Sesamoid bones are small, seed-shaped bones embedded within tendons or joint capsules, positioned at points where a tendon crosses a joint. The kneecap is the most familiar one, but your body contains dozens of others, mostly in your hands and feet. Their defining characteristic is that they are not connected to other bones by typical joints. Instead, they float inside soft tissue and serve a mechanical purpose: they redirect the pull of a tendon, increase its leverage, and protect both the tendon and the underlying joint from excessive friction and pressure.
Where Sesamoid Bones Are Found
The largest and most recognizable sesamoid bone is the patella, or kneecap, which sits within the tendon of the quadriceps muscle group at the front of the knee. It improves the force and velocity of the knee extensors and plays a central role in locomotion and limb stability.1PubMed Central. Sesamoid bones also show functional adaptation in their microanatomy-The example of the patella in Perissodactyla Beyond the patella, the two most commonly discussed sesamoid bones are the pair beneath the big toe, located under the first metatarsal head on the ball of the foot. These are called the tibial (medial) and fibular (lateral) sesamoids, and they bear a remarkable amount of your body weight during walking and running.
The hand is another hotspot. A multicenter radiographic study found that two sesamoid bones were present at the base of the thumb in every single subject examined. Additional sesamoids appeared at the fifth finger’s base joint in about 73% of people, at the index finger’s base joint in about 43%, and at the thumb’s tip joint in roughly 22%. These extra sesamoids were recorded more frequently in women than in men.2PubMed Central. Sesamoid bones of the hand: A multicenter study So while some sesamoid bones are universal, others show up only in certain people, making the exact count different from one skeleton to the next.
What Sesamoid Bones Actually Do
Think of a sesamoid bone like a pulley built into your body. When a tendon wraps around a joint, the sesamoid lifts the tendon slightly away from the joint’s center of rotation. This increases the tendon’s “moment arm,” which is the effective leverage that a muscle has when it pulls. A longer moment arm means the muscle can produce more force or move the joint more efficiently without the muscle itself getting any stronger.
A cadaver study on the foot sesamoids demonstrated this clearly. Researchers measured what happened to the leverage of the flexor hallucis brevis muscle (which bends your big toe) as they progressively removed the sesamoids. Removing just the inner half of one sesamoid or even the entire medial sesamoid did not significantly reduce leverage. But removing both sesamoids caused the muscle’s effective leverage to drop by as much as 29% at certain toe angles.3PubMed. The effect of hallux sesamoid resection on the effective moment of the flexor hallucis brevis The takeaway is that sesamoid bones work as a team. Losing one is usually tolerable; losing both from the same joint changes the mechanics substantially.
Beyond leverage, sesamoid bones also absorb compressive forces and protect tendons from grinding against bone surfaces underneath. In the foot, the sesamoids beneath the big toe glide along grooves on the underside of the first metatarsal head. When the big toe extends during push-off in walking, both sesamoids shift position: the tibial sesamoid displaces by about 30 degrees and the fibular sesamoid by about 36 degrees in the sagittal plane, while the distance between them stays constant throughout the range of motion.4PubMed Central. Displacement of the Metatarsal Sesamoids in Relation to First Metatarsophalangeal Joint Extension They are not passive lumps of bone; they track smoothly with the joint’s movement, maintaining their protective and leverage-enhancing positions.
How Sesamoid Bones Develop
Sesamoid bones form through a process that begins in the embryo and continues well into childhood and adolescence. Early research assumed that mechanical forces from muscles and tendons triggered sesamoid formation, essentially that the tendon’s repeated stress on a joint somehow coaxed bone into existence. More recent developmental work has complicated that picture. Studies on mouse embryos showed that sesamoid bone precursors form from distinct populations of cells that express both cartilage and tendon markers. Critically, these precursors appeared independently of muscle-induced mechanical load, meaning the genetic programming for a sesamoid’s location is already in place before the tendon starts pulling.5bioRxiv. On the Development of Sesamoid Bones
The process of turning from cartilage into mature bone (endochondral ossification) takes its own time depending on the sesamoid. In horses, which are studied because their large sesamoid bones make the process easier to observe, mineralization of the proximal sesamoid bones begins before the articular cartilage and the fibrocartilaginous attachment sites have fully matured. Even at 540 days after birth, the connective-tissue attachment zones on the flexor side of the bone had not yet fully matured.6PubMed. Mineralization of equine proximal sesamoid bones precedes articular cartilage and fibrocartilaginous enthesis maturation in early postgestational development In humans, some sesamoid bones do not ossify until adolescence, which is one reason they serve as useful growth markers.
