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ACL injuries in women: Why the gender disparity and how do we reduce it?
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Women tear their ACLs three to 10 times more frequently than men in similar athletic events, but the reasons for this disparity are not fully known. Several factors — anatomic, neuromuscular and hormonal — may play an interrelated role. Neuromuscular training programs appear successful, in part, in leveling the playing field between men and women.
The impact of size differences in the ACL may be a key factor in injury rates. The cross sectional area of the ACL in women is smaller than in men when normalized by body weight. Thus the same tensile forces placed on a female ACL may be sufficiently high to injure the ACL in women but not in men.
Tensile loads on the ACL may be higher in women. For example, dynamic resistance to tibia femoral translation of the tibia on the femur is termed sagittal plane shear stiffness. This stiffness is defined by co-contraction of the quadriceps and hamstring. Wojtys reported that sagittal plane knee stiffness is less in women. Therefore, one would predict that greater loads are placed on the ACL, which is the major static restraint to anterior tibial translation.
According to Wolff’s Law, one would predict that the increased loads on the ACL should invoke a response in the ligament to increase its strength though tissue remodeling. However, in basic science studies, increasing estrogen concentration in ACL tissue culture decreases collagen synthesis. Also, expression of a protease gene, MMP3, is decreased in women compared to men, possibly signifying an increased degradation of structural elements in female ACLs. These two studies support the clinical study identifying smaller ACLs in women of similar size and define the concept that ligament remodeling plays a role in gender disparity of ACL injury.
Men and women demonstrate different neuromuscular responses to landing from a height. When women land, there is significantly greater medial knee collapse (valgus). The relationship between the medial knee collapse, puberty and menstrual cycle needs further study.
This round table brings together several experts in both clinical and basic science to share their expertise on ACL injury, risk and its prevention.
James R. Slauterbeck, MD
Moderator
Slauterbeck: Dr. Arendt, an epidemic of ACL injuries appears to be occurring in the United States. What is the incidence of ACL injury?
Elizabeth Arendt, MD: In 1985, Dale Daniel reported that the annual incidence of ACL injuries in the United States was 1/3500 or 75,000 per year.
Slauterbeck: Are the rates of ACL tears equal for male and female athletes in similar sports in the NCAA?
Arendt: A review of NCAA data for collegiate basketball and soccer players from 1989-2001 showed that injury rates per 1000 athletes for any ACL injury ranged from 0.03 to 0.13 for male basketball players, 0.20 to 0.37 for female basketball players, 0.001 to 0.16 for male soccer players, and 0.17 to 0.34 for female soccer players. The rates of ACL injuries in male soccer players went down significantly over time while the rate in femaleathletes rose. In basketball the rate in men went up while the rate in women went down.
Slauterbeck: Dr. Beynnon, ACL injury is also common in skiers. Are female skiers at an increased ACL injury risk? If so, why?
Bruce D. Beynnon, PhD: The incidence of ACL injury in competitive alpine ski racers and recreational skiers appears to be two times greater for women compared to men. Even more discouraging is our finding that one in five women alpine ski racers (22%) rerupture their ACL reconstruction, requiring additional surgery to the same knee. The explanation remains unclear. Very little data are derived from good prospective studies. In addition, alpine skiing is a unique sport that not only includes a broad selection of risk factors intrinsic to the skier (eg, neuromuscular, biomechanical, structural and hormonal variables), but it also includes factors extrinsic to the skier (snow conditions, ski design, boot design, binding design and binding settings).
Slauterbeck: Although the incidence of lower extremity injury has decreased in alpine skiing over the last 30 years, why has the incidence of ACL injury increased?
Beynnon: This trend runs counter to the dramatic reduction in lower leg injuries that began in the early 1970s that has lowered the risk of injury below the knee by almost 90%. The primary design objectives of the modern ski boots and bindings have been to protect the skier from tibia and ankle fractures, and they have done a very good job at accomplishing this goal; however, modern ski bindings have not protected the knee from serious ligament trauma.
Slauterbeck: Dr. Beynnon, what risk factors are most likely associated with ACL injury; are any of these factors more prevalent in women?
Beynnon: Many factors have been implicated as predisposing athletes to knee ligament trauma; however, there is little objective data that derive from well-conducted prospective studies and identify the putative risk factors for ACL trauma. There are five prospective studies that have compared male and female athletes who competed in the same sport with the same baseline risk. This body of literature has revealed that women are at 2 to 9.7 times greater risk for ACL trauma in comparison to men. In addition, there is evidence from prospective, observational, cohort studies that an increased incidence of ACL rupture in athletes is associated with a narrower femoral intercondylar notch. There is also preliminary evidence that the phase of the menstrual cycle may explain a portion of the increased risk of ACL injury.
