Two orthopedic surgeons say they’ve confirmed the human knee does indeed have an extra ligament — something that has previously only been speculation.

The traditional model of the knee has four major ligaments: a cruciate ligament at the front and another at the back, and a collateral ligament on the inside and another on the outside, all four running from the femur (thigh bone) to the tibia (shin bone).

“Minor” ligaments run in other places such as to or from the patella (knee bone) to the tibia, or wrapping around the knee from one side of the tibia to the other.

In 1879, French surgeon Paul Segond wrote a paper with a theory that the knee had another major ligament connecting the femur and tibia. Dr Steven Claes and Professor Dr Johan Bellemans of Belgium’s University Hospitals Leuven decided to investigate that theory with the aid of knees from 41 cadavers.

In a paper in the Journal of Anatomy, they revealed that in 40 of the 41 knees, they found a ” well-defined ligamentous structure, clearly distinguishable” from previously charted knee parts. They have called it the anterolateral ligament (ALL), referring to its position on the front and outside of the knee.

The working theory is that the ALL’s role is to stabilize the knee’s rotation. That could in turn solve the mystery of pivot shift, a situation in which people who’ve had anterior cruciate ligament (ACL) injuries repaired later find their knee gives way.

So far surgeons have struggled to explain how a seemingly-fixed ACL should “fail” in this specific way, but it now appears pivot shift could be the result of damage to the ALL, suffered as part of the same injury. Claes and Bellemans are now working on ways to surgically repair the ALL after injury.

Several sources, apparently stemming from Science Daily, have reported that the researchers showed that 97 percent of people (or 97 percent of knees) have the ALL. It appears this is a misunderstanding (or mis-extrapolation) of the fact that the researchers didn’t find the ALL in one of the knees. That may well be because the person the knee came from had suffered a major injury that destroyed it; given the sample size, it’s tough to extrapolate the actual proportion of living people who are missing an ACL.

This is actually the second case of a new body part being discovered this year. In June, researchers discovered that the cornea has six layers rather than five, with the extra layer now dubbed Dua’s layer after researcher Harminder Dua. It’s just 15 microns thick, less than three percent of the cornea’s total thickness. Dua believes tears to this layer, which are difficult to spot given its thinness, may be responsible for a condition involving a build-up of fluid.

(Image credit: University Hospitals Leuven)