Inside the knee, the anterior and posterior cruciate ligaments are positioned to prevent the tibia (shin bone) from shifting forward and backward, respectively, on the femur (thigh bone). These two interior ligaments along with the collateral ligaments spanning the outside of the joint allow the knee to bend through a 120-150-degree arc of motion while keeping the tibia firmly aligned on the end of the femur. The ACL is the more important of the two and is angled from the back side of the femur to the front edge of the tibia. It keeps the tibia from shifting forward. A rough analogy of its function is to imagine that the femur is a kitchen counter and the tibia is a drawer underneath. The ACL keeps the draw from sliding forward too far. Absent the ACL, the drawer flies out. Chaos ensues.
ACL tears draw greater attention from the people injured, their surgeons, investigators, news media, and the public than all other ligament injuries combined. Out of 900 ligaments in the body, why the ACL? The multiple reasons include the fact that an ACL injury can occur in a high-profile athlete, where it is wincingly reviewable on YouTube and sidelines the star for at least eight months, if not permanently. Soccer standout Megan Rapinoe tore her left ACL in college, had it reconstructed, and returned the next season only to tear it again. Eight years later, the one in her right knee gave way. That is not unusual. Research confirms that nearly a third of people with one ACL tear get another one, in that leg or the opposite, within two years after their first. That should make athletes, team owners, fans, and journalists nervous. Notable athletes who have so far escaped with just one ACL injury include footballer Tom Brady, skier Lindsey Vaughn, golfer Tiger Woods, soccer star Neymar, and basketball great Kawhi Leonard. For pro athletes and their team owners, the ACL is big money. The injury also occurs in high school and college athletes.
When the injury comes to attention, it is a grade III (complete, serious) ligament tear, which destabilizes the tibia on the femur and prevents the sharp turns and sudden stops that are integral to many sports maneuvers. Usually these sudden disruptions do not occur in an entirely normal ligament. It is likely that there has been fraying and degeneration for months, but the dramatic snap of the ACL giving way is the first sign of trouble.
Another reason attention is focused on the ACL is because if investigators and surgeons can solve the ACL problem, then restoration of any other ligament would be easy by comparison. Here’s the reason. Inside the joint the ACL has a paltry blood supply, even moreso than most ligaments. Without much nourishment, its potential for healing is virtually nonexistent. When chronic fraying precedes the fateful snap, the collagen fibers are completely disorganized, which compounds the repair problem.
ACL ruptures often result from sharp twisting and sudden starting and stopping maneuvers. For example, in basketball these injuries are more common in players who like to weave through the defense for layups. Several football players have surprised themselves with this injury stemming from a victory dance. Conversely, direct hits on the knee account for a minor percentage of ACL tears. They used to be common in football, but less so following a rule change that outlawed blocking from behind and below the waist.
ACL injuries are two to nine times more common in young women than men. Several factors seem to contribute. Women’s soccer shoes used to be just pink versions of men’s without any thought whether they fit a woman’s foot or if the cleats were too long for their weight. That has changed. What hasn’t changed is varying levels of estrogen through a woman’s menstrual cycle, peaking at the time near ovulation. The hormone weakens collagen, and there seems to be a correlation between ACL tears and where women are in their menstrual cycle. Also, women tend to jump differently than men and are more likely to land with their knee straighter and the foot pointing directly ahead, both of which stress the ACL. By contrast, ballet dancers, who are perpetually jumping and landing on one foot, are taught to “turnout” with their leg before landing. This aesthetically pleasing foot position seems to protect their ACL, which ballerinas rarely injure. So athletic trainers are helping female soccer players relearn how to jump and to strengthen the muscles that protect the ACL.
Regardless of sex, there is a debate whether knee injuries in soccer are more common on turf or grass. Most studies favor grass. (Several that favor turf were sponsored by turf manufacturers.) Artificial turf, especially when it is dry, tends to grab cleats more than grass does, which I learned when attending a Major League Soccer game at the Mercedes Stadium in Atlanta. No, I wasn’t playing. Rather Atlanta United was facing off against Inter Miami and their new superstar Lionel Messi. He didn’t show up. All we disappointed spectators learned later that his contract allows him to skip games that are played on artificial turf for risk of injury. I suppose in deference to the other players, the groundskeepers turned the sprinklers on the field for five minutes at half time, not to make it grow but to make it more forgiving on the players’ knees.
To be continued.
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