The fabella, Latin for “little bean,” is a small bone found near the knee joint. It is classified as a sesamoid bone, meaning it develops within a tendon due to mechanical stress, much like the kneecap. While it is a normal anatomical variant, its presence is not universal in humans. Understanding the fabella requires examining its precise location, its variable occurrence across the population, its proposed biological purpose, and the rare instances when it becomes a source of pain.
Anatomy and Location of the Fabella
The fabella is situated at the back of the knee, embedded within the tendon of the lateral head of the gastrocnemius muscle. This muscle makes up part of the calf, and its tendon crosses the knee joint before connecting to the thigh bone. The fabella rests directly against the posterior surface of the lateral femoral condyle.
As a sesamoid bone, its primary role is to interact with the joint capsule and surrounding ligaments. The bone is relatively small, typically measuring less than a centimeter in diameter. When present, it is often found in both knees, though it can occur unilaterally.
How Common is the Fabella?
Not everyone has a fabella, as the bone is considered an accessory structure. Its prevalence varies widely globally, but recent studies suggest it is present in approximately 39% of the world population. This rate is far from constant, however, and represents a significant shift over time.
The occurrence of the fabella has increased dramatically in the last century. Researchers estimate that the bone is now about 3.5 times more common than it was in the early 1900s. This increase is unique among sesamoid bones, which otherwise maintain stable prevalence rates.
The likelihood of having a fabella also shows geographical and demographic variation. Prevalence rates are highest in populations from Asia and Oceania, while they are lowest in individuals from North America and Africa. Furthermore, if a person does have a fabella, there is a high probability that it will be present bilaterally, rather than in just one.
Why Does This Small Bone Exist?
The existence of the fabella is thought to be a response to the mechanical forces exerted on the knee joint. Sesamoid bones typically function to increase the mechanical advantage of a muscle by acting as a pulley, similar to how the kneecap works. In this case, the fabella helps increase the lever arm of the gastrocnemius muscle, potentially making the calf muscle more efficient.
Its formation is strongly linked to biomechanical stress, which may explain the observed increase in prevalence. Researchers hypothesize that the increase in average human height and weight globally has led to longer shinbones and larger calf muscles. These changes place greater mechanical demands and pressure on the knee, providing the stimulus for the fabella to form and ossify within the tendon.
The fabella is common in many non-human mammals, where it plays a stabilizing role. For humans, however, its function is often described as vestigial, an accessory structure that is not strictly necessary for normal knee function. Its formation seems to be a complex interplay of genetic predisposition and environmental factors like nutrition and body size.
When the Fabella Causes Problems
While most individuals with a fabella remain unaware of its presence, the bone can occasionally become symptomatic. When the fabella causes chronic posterior knee pain, the condition is known as Fabella Syndrome. This syndrome typically involves irritation and friction between the bone and the lateral femoral condyle, often exacerbated by repetitive knee movements.
Symptoms include a dull, aching pain localized to the outer back of the knee, which often intensifies when the leg is fully extended. In some cases, the fabella can exert pressure on nearby structures, such as the common peroneal nerve, leading to radiating pain or numbness. Diagnosis is usually confirmed through imaging studies, such as X-rays or Magnetic Resonance Imaging (MRI), to visualize the bone and assess surrounding soft tissue inflammation.
Initial treatment for Fabella Syndrome is conservative and focuses on reducing inflammation and mechanical stress. This approach includes rest, physical therapy, and the use of nonsteroidal anti-inflammatory drugs (NSAIDs). If conservative management fails to provide relief, the bone can be surgically removed in a procedure known as a fabellectomy.