Surgeons in Belgium have described in detail a previously obscure ligament in the knee that could play a role in anterior cruciate ligament (ACL) tears, according to an article published recently in the Journal of Anatomy.

The anatomical structure was first characterized by a French doctor in 1879, but until now it received scant attention and was little noticed, even by knee surgeons.

“The strange thing is that nobody has ever described it in detail, studied the anatomy in a series of cadavers or provided drawings or photographs of it,” says lead study author Steven Claes, MD, an orthopaedic surgeon at University Hospitals Leuven, Belgium. “The structure itself was probably mentioned – although because of the vague descriptions, we cannot be sure – but both its anatomy and function remained a mystery.”

Ligaments are narrow, flexible bands of tissue that connect bone to bone. This one links the femur (thigh bone) and the tibia (shin bone). It has been dubbed the anterolateral ligament, or “ALL,” because it sits in the front and outer side of the knee. The surgeons believe it may play a role in keeping the knee – a notoriously injury-prone joint – stable when the tibia rotates, as it does in a person pivoting in basketball or football, for example.

For the study, the doctors dissected knees from 41 human cadavers. In all but one, they found “a well-defined ligamentous structure, clearly distinguishable from the anterolateral joint capsule” and “definitely separate from the iliotibial band.” Previously, doctors familiar with the structure thought it might be part of the capsule, which contains the joint, or of the iliotibial band, which runs down the outside of the thigh.

How has the ligament remained an enigma in this age of modern medicine? “You can only find what you are looking for,” says Dr. Claes. “The outer side of the knee has gained little attention until now. Most operations are done through keyholes [arthroscopy], which is very good for both patients and surgeons, but it only allows us to visualize [and treat] the inner side of the joint. Enveloping structures remain under the radar.”

Excitement about the study stems from the authors’ belief that understanding the ligament better could help doctors restore knee stability after an ACL tear, which are notoriously slow to heal. According to the authors, hidden injury to the ALL may be the reason some ACL patients experience a “pivot shift,” or “giving way” of the knee, even after surgery. In theory, repair of this ligament in these cases could help.

“The future will tell whether ALL [injuries] should be treated and how,” says Dr. Claes. As the authors write in the study, “Further research is needed to establish the function of the ALL and to determine its role in clinical knee injuries.”

Elizabeth Matzkin, MD, assistant professor in the department of orthopaedic surgery at Harvard Medical School, agrees that more research is needed. “I think that it is interesting. I’m not sure how important it is at this point,” she says. “This is definitely not going to change clinical practice at this time. A lot of studies, anatomic and biomechanical, need to be performed to figure out if this so-called anterolateral ligament plays a role in knee stability.”

Dr. Claes says he and his team have already conducted some of these studies, which “are currently under review at major orthopedic journals.”

The ALL is unlikely to factor into knee arthritis, says Dr. Matzkin. “We do know that patients who have an ACL tear have a higher incidence of getting some post-traumatic arthritis in that knee down the road. But it’s been pretty well shown that that is secondary to the cartilage injury at the time of the ACL tear. There’s nothing at this point in time to suggest that this [ligament] has anything to do with an arthritic knee.”