Meet Your New Knee Ligament

Scientists revealed online in August and in print in October in the Journal of Anatomy the discovery of what can reasonably be termed, if only for slight dramatic effect, a new body part:  the anterolateral ligament (ALL). Located just in front of the knee's outside collateral ligament, the newly identified ligamentous structure acts to control internal tibial rotation, and therefore affects what is known among physicians and anatomical experts as the “pivot-shift phenomenon." The pivot-shift phenomenon is produced by specific knee joint motions, including medial-lateral tibiofemoral compartment subluxations or partial dislocations. This makes the ALL a particularly important structure to understand in the field of sports medicine.

The new study that gives us the first modern detailed anatomy of the anterolateral ligament sought to affirm the presence and outline the characteristics of what the French surgeon Segond described back in 1879 as a "pearly, resistant, fibrous band" at the anterolateral aspect of the human knee. Until now, the enigma surrounding this structure has been reflected in a variety of confusing names for it, including, as the authors point out, the "(mid-third) lateral capsular ligament," "capsulo-osseous layer of the iliotibial band," or "anterolateral ligament"—and a clear anatomical description had not been forthcoming.

The “new” ligament was investigated in 41 unpaired knees on human cadavers. The study examined the femoral and tibial attachment of the ALL, its course, and its relationship with nearby anatomical structures. In all but one of the 41 cadaveric knees (97%), the ALL was found as a well-defined ligamentous structure, and clearly distinguishable from neighboring structures such as the anterolateral joint capsule. The researchers report that the origin of the ALL is situated at the prominence of the lateral femoral epicondyle; this is the protuberance above the long bone of the femur. This puts the ALL just in front of the origin of the lateral collateral ligament, although connecting fibers between the two structures were observed.

The ALL showed an oblique course to the anterolateral aspect of the proximal tibia, with firm attachments to the lateral meniscus. It envelopes a prominent artery of the lower limbs, the inferior lateral geniculate artery. Its insertion on the outside front of the tibia is roughly midway, not near the tip of the fibular head, which definitely separates it from the iliotibial band (ITB).

By providing such a new and detailed anatomical characterization of the anterolateral ligament, the study clarifies the longstanding confusion surrounding its existence. The ligament essentially connects the femur to the anterolateral tibia. This is the front outside edge of the inner of the two bones of the shin, which makes the ligament a key fibrous structure to understand when diagnosing and treating athletes across an enormous array of sports activity. This is due to the fact that, as noted above, given its structure and anatomic location, the ALL appears to be a key controller of internal tibial rotation. More studies are needed to further investigate its exact biomechanical functions.

Journal of Anatomy, 2013, Vol. 223, No. 4, pp. 321-328, http://onlinelibrary.wiley.com/doi/10.1111/joa.12087/abstract

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