Increased Incidence of Anterior Cruciate Ligament Tears in Adolescent Females Kristin M. Steinert 04

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1 Increased Incidence of Anterior Cruciate Ligament Tears in Adolescent Females Kristin M. Steinert 4 Abstract The potential role of gender and age in the incidence of tears of the anterior cruciate ligament (ACL) of the knee was studied by a bivariate and multivariate analysis of a database of magnetic resonance image scans with the diagnoses of anterior cruciate ligament tears and medial meniscus injury from 199 to Overall, females had a higher risk of ACL injury than males relative to medial meniscus tears (odds ratio 1.22, 95% confidence interval , p=.1). The highest risk age group was age years (odds ratio 1.78, 95% confidence interval , p=.8). The rate of ACL injury increased over time, with a higher rate in for both males and females (p<.5). This analysis shows that females are at a higher risk for ACL tears than males, particularly in the adolescent age group. The rate of ACL tears is increasing for both genders. Interventions to modify extrinsic factors that may be contributing to this excess injury rate are needed. INTRODUCTION Over the past decade, several studies have suggested that females may have an increased rate of ACL injuries compared to males. Possible explanations for a gender difference are classified as either intrinsic or extrinsic. These studies have concentrated on collegiate or Olympic athletes and most are retrospective or utilize historical controls. A literature review found no studies addressing the role of gender in ACL injuries in secondary school athletes. This study utilized bivariate and multivariate statistical analyses of a large magnetic resonance imaging (MRI) database to investigate whether females overall, and secondary school female athletes in particular, are at an increased risk for an ACL tear compared to males. The incidence of MRI diagnosis of another common knee injury, a tear of the medial meniscus, was used as a control. METHODS The computerized database of the Shields Health Care Group, which operates multiple MRI centers in Eastern Massachusetts, was queried for the incidence of the diagnosis of ACL tear (International Classification of Disease, edition 9 (ICD9) code ) and, as a control, the incidence of tear of the medial meniscus (MM) (ICD9 code 836.). Patients with injuries to multiple knee structures in association with the ACL tear were counted only as ACL tears. Diagnoses were sorted by gender, age group (6-13,14-18,19-22,23-3, and over 3 years), and year of the diagnostic scan (199-92, , and 1996-July 2, 1998). A total of 6,83 positive scans were analyzed. Statistical analysis of the total database was Number of scans by bivariate and multivariate analyses, using SAS software (Statistical Analysis System, Cary, North Carolina). RESULTS The number of scans with a positive diagnosis are grouped by gender and subdivided by date and patient age in Tables I-IV and Figure 1. The ratios of male:female cases are given in Tables V-VI and Figure 2. Comparing Table V to Table VI, the higher proportion of ACL tears in females compared to MM tears is evident, particularly in the age group between 14 and 18 years. As shown in Figure 3, the number of ACL tears increased over the three time periods of the study for both males and females. Multivariate analysis demonstrated that this increase is significant and independent of the effects of age and gender (p<.5). Females had a greater rate of ACL tears than MM tears compared to males. Odds ratios are MRI Diagnoses Female ACL Male ACL Female MM Male MM Figure 1. Number of MRI scans with the diagnosis of ACL or MM tears in the years for the three age groups of 14 through 3 years. 32 Dartmouth Undergraduate Journal of Science

