Women’s Health Month: Why Female Athletes are More Prone to ACL Injury

Thanksgiving is coming a little later this year, and so are my blogs on the promised topic of Women’s Health Month. As the weather gets colder and most high schools are starting their basketball seasons around this time of year, I thought a great blog topic would be ACL injury with particular attention to why this injury has a female-athlete predominance.

In 1972, 1 in 27 Varsity sport athletes were female. In 1997, this number drastically increased to about 45%: 55% female: male participation, thanks to Title IX. With this increase, the number of ACL injuries also increased with a notable female predominance to ACL injury. This sex discrepancy in ACL injury begins at puberty and continues into adulthood. Even more so, the number of non-contact ACL injuries is higher in girls than boys, and this is a notable increase in incidence that is also seen from the onset of puberty, age 12 to 14 years in girls. The peak incidence for ACL tear is in the group of 16 to 18 year-old girls, and the highest numbers of ACL injury have shown to be in girls’ soccer: 14 per 100,000 female soccer players vs. 4 per 100,000 male soccer players at the high school level. The second highest incidence is in basketball with the ratio of female: male ACL injuries in this group of high school athletes being 2:1.

So what’s the deal? Why are ACL injuries more common in female athletes? To begin to answer this question, we first have to discuss the biomechanics involved and this boils down to what is an ACL and what does it do…

What is the ACL and what does it do?

The ACL is the anterior cruciate ligament of the knee joint, and it runs from deep within the distal femur (the long bone of the thigh that is above the knee) to the front of the tibia (the bigger bone that makes up the shin). The origin or beginning of the ACL is within the notch of the distal femur and the proximal (beginning) fibers of the ACL fan out along the medial wall of the lateral femoral condyle. The ACL attaches to the tibia on the intercondyloid eminence that is adjacent to the medial meniscus. All of these interactions allow the ACL, along with the PCL (the posterior cruciate ligament) to stabilize the knee joint.

The ACL resists anterior (forward) and medial (sideways toward the middle of the body) movement of the tibia across the femur. In short, it keeps your lower leg from moving forward and rotating inward toward your other leg.

Terrible Triad

Because of the interaction of the surrounding stabilizing structures of the knee joint, a common injury that can occur is called the “Terrible Triad.” You might hear someone say with reference to this injury that they “blew out” their knee. This occurs when there is a tear of the ACL, the MCL (medial cruciate ligament), and the medial meniscus.

So what’s with the ladies?

There are many theories about why ACL injuries are more common in the female athlete. The answers have ranged from differences in female vs. male training, issues with flexibility and ligament laxity, hamstring/quad strength discrepancy, bone structure, and even female hormones. So let’s analyze the data and separate fact from fiction, folks.

Hormones

Like everything that affects women, they’ve tried to blame the female athlete ACL injury predominance on our hormones, ladies, and guess what? The studies are still inconclusive. The most convincing theory is that estrogen receptors on ligaments cause increase ligament laxity in female athletes causing increased risk of ACL tear as opposed to male athletes.

                What supports this theory:

Some studies have found that there are an increased number of ACL tears around the time of ovulation (in the pre-ovulatory and ovulatory phase where estrogen levels peak). Moreover, some studies have shown that oral contraceptive pills have decreased the risk of ACL tear in female athletes. The theory behind this would be that OCPs prevent ovulation, maintain steadier hormone levels, and prevent the changes that make women more susceptible to ACL tear around the time of ovulation. This theory has been supported by evidence showing that early on in puberty there is not as much of a female predominance to ACL tear. Folks have postulated that this is because estrogen does not vary much at this time, i.e. estrogen is not exerting its effects on the neuromuscular system.

                What goes against this theory:

Men have estrogen too. What? Well, they do. But they don’t get the increased joint laxity that is often seen in female athletes. It may be that their receptors respond differently, but the fact is that in both boys and girls after puberty, peripheral testosterone is converted to estrogen, so both sexes have this hormone, which could exert effects on the neuromuscular system. Other studies have also come about to debunk the idea of an ovulatory association with ACL tear. They have found a greater number of ACL tears on cycle days 1 and 2 of the menstrual cycle. These days are during the first part of menstruation or during the early follicular phase of the cycle when levels of estrogen and progesterone are low. So what gives? Maybe this one isn’t actually something we can blame on our hormones, ladies.

Athletic Training and Conditioning

This argument really used to be something to think about back in the day when women were just getting into competitive sports, but now look at us! Go ladies! So proud of where we have taken female sport!!! :) With female athletics taking off and with Title IX giving women the same resources to trainers, physical therapists, strength and conditioning coaches, etc. that male athletes have, we really can’t prove with evidence-based studies that there are specific differences in the type of training that male and female athletes are doing which predisposes women to ACL tears. However, it may be more about what we are NOT doing in our training of female athletes to compensate for the differences in male and female musculoskeletal structure and biomechanics. This idea has been linked to hormones in one study showing that changes in knee joint laxity during the menstrual cycle does affect knee joint loading during movements, which could be important in ACL injury prevention programs and how we are training our athletes. This is just one study done in 2009. So, we need more evidence to come up with better training programs. But up next, what it is about the female musculoskeletal  system that predisposes to ACL injury?

Musculoskeletal Biomechanics

Female athletes are more quad and leg dominant; they rely more on ligaments for stability, and they have increased ligament laxity, especially after puberty. The intercondylar notch of the femur is more A-shaped in the female femur vs. the male femur. Women also have a wider pelvis, which increases knee valgus and causes a greater Q angle.

