Does the fabella contribute to the reinforcement of the posterolateral corner of the knee by inducing the development of associated ligaments?

The fabellofibular (FF) and arcuate ligaments are closely colocated at the posterolateral corner of the knee. However, a bony fabella is often absent in Japanese subjects. We investigated these structures morphometrically using 212 knees. A thick FF ligament was found in 38.1% (40/105) of knees with a hard fabella. When identified histologically, a bony fabella accompanied a thick FF in 82.8% (24/29). Conversely, a thin or indefinite FF ligament and an elastic-textured fabella co-existed in 77.9% (60/70) of knees. There were no strong correlations between the morphology of the fabella and the arcuate ligament. Thus, a hard or bony fabella seemed to relate to thickening of the FF ligament. In contrast, the thicknesses of the FF and arcuate ligaments were negatively correlated. When the FF ligament was more than 5 mm thick, indicating that it was as strong as the lateral collateral ligament, no arcuate ligament could be identified. Conversely, a thin or indefinite FF ligament often accompanied a well-developed arcuate ligament (61.9%, 96/155). Our observations suggest that these ligaments act in a complementary manner to stabilize the posterolateral corner. A possible common function is fixation of the popliteal tendon on the joint capsule and lateral meniscus, rather than direct stabilization against rotation stress.