How Many Sesamoid Bones Are in the Body?

Most people have somewhere around 42 sesamoid bones, but the real number in any individual can range from the low 30s to over 50. That wide spread exists because, apart from a handful of sesamoids that are present in virtually every human skeleton, the rest form unpredictably, influenced by genetics, mechanical stress, age, sex, and even population background. The patella is the one sesamoid almost everyone can name, yet it represents just a fraction of these small, tendon-embedded bones scattered throughout your hands, feet, and knees.

What Sesamoid Bones Actually Do

A sesamoid bone sits inside a tendon or a joint capsule, usually near a joint where the tendon changes direction or endures heavy friction. The name comes from the Latin for “sesame seed,” a nod to the small, rounded shape most of them share. Their job is mechanical: they act as pulleys, redirecting the line of pull of a tendon so the attached muscle can generate force more efficiently. They also reduce friction between the tendon and the underlying bone, and they protect tendons from being ground down by repetitive motion.

The patella is the textbook example. It sits in the tendon of the quadriceps muscle, in front of the knee, and dramatically increases the leverage of the muscles that straighten your leg. Without it, extending your knee against resistance would require far more muscular effort. Research on the patella across species confirms that it improves both the output force and velocity of the knee extensors, making it central to locomotion and limb stability.1PubMed Central. Sesamoid bones also show functional adaptation in their microanatomy-The example of the patella in Perissodactyla The same principle applies on a smaller scale to the tiny sesamoids in your thumb and big toe: they help tendons glide smoothly and give you a stronger grip or push-off.

The Sesamoids That Are Always There

A small set of sesamoids shows up in essentially every adult human body. These “constant” sesamoids are so reliable that their absence on an X-ray is considered abnormal and can even point to an underlying condition. The list is short:

That gives you a baseline of about eight constant sesamoids: two patellae, two pairs in the thumbs, and two pairs under the big toes. Some anatomists also count the pisiform bone in the wrist as a sesamoid because it develops inside a tendon, though most modern classification treats it as a carpal bone. Either way, the constant set is small. Everything beyond it is where individual variation explodes.

The Variable Sesamoids of the Hand

Your hands are the single richest territory for variable sesamoids. Beyond the near-universal pair at the thumb’s base joint, smaller sesamoids can appear at every other finger joint, and their prevalence follows a surprisingly consistent pattern across studies. In the hand, the little finger’s base joint has a sesamoid roughly 70% of the time, the index finger’s base joint about 50%, and the interphalangeal joint of the thumb about 62%.4SpringerLink / PubMed Central. Anatomy and pathology of the sesamoid bones. The hand compared to the foot The middle and ring fingers trail far behind, at around 3% and 1% respectively.

Population background matters here. A study of the Chinese population found higher rates of sesamoid bones at the second and third finger joints compared to other ethnic groups.5Journal of Orthopaedics, Trauma and Rehabilitation. Distribution of Sesamoid Bones in the Hand—A Study in the Chinese Population A multi-center radiographic study also noted that even the “constant” thumb sesamoids can show morphological variation, with about 30% displaying variant forms such as bipartite or multipartite configurations, where the bone develops with multiple ossification centers instead of fusing into a single piece.6PubMed Central. Prevalence and variation of sesamoid bones in the hand: a multi-center radiographic study That variation matters clinically because a bipartite sesamoid on an X-ray can easily be mistaken for a fracture.

Variable Sesamoids Elsewhere in the Body

The feet mirror the hands in having numerous variable sesamoids beyond the constant hallux pair. Small sesamoids can appear beneath the lesser toes, at the interphalangeal joint of the big toe, and within tendons running along the sides of the foot. One of the better-known foot sesamoids is the os peroneum, a small round or oval bone embedded in the peroneus longus tendon near the cuboid bone on the outer edge of the midfoot.7PubMed Central. Os peroneum imaging: normal appearance and pathological findings It appears in a substantial fraction of the population and is usually harmless, though its presence can predispose the surrounding tendon to rupture due to increased friction with adjacent structures.8PubMed Central. Painful Os Peroneum Syndrome: Underdiagnosed Condition in the Lateral Midfoot Pain