2 Ratio shown in Figure 4. Overall, the risk of ACL tears is 1.22 times greater for females than males (p=.1, 95% confidence interval, 1.8 to 1.37). The risk of ACL tears in females was higher for some age groups than others. The highest risk group was ages 14 to 18 (odds ratio 1.78, 95% confidence interval 1.16 to 2.71, p=.8). The next highest risk age group was 23 to 3 years (odds ratio 1.65, 95% confidence interval 1.21 to 2.32, p=.1). Multivariate analysis showed independent effects of gender, age, and year (each significant at p<.5). DISCUSSION Ratios male:femal These findings confirm and extend previous observations on the role of gender in ACL injuries. The data support the position that female athletes are at increased risk for ACL tears compared to males. This investigation produced a new finding, showing that the highest excess female risk of ACL tears is in the adolescent years. Previous studies have concentrated on college age athletes (Huston & Wojtys, 1996). Only one study examined ACL tears in adolescent females; while this study suggested a higher rate of ACL tears in adolescent female basketball players, only total numbers of injuries were available for analysis (Gray et al., 1985). Calculation of incidence of injury and comparison to control groups was not made. The present study also suggests that the rate of ACL injuries may be increasing for both male and female athletes. ACL MM Figure 2. Ratios of male to female positive scans for ACL and MM tears. Males have more injuries to both ACL and MM than females, most likely due to larger numbers of males participating in sports with potential knee injury. Review of the literature does not reveal any studies on ACL injury that control for differential exposure of males and females to activities that lead to knee injury. The statistical methodology of utilizing MM injury as a control population attempts to meet this need. The ideal control is a sports-related injury to the same portion of the anatomy, diagnosed with the same methodology, but where a gender bias might not be present. The methodology of this study cannot control for all possible variables impacting on ACL injuries. MM injury was chosen as the control because it is the most frequent knee injury diagnosed by MRI scans and because it is usually a sports-related injury. By using this type of control, other biases inherent in most epidemiological surveys are eliminated. Gender differences in the types of sports played (rate of exposure to risk), intensity of athletic activity (severity of risk), and rate of referral for diagnosis (potential gender bias in medical care) are all controlled through the use of the MM injury in the same groups as ACL tears. MM injury itself, in contrast to ACL tears, has not been observed to have a gender bias in epidemiological studies, but an undetected gender bias may be present. If females also have a higher predilection for MM tears, the risk calculations in this study would be lower than the true ratios. Indeed, the risk ratios for ACL tears in our study are generally lower than in earlier studies (Arendt & Dick, 1995). Conversely, the methodology of this study cannot distinguish between a gender bias of increased female risk of ACL tears and increased male risk of MM tears. The use of MM tears as a control may also include flaws due to the Number of scans Positive Scans Over Time Years Female ACL Male ACL Female MM Male MM Figure 3. Positive scans for all ages over three time intervals. Note the decline in medial meniscus tears in the timeframe, while ACL tears have been increasing. Vol. III, No. 1, Fall 2 33

3 Ratio Odds Ratio Overall Figure 4. Odds of female ACL tears compared to males for three age groups and for all ages combined. In the adolescent group, ages 14-18, the risk of an ACL tear is almost double the risk of a male. diagnoses of patients through clinical examinations and arthroscopy instead of MRI scans. However, no other knee injury is diagnosed at a high enough rate by MRI scans to serve as a second control. This control of MRI-diagnosed MM tears, although not ideal, nevertheless provides the best method to date to analyze relative risk of ACL injury while reducing or eliminating the impact of biases in the database such as changing area demographics, physician referral patterns, and, most importantly, the rate of female participation in high-risk sports (a factor which has not been controlled in previous studies). Unidentified variables may be present, however. The statistical analysis of risk of ACL tears is, at all times, relative to medial meniscus injury. While our analysis shows that females, particularly adolescents, are at higher risk for ACL injury than males, our data could be interpreted as showing that males have an increased risk for medial meniscus injuries. The sports medicine literature, however, indicates that it is the rate of ACL injuries in females which is disproportionately high. Medial meniscus tears are not always sent for MRI scans, and biases may exist in the pattern of referral for MRI scans when medial meniscus injury can be diagnosed clinically or by arthroscopy. The literature shows that the theories as to why women seem to be significantly more predisposed to tearing an ACL have been classified as either extrinsic or intrinsic in nature. The extrinsic theories include physical conditioning (Cox, Heinz, 1992), body movement (Bjordal, Frode, et. al., 1997), and muscular strength (Acasuso-Diaz, Collantes-Estevex, Sanchez-Guijo, 1993). Intrinsic factors include increased ligamentous laxity in women (Huston, Wojtys, 1996), possibly due to hormones, especially estrogen, limb alignment (Ireland, 1994), and size of the intercondylar notch (Souryal, Freeman, 1993). The intercondylar notch is the valley in the middle of the knee through which the anterior and posterior cruciate ligaments pass; if this space is narrow, the knee bones could possibly sever the ligament as the knee flexes (Good, Odensten, Gillquist, 1991). However, other studies have questioned the role of these factors (Godshall, 1992). CONCLUSIONS Females are at a higher risk for ACL tears than males. This risk is particularly high for the adolescent age group, but the rate of ACL tears is increasing for both genders. Interventions to modify extrinsic factors that may be contributing to this excess injury rate are needed. The finding of an increased incidence of ACL tears for both males and females in recent years lends support to theories that extrinsic factors may play a major role in ACL tears in both genders, but may not necessarily be responsible for the differential risk in females. The methodology of this study does not have the ability to identify the reasons for the increased female risk. The findings do suggest, however, that studies to determine the reasons for this increased risk are necessary. In addition, steps to reduce the increased risk of this major injury for all athletes, and particularly females, are needed. Pending more specific delineation of the causes of this problem, suggested intervention has been directed at the possible contributing factors that can be addressed immediately. The following steps may be helpful: 1) increase muscular strength in the thigh with emphasis placed on the hamstrings; 2) work on developing proper aerobic conditioning before participating in high-risk sports; 3) modify the regular plant and cut maneuver as a method of changing direction to a three step technique where the knee is never fully extended; 4) improve training of coaches in evaluation of an athlete s skill, condition, and readiness to progress. ACKNOWLEDGEMENTS This study was supported by a grant from the Brace Center for Gender Studies, Phillips Academy, Andover, Massachusetts. Thomas Shields kindly made available the resources of the 34 Dartmouth Undergraduate Journal of Science

