Sesamoid bones are small, often overlooked skeletal elements embedded within tendons or ligaments near joints, named for their resemblance to sesame seeds. The most familiar example is the patella, or kneecap, but the human body contains dozens of these bones, and their roles range from redirecting the pull of a muscle to protecting tendons from wear. Despite their modest size, sesamoid bones have attracted growing scientific attention for their surprising evolutionary tricks, their puzzling variation from person to person, and their ability to cause real clinical headaches when they go wrong.
Where Sesamoid Bones Sit and What They Do
Sesamoid bones are variably present skeletal elements found in tendons and ligaments near joints throughout the body.1PubMed Central. Easy to gain but hard to lose: the evolution of the knee sesamoid bones in Primates—a systematic review and phylogenetic meta-analysis The patella is the largest and most consistent, but the pair you are most likely to injure sits under the ball of your foot, just behind the big toe. These two pea-sized bones, the tibial (medial) and fibular (lateral) hallux sesamoids, are embedded in the tendons of the flexor hallucis brevis, the short muscle that helps you push off the ground with every step. They act like tiny pulleys: by lifting the tendon slightly away from the joint’s axis, they increase the muscle’s mechanical leverage.
A cadaver study quantified exactly how much these foot sesamoids matter. With both sesamoids intact, the effective tendon moment arm, essentially the muscle’s leverage, was at its peak. Removing the inner sesamoid alone made no statistically meaningful difference. But removing both sesamoids cut the muscle’s effective leverage by roughly a quarter to nearly a third at higher angles of toe extension.2PubMed. The effect of hallux sesamoid resection on the effective moment of the flexor hallucis brevis In practical terms, this means one sesamoid can compensate for the other, but losing both leaves the toe significantly weaker during push-off.
Beyond the foot, sesamoid bones appear in the hand, the wrist, and around the knee. Some are nearly universal in humans, while others show up in only a fraction of the population. This variability is part of what makes them so interesting, and so easy to mistake for something else on an X-ray.
How Sesamoid Bones Form During Development
For a long time, researchers assumed that sesamoid bones formed primarily in response to mechanical stress. The logic seemed straightforward: wherever tendons experienced high friction or pressure, a bony nodule would crystallize in response. This turns out to be only part of the story. Mouse studies have shown that sesamoid bones, including the patella and several smaller sesamoids in the digits and wrist, form even in embryos whose muscles are non-functional and produce no mechanical load.3PubMed Central. Common cellular origin and diverging developmental programs for different sesamoid bones The initial induction of these bones is genetically programmed, not mechanically forced.
Specifically, all three types of sesamoid bones examined in mouse limbs originate from progenitor cells that express both Sox9 and Scx, markers shared by cartilage and tendon precursors. Their formation is regulated by TGFβ signaling: when the TGFβ receptor was knocked out in limb mesenchyme, the patella, the lateral fabella, and a digit sesamoid all failed to develop.4Development. Common cellular origin and diverging developmental programs for different sesamoid bones BMP signaling also plays a role, though the details differ between sesamoid types. BMP2 regulates growth across all examined sesamoids, while the lateral fabella and digit sesamoids also depend on BMP4 in a partially redundant way. Knocking out both BMP2 and BMP4 eliminated all three sesamoid bones entirely.5PubMed Central. Common cellular origin and diverging developmental programs for different sesamoid bones
So the picture is layered: genetics and signaling pathways lay the groundwork, but whether a sesamoid fully ossifies into hard bone or remains as soft cartilage can depend on the mechanical environment later in life. This dual influence, genetic initiation plus environmental modulation, helps explain why sesamoid presence varies so much between individuals.
The Fabella Is Getting More Common
The fabella is a small sesamoid bone that sits behind the knee, embedded in the tendon of the lateral head of the gastrocnemius muscle. In many people it never ossifies at all and exists only as a nub of cartilage, invisible on X-rays. But a systematic review found something unexpected: the fabella has become dramatically more common over the past century. The median prevalence around 1900 was about 7.6%, while by the year 2000 it had climbed to roughly 31%, a roughly 3.5-fold increase.6PubMed Central. Fabella prevalence rate increases over 150 years, and rates of other sesamoid bones remain constant: a systematic review In every country with data from both before and after 1960, the post-1960 rates were higher.