The knee, despite already housing the patella, can contain additional sesamoids. The fabella is a bean-shaped bone that forms in the tendon of the lateral head of the gastrocnemius muscle, behind the knee. The cyamella is even rarer, sitting in the proximal tendon of the popliteus muscle. A meta-analysis found cyamellae in only about 1.4% of knees examined, making it an anatomical curiosity that most people will never know they have (or don’t have).9PubMed. Cyamella (a popliteal sesamoid bone) prevalence: A systematic review, meta-analysis, and proposed classification system Both the fabella and the cyamella are generally asymptomatic, though a cyamella can occasionally be injured alongside other structures in the back of the knee.10PubMed Central. The cyamella, a lost sesamoid: Normal variant or posterolateral corner anomaly?

Why the Fabella Is Getting More Common

One of the stranger findings in sesamoid research is that the fabella appears to be far more prevalent today than it was a century ago. A systematic review using a Bayesian model estimated that the median prevalence in 2000 was about 31%, roughly 3.5 times higher than the estimated 7.6% prevalence in 1900.11PubMed Central. Fabella prevalence rate increases over 150 years, and rates of other sesamoid bones remain constant: a systematic review No other sesamoid bone in the body showed a similar trend over the same period, which makes this something more than a statistical quirk in detection methods.

The leading hypothesis ties the increase to improved global nutrition. People today are taller and heavier on average than people in 1900. A longer tibia creates a larger mechanical moment arm at the knee, and greater muscle mass in the calf means more force passing through the gastrocnemius tendon. Together, those forces could supply the mechanical stimulus that triggers the fabella to form or ossify. A detailed study of biological variation in fabella prevalence found that rates are highest in Asia, followed by Oceania, South America, and Europe, and lowest in Africa, with an overall worldwide average of about 37% of knees having an ossified fabella.12PubMed Central. Human biological variation in sesamoid bone prevalence: the curious case of the fabella That same study found fabellae were slightly more common in men than women and that prevalence increased in people over 70, suggesting that ossification can happen late in life.

When Sesamoids Start to Ossify

Sesamoid bones do not appear on X-rays from birth. Most of them begin as cartilaginous nodules and gradually ossify during childhood and adolescence. The hallux sesamoids, for instance, enter a pre-ossification stage from about age one to seven in both sexes, then begin actively ossifying between ages 8 and 29 in males and 8 and 26 in females.13PubMed. Ossification timeline of sesamoid bones at metatarsophalangeal joints For the fabella, ossification can begin as early as age 12 or as late as the 70s.12PubMed Central. Human biological variation in sesamoid bone prevalence: the curious case of the fabella

This protracted timeline helps explain why sesamoid counts differ so much between studies of children and studies of adults. A radiographic survey of teenagers will miss sesamoids that have not yet ossified, underestimating the true count. It also means a single person’s sesamoid count can literally change over their lifetime as new bones ossify from previously cartilaginous precursors. Research on the developmental biology of sesamoids has shown that the initial formation of these bones does not depend on mechanical load from muscles; they originate from progenitor cells under the regulation of growth-factor signaling pathways, and their further development and ossification are then shaped by both genetic programs and mechanical forces.14Development. Common cellular origin and diverging developmental programs for different sesamoid bones

When Sesamoids Cause Problems

For most people, sesamoid bones go unnoticed for a lifetime. The exceptions tend to involve the hallux sesamoids beneath the big toe, which absorb substantial force during walking, running, and jumping. Runners, dancers, and anyone who spends time on the balls of their feet can develop sesamoiditis (chronic inflammation), stress fractures, or avascular necrosis of these tiny bones. Because they sit right in the weight-bearing path, hallux sesamoid injuries can be surprisingly debilitating and notoriously slow to heal.15PubMed. The hallux sesamoids revisited

When conservative treatment fails, surgical removal of the affected sesamoid (sesamoidectomy) is an option. A study tracking outcomes at two years found that patients who had a hallux sesamoid removed showed meaningful improvements in daily-activity function scores, pain reduction, and both physical and mental health measures. Athletes in that study saw particularly large gains in sports-function scores after surgery.16PubMed. Functional Outcomes Following Hallux Sesamoidectomy in Athletic and Non-Athletic Populations at Two-Year Follow-Up These results are encouraging, though removing a sesamoid does alter the biomechanics of the toe joint, and surgeons try to exhaust non-surgical approaches first.

