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Beginners' Guide To ACL Injuries: How Gender and Contact Influence Injury

Contact vs Non-contact

When injuring the ACL there are two major categories the injury falls into: contact or non-contact. These may sound completely obvious to you, but we are going to describe them anyways. In a “contact” scenario, an athlete has a collision with another athlete that forces the lower leg into an unnatural position and this causes the bones to move past the limits the ligament allows. Think of a collision during football. The receiver plants his foot to catch the ball as the defensive lineman collides with him shoving him in the opposite direction. The foot remains planted, but the upper leg keeps moving, causing the injury to the ligament. This can also happen during slide tackles in soccer, or sliding into base in baseball or softball. The standing player has a planted foot. The sliding player, moving from the side, contacts the standing player’s knee pushing it toward the middle and rupturing the ligament. Another more violent form of contact injury is when the knee is forced into valgus stress by another player. For example, when a group of basketball players jump for the rebound and one player falls onto the other’s knee while they are landing. They key point is that contact with another person is made.

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Non-contact injuries happen when the athlete’s mechanics place more stress on the ligament rather than less stress. Repetitive stress on the ligament leads to weakening. In one moment of a game, while slowing down to make a cut, the body places stress on the ligament during a change in direction, and due to weakening, the ligament is unable to hold up. In this case, the athlete has developed patterns of movement that place more stress on the ligaments rather than the muscles. When good mechanics are developed, the muscles position the body in a specific way that causes a higher amount of the stresses to be absorbed by the muscles. This is how ACL Injury Prevention programs work. These programs help athletes develop good mechanics, strengthening the muscles that support and absorb the forces related to jumping, cutting, and twisting, so that the forces are transferred to the large strong muscles and do not overwhelm the ligaments.


Female Vs Male

When assessing middle school, high school, and collegiate athletes, boys and men have a greater total number of ACL injuries. However, when comparing athletes in the same sport, girls and women injure their ACL at greater rates. What does this mean?

This is likely related to the high number of men that are on football teams. In most places, women do not participate in football. (But that might change in the future thanks to Sam Gordon in Kansas City: read more about her here)  Additionally, women do not have a sport, unique from men, that involves as much physical contact. So what this means, is that men have higher rates of contact related ACL injuries and women have a higher rate of non-contact ACL injuries. Football teams also have a higher proportion of players overall. The typical high school varsity team would have 35 players between the 1st, 2nd, and 3rd string. When you consider that the school also would have a junior varsity team, you are looking at up to 60 kids in any given high school football practice.

So this difference between injuries in men and women also needs to account for the high quantity of football players, a number that is not matched by any other female sport. This means, that if we removed all ACL injuries due to football, women would have a higher rate of injury and higher number of injuries compared to men. Men in sports other than football have ACL injuries, and men in sports have non-contact ACL injuries. Women just have the injury occur more frequently than men. Why would this be true?

There are a few reasons. Unfortunately, most of them are due to differences in the structure and function of women’s bodies, and are unable to be changed. (While you read below about how women are at higher risk of injury, try not to get discouraged. Because we know women are at a higher risk of injury, we know to implement rehab programs that improve mechanics to reduce these risks. Because we know the risks, we can be empowered to prevent injuries through strength and agility training.)

One of the biggest predictors of ACL injury is generalized joint laxity, for both men and women. However, women are more likely to have joint laxity than men. This is thought to be due to women’s reproductive hormones and how they change. It is not the presence of the hormones but rather the fluctuation throughout the month that puts women at risk. When these hormones are highest, they inhibit the ligaments from building collagen, which is the main load bearing structure of ligaments. So the risk of injury would be highest during specific times of the menstrual cycle and then reduced during other times of the cycle. The exact times of the cycle the risk is highest is currently debated and not well understood.

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Another well-known, but debated, factor in women’s ACL injury risk is the Q-angle. The Q-angle, or quadriceps angle, refers to the angle of force production of the quadriceps muscles. The angle is measured using the origin of the rectus femoris muscle (the longest of the quadriceps muscles) at the ASIS (or anterior superior iliac spine, in photo), and follows the length of the muscle to the midpoint of the patella, or kneecap. The reason the Q-angle was “so important” as a risk factor for women was due to the angle difference when comparing men to women.

Due to future birthing needs, women are born with slightly wider hips than men. This means the starting points for the measurements are farther apart, and the Q-angle is increased. In the past, the Q-angle was seen as a large risk factor of ACL injuries, but recently, studies show that is not the case. Q-angle is more likely to cause patellofemoral pain than ACL injury.

Moving to the next factor, differences in how the knee is shaped. There is a notch where the ACL is located that is referred to as the intercondylar notch. All that means is there is a gap between two rounded parts of a bone. Both the ACL and PCL camp out in this notch, and can be stressed when either side of the notch begins to touch the ligaments.

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Since this notch is inside of the knee joint, we can’t see it. Not unless an x-ray is performed. Then again, an x-ray doesn’t show any ligaments. That means an MRI is needed. Unfortunately, MRIs can be misread. That means the best way to tell if the intercondylar notch is going to lead to an ACL injury is to get an arthroscopy. For those of you who don’t know, an arthroscopy is a minimally invasive surgery in which the surgeon looks inside a joint with a camera for a real-time view. What I’m saying is things are going to be costly.

When the notch is narrow, there is less room for the ACL and PCL. When there is valgus stress (inward angle) at the knee, the outside condyle comes in contact with the ACL. With a smaller notch, it takes less valgus stress to cause contact. Think of a taffy being stretched. The point when the taffy stretcher makes contact to stretch the taffy is the point when the outside condyle makes contact with the ACL. As the stress continues, the ACL becomes loose and allows for more motion. At some point, the ACL will give out. *Methods for reducing the notch contact will be discussed in a later post.*