Anatomical and hormonal differences are factors that explain the increased incidences of ACL injuries in women compared with men performing the same type of activity

There is a growing number of females who are actively participating in recreational, high school, collegiate, and professional sports. All this activity and success has greatly increased attention toward women’s health issues regarding athletic activities.

Thousands of individuals sustain anterior cruciate ligament (ACL) injuries annually. Unfortunately, women are two to eight times more likely to sustain an ACL injury than their male counterparts doing the same activity or sport.¹ The National Collegiate Athletic Association (NCAA) estimates that one in 10 female college athletes will have a serious knee injury.¹ That equates to 13,000 females with a knee injury on the college level and another estimated 25,000 on the high school level.

Back in 1997, the average cost of surgical care and rehabilitation for an athlete with an ACL injury was $17,000.¹ By today’s figures, we are looking at approximately $650 million annually for high school and college female athletes.

Work the Q Angle
From a mechanical perspective, alignment of the lower extremity contributes to the overall stability of the knee. The most apparent difference in the lower extremity between male and females is pelvic width. The increased width of the female pelvis along with a shorter femur length causes increases in the tibiofemoral angle and the quadriceps femoris angle (Q angle). The Q angle is the acute angle formed by drawing one line from the anterior superior iliac spine (ASIS) and the midpoint of the patella intersecting another line drawn from the tibial tubercle and the same midpoint on the patella.

Theoretically a larger Q angle increases the lateral pull of the quadriceps on the patella and puts increased medial stress on the knee. Also, if the foot is pronated and flattened this will also increase the Q angle.² Recent research has shown that custom-made, flexible orthotics will reduce pronation, put less stress on the knee, and improve the Q angle.³

Effects of the menstrual cycle. A significant statistical association has been found between the menstrual cycle and the likelihood for an ACL injury. A 1998 study reported a statistically significant increase in ACL injuries in women who were in the ovulatory phase of their menstrual cycle.⁴ In contrast, there was a statistically significant decrease of ACL injuries in women who were in the follicular phase of their cycle. A later study, using better lab tests to clearly identify the phase of the menstrual cycle, also confirmed the increase in ACL tears during the ovulatory phase, but it found that there were fewer ACL tears during the luteal phase.⁵

Looking at the menstrual cycle, days 1–9 are called the follicular phase. During this time, levels of both estrogen and progesterone are low. The next phase is the ovulatory phase. It includes days 10–14 where there is a midcycle surge in estrogen levels. The last phase is called the luteal phase and includes days 15 to the end of the cycle.

During this final phase, progesterone levels significantly increase and relaxin levels also rise. In general, estrogen affects soft tissue strength, muscle function, and the central nervous system function. The estrogens also give one more endurance, which allows women to put longer periods of stress on their joints. Increased estrogen causes a lack of circulating androgens in the body that are needed to stimulate the development of large and more powerful muscles, which act as a protective mechanism for the joints.

Progesterone can act as a central nervous system anesthetic, and relaxin can drastically diminish collagen tension. Also because of the increased body fat ratio of a female, the stress on the joints of the lower extremity is greater, which may equate to carrying a heavy backpack.

The notch factor. The size of the femoral intercondylar notch in men and women has also been studied as a possible factor for an increased number of ACL injuries in women.⁶ The volume of the femoral notch was found to be statistically smaller in women compared to men. This is also related to differences in height and weight. This study was performed using MRI scans of the knee. The results of this study clearly show that men do have larger femoral notches, but when putting everything into prospect the larger femoral notch was shown to be more due to the fact that men are taller and also weigh more.⁶

Statistical observations. A study of male and female Texas high school basketball players examined the incidence, rate, risk, and types of injuries commonly found in those sports.⁷ This study found that boys had a slightly higher risk of injury, and that the most common injury was to the ankle. Girls on the other hand, had a significantly higher rate of knee injuries, and were almost four times as likely to injure of their their knees than boys. The girls’ knee injuries tended to be more severe in nature and more often required surgical intervention than did knee injuries in boys. The study also noted that there was a statistically higher rate of injury during a game than during a practice session.

