Learn about the silent epidemic affecting girls who play sports, from Warrior Girls: Protecting Our Daughters Against the Injury Epidemic in Women’s Sports, by Michael Sokolove.
The human knee is a wondrous thing, at once the most athletic part of the musculoskeletal system — without its bend the muscles could not thrust for running and jumping — and often the most fragile. It is a jointed column, formed by the intersection of four bones. The femur, the large bone in the thigh, connects by ligaments to the tibia, the main bone in the lower leg.
Next to the tibia and running parallel to it is a smaller bone, the fibula. Between the femur and the tibia are the menisci and the articular cartilage. The menisci act as a natural shock absorption system, and the articular cartilage, which lines the end of the bones, is similar to a layer of ice providing a near frictionless surface during motion. The patella, what we commonly call the kneecap, rides on the joint as the knee bends. Running through, over, and around this mechanism is a network of muscles, ligaments and tendons.
The anterior cruciate ligament is one of two ligaments that form a cross (hence cruciate) in the joint, under the kneecap. The posterior cruciate ligament, or PCL, which tears very rarely, is in the back and the ACL is in the front of the knee.
Considering the size of the job it performs, which is to stabilize this large, important joint, and all the mischief its failure can cause, the ACL is tiny, about thirty millimeters long, smaller than a little finger and less than ten grams in weight. Its shape is rectangular, as if you took a section of a rubber band and stretched it against a flat surface. The ACL’s main job, says William Garrett, is to “restrain abnormal motion. It keeps the lower leg, the tibia, from sliding forward, and to a lesser degree, prevents the tibia from rotating internally.”
An ACL tear happens on the field of play in one of two ways: contact, which is the most frequent cause in football (a shoulder pad or a helmet crashes into the knee, buckling it), and noncontact, which is the predominant cause in other sports. In a noncontact injury, an athlete lands from a jump — a rebound, a headed ball — and then crumples to the ground. Or she is sprinting and suddenly folds to the ground just as she decelerates to turn.
The mythology is that athletes blow out their ACLs while cutting, but the injury usually occurs just before that, in the slowing-down phase. The leg in the direction of the turn, or the plant leg — the left leg when cutting to the left, the right when going right — is always the one that suffers the injury. It is usually painful, especially if there is other, collateral knee damage. But what an athlete is often most immediately cognizant of is a sudden sense of instability, a feeling that something — it’s not clear what — has collapsed under her.
An ACL does not tear so much as it utterly disintegrates. It pulls away from the femur and turns into a viscous liquid. Researchers know the force required for this to occur: 2,000 newtons, a newton being a unit of measurement. They could not learn this, of course, from volunteer medical subjects — you can’t ask someone to tear an ACL to further medical knowledge — so they used cadaver knees. They increased the force on these cadaver tibias to determine the level that causes ACLs to rupture.
But what actually causes that force of 2,000 newtons to settle in the ACL, rather than being more evenly absorbed in the lower leg and up the trunk — and why it happens sometimes but not most of the time — is a source of ongoing research and debate. Since its cause is not clear, an ACL tear is a perplexing injury, not easy to prevent. “I’m an injury epidemiologist, and I’ve been doing this for a while now,” Steve Marshall, the lead researcher on a vast ACL research project funded by the National Institutes of Health, told me. “This is the first time I’ve studied something where I can’t show you what did the damage. If we were reconstructing an incident where a child fell down a staircase, I could say, OK, he got a laceration here because that’s where he hit the handrail. Or he rolled his ankle, or whatever. If it’s a car crash, you say, OK, the road was slick, a crash occurred, and a loose object in the car came up and hit someone on the head. But here, you can look at a video of an injury all day long, and what you see is people in the air. People landing. People cutting. What we can’t actually see is what tears the thing apart. No other acute injury is analogous, where you cannot clearly see the cause and effect.”
Why are female athletes so much more likely to suffer the injury? There are suspicions, all of them under study: Women have wider Q angles, the measurement taken of the line between the middle of the kneecap and the center of the hip. It is more common for them to have knee valgus (knock-knees), meaning their knees collapse inward when they run. They are more upright when they run and cut. Some women have quadriceps muscles that are out of balance with their hamstrings: the quads are stronger and may “overfire” and rip the ACLs. There could be hormonal reasons or even causes related to shoe design. (Most women’s athletic shoes are simply smaller versions of men’s shoes, despite what is known about differences in anatomy, gait, and even the shape of the foot — and despite the intense efforts of Nike and other sporting goods companies to both celebrate the female athlete and capture her share of the market.) There could, as well, be a neurological component, a garbled message — an error in the motor program — that gets passed from the brain to the knee.
When the injury does occur, however, it is one of the easiest orthopedic events to diagnose. An MRI examination is generally just a formality. For starters, the injured athlete often hears a loud pop as the ACL blows, and if she doesn’t hear it, others on the field often do, even amid the hurly-burly of an ongoing game. And almost any experienced athletic trainer can make the diagnosis as soon as he puts his hands on the injured joint. It feels mushy to the touch. When the athlete stands, the knee feels unstable under her, as if the injured leg were planted in a bowl of jelly.
An ACL cannot be stitched back together or otherwise repaired. Surgeons must graft a new ACL in its place, usually from the patient’s own tissue, by taking a snippet of the patellar tendon or part of the hamstring tendon. Occasionally an ACL from a cadaver is used, especially if the patent has had multiple ACL tears and no more of her own tissue is available to harvest.
A knee, obviously, has no intelligence of its own. It cannot know what team it is playing for, or what coach. But it is sensitive to speed of play, intensity of play, and frequency of play. One of the cruelest things about the ACL injury is that it devastates the ranks of the best female athletes, those who compete with the most skill and passion.
Just kicking the ball around in a rec league with some semiskilled, semifit players? An ACL tear is probably not an issue. You are probably moving too slowly and taking too much care to stay out of one another’s way to be at a high risk. But playing on a go-go club team and seeing some kind of future for yourself in your sport — a spot on the high school varsity or maybe even a chance to play in college? Your odds are far worse.
ABOUT THE AUTHOR
Michael Sokolove, author of Warrior Girls: Protecting Our Daughters Against the Injury Epidemic in Women’s Sports (Copyright © 2008 by Michael Y. Sokolove), is a contributing writer for The New York Times Magazine and the author of and Hustle: The Myth, Life, and Lies of Pete Rose and The Ticket Out: Darryl Strawberry and the Boys of Crenshaw. He and his family live in Bethesda, Maryland.
- Read Chapter 1 of Warrior Girls: Protecting Our Daughters Against the Injury Epidemic in Women’s Sports
- See the book’s Table of Contents