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COURTESY OF THE CINCINNATI CHILDREN’S SPORTS MEDICINE BIODYNAMICS CENTER |
Slauterbeck: Dr. Hardy, is there a relationship between women’s ACL injuries and the menstrual cycle?
Daniel M. Hardy, PhD: A few studies have looked at this question. Dr. Wojtys’ group at the University of Michigan found an increased injury rate in the ovulatory phase among female skiers. Our group at Texas Tech Health Sciences Center studied soccer and basketball players and found a significant increase in ACL injuries immediately preceding and in the first two days of menses. A recent report on Norwegian handball players also found an increased injury rate in the menstrual phase. Based on this limited number of studies, it seems clear that the menstrual cycle does affect susceptibility to ACL injury, but the effect varies depending on injury mechanism.
Slauterbeck: What relationship does the menstrual cycle have to ACL injury?
Beynnon: Well-conducted prospective studies using saliva or urine analysis of hormone levels to confirm phase of menstrual cycle at the time of ligament injury have presented evidence that there is an increased prevalence of injuries in the preovulatory phase of the menstrual cycle in contrast to the postovulatory phase.
Slauterbeck: Where is research on the menstrual cycle’s effect on ACL injury headed?
Hardy: It’s pretty clear that we need to understand how hormones affect the ACL. Biomechanical studies on ACLs from nonhumans have yielded significant but conflicting results. Some have found that certain hormone regimens weaken the ligament, whereas others found no effect. Interpreting the results of such model studies is difficult because reproductive processes are astonishingly variable between species. Ultimately, of course, we want to understand ACL injury in humans, which are the only bipedal animals with a menstrual (as opposed to estrous) cycle.
So we think it is essential to study the human directly. For biomechanical experiments there is still plenty to be done in model species because of limitations in availability of suitable human materials. However, available genome sequence data make the human the ideal species for gene expression and other molecular studies of the ACL.
Slauterbeck: What additional research is necessary to better define how the menstrual cycle relates to ACL injury?
Beynnon: Additional research is needed that uses standardized time intervals to establish the menstrual phases, investigates subjects with different cycle lengths, measures actual hormone levels present in either urine, saliva or serum, and documents hormone levels at the time of injury and the onset of the following menstrual period to determine menstrual cycle phase and cycle length in which the injury occurred. These studies should also include an adequate distribution of control subjects that represent the population at risk for this injury.
Slauterbeck: What kind of molecular studies should be done to better understand ACL injury?
Hardy: One of the biggest gaps in our knowledge is in the basic biochemistry of ligament remodeling and the effects of gender on that process. Our group is studying how gender and sex hormones affect the expression of tissue remodeling genes in the human ACL. Other groups are finding that mechanical load affects ACL gene expression. Links between the loads placed upon the ACL, the material properties of the ACL, ACL remodeling and the effects of hormonal control on the ACL need to be established. Many such fundamental questions need to be answered before we will fully understand ACL injury.
Slauterbeck: Gender differences in anatomy place some at risk for injury. Many researchers report that injury risk with smaller size of the notch is increased and others do not. Can you provide an explanation why the conclusions vary so much?
Arendt: Numerous articles explore a possible relationship between the width of the intercondylar notch and the risk of noncontact ACL injury. A review article considered 15 papers on the relationship between intercondylar notch size and noncontact ACL injuries. These research articles varied considerably with regard to population groups studied and measurement techniques. Only two prospective studies were identified and only one study rigorously controlled rotation of the limb while taking the x-rays. None of the articles state any error in the measurement techniques themselves or inter/intra variability in measurement techniques.
Slauterbeck: Dr. Arendt, could you synthesize the information on notch size and shape on ACL injury risk.
Arendt: Despite the limitations, all the papers have collectively supported the following statements:
- Notch width of bilateral ACL injuries is smaller than that of a unilateral ACL-injured knee.
- Notch width of bilateral and unilateral ACL-injured knees is smaller than notch width of normal controls. In studies that group study populations by sex, the width of a female notch is smaller than the width of a male notch, as well as the ratio of the notch width to the bicondylar width.
- A relationship exists between the width of the bicondylar femur and the width of the notch. This is true for both sexes. The smaller the width of the femur, the smaller the notch.
