A sesamoid bone is a small bone embedded within a tendon or muscle, usually near a joint. Unlike most bones in your body, which connect to other bones, sesamoid bones float within soft tissue and act like tiny pulleys. They redirect forces, reduce friction, and help tendons glide smoothly over joints. The most familiar example is your kneecap.
How Sesamoid Bones Work
Think of a sesamoid bone as a built-in mechanical advantage for your tendons. When a tendon wraps around a joint, it experiences friction and pressure at the bending point. A sesamoid bone sitting inside that tendon lifts it slightly away from the joint surface, changing the angle of pull and spreading the force over a wider area. This lets the tendon handle heavier loads without wearing down or tearing.
The result is less friction, better leverage, and greater weight-bearing capacity. Without sesamoid bones, the tendons in your feet and knees would grind directly against bone surfaces during movement. Over time, that would cause significant damage. These small bones are a key reason you can push off the ground when you walk, run, or jump without shredding the tendons involved.
Where Sesamoid Bones Are Found
The largest and most well-known sesamoid bone is the patella, your kneecap. It sits within the tendon of your quadriceps muscle and acts as a pivot point, letting that powerful muscle group straighten your leg with less effort. When your knee fully extends, the patella is the only structure making contact with the thighbone, creating a pulley effect that multiplies the force your quadriceps can generate.
Beyond the kneecap, the most common location for sesamoid bones is the ball of your foot, just beneath the big toe joint. Two small, pea-sized sesamoid bones sit under this joint and bear your weight every time you push off during a step. They help the tendons that flex your big toe work efficiently and absorb the repeated impact of walking.
Sesamoid bones also appear in the hands, particularly near the base of the thumb, and occasionally near other joints. The exact number varies from person to person. Some people have extra sesamoid bones in places where others don’t, and this is completely normal.
Bipartite Sesamoids: A Common Variation
In roughly 7% to 30% of people, one of the sesamoid bones under the big toe develops in two separate pieces instead of one. This is called a bipartite sesamoid, and it’s not an injury. It’s simply how the bone formed during development. The two pieces have smooth, rounded edges where they meet.
This matters because a bipartite sesamoid can look a lot like a fracture on an X-ray. The key difference is that a true fracture shows jagged, irregular edges between the fragments, while a bipartite sesamoid has smooth borders. If you ever injure the ball of your foot and get imaging done, your doctor may check the other foot for comparison, since bipartite sesamoids tend to occur on both sides.
Sesamoiditis: When These Bones Cause Pain
Sesamoiditis is inflammation of the sesamoid bones under the big toe, and it’s the most common problem people experience with these bones. It causes pain on the ball of the foot, right beneath the big toe joint. The pain can come on suddenly after an injury or build gradually from repetitive stress, like running, dancing, or wearing high heels that force extra pressure onto the forefoot.
The hallmark symptom is tenderness when pressing directly on the sesamoid area. You might also notice pain when pushing off during walking, difficulty doing single-leg calf raises, or a dull ache that worsens with activity and improves with rest. The pain tends to be worst during the push-off phase of each step, since that’s when these bones are under the most load.
Sesamoiditis typically responds to conservative treatment: reducing activity, wearing shoes with more cushioning, and using pads or orthotic inserts that offload pressure from the ball of the foot. Recovery timelines vary, but most cases improve with consistent offloading over several weeks.
Sesamoid Fractures
Because sesamoid bones bear so much force, they can fracture. This happens in two ways. An acute fracture occurs from a sudden injury, often when the big toe is forcefully bent backward. You’ll feel immediate, sharp pain at the ball of the foot. A stress fracture, on the other hand, develops gradually from repetitive overuse. The pain starts mild and gets worse over weeks, sometimes making it hard to distinguish from sesamoiditis early on.
Treatment for sesamoid fractures starts conservatively, similar to sesamoiditis but often with more aggressive immobilization. Stress fractures in particular can be slow to heal because these bones have a limited blood supply and are under constant pressure during normal walking.
When Surgery Becomes an Option
If months of conservative treatment fail to resolve chronic sesamoiditis or a fracture that won’t heal, surgical removal of the affected sesamoid bone (called a sesamoidectomy) may be considered. Research on the procedure shows it can effectively reduce pain and help heal chronic problems that haven’t responded to anything else.
One common concern is whether removing a sesamoid bone will change the alignment of the big toe. Studies tracking patients after surgery found no significant change in toe angles at follow-up, which is reassuring. However, about 17% of patients experienced complications including nerve irritation, pain shifting to nearby areas of the foot, or persistent changes in toe position. For this reason, surgery is generally reserved for cases where conservative approaches have been thoroughly exhausted.