4 Total Table I. Number of MRI scans with diagnosis of anterior cruciate ligament tears in females Total Table II: Number of MRI scans with diagnosis of anterior cruciate ligament tears in males Total Table III: Number of MRI scans with diagnosis of medial meniscus tears in females Total Table IV: Number of MRI scans with diagnosis of medial meniscus tears in males Total Total Table V: Ratio of male:female incidence of ACL tears Shields Health Care Group and Maura Switzer assisted in the database search. Lawrence Bernstein, Ph.D., was the statistical consultant. Lyle Micheli, M.D., Karen Kennedy, Diane Moore, Ph.D., and Richard Keller, M.D. each reviewed the manuscript and made valuable suggestions. REFERENCES Acasuso-Diaz, M., Collantes-Estevex, E. & Sanchez- Guijo, P. (1993). Joint hyperlaxity and musculo-ligamentous lesions: study of a population of homogeneous age, sex, and physical exertion. British Journal of Rheumatology, 32, Arendt, E. & Dick, R. (1995). Knee injury patterns Table VI: Ratio of male:female incidence of medial meniscus tears among men and women in collegiate basketball and soccer. American Journal of Sports Medicine, 23, Bjordal, J., et al. (1997). Epidemeology of anterior cruciate ligament injuries in soccer. American Journal of Sports Medicine, 25(3), Cox, J. & Heinz, W. (1984). Women midshipmen in sports. American Journal of Sports Medicine, 12, Ferretti, A., et al. (1992). Knee ligament injuries in volleyball players. American Journal of Sports Medicine, 2, Garrick J. & Requa R. (1978). Girls sports injuries in high school athletics. Journal of the American Medical Association, 239, Vol. III, No. 1, Fall 2 35

5 Godshall, R. (1992). The predictability of athletic injuries: An eight-year study. Journalof Sports Medicine, 2, Good, L., Odensten, M., & Gillquist J. (1991) Intercondylar notch measurements with special reference to anterior cruciate ligament surgery. Clinical Orthopedics, 263, Gray, J., et al. (1985). A survey of the injuries to the anterior cruciate ligament of the knee in female basketball players. American Journal of Sports Medicine, 6, Hutchinson, M. & Ireland, M. (1995). Knee injuries in female athletes. Sports Medicine, 19, Huston, L. & Wojtys, E. (1996). Neuromuscular performance characteristic in elite female athletes. American Journal of Sports Medicine, 24, Ireland, M. (1994). Special concerns of the female athlete. In: Fu FH, Stone DA, eds.. Sports Injuries: Mechanism, Prevention, and Treatment. 2nd ed. Philadelphia, Pa: Williams & Wilkins, LaPrade, R. & Quinter, M. (1994). Femoral intercondylar notch stenosis and correlation to anterior cruciate ligament injuries: a prospective study. American Journal of Sports Medicine, 22, Souryal, T. & Freeman, T. (1993). Intercondylar notch size and anterior cruciate ligament injuries in athletes: a prospective study. American Journal of Sports Medicine, 21, Souryal, T., Moore, H., & Evans, P. (1988). Bilaterality in anterior cruciate ligament injuries: associated intercondylar notch stenosis. American Journal of Sports Medicine, 16, ABOUT THE AUTHOR Kristin Steinert 4 is a Massachusetts native and graduate of Phillips Academy Andover. She hopes to major in economics and minor in German. She is a member of the studentrun organization SAFE and her interests include skiing, hiking and community service. Feedback? Feedback? Questions? Submissions? Questions? Submissions? dujs@dartmouth.edu 36 Dartmouth Undergraduate Journal of Science