Do some individuals have a tiny bone?

September 27, 2025 •

4 min read

The human skeletal structure is generally consistent, but small anatomical variations are common across the population. Some individuals do, in fact, have a tiny bone that others lack, which typically arises from either an unfused developmental center or a bone that forms within a tendon.

Quick Summary

Yes, some people have extra bones, known as accessory or sesamoid bones, which are normal anatomical variants that can occur in different parts of the body, though most frequently in the feet, and often cause no symptoms unless irritated.

Key Points

Anatomical Variation: Many individuals have tiny, extra bones called accessory or sesamoid bones, which are normal anatomical variants and not a medical defect.
Common Locations: These extra bones are most frequently found in the feet, ankles, and hands, such as the accessory navicular or fabella.
Sesamoid vs. Accessory: Sesamoid bones are embedded in tendons to improve mechanical function (like the patella), while accessory bones result from unfused ossification centers and typically have no function.
Pain is Not Universal: Most extra bones are asymptomatic and are discovered by chance during X-ray imaging for other reasons.
Symptomatic Cases: In some instances, trauma, overuse, or chronic irritation can cause pain, a condition known as a painful ossicle syndrome, which can be treated conservatively or, rarely, surgically.
Cervical Ribs: A rare type of extra bone is a cervical rib, which can sometimes cause compression of nerves and blood vessels, leading to thoracic outlet syndrome.

The fascinating world of accessory and sesamoid bones

Did you know that not everyone has the same number of bones? While the typical human skeleton has around 206 bones, this number can vary from person to person due to the presence of small, extra bones. These tiny bones are not signs of a medical disorder but rather normal anatomical variations that are relatively common.

What are accessory bones?

Accessory bones, or supernumerary bones, are extra bones that develop from secondary ossification centers that fail to fuse with the main bone during childhood development. They are most frequently found in the foot, ankle, and wrist. For many people, these extra bones cause no issues and are only discovered incidentally during an X-ray for an unrelated problem. In some cases, however, they can become symptomatic due to trauma, overuse, or chronic irritation.

Some of the most common accessory bones include:

Accessory Navicular (Os Tibiale Externum): An extra piece of cartilage or bone on the inner side of the foot, located near the navicular bone and incorporated within the posterior tibial tendon. This occurs in about 10–15% of the population.
Os Trigonum: A small bone that forms behind the talus (ankle bone). It is relatively common, with some studies showing its presence in 15–25% of individuals.
Os Peroneum: A sesamoid bone embedded within the peroneus longus tendon near the cuboid bone. It is fully ossified in up to 26% of feet.
Cervical Rib: An extra rib that grows from the seventh cervical vertebra at the base of the neck. It is found in a small percentage of the population (0.5–1%) and can sometimes cause issues like thoracic outlet syndrome by compressing nerves or blood vessels.

What are sesamoid bones?

Sesamoid bones are small, nodular bones embedded within a tendon where it passes over a joint. Their primary function is to protect the tendon from friction and wear, increase the mechanical leverage of the muscle, and change the direction of tendon pull. The most well-known sesamoid bone is the patella, or kneecap, which is present in everyone. However, many other, smaller sesamoids exist and are inconsistent among individuals.

Hallux Sesamoids: Two small, pea-shaped bones found under the head of the first metatarsal bone (at the base of the big toe). They act as a pulley for the flexor hallucis brevis tendon, which controls the big toe. These are constant in humans.
Fabella: A small sesamoid bone located within the tendon of the lateral head of the gastrocnemius muscle at the back of the knee. Once considered rare, its prevalence has reportedly increased over the past century. While often asymptomatic, it can cause pain known as fabella syndrome in some individuals.

Comparison of Accessory and Sesamoid Bones

To better understand the distinction, here is a comparison of their key differences:


Feature	Sesamoid Bone	Accessory Bone
Definition	Embedded within a tendon or muscle, serving a specific biomechanical purpose.	An extra bone resulting from an unfused ossification center, typically lacking a defined function.
Function	Increases leverage, reduces friction, and protects tendons.	Generally considered a normal variant with no specific function, but can occasionally serve as a point of attachment for ligaments or tendons.
Prevalence	Many are consistently present (e.g., patella, hallux sesamoids), while others (e.g., fabella) vary.	Prevalence varies widely depending on the specific bone, with some present in 10-20% of the population.
Symptoms	Can cause pain if injured or subjected to repetitive stress, a condition known as sesamoiditis.	Often asymptomatic, but can become painful due to trauma, overuse, or direct pressure from footwear.
Example	Patella (kneecap), hallux sesamoids (in foot).	Accessory navicular (in foot), cervical rib (in neck).

Potential issues and clinical significance

Although most extra bones are benign and go unnoticed, there are instances when they can become problematic. For example, conditions like accessory navicular syndrome or os trigonum syndrome occur when the extra bone becomes inflamed or irritated, causing pain and swelling. These painful syndromes are often triggered by trauma, repetitive activity, or pressure from footwear.

Diagnosis and treatment

Diagnosing a symptomatic accessory or sesamoid bone often requires medical imaging, such as an X-ray, to confirm its presence and rule out other conditions like fractures. In cases where the bone is causing pain, treatment typically begins with conservative methods, including rest, ice, anti-inflammatory medication, physical therapy, and modifying footwear or activities. If these measures are ineffective, a doctor may recommend custom orthotics to alleviate pressure or, in rare cases, surgical removal of the tiny bone.

Conclusion

In summary, the presence of an extra tiny bone is not a defect but a normal variant of human anatomy. It is a testament to the fact that every individual is slightly different, even at a skeletal level. For the vast majority, these extra bones are completely harmless and are never a cause for concern. However, in the rare instances that they do cause pain, effective treatment options are available to manage the symptoms and provide relief. Awareness and proper diagnosis are key to differentiating an incidental anatomical variation from a painful syndrome. For more information on bone health, consult resources like the National Institutes of Health for reliable and detailed guidance.

Frequently Asked Questions

Yes, tiny, extra bones—known as accessory ossicles or sesamoid bones—are relatively common. Their prevalence varies, but some are found in up to 10-20% of the population, with many people having no symptoms.

Sesamoid bones are functionally embedded within tendons to reduce friction and increase leverage, while accessory bones are extra bones that result from unfused bone development centers and often have no function.

No, most individuals with an accessory or sesamoid bone live their entire lives without experiencing any symptoms. They are often discovered accidentally during routine medical imaging, such as an X-ray.

Yes, repetitive stress, trauma, overuse, or pressure from footwear can irritate an extra bone and the surrounding soft tissue, leading to pain and swelling. Conditions like sesamoiditis or accessory navicular syndrome are examples of this.

The fabella is a small, sometimes-present sesamoid bone located in the tendon at the back of the knee. Though often harmless, it can cause pain if it impinges on the knee joint or surrounding nerves.

Painful extra bones are typically treated with conservative methods first, including rest, ice, anti-inflammatory drugs, physical therapy, and supportive footwear. In rare, persistent cases, surgical removal may be an option.

A cervical rib is a rare extra rib that originates from the seventh cervical vertebra in the neck. In some cases, it can compress nerves and blood vessels, causing a painful condition called thoracic outlet syndrome.

No, accessory and sesamoid bones are not considered birth defects. They are normal, albeit infrequent, anatomical variations resulting from the natural process of bone development.