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Researchers at the Annual Meeting of the American Orthopaedic Society for Sports Medicine Find That High Rate of Serious Knee Injuries Among Female Athletes May Be Influenced by Numerous Factors
June 22, 1999-Traverse City, Michigan-The incidence of serious knee injuries among female athletes at the high school and college level is four to six times higher than that of their male counterparts. Most of these injuries involve tears of the anterior cruciate ligament, or ACL, which is the central ligament in the knee and provides strength and stability to the joint. While the exact cause or causes of this increased risk of knee injury remains unknown, new studies presented today at the Annual Meeting of the American Orthopaedic Society for Sports Medicine at the Grand Traverse Resort in Traverse City, Michigan may help get to the root of the problem.
"There are many pieces to this puzzle," said Edward M. Wojtys, MD, 1999 Program Chairman. "Certainly, there are many factors that influence the risk and these studies highlight several of them."
Biomechanical and Neuromuscular Factors
Two researchers presented scientific papers that addressed biomechanical and neuromuscular factors that play important roles in the prevention of knee injuries.
In his paper, "Differences in Single Leg Balance on An Unstable Platform between Female and Male Normal and ACL Deficient and ACL Reconstructed Knees", Timothy E. Hewett, PhD, from the Cincinnati Sportsmedicine Research and Education Foundation in Cincinnati, Ohio, and his co-researchers, Mark V. Paterno, MS, PT, and Frank R. Noyes, MD, studied 24 subjects: twelve men and women with normal knees and twelve men and women with injuries to their anterior cruciate ligaments who were scheduled for surgery to reconstruct the ligament.
They discovered that women without any injury had much better neuromuscular control and balance than the men without injury. However, injured women had much worse control and balance than the injured men, both before and after surgery. Not until almost a full year postoperatively do women regain their superior control and balance.
Laura J. Huston, MS, and her co-researchers Brady Vibert, BS, James A. Ashton-Miller, PhD and Edward M. Wojtys, MD, at the University of Michigan in Ann Arbor, Michigan presented "Gender Differences in Knee Angle When Landing from A Jump", a study that looked at landing mechanics and their effect on knee injury.
One of the most frequent causes of non-contact ACL injury in women is when landing from a jump. Knee angle is an important determinant of the force of impact on the leg when landing. The study looked at 24 subjects, twelve men and women, when they jumped down from a height of 20, 40 and 60 centimeters. Using reflective markers and 2-D motion analysis, the researchers determined that the women landed with a much straighter knee than the men and therefore put much more load on their knees than men, relative to body weight.
It has been shown that increased force upon impact when landing increases the risk of injury to the ACL. The researchers speculate that landing with a straighter knee strains the ACL unnecessarily and may be a contributing factor in ACL injuries among women.
Hormonal Factors
The mechanical properties of ligaments may be influenced by the normal fluctuations of the menstrual cycle, although at a smaller level than hypothesized in other studies, according to study conducted by Michael J. Belanger, MD, from the Brown School of Medicine/Rhode Island Hospital in Providence, Rhode Island, and his co-researchers, Robert D. McGovern, BS, Douglas C. Moore, MS, Paul D. Fadale, MD, Michael J. Hulstyn, MD, Joseph J. Crisco, PhD and Michael J. Erlich, MD.
Estrogen receptors have been identified within the ACL and ACL fibroblasts have been shown to respond to estrogen by decreasing both collagen synthesis and cellular proliferation. In women, knee laxity, or looseness, has also been shown to increase during pregnancy and during mild exercise.
The researchers followed 30 college age women between 18 and 33 for ten weeks through two menstrual cycles. None had a previous ACL injury or previous knee surgery. Laxity testing was done twice a week before and after riding an exercise bicycle for 20 minutes (enough time to allow the muscles to warm up but not to become fatigued). There was no simple cyclic relationship between pre-exercise laxity and day of menstrual cycle, although there was a slight trend toward laxity in the middle of the menstrual cycle.
William P. H. Charlton, MD, from Thomas Jefferson University in Philadelphia, Pennsylvania, and his co-researchers, Lynne M. Coslett-Charlton, MD, and senior researcher Michael G. Ciccotti, MD, from the Rothman Institute in Philadelphia also presented a study of the clinical effects of estrogen on knee ligaments, "The Effect of Endogenous Estrogen on the Instrumented Measurement of the Anterior Cruciate Ligament".
The researchers evaluated the laxity of the ACL in 40 knees of twenty pregnant women in their third trimester, when the levels of endogenous estrogen, specifically estradiol, are highest. All patients underwent a first examination at an average of 29.8 weeks of pregnancy to measure both hormone levels and knee laxity using a knee athrometer. Approximately six weeks after delivery, all subjects were again tested for hormone levels and knee laxity.
The first exam showed that 38 of 40 the subject's knees had increased laxity and that their serum levels of estradiol were high. The second exam revealed that the laxity decreased along with the level of serum estradiol. The researchers demonstrated that high serum estradiol levels correlated with increased laxity of the ACL in pregnancy. The researchers believe that this relative laxity may be a significant factor in the higher ACL injury rates in females.
Anatomical Factors
Is the higher incidence of ACL injuries among female basketball players due to an anatomical difference between males and females? In a prospective study to determine whether the fact that women have a smaller ACL compared to men, may have a mismatch between the size of the ACL and the notch in the bone of the knee that it grows out of (large ligament, small notch), or have differences in strength relative to body weight may predispose women to ACL tears, Allen F. Anderson, MD and co-researchers David C. Dome, MD, Shiva Gautam, PhD, Mark H. Awh, MD, Gregory W. Rennirt, MD and Michael J. Pagnani, MD of the Lipscomb Clinic Sports Medicine Center in Nashville, Tennessee, studied 100 high school basketball players.
Fifty males and 50 females were measured and compared against each other for body fat analysis and strength evaluation and underwent MRI measurements of their ACL structure. The most prominent finding was that the females had statistically smaller ACLs than the males. They also found that the females had lower strength levels. The researchers believe that a smaller ACL may be an intrinsic risk factor for females.
Statistical Data
In an effort to document the incidence of ACL injuries among male and female high school athletes who play soccer or basketball in one state, Jay Kimmel, MD and co-researcher David G. Yeo, DPE, ATC of Eastern Connecticut State University in Willimantic, Connecticut, developed a questionnaire that was sent to all high schools in the state of Connecticut to document injuries sustained in 1997-1998.
Of 198 schools with boys' soccer, 148 responded and 125 of the 164 schools with girls' soccer responded. Among the male athletes, there were 20 ACL injuries, 0.44%; among the females, there were 23 injuries, 0.67%. Ninety four schools that offered boys' basketball and 89 that offered girls' basketball also responded. They documented 9 tears in male athletes, .39% and 24 tears in females, 1.2%.
This data confirms that the female athletes are at an increased risk of ACL injury, and that there is also an increased incidence depending upon sport. The researchers stress that higher injury rates should not discourage women from participating in sports and that further research needs to be done to diminish the increased risk.
The American Orthopaedic Society for Sports Medicine (AOSSM) is an international medical specialty association of allied health professionals and physicians, primarily orthopaedic surgeons, who have demonstrated a significant and long-term commitment to sports medicine. The AOSSM promotes and supports education and research programs in sports medicine, including those concerned with physical fitness, as well as programs designed to advance our knowledge in the recognition, treatment, rehabilitation and prevention of athletic injuries.
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