However, if one accepts that there is a relationship between the size of the intercondylar notch and the risk of noncontact ACL injury, it is difficult to conclude exactly what that risk is. In a 1997 article (Shelbourne, Facibene, Hunt), the authors reported a smaller notch width index in women than in men. However, although 66% of female controls had a notch width less than or equal to 15 mm and 34% of male controls had notch width less than or equal to 15 mm, these athletic young adults did not sustain an ACL injury. Therefore, the actual risk of a small notch and the likelihood of sustaining a noncontact ACL injury are unclear.
Slauterbeck: Does the ACL size play any role in ACL injury in female athletes?
Arendt: More recently, it appears that a small notch may house a smaller ACL, and size (and therefore strength) of the ACL alone may be the primary risk factor leading to risk of injury. Three authors in separate studies, Staubli, et al (Arthroscopy, 1999), Jackowski, et al (Graduate student thesis, 2001), and Willitis et al (AOSSM, 1999) measured ACL width or area and found that women had smaller ACLs than men or controls. Based on the data to date, it appears that the increased rate of noncontact ACLs seen in patients with narrower notches may simply be a manifestation of a smaller ACL. It would be prudent to try to look at this across a larger-sized population group.
Slauterbeck: Differences in the way one lands, runs and cuts plays a role ACL injury. Dr. Hewett, is there a gender difference in neuromuscular control during those athletic maneuvers?
Timothy E. Hewett, PhD: Yes, there is a gender difference in neuromuscular control of the knee joint. Three neuromuscular control imbalances are common in women athletes — ligament dominance, quadriceps dominance and leg dominance.
Slauterbeck: Please define the three imbalances.
Hewett: Ligament dominance occurs in women because they allow stress on ligaments prior to muscular activation to absorb ground reaction forces. The lack of dynamic muscular control of the joint leads to increased valgus motion, increased force and high torque that can stress the passive restraints of the knee joint.
Quadriceps dominance occurs in female athletes because they activate their knee extensors preferentially over their knee flexors during sports movements to stabilize their knee joint. At low knee flexion angles, quadriceps activity stresses the ACL, while flexor activity decreases ACL stress. ACL injury nearly always occurs when the knee is near full extension, where quadriceps dominance may put the ligament at increased risk.
Leg dominance is the imbalance between muscular strength and coordination on opposite limbs with the dominant limb often demonstrating greater strength and coordination. Limb dominance may place both the weaker, less coordinated limb and the stronger limb at increased risk of ACL injury. The weaker limb is compromised in its ability to manage even average forces and torques, while the stronger limb may experience exceptionally high forces and torques due to increased dependence and increased loading on that side in high-force situations.
Slauterbeck: Are neuromuscular training programs successful at decreasing ACL injury?
Hewett: Yes, they appear to be successful in the high-risk female sports population. I led an epidemiology study to prospectively evaluate the effect of neuromuscular training on ACL injury rates in female athletes (Hewett et al, 1999). Trained women had a 72% lower incidence of ACL injury than untrained women. Trained women were not different than untrained men. The above chart demonstrates that training resulted in significant differences in noncontact ACL injuries between the female groups. These results indicate that neuromuscular training decreases injury risk in female athletes.
Slauterbeck: Researchers at the Oslo Sports Trauma Research Center, Norwegian University of Sport and Physical Education have been studying athletic injuries and have been instrumental in implementing injury prevention strategies for the elite Norwegian athletic teams. A prospective study has recently been published on ACL injury prevention in team handball players. Dr. Engebretsen, can you describe the athletes studied?
Lars Engebretsen, MD: Yes, we recently published a prospective study assessing the effect of a neuromuscular training program on the incidence of ACL injuries in women handball players. Women players from the three top divisions were studied for three years (1998-99, 2000-01, 2001-02). The control season included 942 players; the first intervention season, 855; the second intervention season, 850.
Slauterbeck: How were the athletes taught the program?
Engebretsen: The teams were taught a five-phase program that used three balance exercises focusing on neuromuscular control and planting/landing skills. Each team was supplied with an instructional video, poster, balance mats and wobble boards. Program success was measured by identifying the number of ACL injuries during the three seasons and compliance with the program.
Slauterbeck: What were the results of the study?
Engebretsen: The athletes sustained 29 ACL injuries during the control season, 23 during the first intervention season (P=.62), and 17 during the second (P=.15). In the elite division, there were 13 injuries during the control season, six in the first intervention season (P=.17), and five in the second (P=.06). In the elite division, the risk of injury was reduced among those who completed the ACL injury prevention program.