The os peroneum can also become a source of pain. A condition called painful os peroneum syndrome (POPS) covers a cluster of problems on the outer midfoot, from fractures of the os peroneum itself to tears in the surrounding peroneus longus tendon.8PubMed Central. Painful Os Peroneum Syndrome: Underdiagnosed Condition in the Lateral Midfoot Pain Because the os peroneum is not universally present and not always visible on standard X-rays, POPS is considered underdiagnosed.

Sesamoids in Other Animals

Sesamoid bones are not unique to humans. They show up throughout the vertebrate world, and in some lineages they have been co-opted for purposes far beyond tendon protection. The most famous example is the giant panda’s “false thumb,” an enlarged radial sesamoid in the wrist that functions as an opposable digit for gripping bamboo stalks. The red panda independently evolved a similar structure, making this one of the more striking cases of convergent evolution among mammals.17PubMed Central. Implications of the functional anatomy of the hand and forearm of Ailurus fulgens (Carnivora, Ailuridae) for the evolution of the ‘false-thumb’ in pandas Fossil evidence suggests the giant panda’s false thumb likely evolved specifically for bamboo manipulation, while the red panda’s version initially aided arboreal climbing and only later was recruited for food handling.18PubMed Central. Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas

The giant panda’s false thumb has not grown beyond a certain size, and the reason appears to be a biomechanical trade-off. Pandas are plantigrade, meaning they walk with their palms flat on the ground. A longer radial sesamoid would be better for grasping bamboo but would jut out and interfere with walking. The solution pandas evolved is a hook-shaped distal tip for gripping, paired with a flattened outer surface for weight bearing, a compromise between feeding and locomotion.19Scientific Reports. Earliest giant panda false thumb suggests conflicting demands for locomotion and feeding

Elephants offer another example. Their fore- and hindfeet contain expanded sesamoid structures embedded in fat pads that function like extra digits, giving the animals additional support as they bear their enormous weight. These “predigits” evolved as elephants shifted from flat-footed posture to a more tip-toed stance, and fossil evidence shows the transformation unfolded gradually over millions of years.20PubMed. From flat foot to fat foot: structure, ontogeny, function, and evolution of elephant “sixth toes” Meanwhile, in reptiles, the patella itself appears to have evolved independently in lizards, and mineralized patellar structures have been identified even in tuatara, a lineage that diverged from other reptiles hundreds of millions of years ago.21PubMed Central. Anatomy, morphology and evolution of the patella in squamate lizards and tuatara (Sphenodon punctatus) Some reptiles that lack a patella possess other knee sesamoids like the fabella and cyamella instead, suggesting that the mechanical problem these bones solve is ancient and has been addressed by different skeletal solutions across lineages.22PubMed Central. Knee tissue diversity and evolution in different reptile taxa

Putting a Number on It

The figure of 42 that shows up in many anatomy references is a rough average for an adult, and it assumes a fairly typical set of variable sesamoids in the hands and feet. But “typical” is doing a lot of work in that sentence. A person who happens to lack sesamoids at most of their finger joints, has no fabella, and has no os peroneum could have closer to 30. Someone on the other end of the distribution, with bilateral fabellae, os peronea, and a generous spread of hand and foot sesamoids, might be well into the upper 40s or beyond. The patellae alone account for two; the constant hand and foot sesamoids add another six to eight; everything else depends on the individual.

This variability is part of why the “206 bones in the human body” figure you learn in school is itself an oversimplification. That count typically includes the patellae but handles the remaining sesamoids inconsistently, sometimes ignoring them and sometimes folding in a standard set. If every ossified sesamoid were counted, many adults would clear 250 bones. The honest answer to “how many sesamoid bones do I have?” is that you would need X-rays of your hands, feet, and knees to know for sure, and even then, the count might change as you age.