The fabella is a small bone located near the knee joint. It is classified as a sesamoid bone, meaning it is embedded within a tendon, much like the kneecap. Its name, derived from the Latin word for “little bean,” reflects its typical size and shape. Despite its small stature, the presence of the fabella is highly variable, with a significant portion of the population not possessing it.
The Fabella’s Place in Knee Anatomy
The fabella is situated on the posterior aspect of the knee, behind the lateral femoral condyle. It is found within the tendon of the lateral head of the gastrocnemius muscle, one of the main calf muscles. This location allows it to articulate directly with the femur, particularly during knee extension.
The incidence of the fabella varies significantly across different studies and populations. It is generally reported to be present in approximately 10% to 30% of the worldwide population, though some analyses suggest the global average may be closer to 39% today. When present, the fabella is often found in both knees, with bilateral cases reported in over 60% of individuals. Ossification of this bone typically occurs by the age of 12.
Evolutionary Role and Biomechanical Function
The fabella is common in many non-human mammals, where it increases the mechanical leverage of the muscle. In humans, its function is less clear, and it is frequently considered an accessory or vestigial structure, a remnant from an earlier evolutionary stage. When present, the fabella may serve a biomechanical role by reducing friction within the gastrocnemius tendon or by slightly increasing the force the muscle can exert.
The bone’s increasing prevalence in modern humans, rising significantly since the early 1900s, suggests an environmental or mechanical trigger for its formation. One hypothesis suggests that increased human height and weight has led to greater mechanical stress on the gastrocnemius tendon, stimulating the bone’s growth. Despite its potential function, the vast majority of people without a fabella experience no impairment in knee function, confirming it is not necessary for bipedal locomotion.
Understanding Fabella Syndrome
Fabella Syndrome describes pain originating from a symptomatic fabella, characterized by discomfort in the posterolateral, or outer-rear, region of the knee. This condition arises from the fabella’s mechanical interaction with the lateral femoral condyle and the surrounding soft tissues. The mechanism of injury often involves repetitive friction, irritation, or impingement as the knee moves.
Patients typically report a slow onset of pain that may become sharper over time. The pain is often exacerbated by activities that involve full extension of the knee, such as walking, running, or climbing stairs. Localized tenderness is usually present when pressure is applied directly over the fabella. Mechanical symptoms like snapping, clicking, or catching may be felt with repeated knee movement. Continuous rubbing can lead to degenerative changes in the tendon and cartilage, sometimes contributing to common fibular nerve palsy due to compression.
Diagnosis and Management Options
Diagnosing Fabella Syndrome begins with a thorough physical examination, where a clinician can often pinpoint the pain to the small bone itself, especially with the knee straightened. Because symptoms can mimic other common knee issues, such as meniscal tears or biceps femoris tendinopathy, imaging studies are necessary to confirm the diagnosis and rule out other pathology.
Standard X-rays are typically sufficient to confirm the presence of the fabella, appearing as a distinct ossified structure behind the femur. MRI or ultrasound may be used to visualize soft tissues and identify signs of inflammation or tendon impingement. Initial management is conservative, focusing on reducing inflammation and mechanical stress through rest, ice, NSAIDs, and physical therapy. If conservative treatment, which may include steroid injections, fails to provide lasting relief after a trial period, surgical intervention is considered. The definitive surgical treatment is a fabellectomy, which involves the removal of the fabella bone.