Slauterbeck: Dr. Mandelbaum, the PEP (Prevent Injury, Enhance Performance) ACL Prevention Program has been used in young soccer players to decrease ACL injury. Please describe the program.
Bert Mandelbaum, MD: The PEP program focused on biomechanical risk factors, stressing avoidance of high-risk behaviors and increasing kinesthetic awareness of athletes. It is a comprehensive alternative warm-up program consisting of an educational videotape and a supplemental literature packet focusing on biomechanical strategies to reduce ACL injuries in soccer athletes. The program was completed two to three times a week over a 12-week soccer season.
Slauterbeck: What exercises are included in the PEP program?
Mandelbaum: The program includes stretches for the trunk and lower extremities, strengthening of core stability and lower extremity, plyometrics (single and double leg stance), and sports-specific agility drills.
Slauterbeck: What was the incidence of ACL injury in soccer athletes you studied before and after the PEP program?
Mandelbaum: Epidemiological data for 2000 include two ACL tears in the intervention group (incidence rate of 0.2 ACL injuries/athlete per 1000 exposures) and thirty-two ACL tears in the control group (incidence rate of 1.7 ACL injuries/athlete per 1000 exposures). These results indicate an 88% overall reduction of ACL injury per individual athlete compared to a skill- and age-matched control athlete.
Slauterbeck: Researchers at Cincinnati children’s hospital have made tremendous strides in implementing injury prevention programs for athletes. What is the Dynamic Neuromuscular Analysis (Sports DNA) Training Program all about?
Hewett: DNA training is a synthesis of the most relevant data derived from published research studies and prevention techniques developed through more recent empirical and analytical evaluations of neuromuscular deficits and on-field play. The three components of Sports DNA training are dynamic sport-specific movement skills, neuromuscular patterning drills and constant biomechanical analysis by the instructor and feedback to the athlete during training. Correction of neuromuscular imbalances in three planes of movement — sagittal, coronal and transverse — is critical for improving the biomechanics of athletic movements and reduction of ACL injury incidence.
Slauterbeck: In your opinion, how does DNA training work?
Hewett: DNA training addresses the neuromuscular imbalances present in female athletes. The exercises challenge the dynamic joint restraints (muscle-tendon units) that maintain limb and joint position in response to changing loads. The dynamic component helps the female athlete progress to high-risk sport-specific maneuvers that are performed in a safe and controlled manner. A properly trained athlete is prepared to handle the high joint forces generated during athletic competition to reduce the risk of injury and to achieve peak levels of performance.
The enhanced neuromuscular control can protect the athlete from ground reaction forces encountered in competitive play when jumping, landing and cutting. Exercises that emphasize co-contraction of the knee flexor and extensor muscles can decrease quadriceps dominance. The training develops a more appropriate firing pattern for the knee flexors while utilizing exercises that also strongly activate the knee extensors. At knee flexion angles greater than 45°, the quadriceps is an agonist to the ACL. Therefore, the training uses deep knee flexion angles to put the quadriceps into an ACL agonist position.
Slauterbeck: How is the athlete taught to correct ligament dominance and to prevent medial knee collapse during landing maneuvers?
Hewett: The athlete is taught to control dynamic knee motion in the coronal (valgus) plane. Additionally, the athlete is taught that the knee is a single-plane hinge, not a ball-and-socket joint. Education of the female neuromuscular system away from multi-planar motion of the knee to dynamic control of knee motion in primarily the sagittal plane only is achieved through a progression of single to multi-planar plyometric, strength and agility exercises.
Slauterbeck: Are there any other studies that demonstrate significant decreases in ACL injury with neuromuscular training specifically in the female athlete?
Hewett: Considered together, the studies we examined (Ettlinger, 1995; Caraffa, 1996; Hewett, 1999; Wedderkopp, 1999; Heidt, 2000; Mandelbaum , 2002, Myklebust, 2003) provide strong evidence that neuromuscular training is likely to be an effective solution to the problem of gender inequity in ACL injury.
Slauterbeck: In summary, ACL injury is frequent among our youth today. Women are at greater risk of ACL injury. Many factors including anatomy, sex hormones, and neuromuscular control are related to the increased ACL injuries in females. Neuromuscular training programs both in the United States and abroad are successful at decreasing ACL injury.
I thank the panel for their time and enthusiasm in preparing thoughtful and thought-provoking answers.