenatal and postnatal development. This is referred to as the ontogenetic theory. Some studies even suggest a post-traumatic origin for accessory bones. The exact etiology remains to be discovered in most cases.

What Is the Clinical Relevance?

In most cases, the accessory bone is asymptomatic. Still, it can develop symptoms like pain due to various reasons like infections, trauma, fracture, dislocation, tumor, irritation, osteoarthritis, osteonecrosis, degenerative diseases, and impingements. In some cases, these accessory bones may be misdiagnosed as a fracture or non-union and thus overtreated.

Dislocations and fractures are the most common causes of accessory bone disorders. Due to fractures, these accessory bones may get infected or dislocated. These can restrict the range of motion and stimulate other fractures.

What Are the Diagnostic Methods?

The most convenient and effective way to make a diagnosis is through a radiologic examination. Each specific bone has a unique projection that aids in its best visualization. CT (computed tomography) scans help visualize these accessory ossicles and their relation to the adjacent bone.

Ultrasound helps visualize non-ossified bones and evaluate soft tissue injuries or inflammation. MRI (magnetic resonance imaging) and CT scans help differentiate accessory bones from fractures and help understand their clinical relevance. A technetium bone scan is useful for symptomatic accessory ossicles, as it helps differentiate them from acute injuries.

What Are the Different Accessory Bones of the Foot and Ankle?

Around 40 accessory bones have been described, and the common ones in the foot and ankle are described below:

1. Os Tibiale Externum:

• One of the most common accessory foot bones.

• The prevalence is 4 to 34 percent.

• Synonyms - accessory navicular, accessory scaphoid, os naviculare secundarium.

• Located in the plantar, medial, and proximal aspects of the navicular bone (the wedge-shaped bone in the midfoot region that articulates with the tarsal bones).

2. Os Peroneum:

• This bone lies in the peroneus longus tendon.

• The prevalence ranges from 9 to 26 percent.

• Is oval in shape and situated lateral to the cuboid bone.

• In 60 percent of cases, the bone is seen bilaterally and is radiologically visualized by an oblique view of the feet.

3. Os Trigonum:

• Synonyms - talus secundarius, Os intermedium.

• Usually seen unilaterally.

• The prevalence is 7 to 25 percent.

• Oval, round, or triangular.

• It is connected to the lateral tubercle through a fibrocartilagenous synchondrosis.

• Forced plantarflexion can lead to fracture of the posterior process of the talus, or Os trigonum.

4. Calcaneus Secundarius:

• Synonym - secondary Os calcis.

• These accessory bones are located in the anterior facet of the calcaneus bone (The calcaneus is the large heel bone articulated with the talus bone of the ankle and the cuboid bone of the feet).

• The prevalence is 0.4 to 11 percent.

• It can result in restricted midtarsal motion and pain.

• In the case of painful ossicles, surgical treatment provides full relief.

• Radiological imaging confirms the diagnosis.

5. Os Supranaviculare:

• Synonym - Os talonaviculare dorsale.

• Located in the proximal dorsal aspect of the talonavicular joint or navicular bone.

• An uncommon incidental skeletal variant with a prevalence of 1 percent.

• Symptomatic cases are rare.

• Surgical resection relieves the symptoms.

6. Os Supratalare:

• Located on the dorsal side of the talar neck.

• It may either remain a free bone or fuse with the talus.

• The prevalence is 0.2 to 2.4 percent.

• In most cases, they are asymptomatic.

• Degenerative changes or pain can develop due to trauma to the talonavicular joint.

7. Os Subfibulare:

• Located below the lateral malleolus (prominent bone on the outer side of the ankle).

• The prevalence ranges from 0.2 to 6.6 percent.

• Two theories are suggested related to its origin; one suggests these are remnants of accessory ossification centers in the distal fibula. Another theory suggests these are avulsion fractures of the anterior talofibular ligament.

• In symptomatic cases, the patient presents with pain or lateral ankle instability.

• In cases where non-surgical treatment fails, surgical treatment is done.

• Excision is done in cases of small, painful ossicles without ankle instability.

8. Os Subtibiale:

• The prevalence is 0.7 to 1.2 percent.

• Located below the medial malleolus.

• Frequently rounded triangular shape.

9. Os Intermetatarseum:

• Situated between the base of the first and second metatarsal and medial cuneiform.

• The commonest accessory bone in the forefoot region.

• The prevalence is 1 to 7 percent.

• Either found as a separate bone or fused with the second or first metatarsal base.

10. Os Vesalianum:

• Uncommon accessory bone located in the fifth metatarsal base.

• The incidence is less than 1 percent

11. Os Sustentaculi:

• A small ossicle mostly connected to sustentaculum tali.

12. Calcaneus Accessorius:

• Located on the lateral side of the calcaneus.

• In symptomatic cases, surgical resection is done.

13. Os Talus Secundarius:

• Located on the lateral side of the talus.

• Size and shape vary.

• Mostly asymptomatic.

• An ankle sprain may result in pain.

• Symptomatic cases are treated with surgical resection.

14. Os Intercuneiforme:

• Located distal to the navicular bone between the intermediate and medial cuneiform.

• Mostly of triangular shape and treated by conservative methods.

15. Os Paracuneiforme:

• Uncommon accessory bone similar to Os intercuneiforme.

16. Os Retinaculi:

• Uncommon ossicle, mostly flat in shape.

• Located on the lateral side of the distal fibula.

17. Os Apponuerosis Plantaris:

• Found enclosed in the plantar aponeurosis.

• Mostly flat or oblong.

18. Os Achille:

• Found in the Achilles tendon.

• No clinical significance has been reported to date.

19. Os Subcalcis:

• Extremely uncommon ossicle.

• Situated in plantar aspect of calcaneal tuberosity.

20. Os Uncinatum:

• Located in the plantar aspect of lateral cuneiform.

21. Os Supracalcaneum:

• Located posterior to Os trigonum.

• Results in pain and swelling related to the posterior aspect of the foot.

Conclusion

Accessory bones, which are normal skeletal variants, are most prevalent in the foot and ankle regions. The etiology remains controversial, with different theories like ontogenetic theory, evolution, or posttraumatic origin suggested. Clinical symptoms, along with radiologic imaging, help in a proper diagnosis. A proper diagnosis is clinically relevant to prevent overtreatment.