Why Anterior Cruciate Ligament Tears Are More Common in the Female Gender
October is a busy month for sports, with professional major league baseball playoffs, regular professional NFL football and NHL hockey season, men’s professional NBA basketball preseason, women’s professional WNBA basketball championship, and collegiate football games among the most popular in Northern America. However, it seems knee injuries among college and professional athletes are in the weekly or daily national headlines.
Some knee injuries are considered minor, but many athletes will suffer anterior cruciate ligament (ACL) tears, which can end a career or at least the rest of the season. Many collegiate and professional athletes have undergone tough rehabilitation after surgery to repair the tear, but few are able to perform successfully at their pre-injury athletic level.
Few people realize ACL tears are more common in female athletes, from adolescents to adults, with about three out of four ACL surgeries performed on the female gender. Many hypotheses and speculations have been made to explain this phenomenon, with anatomical and hence biomechanical reasons at the top of the list.
Females have wider pelvis and thus hips, which increases the force on the knees during athletic maneuvers, such as jumping and cutting (rapid change in directions). The female gender also has a smaller end of the femur (thigh bone) where the ACL is connected to. Thus, the ACL room or clearance during movement is reduced and is believed to increase the chance of an ACL tear.
In addition, women tend to have more laxity in their ligaments and joints because of genetics and probably also due to the higher level of particular hormones, such as estrogen, and the increased laxity is believed to elevate the odds of ACL tissues to tear. So, as the hormonal level increases during puberty, female high school athletes are at higher risk than their male counterparts to suffer from serious knee injuries.
As the high school sports season gets back into full-swing, sports medicine physicians and orthopedic surgeons see an influx of young athletes, predominantly females, who suffer ACL tears that require surgery. Many athletes wonder why the injury happened to them, and ask if they did something wrong, or whether it was due to intrinsic reasons beyond their control.
Sometimes ACL tears are related to just bad luck, but the sports medicine community has known for quite a while now that several specific risk factors may predispose an athlete to sustain an ACL tear. Some of these risk factors can be environmentally controlled, while others cannot. These risk factors are often classified as either intrinsic or extrinsic.
The anatomical reasons mentioned above to explain the much higher rate of ACL tears in the female gender are examples of intrinsic risk factors. Other common intrinsic risk factors are: proprioception and neuromuscular control, landing, core strength, and the quadriceps/hamstrings (Q/H) strength ratio.
The nerves in the human body transmit sensory input and motor output. Different sensory nerves exist, that is nerve fibers transmit different sensory information. Light touch, temperature (hot, cold), vibration, and pain nerve fibers are well-known, but the transmission of proprioceptive information is less known to laypeople, yet proprioception (also known as joint position sense – JPS) is vital for carrying out daily activities and is especially important for athletes.
Some specialized nerve fibers transmit JPS, to allow us and our brain to know what position different parts of our body are at any given moment, without requiring direct visual input. In other words, proprioceptive feedback makes it possible for us to know where our body parts are located, without having to use our eyes to actually see the location of our feet and arms. Thus, JPS is unconscious and ‘automatic’ (we simply know the location of our body parts at all times), and it frees up our eyes for other important tasks.
Having JPS, or the ability to identify where our body and its parts are in space at any given time, provides automatic feedback that is vital for optimal use of motor output, to be able to control our muscles and make gross or fine adjustments, which is a vital unconscious task that allows for maximum athletic performance, such as in the skills of jumping and landing.
The landing phase is when many ACL injuries take place, and athletes with poor proprioception and/or neuromuscular control are prone to experience higher rates of ACL tears. The good news is that particular drills and exercises are designed to improve proprioception and neuromuscular control, and thus reduce the chance of tearing ACL. Also, JPS nerve fibers may be injured and subsequent neuromuscular control is out of sync after knee injuries and ACL surgical repairs, and thus an important part of knee injury and post-op rehabilitation is to regain and maximize proprioception and neuromuscular control before returning to playing competitive sports – to prevent early re-injury.
Landing with locked knees and/or knock-kneed (with the knees pointing inward towards each other), especially with flat feet drastically elevates the rate of serious ACL injury. Luckily, training exercises and practice can improve our landing techniques to avoid knee injuries.
“Core strength” is a common buzzword these days and it basically refers to the strength of our trunk muscles, which are abdomen, especially the lower abs, low back or lumbar muscles, and even the muscles that attach to the pelvis and hips. Athletes with poor core strength have difficulty stabilizing their trunk/hip region, which then places more force on their knees and strain on the ACL. Fortunately, core strength is an intrinsic risk factor that can be dramatically reduced with the proper training program and hard work.
Another speculated risk factor for ACL injury is the Q/H strength ratio. Many people perform strengthening exercises that target the quadriceps or quads muscles, which are the bulky muscles located along the front of the thigh, but don’t strengthen the hamstrings (muscles located along the back of the thigh) nearly to the same degree. Unfortunately, the massive quads muscles pull the tibia (shin bone) forward relative to the femur, which is the opposing function of the ACL!
Many sports medicine practitioners believe an excessively high Q/H ratio increases the rate of ACL tears, with the massive quads placing enormous strain on the ACL because the weaker hamstrings cannot match with (near) equal amount of force – to pull the shin bone back relative to the femur in order to oppose or reduce the straining force on the ACL. So, the preventative goal of training is to lower the Q/H ratio by focusing on hamstring strengthening to keep pace with quads strengthening.
Common extrinsic factors are more easily understood and include direct blow, dry weather, and artificial surfaces. Regarding direct blow, you can’t do much about the outcome if someone strikes your leg. Fortunately, direct blows to the leg accounts for only about 30% of ACL injuries. The rest happens without direct contact, such as cutting, changing directions quickly, or landing awkwardly.
Professional trainers keep tabs on the weather on game day because dry weather increases friction between the athlete’s shoe and the surface (e.g., grass, artificial turf, sand, mats, and a variety of indoor flooring). So, athletes are more likely to get stuck while pivoting, cutting, and performing other agile and evasive maneuvers during dry weather, and hence sustain dreaded ACL tears.
Artificial surfaces are notorious for their high friction between shoe and ground, and thus lead to higher rate of ACL tears. Wet grass reduces friction and hinders an athlete from sudden direction changes, leading to ‘sloppy games’, but reduces the chance of an athlete suffering a serious knee injury that requires surgery.
In summary, ACL injuries that curtail a competitive athlete’s career or at least sideline her for the rest of the season are not always random events. There are many other risk factors that are beyond the scope of this article, but they can aid in the prediction of which athlete and when the athlete will probably suffer a career-ending ACL tear. The good news is that a wise athlete at the high school or higher level can focus on the risk factors that are modifiable to minimize her chance of undergoing knee surgery for an ACL tear.
Tobey Leung, M.D.