This increase appears to be unique to the fabella. Prevalence rates for ten other sesamoid bones in the human body showed no similar trend over the same period. Because the change happened too fast to be genetic, the leading hypothesis points to environmental factors, specifically improved nutrition driving increases in human height and weight. Taller people have longer tibias, which create a larger moment arm at the knee. Combined with greater muscle mass, this produces more mechanical stress in the gastrocnemius tendon, potentially triggering the fabella to ossify.7PubMed Central. Fabella prevalence rate increases over 150 years, and rates of other sesamoid bones remain constant: a systematic review
The fabella also varies by geography. Worldwide, about 37% of knees have an ossified fabella detectable by dissection, but rates are highest in Asia, followed by Oceania and South America, and lowest in Africa. Men are a few percentage points more likely to have one than women, and prevalence climbs in old age, suggesting that the bone can continue to ossify well into later decades of life. About 73% of the time, when a person has a fabella, they have one behind each knee.8PubMed Central. Human biological variation in sesamoid bone prevalence: the curious case of the fabella
Having a fabella is usually harmless, but it is not always benign. It can cause posterolateral knee pain, irritate surrounding tendons, or compress the nearby common peroneal nerve. In patients undergoing total knee replacement, a large fabella can interfere with the prosthetic components. The rise in fabella prevalence means orthopedic surgeons are encountering it more often and need to consider it in differential diagnoses of unexplained knee pain.
Sesamoid Bones in the Animal Kingdom
Sesamoid bones are not a human peculiarity. They appear across vertebrates, and in some lineages they have been co-opted for remarkable new functions. The patella, for instance, plays a crucial role in hindlimb function wherever it appears in mammals, enhancing the lever system of the knee joint to help resist gravity.9PubMed Central. Evolution of the patellar sesamoid bone in mammals But the most dramatic evolutionary stories involve sesamoid bones that have been repurposed far beyond their original pulley role.
The giant panda’s famous “thumb” is actually a radial sesamoid, a wrist bone that has enlarged over millions of years to function as an opposable digit for gripping bamboo stalks.10PubMed Central. Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas This is an elegant workaround: mammals are constrained to five true digits, so the panda evolved a sixth functional finger from existing hardware. The red panda has a similar structure, though proportionally smaller, less compressed, and with less grasping ability than its giant cousin.11PubMed 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
The giant panda’s false thumb has not grown as large as it theoretically could, and the reason reveals how evolution manages tradeoffs. Because all bears walk with their palms flat on the ground, an overly elongated radial sesamoid would jab into the earth with every step. Fossil evidence shows that the false thumb reached roughly its current proportions millions of years ago and then stopped growing. Instead, the bone’s tip curves inward as a hook for gripping bamboo, while its outer surface is flattened to serve as a weight-bearing pad during walking.12Scientific Reports. Earliest giant panda false thumb suggests conflicting demands for locomotion and feeding The panda’s thumb is a compromise frozen in bone.
