There's A Sneaky Bone Hiding Behind Your Knee
Category: Human Anatomy 14th May 2026
Oddly enough, most of us assume knees are simple hinges: bone, joint, cartilage, swear words when stairs appear. But tucked behind the lateral side of the knee, inside the tendon of the lateral head of the gastrocnemius, some people have a little sesamoid bone called the fabella. Think of it as a microscopic pebble a cobbler forgot in your shoe and then decided to keep.
The fabella is not a fluke. It can be cartilage in younger folks or fully ossified bone in adults. It sits where tendons rub over bone, the sort of place evolution thought, "We could put a tiny fulcrum here," and some bodies obliged. Not everyone gets one, which makes it an amusing anatomical lottery prize: present in a minority, absent in most, and maddeningly variable between families and populations.
Why does it matter? For the most part it doesn't. Plenty of people live their whole lives with a fabella and never notice it. But it can cause actual medical mischief. It has been blamed for a distinct posterior knee pain syndrome, can be fractured, and sometimes irritates nearby structures, complicating knee replacement surgery. Surgeons occasionally have to remove it like a stubborn splinter.
Researchers have noticed something else: the fabella appears to be turning up more often than it used to. Studies comparing skeletons from a century ago to modern scans suggest prevalence has risen, for reasons not fully nailed down. Theories range from dietary and developmental changes to the simple fact that we now live longer and more imaging reveals more oddities. In other words, your grandparents probably had fewer of these little freeloaders.
If you ever get an odd, sharp twinge behind the knee, the fabella might be the culprit. If a surgeon mentions finding a tiny bone behaving badly, try to look surprised and offended on principle. Personally, I like the idea that the body occasionally installs a spare part on its own initiative; it is both helpful and mildly cheeky, like a houseguest who does the washing up and then borrows your socks without asking.