Last month, knee surgeons from the University Hospitals Leuven in Belgium announced that they had found a new knee ligament, one that had not previously been specifically identified despite untold numbers of past knee dissections and scans. This surprising announcement, in The Journal of Anatomy, should improve our understanding of how the knee works and why some knee surgeries disappoint and also underscores the continually astonishing complexity of human anatomy.
To find and characterize this new knee part, the orthopedic surgeons Dr. Steven Claes and Dr. Johann Bellemans and their colleagues gathered 41 knee joints from human cadavers and began minutely dissecting them.
The knee, as those of us who own and operate a pair know, is complicated and somewhat fragile, an intricate construction of bones, cartilage, fluids, ligaments (which attach bones to bones) and tendons (which attach muscles to bones). Ideally, the various parts move together smoothly, but they can tear, rupture or fracture if the knee abruptly twists or overpivots. Knee injuries and pain drive millions of people to doctors every year and result in millions of knee exams, scans and surgeries.
So most of us probably have assumed that the entire internal structure of the knee has been fully mapped and delineated.
But knee specialists themselves have long been less sanguine. As far back as 1879, a French surgeon named Paul Segond first speculated that, in addition to the four obvious structural knee ligaments known then — the anterior cruciate, medial collateral, posterior cruciate and lateral collateral, which loop around and through the joint — other ligaments must exist in the knee or it would not be stable. He wrote that during dissections he had noticed a "pearly, resistant fibrous band" originating at the outside, front portion of the thighbone and continuing to the shinbone, which, in his estimation, must stabilize the outer part of the knee, preventing it from collapsing inward.
He did not, however, give this pearly band a name and somehow, in the decades that followed, its existence was forgotten or ignored. While some surgeons noted that a ligament seemed to exist there, none named and systematically studied it, and many came to consider it a continuation of other tissues, such as the nearby iliotibial band.
But a few years ago, Dr. Claes and his colleagues began to suspect otherwise. Their interest had been piqued by a problem that occurred in some patients who had undergone reconstructive surgery for an injured anterior cruciate ligament, or A.C.L. Despite the repaired knees' appearing afterward to be healthy, the joint would sometimes give way as people moved.
"We thought, something is still not right" in that knee, said Dr. Claes, who wondered whether additional, untended knee injuries might be to blame, and if so, whether they were occurring in uncharted knee parts. "I know it probably sounds crazy to say that we thought there might be this new ligament," he said.
But, like Dr. Segond so long before him, Dr. Claes became convinced that such a tissue must exist. Positioned at the front of the knee, it would be vulnerable to tearing when an A.C.L. was injured; the same forces would move through both ligaments. But, unlike torn A.C.L's, this new tissue's injuries would remain untreated, potentially leading, Dr. Claes considered, to knee instability and buckling.
So, knowing it should be there, he and his colleagues began to search for this chimerical ligament in the 41 donated knees. And there it was, a narrow band of tissue, clearly separate from the illiotibial band and neatly linking the femur and the tibia. Because it was located on the outside, front portion of the knee, they named it the anterolateral ligament, or A.L.L.
They subsequently identified, measured and scanned the A.L.L. in all but one of the donated knees, and even in that final knee, Dr. Claes said, he suspects there may once have been an A.L.L, but it possibly ruptured and withered at some point.
Whether a similar process occurs in living people who injure and don't treat an A.L.L. — because they don't know they have one — is unknown, Dr. Claes said, but is potentially the weightiest question raised by this new research. "We think that it's quite likely many people who tear an A.C.L. also tear an A.L.L," he said, and that lingering injury or weakness in this overlooked ligament could leave joints unstable.
But at the moment, that possibility is speculative, although Dr. Claes said that he and his colleagues had re-examined scans of some of the knees that they had operated on to repair A.C.L. injuries and identified concomitant A.L.L. tears in many of them.
He and his colleagues have begun planning and practicing surgical procedures for treating A.L.L. tears, but at the moment, so much remains unknown about the ligament, including whether it can heal without surgery.
"We still have a lot of work to do," said Dr. Claes, who, with his colleagues, will be presenting continuing results at orthopedic surgery conventions in the coming months. But the fact that the A.L.L is now recognized is already "an important step forward," he said, in understanding and potentially treating injured knees, at least in those parts of which, to date, we are aware.
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