Moles have pulled off a similar trick. Talpid moles across the Northern Hemisphere have a large, sickle-shaped radial sesamoid bone that functions essentially as a sixth digit, widening the paw for digging. This is always paired with a smaller tibial sesamoid in the hind foot. Because mammals cannot easily evolve a true extra finger, moles circumvented this constraint by recruiting wrist sesamoids into the digit series through a novel developmental pathway.13PubMed Central. Circumventing the polydactyly ‘constraint’: the mole’s ‘thumb’
Ostriches take things in a different direction entirely. They possess two patellae, a standard proximal one and a second distal patella unique to their lineage. Researchers have used computer modeling to test whether the mechanical environment in the ostrich patellar tendon favors the formation of multiple kneecaps, exploring how compressive forces within the tendon encourage bone or cartilage tissue to form in specific locations.14Royal Society Open Science. Finite-element modelling of mechanobiological factors influencing sesamoid tissue morphology in the patellar tendon of an ostrich
When Foot Sesamoids Cause Trouble
The hallux sesamoids under the big toe are the sesamoid bones most likely to send you to a doctor. They sit right at the point where the forefoot bears the most load during walking, running, and jumping. In a study of pediatric and adolescent athletes who visited a sports medicine clinic, the most common diagnosis was sesamoiditis, an inflammation of the bone or surrounding tissue, which accounted for about 30% of sesamoid injuries. Stress fractures were the next most frequent, at roughly 13%.15Orthopaedic Journal of Sports Medicine. Sesamoid Injuries in Pediatric and Adolescent Athletes Presenting to Sports Medicine Clinic
Sesamoiditis tends to come on gradually and causes aching pain under the ball of the foot, especially during activities that load the forefoot, like running, dancing, or wearing high heels. Stress fractures are more acute, sometimes following a sudden increase in training intensity. A more serious but rarer condition is avascular necrosis, where the blood supply to the sesamoid is disrupted and the bone tissue begins to die. One case report described a 17-year-old football player with avascular necrosis in both hallux sesamoids alongside a stress fracture, illustrating how these problems can overlap.16PubMed Central. Sesamoid Avascular Necrosis and Stress Fracture Treated with Core Decompression and Biologic Augmentation
Telling these conditions apart on imaging can be tricky. A plain X-ray may show a fragmented sesamoid, but that could be a fracture, avascular necrosis, or simply a bipartite sesamoid, a normal anatomical variant where the bone develops in two pieces and never fuses. Bipartite sesamoids occur naturally in a significant percentage of people and are usually painless. CT and MRI help sort things out: a fracture tends to show irregular, poorly corticated fragment edges, while avascular necrosis shows up as bone marrow edema localized to the sesamoid.17MedAlliance. Differential diagnosis of aseptic (avascular) necrosis and stress fracture of the sesamoid bone (review) Ultrasound can also be useful for evaluating forefoot pain and visualizing both sesamoiditis and surrounding soft-tissue problems.
Surgery on Sesamoid Bones
Most sesamoid problems in the foot respond to conservative treatment: offloading the area with custom orthotics or pads, modifying activity levels, anti-inflammatory medication, and occasionally immobilization in a walking boot. When these measures fail, surgery may be considered. The most common procedure is sesamoidectomy, removing one of the two hallux sesamoids.
The concern with sesamoidectomy has always been whether removing a sesamoid would destabilize the big toe and push it into a bunion-like deformity. A systematic review of outcomes after sesamoidectomy found that the hallux valgus angle and the intermetatarsal angle did not increase significantly after surgery.18PubMed. Sesamoidectomy for Hallux Sesamoid Disorders: A Systematic Review A separate study focused on removal of the fibular (lateral) sesamoid alone through a plantar approach confirmed similar results: neither the hallux valgus angle nor the intermetatarsal angle changed meaningfully, and flexion strength of the big toe was preserved, with no significant difference between the operated and unoperated foot.19PubMed. Efficacy, Outcomes, and Alignment Following Isolated Fibular Sesamoidectomy via a Plantar Approach
These findings align with what the cadaver study mentioned earlier would predict: removing one sesamoid does not appreciably weaken the muscle’s leverage. Removing both, however, is a different story, which is why surgeons avoid bilateral sesamoidectomy whenever possible. The remaining sesamoid takes over as the primary fulcrum, and in most patients, toe alignment and strength stay intact after a single removal.
