The fabella is a small, usually bony or cartilaginous sesamoid bone found in some people at the back of the knee joint. More precisely, it sits within the tendon of the gastrocnemius muscle, which is part of the calf muscles. It belongs to a group of bones called sesamoid bones. These are small bones embedded in tendons that help support certain movements or protect tendons from too much strain.
Where is the fabella and how common is it?
Not everyone has a fabella in their knee. It is typically found on the outer side of the back of the knee, where the tendon of the outer head of the gastrocnemius muscle runs. Interestingly, the fabella is not present in everyone. Studies show it appears in around 10 to 30 per cent of people, though how often it is found varies depending on the region and the method used to look for it. Some people have it on both sides, others only on one side, and many people do not have one at all.
What does this small sesamoid bone do?
The exact role of the fabella has not been fully established. Like other sesamoid bones, it may help improve the direction of pull on tendons and optimise how force is transferred through the knee joint. It also protects the tendon from too much friction against the bone. In most cases, having a fabella has no noticeable effect on the knee or on movement.
When does the fabella show up in medical practice?
Most of the time, the fabella is discovered by chance, for example during X-rays, MRI scans, or surgery on the knee. In reports or medical letters, it often appears as an incidental finding, without causing any symptoms. For many people, it goes unnoticed throughout their entire life and causes no problems at all. It is simply a normal anatomical variation, similar to small extra bones found in other parts of the body.
Can the fabella cause problems?
In very rare cases, a fabella can actually cause symptoms. This may happen if it is particularly large, positioned awkwardly, or presses on nerves or other structures during certain movements. Some people then experience pain at the back of the knee, especially when bending or straightening the leg. Doctors refer to this as "fabella syndrome". After knee injuries or surgery, the fabella can occasionally play a role too, for example if it comes into contact with other structures or becomes inflamed.
However, such problems are the exception. In the vast majority of cases, the fabella is harmless and only discovered by chance. Treatment is only needed when symptoms genuinely occur and can be clearly traced back to the fabella.
What happens if the fabella causes symptoms?
If this small sesamoid bone does cause pain or restricted movement, the next steps depend on the exact cause. Sometimes targeted physiotherapy, rest, or pain-relieving measures are enough to help. Only very rarely is the fabella surgically removed, for example when all other treatments have not worked and the symptoms are clearly coming from it.
In summary: what does a finding of "fabella" mean?
If you come across the word "fabella" in a report, a medical letter, or on an X-ray, there is generally no need to worry. It is usually an incidental, harmless finding and is one of the normal anatomical variations in the human body. Only if symptoms genuinely appear at the back of the knee and other causes have been ruled out can the fabella become medically significant. In the vast majority of cases, however, it simply remains an interesting detail, and a fine example of just how varied and individual the human body can be.
