Good news for anyone who has ever had an ACL tear that never seemed to heal properly: You may have actually torn a different ligament. After examining the knees of 41 cadavers, a team of Belgian doctors found a fifth kind of knee ligament (after the ACL, PCL, MCL, and LCL), which they named the anterolateral ligament, or ALL. The ligament (which some bloggers and journalists noted was first described more than a hundred years ago) likely helps control and stabilize lower-leg rotation, and like the others, it can be stretched and torn — most likely, the researchers think, at the same time that the ACL is.
The ALL may be behind a condition called pivot shift, in which a person who has had an ACL tear will find that their knee sometimes still sags or gives way even after the damaged ligament has been repaired. "This requires further investigation, but in some people, the ALL may benefit from more surgery in addition to the ACL," says Dr. Robert Marx at the Hospital for Special Surgery in New York. In other words, people who have had ACL surgery that didn't work may want to hold off on more ACL surgery and find out if it's a different part of the knee entirely that needs repair.