Accessory Bones That Mimic Fractures
Sesamoid bones are not the only small skeletal extras that can complicate a diagnosis. The foot contains several accessory ossicles, tiny bones that form from secondary ossification centers and never fuse to their parent bone. One of the rarer examples is the os vesalianum pedis, a small bone found near the base of the fifth metatarsal on the outer edge of the foot. It is usually discovered by accident on an X-ray taken for another reason and causes no symptoms.20PubMed. Lateral foot pain due to os vesalianum pedis in a young football player; a case report and review of the current literature
The trouble arises when someone twists an ankle and the radiologist spots what looks like a bone fragment at the base of the fifth metatarsal. An avulsion fracture at that location is a common sports injury, and an os vesalianum can look nearly identical on a standard X-ray. Mistaking one for the other could mean unnecessary surgery or, conversely, missed treatment. A case report described this exact scenario, where a painful os vesalianum masqueraded as a fracture and led to initial diagnostic confusion.21PubMed Central. Red Herring in Orthopedics: A Case Report on Painful Os Vesalianum Pedis Masquerading as an Avulsion Fracture of 5th Metatarsal and Review of Literature A meta-analysis confirmed that while the os vesalianum rarely causes lateral foot pain on its own, it must be differentiated from fifth metatarsal fractures and from Iselin’s disease, a growth plate injury in adolescents.22PubMed. The prevalence and clinical considerations of Os Vesalianum Pedis: A meta-analysis
The clinical lesson extends beyond this one bone. The foot has over a dozen recognized accessory ossicles, and their presence varies between individuals. If you have ever been told you have a “chip fracture” on an X-ray that did not match your level of pain, an accessory bone is worth considering. Comparing the other foot’s X-ray often settles the question, since these extras tend to appear symmetrically.
Intermuscular Bones in Fish and the Frontier of Gene Editing
The word “sesamoid” usually brings mammals to mind, but bony fishes have their own version of the problem: intermuscular bones, or IMBs. These are slender, Y-shaped or needle-like bones scattered through the muscle tissue of carp, herring, pike, and many other freshwater species. They are a major nuisance for fish consumers and a real obstacle for the aquaculture industry, since their presence makes fillets harder to eat and process.
Researchers identified a gene called bmp6 as essential for intermuscular bone development. Knocking out bmp6 in zebrafish eliminated these bones entirely without affecting the fish’s growth or reproduction. Building on that work, a team applied the same approach to crucian carp, a commercially farmed species, and generated a new strain in which both copies of the bmp6 gene were knocked out. In the resulting fish, bone staining and X-ray imaging confirmed that no intermuscular bones were present.23Aquaculture. Generate a new crucian carp (Carassius auratus) strain without intermuscular bones by knocking out bmp6 The strain, dubbed WUCI, represents the first successful production of a bone-free cyprinid line through gene editing.
This work is a striking example of how understanding the developmental genetics of small skeletal elements can translate into practical applications far removed from human medicine. The BMP signaling pathways that regulate intermuscular bones in fish overlap conceptually with the BMP pathways that control sesamoid formation in mammals, a reminder that evolution often reuses the same molecular toolkit for very different purposes in different lineages. Whether gene-edited boneless carp will reach consumers depends on regulatory approval and public acceptance of gene editing in food animals, but the biological proof of concept is clear.
Tissue Engineering and What Sesamoid Bones Can Teach Us
Sesamoid bones sit at interfaces where tendon meets bone, and these junctions are among the most mechanically demanding zones in the body. The tissue at such interfaces is not a clean boundary but a gradual transition: tendon fibers give way to fibrocartilage, then mineralized fibrocartilage, and finally bone. Research into the microstructure beneath the tendon-bone interface has revealed that both mineralized fibrocartilage and the underlying bone show strong directional alignment and finger-like interdigitations at the transition zone.24Scientific Reports. Local anisotropy in mineralized fibrocartilage and subchondral bone beneath the tendon-bone interface These features help distribute stress across the junction rather than concentrating it at a single plane.
For bioengineers trying to repair torn tendons or design artificial grafts, these details matter. A tendon graft that is bolted to bone without mimicking the natural gradual transition tends to fail at the attachment site. Understanding how sesamoid bones and their surrounding tissues manage the handoff between soft tendon and rigid bone is informing the design of next-generation surgical repairs and lab-grown tissue scaffolds. The sesamoid, once dismissed as a minor anatomical footnote, turns out to be a useful model for one of the trickiest problems in orthopedic engineering: making two fundamentally different tissues work together under high, repeated loads.