The United States Naval Academy conducted a study to evaluate the relative risk of ACL injuries in female versus male midshipmen.⁸ In this study, they looked at recent studies of NCAA Division I collegiate athletes who have shown a significantly increased incidence of ACL injuries in female athletes when compared to male athletes.⁹

When combining the statistics from intercollegiate soccer, basketball, and rugby, women had almost four times as many ACL injuries as men doing the same sport. When looking at military training activities that were required of all midshipmen, women were more than nine times as likely to suffer an ACL injury than men doing the same activity. These activities included the obstacle course, wrestling, and physical fitness class. Most of those injuries were incurred by jumping from an elevated obstacle, especially a wall. Previous data had indicated that most noncontact ACL injuries happen from one-step stop, one-legged landing, and pivot shift movements. As a side note, the Naval Academy does not accept candidates with an ACL-deficient knee because of the vigorous physical training and athletic demands. However, those who have had a successful reconstruction of their ACL are qualified.

Options for Care
When working with a female athlete, remember to stress the importance of strength, endurance, and agility training in an effort to decrease the incidence of ACL tears.

Figure 1: Hamstrings exercises that protect the anterior cruciate from excessive strain. Special emphasis needs to be put on hamstring and gastrocnemius exercises for strength and limb control.

Weight training should be used with an emphasis to increase the strength of the quadriceps and hamstrings, which can act as a protective mechanism for the knee joint. The hamstrings protect the anterior cruciate from excessive strain (and female athletes tend to be more quadriceps dominant), so special emphasis needs to be put on hamstring and gastrocnemius exercises for strength and limb control (Figure 1). Also, because there are inherent differences in coordination of male and female athletes, proprioceptive training using a device like a wobble board is valuable in causing the hamstring and quadriceps muscles to work together.

Typically, young girls do not decide they want to play sports until an older age, and they have not spent as much time doing activities that encourage coordination and agility. Also, because women’s hormones are constantly changing, it is important that they are getting the proper nutrients. They need to be educated in the proper use of supplements that support their normal hormone cycles and reduce the changes of elevated hormones levels.

To assess the effectiveness of an ACL treatment plan, both objective and subjective data on patient results on outcomes must be collected and documented. The data typically focus upon the physical changes noted at the time of consultation and in subsequent visits. Ongoing outcome assessment data using the knee score questionnaire with comparative graphs over treatment time (available at www.outcomesassessment.org) documents the long-term results and effectiveness of the rehab procedures.

Lastly, female athletes should have a complete postural screening exam, with emphasis on stability and stress reduction to the knee joint. When all these techniques are followed, women can participate in athletics with less susceptibility to injury. CP

Kim D. Christensen, DC, DACRB, CCSP, CSCS, founded the SportsMedicine & Rehab Clinics of Washington. He is a team doctor to high school and university athletic programs, a postgraduate faculty member at numerous chiropractic colleges, president of the ACA Rehab Council, and a lecturer and the author of many musculoskeletal rehabilitation and nutrition texts. He can be reached at PeaceHealth Hospital via email: kchristensen@peacehealth.org.

References
1. Huston LJ, Greenfield ML, Wojtys EM. Anterior cruciate ligament injuries in the female athlete. Potential risk factors. Clin Orthop. 2000;372:50-63.
2. Nicholas JA, Hirshman EB. The Lower Extremity and Spine in Sports Medicine. Vol 2. St Louis, Mo: CV Mosby; 1995:1540,1567.
3. Kuhn DR, Yochum TR, Cherry AR, Rodgers SS. Immediate changes in the quadriceps femoris angle after insertion of an orthotic device. J Manipulative Physiol Ther. 2002;25(7):465-470.
4. Wojtys EM, Huston LJ, Lindenfeld TN, Hewett TE, Greenfield ML. Association between the menstrual cycle and anterior cruciate ligament injuries in female patients. Am J Sports Med. 1998;26(5):614-619.
5. Wojtys EM, Huston LJ, Boynton MD, Spindler KP, Lindenfeld TN. The effects of the menstrual cycle on anterior cruciate ligament injuries in women as determined by hormone levels. Am J Sports Med. 2002;30(2):182-188.
6. Charlton WP, St John TA, Ciccotti MG, Harrison N, Schweitzer M. Differences in femoral notch anatomy between men and women. Am J Sports Med. 2002;30(3):329-333.
7. Messina DF, Farney WC, DeLee JC. The incidence of injury in Texas high school basketball. A prospective study among male and female athletes. Am J Sports Med. 1999;27(3): 294-299.
8. Gwinn DE, Wilckens J, McDevitt ER, Ross G, Kao TC. The relative incidence of anterior cruciate ligament injury in men and women at the United States Naval Academy. Am J Sports Med. 2000;28(1):98-102.
9. Arendt E, Dick R. Knee injury patterns among men and women in collegiate basketball and soccer. Am J Sports Med. 1995;23(6):694-701.