Sesamoid Bones as Growth Markers in Children
One of the more practical uses of sesamoid bones in medicine has nothing to do with treating the bones themselves. Orthodontists and pediatric endocrinologists have long used the appearance of the thumb’s sesamoid bone on a hand X-ray as a signal that a child’s adolescent growth spurt is approaching. The first radiographic appearance of this bone is considered one of the most reliable and commonly used indicators of the puberty growth spurt.7PubMed. Assessment of puberty growth spurt in boys and girls–a dental radiographic method
The timing is not perfectly precise for every individual. One longitudinal study found that in most people, the appearance of the ulnar sesamoid bone (the sesamoid at the thumb’s base joint visible on the pinky side of the hand X-ray) meant peak height velocity was imminent, but the relationship varied from about three years before to about half a year after the growth spurt in the study group.8Australasian Orthodontic Journal. Sesamoid Bone Appearance as an Indicator of Adolescence Despite this variability, the presence of sesamoids in the thumb’s joint on a hand radiograph can help predict puberty timing in both boys and girls, making it a useful tool when investigating conditions that cause early or delayed puberty.9PubMed Central. The Relationship Between Ossification in Metacarpophalangeal Sesamoids of the Thumb and the Period of Puberty: A Radiographic Study
Uncommon Sesamoid Bones and Anatomical Variation
Not all sesamoid bones are present in every person, and some are outright rare. The fabella, a small sesamoid sometimes found in the tendon behind the lateral side of the knee, is one of the better-known variable sesamoids. When it is present, it usually causes no trouble, but in certain situations it can become a source of pain. One case report described a 65-year-old woman who developed persistent posterolateral knee pain and a clicking sound during knee flexion after a total knee replacement. After eight months of unsuccessful conservative treatment, imaging revealed progressive arthritic changes in her fabella, and surgical removal resolved the problem.10PubMed Central. Fabella Syndrome as an Uncommon Cause of Posterolateral Knee Pain after Total Knee Arthroplasty: A Case Report and Review of the Literature
Even rarer is the cyamella, a sesamoid bone found in the tendon of the popliteal muscle behind the knee. It is ubiquitous in certain other mammals, present in 100% of cats and 84% of dogs.11PubMed Central. The cyamella, a lost sesamoid: Normal variant or posterolateral corner anomaly? In humans, though, the ossified version shows up in fewer than 2% of knees based on the limited studies available. A systematic review and meta-analysis found cyamellae in about 1.4% of examined knees, with no link to height, age, or sex. Ossification could occur as early as age 14. Researchers proposed a classification system based on the cyamella’s three distinct locations, hypothesizing that each location may correspond to a different developmental pathway.12PubMed. Cyamella (a popliteal sesamoid bone) prevalence: A systematic review, meta-analysis, and proposed classification system Whether a cyamella or fabella causes symptoms seems to depend largely on whether the bone impinges on surrounding structures, which is uncommon but not impossible.
Sesamoid Bones Across the Animal Kingdom
The patella’s evolutionary history alone hints at how versatile sesamoid bones can be. Among living mammals, an ossified patella is found in monotremes (like the platypus), almost all placental mammals, and most lizards and birds. Yet it is absent in most marsupials and in bats. Researchers reconstructing the patella’s evolutionary history have concluded that bony kneecaps most likely evolved independently between four and six times in the mammalian family tree, and were lost several times as well, including at least once in bats and one or more times in marsupials (with some later re-acquisition).13PubMed Central. Evolution of the patellar sesamoid bone in mammals That pattern of repeated gain and loss tells us something about sesamoid bones generally: they evolve when the mechanical demands of a joint favor them, and they can be lost when those demands change.
Perhaps the most famous non-human sesamoid bone story involves pandas. Both the giant panda and the red panda have an enlarged radial sesamoid bone in the wrist that functions as an opposable “false thumb,” allowing them to grip bamboo stalks.14PubMed Central. Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas What makes this especially striking is that the giant panda is a bear and the red panda is an ailurid; the two are not closely related. The fact that both independently evolved the same solution, an enlarged sesamoid repurposed as a gripping digit, represents a remarkable case of convergent evolution.15PubMed 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 It also illustrates a broader point: sesamoid bones are not evolutionary afterthoughts. Under the right selective pressures, they can be dramatically reshaped into entirely new functional structures.
