What exactly is the ACL, and what role does it play in sports?
Dr. Souryal: The ACL is one of the four main ligaments in the knee and is the primary stabilizer. It’s the smallest of the four, but it serves the most important function: It stabilizes the knee for rotational movement. When you cut to change direction, that’s when the ACL comes into play. For most straightforward activities, such as jogging, the ACL isn’t involved. The most common sports in which we see ACL tears include soccer, basketball, football, and tennis. Back in the old days, before we really understood the function of the ACL, having an ACL injury used to be called a “trick knee.” The reason a knee without an intact ACL gives out occasionally is that it’s lost that stabilizer, and although it does not give out every single time, you never know when it will and when it won’t.
This injury is rampant. There are between 250,000 and 300,000 ACL injuries per year, and they’re almost exclusively happening to athletes. The chances of a nonathlete suffering an ACL injury are 1,000 to 1.
How do ACL tears occur?
Dr. Souryal: The most common mechanism of injury is non-contact and caused by cutting. You’re running along, you plant to change direction, and your knee buckles. The next most common is a contact injury such as being clipped from behind in football and hockey. A skier can also tear an ACL when the ski catches awkwardly in the snow or the carving of the ski twists the leg.
Are certain athletes predisposed to the injury?
Dr. Souryal: My study was the first to describe the link between bony anatomy and ACL tears and to describe the way of measuring it. We found that a certain segment of the population has a bone structure that predisposes them to an ACL tear. The ACL and posterior cruciate ligament (PCL) live in a tunnel at the end of your thigh bone. If that tunnel is very narrow (which you can detect by X-ray), then there’s no room for the ACL to maneuver in cutting activity, and those people are at 26 times more risk of tearing an ACL.
In 1992, we took X-rays of 1,000 high school athletes’ knees and followed them for the next two years. Whenever a player went down with an ACL injury, we looked at his notch. And it was phenomenal: The kids who were blowing out their ACLs all had those narrow notches. We stopped the study early because the numbers were so dramatic. I’m sure that there are a lot of factors that go into ACL injuries — the notch is just one of them — and the biggest question right now is, what lives inside the narrow notch? Does a small ACL live in a narrow notch and tears because it’s never had the room to grow properly? Or does a normal-sized ACL live in the small notch and ruptures because it doesn’t have the room? We don’t have answers because we’re not doing surgery on uninjured people and measuring their ACLs.
As a next phase, we’re waiting for MRIs to get a bit clearer to allow us to measure the normal ACLs in the notches. Maybe in the next couple of years, we’ll be able to answer the question. If it is a normal-sized ACL in the notch, then maybe we can go in there surgically and widen the notch, and perhaps save that person from an ACL rupture. But if it’s an underdeveloped ACL in that narrow notch, you can widen the notch all day long and you’re not going to fix it. Once we get this question answered, maybe we can do something about people with the predisposition.
In ’92, we found that girls have proportionally narrower notches than boys. Does that have anything to do with the fact that ACL injuries are epidemic among females? I don’t know, but I certainly do think that it’s one factor. Other factors have to do with muscle development and neuromuscular coordination, because the boys are encouraged to be active athletically almost from birth. They may develop better neuromuscular coordination and therefore are somewhat protected as they get into high school and college. Girls are not necessarily directed into athletics early in life, and as they become more athletically active in their teens, they may not have the neuromuscular coordination that boys have. The bottom line is that nobody knows for sure. But there’s definitely a higher incidence of ACL ruptures in female athletes than in male athletes.
How is a torn ACL diagnosed?
Dr. Souryal: By medical history, examination, and nowadays MRI scans. History and examination in most cases are sufficient. MRI scans are only indicated when you really cannot quite distinguish whether it’s torn or not. We fans have been programmed through following the NFL, NBA and other sports that when an athlete has an injury, he gets an MRI. In the case of ACL tears, it’s nice to get an MRI, but it’s not mandatory.
Lachman’s test and the pivot shift test are used. Both are about 90 percent diagnostic. Lachman’s test like a drawer test, but instead of the knee being bent 90 degrees, it’s just slightly bent, and you attempt to pull the tibia forward with one hand while holding the thigh just above the knee with the other hand. If the ACL is intact, the tibia doesn’t come forward. If the ACL is torn, it keeps coming. That test is very diagnostic and conclusive when performed by an expert.