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Why are women more susceptible to ACL injuries?

Every year more women discover the rewards of participating in athletics. Unfortunately, an increased number of anterior cruciate ligament (ACL) injuries have accompanied the increased participation. ACL injuries in female athletes are an epidemic problem facing women, coaches, and the sports medicine community.(6) The injuries can occur from sudden stops and turns, an awkward landing, and from a wide variety of other instances. These injuries most often occur in athletes while participating in basketball, soccer, or gymnastics. It has been found that female athletes are four to ten times more likely to have ACL injuries than male athletes (6). The statistics of ACL tears in women athletes are troubling. A 1998 NCAA report found that women suffered 39 ACL injuries playing college basketball, compared to 16 injuries in men, during the 1997- 98 season (5). The following list contains some theories to why women are more susceptible to ACL injuries.(5)

·Joint laxity — Women have more limber joints, they can more easily bend out of the knee’s range of motion.

·Wider hips — Women’s alignment between hip and knee may putstrain on the ACL.

·Fluctuating hormones — Different levels of female hormones potentially weaken knee ligaments.

·Narrow notch — At the end of the thigh, a bone indentation called the intercondylar notch houses the ACL; a narrow intercondylar notch predisposes athletes to suffer an ACL injury.

·Ligament size — A smaller ACL puts a person at greater risk of ACL injury.

·Weaker hamstrings — Strong hamstrings help stabilize the knee when athletes jump, twist, and pivot.

·Neuromuscular control — Women’s muscles tend to fire at a slower rate than those of men.

·Body movement — Women tend to play sports in a straighter posture, which may strain their ACL.

The following information is a background on the ACL and its functions. In the knee joint, an intercondylar notch lies between the two rounded ends of the femoral condyle (femur). The ACL moves within this notch, connecting the femur and tibia to provide stability to the knee.(6) The ACL is a dynamic structure whose main function is to provide primary restraint to anterior tibial subluxation. This prevents the tibia from moving too far forward. It provides secondary restraint limiting internal rotation and restraint to varus and valgus angulation with the knee in full extension. This keeps the tibia from rotating too far inward under the femur. Along with the posterior cruciate ligament, it provides the axis for knee rotation and links rotation with flexion and extension. The ligament is primarily made up of two bands, the anteromedial and posterolateral. The ACL runs from the posteromedial portion of the lateral femoral condyle (thigh) to an area just lateral to the medial tibial eminence (shin).(1)

Studies have shown that men and women’s knees have different anatomical structures, which could lead to an increase in knee injuries. Women have a narrower intercondylar notch than men have. Therefore, the space for ACL movement is more limited in women than in men. Within this restricted space, the femoral condyles can more easily pinch the ACL as the knee bends and straightens out, especially during twisting and hyperextension movements. In the knee, the femur meets the tibia at an angle (called the quadriceps, or Q, angle). The width of the pelvis determines the size of the Q angle. Women have a wider pelvis than men, so the Q angle is greater in women than in men. At a greater angle, forces are concentrated on the ligament each time the knee twists, increasing the risk for an ACL tear. A twisting injury in a man's knee may only stretch his ACL; however, because of the greater Q angle, the same type of twisting injury in a woman's may cause a complete ACL tear.(6)

An extrinsic factor that may help explain women's ACL injury rate is the imbalance between hamstring and quadriceps muscle strength. Men tend to have more developed thigh muscles than women, and stability of the knee is thought to be more muscle-dominant in men and ligament-dominant in women. For female athletes, the quadriceps is the dominant muscle group contributing to knee joint stability, while the hamstring dominates in male athletes. Research indicates that women have decreased hamstring-to-quadriceps strength ratios relative to men.(1) Women rely more on the quadriceps muscles in the front of the knee, rather than the hamstrings in back. “There’s a chain, a cascade of responses from the nervous system, to muscle, to coordination, all of which we call proprioception — the ability to respond to the stresses of motion at an instantaneous moment,” says Bert R. Mandelbaum, M.D., team physician for U.S. Soccer. A large part of proprioceptive muscle control is unconscious coordination. To protect the ACL from injury, muscles surrounding the knee must work together with precise timing. In the knee the quadriceps directly pulls on the ACL and the hamstring opposes the pull of the quadriceps. Women tend to have a proprioception deficiency in which their hamstrings typically react slower than men’s. This causes all the instantaneous force of a rebound, jump, or pivot to more directly strain women’s knee ligaments.(5)

In a new study that was presented at the American Academy of Orthopaedic Surgeons in California, researchers saw that women run differently than men. Men run in a more crouched position, while women's knees and hips tend to be straighter when they run. Edward G. McFarland, M.D., director of the Division of Sports Medicine and Shoulder Surgery for the Johns Hopkins Medical Institutions says, “Two recent studies show that women do indeed have different body mechanics than men when they run. They don't have as much flexion in the knee when they do cutting maneuvers, and they put more stress on the inside of the knee when changing direction.”(2) Another reason behind ACL injuries is how women tend to jump and land. Women tend to jump and land flat footed. They keep their knees straight and their hips in the same position, which puts more strain on the ACL. Anatomical differences may incline women to rely on their ligaments to decelerate their momentum, and letting their knees come together in a “knock kneed” motion, creating rotational stress on the ACL. Men tend to land using their stronger muscle groups such as the glutes and hamstrings to absorb the momentum, putting less stress on their knees.(4)

Dr. Edward Wojtys of the University of Michigan headed a study done on women and ACL tears, where they found that women are three times more likely to tear an ACL during ovualtion than at other times during the month (3). During the menstrual cycle, hormone levels vary, which may have an affect on knee stability. Recent studies have shown that, at specific points within the menstrual cycle, the knee becomes looser than normal, and ACL rupture is more common. Female hormones allow for greater flexibility and looseness of muscles, tendons, and ligaments. This looseness helps prevent many injuries because it enables certain joints and muscles to absorb more impact before being damaged. However, this looseness does not necessarily prevent an ACL injury in a woman's knee. If the other ligaments and muscles around the knee are too loose they may not be able to absorb the stresses put on them, causing forces to be transferred directly to the ACL. In this sense, the ACL not only has to maintain stability about the knee, but it also must make up for instability in a generally loose knee.(6)

These are only a few of the factors that could lead to women being more susceptible to ACL injuries. These factors may or may not be what causes every ACL injury; it is usually a combination of things that causes the injury. ACL injuries in women are occurring more frequently and it is gaining more attention. Research is being done continually to hopefully find what makes women more susceptible to ACL injuries. There are workout plans and strengthening programs that can be done to help prevent the risk of an ACL injury, but when the cause is figured out then women will be better able to take the necessary precautions to prevent ACL injuries.

References

1. www.physsportmed.com/issues/1997/04apr/moeller.htm

2. www.knee1.com/news/tech.efm/42/1

3. www.jsonline.com/alive/sports/jun99/running060799.asp

4. www.backtoactionphysicaltherapy.com/acl_tears.htm

5. www.proteamphysicians.com/article/index.asp?showarticle=yes&articleid=173&articletype=13

6. www.hughston.com/hha/a_11_3_2.htm