When Sesamoid Bones Cause Problems
Sesamoiditis is the most common clinical issue involving these bones, and it overwhelmingly affects the pair beneath the big toe. The condition involves inflammation and pain in and around the sesamoid bones, typically brought on by repetitive pressure, force, or tension.16PubMed. Hallucal sesamoiditis manifested on bone scan Runners, dancers, and anyone who spends a lot of time on the balls of their feet are particularly susceptible. The pain tends to be focused under the big toe joint and can make walking, let alone running, genuinely miserable.
A systematic review of conservative treatments for sesamoiditis found that orthotics and corticosteroid injections appeared beneficial, though high recurrence and limited functional recovery persisted in some cases.17PubMed Central. Conservative Treatment of Sesamoiditis: A Systematic Literature Review with Individual-Level Pooled Data Analysis The pooled data from individual patients showed that about two-thirds experienced a reduction in pain scores with conservative care. However, the picture for athletes was more sobering: among those with persistent pain, none could return to sports pain-free. In the group whose pain resolved, about 45% got back to their previous level of sports participation without pain, while the rest continued to have at least mild discomfort during activity.17PubMed Central. Conservative Treatment of Sesamoiditis: A Systematic Literature Review with Individual-Level Pooled Data Analysis
Beyond simple inflammation, sesamoid bones can suffer stress fractures and avascular necrosis, a condition where the bone’s blood supply is disrupted and the bone tissue begins to die. One case report documented a 17-year-old football player who developed avascular necrosis in both of his hallux sesamoids along with a stress fracture. He was treated with a surgical procedure called core decompression combined with biologic augmentation and returned to full athletic activities by six months after surgery.18PubMed Central. Sesamoid Avascular Necrosis and Stress Fracture Treated with Core Decompression and Biologic Augmentation
The Diagnostic Challenge of Bipartite Sesamoids
One of the trickier aspects of sesamoid bone problems is distinguishing a fracture from a normal anatomical variant called a bipartite sesamoid. In a bipartite sesamoid, the bone develops as two separate pieces rather than fusing into one. This is a normal variant that is relatively common, and on an X-ray it can look a lot like a broken bone. The key difference is that a fracture tends to show irregular, jagged margins, while a bipartite sesamoid has smooth, well-rounded edges where the two pieces meet. Looking for callus formation (new bone laid down during healing) on follow-up imaging can also help confirm a fracture. When plain X-rays are not conclusive, MRI is useful because it can reveal inflammation in the bone, surrounding soft tissues, and the nearby tendons.19PubMed Central. Hallucal sesamoiditis on a bipartite sesamoid bone: An uncommon cause of chronic great toe pain Getting the distinction right matters because treatment decisions are quite different. A bipartite sesamoid causing pain might respond well to offloading and rest, while a displaced fracture may need more aggressive intervention.
When Surgery Becomes Necessary
Surgical removal of a sesamoid bone, called a sesamoidectomy, is typically reserved for cases where conservative treatment has failed over months or where a displaced fracture or nonunion has occurred. A study of 26 sesamoidectomies in athletically active patients found that athletes at the professional or varsity level returned to activity in an average of about 7.5 weeks, while less competitive but still active individuals took about 12 weeks. Complications included one case of the big toe drifting inward (hallux varus), two cases of scar-related nerve-like pain (both after fibular sesamoid removal), and one case of the big toe drifting outward (hallux valgus) after tibial sesamoid removal.20PubMed. Return to activity after sesamoidectomy in athletically active individuals
Newer arthroscopic techniques aim to minimize surgical trauma. A retrospective study of arthroscopic sesamoidectomy reported significant improvements in pain scores and functional scores after the procedure. However, the range of reported complications was broad, including wound pain, numbness, discomfort in severe weather, pain during toe extension, and occasional gait changes.21PubMed. Arthroscopic Sesamoidectomy for Hallux Sesamoid Disorders The earlier cadaver data on leverage loss helps explain why surgeons are cautious about removing sesamoid bones. Removing one sesamoid from the big toe may be mechanically tolerable, but removing both carries a real risk of compromising the toe’s push-off strength. For that reason, surgeons almost always try to preserve at least one sesamoid if possible.
Recovery expectations after sesamoidectomy also depend on what you plan to do with your feet. If you are returning to high-impact sport, the timeline is tighter and the stakes for complications like toe drift or scar sensitivity are higher. If you simply want to walk comfortably, outcomes tend to be more reliably positive. Either way, conservative measures like stiff-soled shoes, custom orthotics, activity modification, and sometimes steroid injections remain the first line of treatment, and surgery enters the conversation only when those have genuinely been exhausted.