Philadelphia Eagles quarterback Donovan McNabb tore his anterior cruciate ligament (ACL) in week 11 of the 2006 season, missed the remaining six regular season games and the playoffs, and still wasn’t 100 percent at the start of the 2007 season, nearly a year later.

New Orleans Saints running back Deuce McAllister tore his right ACL in 2005, and then tore his left one in 2007, missing a total of 24 games.

Less than 10 minutes into the first game of his 2008 season, reigning MVP Tom Brady of the New England Patriots suffered a torn ACL in his left knee. He faces months of rehab and will miss the remainder of the season.

And at the University of Florida, five Gators were lost to season-ending ACL tears in a span of just two weeks this summer—before the season, and heavy hitting, even began.

So what exactly is the ACL, and why are ACL tears apparently on the rise? Brian J. Sennett, director of the Penn Sports Medicine Center, recently spoke with the Current about this increasingly infamous ligament and why it brings so many athletes to their knees.

Q. It seems as if ACL tears are most prevalent in football. Is this true or are they just as common in other sports?
A. They are common in a lot of sports. We hear about them more in football just because of the notoriety of television coverage with sports. At the college level, we see a lot of them also in soccer, in gymnastics, wrestling and basketball. Obviously, we don’t have professional wrestling the same as collegiate wrestling. We don’t have professional soccer that is covered the same for the media. It’s really the media coverage of it that makes it much more commonplace in our minds when it comes to the professional level. So while it appears that people tear their ACL more in football, it happens very commonly as well in women’s soccer, men’s soccer, women’s basketball, men’s basketball, wrestling and gymnastics.

Q. Football is a violent, physical game, but is there something about the sport that makes it so hard on the knees?
A. The majority of individuals that tear their ACL playing football [do so because of] a non-contact injury. But there are people that do get hit and they tear their ACL as a result. It sounds like Tom Brady has a torn anterior cruciate ligament, and he was actually struck on that play. But when Donovan McNabb tore his ACL, he just kind of twisted his knee running out of bounds and was never actually hit. So many of the injuries occur from a non-contact mechanism, but they also do occur from contact. But that’s why we see it a lot in those other sports that I mentioned … because it’s very commonly a non-contact injury.

Q. Is one of the problems that these athletes are pushing their bodies beyond their natural limit?
A. The reason you tear your anterior cruciate ligament is that the ligament gets subjected to more loads than it can handle. Sometimes that occurs as a result of getting hit by another individual, and sometimes because their own body can’t respond to some of the cuts and the maneuvers they’re doing. But it’s clearly that they’re subjecting their joints to high level of loads that their muscles just can’t dissipate those forces.
The thing is, when they tear a ligament, many times it’s something that they’ve done 10,000 times before but never had a problem. It’s really at that moment in time, the muscle wasn’t ready to handle that cut or that pivot and the ligament tears as a result of it. So I wouldn’t go along the lines of blaming the human body for this problem. It happens that with certain cuts and maneuvers, the ligaments are put at high loads, which, most of the time, the muscle dissipates those forces and the ACL does not see a load that could tear it. But occasionally, the muscles aren’t quite ready and the ligament does see it and the ligament will ultimately tear.

Q. A lot of NFL players don’t wear knee protection. Do you think this factors at all into these injuries?
A. There’s never been a study that shows that bracing prevents ACL tears in football. There’s a study that shows that bracing protects MCL [medial collateral ligament] tears in football, but not ACL tears. The only sport in which braces have ever been shown to protect ACL tears is skiing.

Q. It sounds like the recovery time for ACL tears is at least a year. Why does it take so long to recover compared to a broken bone?
A. The earliest that they will return is six months. [Often] it takes a full year really to get back into full playing shape. It’s not only the ligament itself, it’s the joint stiffness and it’s the muscle. When you have these injuries and you go through surgery, you lose a fair amount of what we call muscle strength, which is through atrophy. Athletes actually have to build that strength back up. They still have to get back into playing condition. The last thing that happens is the ligament, when we reconstruct them and put new ligaments in there, have to kind of go through a remodeling process and develop the new ligament. And so that’s a whole different process than two bones healing end to end.

Q. San Diego Chargers linebacker Shawne Merriman tried to play this season with ligament tears in his left knee. Tiger Woods played golf for at least 10 months with a torn ACL in his left knee and even won the U.S. Open. How is it possible that some athletes can play with these injuries but others cannot?
A. Tiger actually had this sharp shot of pain that [caused him to] grimace when he would hit a certain shot. And what was happening is his knee was actually shifting and moving abnormally at that time, and that’s why he was getting the pain. And that’s why he couldn’t complete the season and didn’t do the British Open or the PGA Championship and he essentially ended his season. So he tried but he was unsuccessful. If you really look at the timeline of it, he tore it I believe after the British Open last year. He really played with some of it during the fall but when he came back this spring, he had some difficulties with the Masters. He took six weeks off or so between the Masters and the U.S. Open and still was having difficulties and he had surgery. So I think, yes, Tiger won the U.S. Open, which is pretty amazing, but I wouldn’t say he did it without having significant difficulties and ultimately it actually required him to have surgery. So he was having difficulties; he just played through his difficulties. Shawne Merriman is not an ACL tear. The PCL [posterior cruciate ligament] is a ligament that is not used as much in sports and can get away with bracing much more.

Q. Is there anything these athletes can do to prevent these types of injuries?
A. There have been some studies in young female soccer players where a warm-up and conditioning program has actually been able to show a decrease in ACL tears in soccer players. So the concept of having the muscles in what we call ‘playing shape’ by warming up appropriately prior to participation can potentially decrease tears in the ACL.

Originally published Sept. 18, 2008