Bone structure

So what the heck is knee valgus and Q angle? Knee valgus is when the knee deviates inward (knock-knees). In many female athletes, when they jump and land, when they do a deep squat, or when they make a cut to change direction, their feet are externally rotated, and the hip and knee are not very flexed. This posturing causes an increase in Q angle, the angle between the pelvis and the knee. The male Q angle is typically about 14 degrees. The normal female Q angle is about 17 degrees. In persons with patellofemoral pain syndrome (something we can talk about another time), that angle is increased to greater than 22 degrees.

As puberty increases, the female pelvis is becoming and wider and flatter, causing the change in Q angle that we see between male and female athletes. Knee valgus and increased Q angle place increased strain on the ACL because the ACL has to work harder to pull the knee back into normal position and stabilize the knee joint. Taking all of this into consideration, it really does make sense to teach our athletes proper squatting and jumping/landing techniques.

Ligaments

The female ACL may also have a lower percent surface area of collagen.

Muscles

Another consideration is the difference in strength between genders. By age 15-16 years, girls have 75% of the strength that boys have, secondary to increase in lean body mass that is much greater in boys. During puberty, boys increase their hamstring strength by 179% and their quad strength by 148%. On the other hand, girls become more quad dominant than boys do with these changes in strength during puberty.

Moreover, when women experience stress on their ACL, they naturally compensate by activating their quads and lateral hamstrings, which causes increased Q angle. Yikes!

Boys/men seem to fire their semimembranosus (the medial hamstring) later than girls/women do, and this action is thought to be more beneficial in protecting the ACL from injury. It seems that because of these differences, pubescent girls are at greater risk for non-contact ACL injury compared to boys.

Immediate Assessment: Does my child need an MRI?

So we all know that this is my favorite question for athletes and for all pediatric issues. Parents always want to know if their children need cat scans, x-rays, and MRIs. With any injury, it is important to get an assessment from a doctor as close to the injury as possible. Often this does mean going to an Urgent Care Center or Emergency Room as most sporting events take place after regular pediatric or orthopedic office hours. The opportune time for assessment of ACL tear is actually immediately after the injury occurs, so if your team has a Sports Medicine doctor available at the game, this is your best bet. Even waiting until you get to the ER after the game can sometimes make assessing an ACL tear more difficult because the more swelling that occurs, the more difficult it is on physical exam to evaluate the ACL.

Anterior Drawer Test - one of the physical exam tests
to evaluate for ACL tear.

The most important thing is to be evaluated shortly after the injury has occurred. You will not be able to get an emergent MRI, and an MRI is not always necessary in the case of knee injury. Depending on the mechanism of injury and the physical exam, your doctor may think an x-ray is necessary to rule-out fracture. The next step may be an MRI to evaluate ligaments and other soft-tissue damage, depending on the physical exam. Your child does not need a CT or cat-scan. This test is emergently available, but it is not as definitive as MRI and has, as we have previously discussed, a large amount of radiation, and should not be done unless absolutely necessary. Listen to your doctor and ask questions if you do not understand the tests ordered or the plan for the care of your child.

Treatment

Interestingly enough, these days, for an ACL tear, depending on the severity, and depending on the future goals of the athlete, you do not always need surgery! Isn’t that great news? It depends on how severe the tear is and how competitive and lengthy the athlete’s future career will be. It is often helpful to get the opinion of two different orthopedic surgeons before making your decision on surgery. This injury used to be a career-ending injury for many athletes, and now recovery times and strong commitments to rehab have athletes back to jogging in 6 weeks and back to their sport within the same season, depending on when the injury occurs.

Prevention

I hope that what this short article has shown you is that we actually need more studies to show what we should be doing to protect our athletes, what strength and conditioning programs would best help prevent ACL tear. Studies have shown that the types of prevention programs that are in the mix currently have made no difference for kids age 9-12 years in preventing ACL tear; however, they have noted improved technique and better prevention with programs in the age range of 14- to 17-year-olds. These kids may be more susceptible to feedback and coaching at this age, but 9- to 12-year-olds are kids already learning sport-specific skills, and we need to find a program that can work for improving their techniques in order to prevent ACL tear and other injuries. One thing is for sure, our treatments and our rehab have come a long way in helping athletes recover from these injuries, but prevention is definitely the future for our athletes.

References:

Bell DR, et al. The effect of menstrual-cycle phase on hamstring extensibility and muscle stiffness. J Sport Rehabil. 2009 Nov;18(4):553-63.

Bell DR, et al. The effects of oral contraceptive use on muscle stiffness across the menstrual cycle. Clin J Sport Med. 2011 Nov;21(6):467-73.

Dragoo JL, Castillo TN, et al. Serum Relaxin Concentrations and ACL Tears. Journal of Sports Medicine. 2011. 39:2175-2180.

Lefevre N, et al. Anterior cruciate ligament tear during the menstrual cycle in female recreational skiers. Orthop Traumatol Surg Res. 2013 Sep;99(5):571-5.

Park SK, et al. Alterations in knee joint laxity during the menstrual cycle in healthy women leads to increases in joint loads during selected athletic movements. Am J Sports Med. Jun;37(6):1169-77.

Slauterbeck, et al. The Menstrual Cycle, Sex Hormones, and Anterior Cruciate Ligament Injury. Journal of Athletic Training 2002;37(3):275–280.

Wild CY, Steele JR, Munro BJ. Why do girls sustain more anterior cruciate ligament injuries than boys?: a review of the changes in estrogen and musculoskeletal structure and function in puberty. Sports Med. 2012 Sep 1;42(9):733-49.