The normal human foot contains twenty-eight bones. These bones help to give shape and function to the foot, and help provide the framework for shock absorption and mechanics that make movement possible. Sometimes, though, a foot contains more than twenty-eight bones. A number of accessory bones may be present in the foot as well, which may or may not be pathologic.
Accessory bones can be divided into two categories – ossicles and sesamoids. Sesamoids are small, round bones found within tendons in the foot. The two common sesamoids are within the flexor hallucis brevis, and help comprise the sesamoid apparatus. The sesamoid apparatus aids in function of the first metatarsophalangeal joint.
Some of the more common accessory sesamoids include:
- Os peroneum – located within the tendon of the peroneus longus, commonly seen near the peroneal notch of the cuboid.
- Os interphalangeus – Located within the long flexor tendons, adjacent to the interphalangeal joints of the digits.
- Capsular sesamoids – these are located within the tendons crossing the metatarsophalangeal joints, typically in the long flexors
Accessory ossicles differ from sesamoids in the way they develop. They develop from a separate center of ossification in close proximity to the normal anatomy. Some of the more common accessory ossicles include:
- Os Trigonum – develops as a separate ossicle posterior to the lateral tubercle on the posterior surface of the talus. When the ossicle is fused, it is known as a Steida’s process. These can often become pathologic as they can cause friction on the tendon of the flexor hallucis longus, which can cause irritation and pain.
- Os tibiale externum – located on the medial side of the foot in close proximity to the navicular. There are several variations of this accessory bone that exist, including those that have fused to the navicular. These are often symptomatic, and may also be referred to by some clinicians as a pre-hallux
- Os supranaviculare – also referred to as os talonavicular dorsale, it is found on the dorsal aspect of the talonavicular joint.
A number of accessory bones exist in the foot, which may or may not become symptomatic. Clinically, accessory bones can often appear as fractures on x-ray, and may even mimic them in their symptoms. Sesamoids may appear as bipartite, meaning they are split in two pieces, or tripartite, meaning they are split into three pieces. Bipartite and tripartite sesamoids can often resemble fractures on x-ray.
Accessory bones, when symptomatic, can be treated by offloading the area, debridement of callus that forms over them, or even surgical resection for some.
