Rupture of the knee's anterior cruciate ligament, or ACL, is one of the more common—and painful—sports injuries. It occurs most often in women, who experience ACL tears at four to eight times the rate that men do. Scientists have developed a few theories about why this is so: The structure of the knee may be different in the sexes, men may be protected by stronger leg muscles, or fluctuations in hormones that prepare the pelvis for birth may make the ligament "looser" in women. One way to test the third theory is to compare ACL laxity in women with different levels and frequency of female hormones.

What the researchers wanted to know: Do women using oral contraceptives have a more lax ACL than women not on the pill?

What they did: Researchers at McGill University in Montreal studied 78 female varsity athletes at the school, 42 of whom were on the pill and 36 who weren't. (All were screened to be sure they had regular menstrual cycles, weren't taking other medications that might interfere with the ACL, and didn't have a history of knee injury or pain.) Scientists tested the laxity of the ACL using a device called an arthrometer without knowing in which group each woman belonged.

What they found: Women not on the pill had significantly looser ACLs than those using oral contraceptives. There is a medical explanation for this, since receptors for estrogen and progesterone have been found in ACL cells. Researchers hypothesize that the pill might provide a protective effect by preventing the normal hormonal variations of the menstrual cycle. In support of this theory, they note that excluding women on the pill who were menstruating (and not receiving hormones, since the last seven pills of a 28-day packet of birth control pills are placebos) produced even more noticeable differences in the two groups. It's possible, they say, that this hormonal variation found in the natural cycle and at the end of the cycle regulated by the pill is what makes the ACL so vulnerable.

What the study means to you: It's too early to tell all female athletes in sports particularly prone to ACL injuries—basketball, volleyball, and soccer—to take the pill. Oral contraceptives do have risks, and it's not yet clear whether soft-tissue injuries can be prevented by the pill or, if you are on the pill, by omitting the placebo pills and starting your next packet of hormone pills (which means you skip your period‑something you might to do if you have a game or major event). But if you're an athlete and considering birth control, you may want to ask your doctor whether the pill is right for you.

Caveats: It's not clear whether a less lax knee means a smaller chance of ACL injury or whether structural or muscular factors may play a bigger role. In addition, because the study didn't include women with previous knee injuries, it's possible the study group was inherently less prone to those problems.

Find out more: To read about treatment options for ACL injuries, see the Mayo Clinic's decision guide.

Read the article: Martineau, P.A. et al. "Effect of the Oral Contraceptive Pill on Ligamentous Laxity." Clinical Journal of Sports Medicine. September 2004, Vol. 14, No. 5, pp. 281–286.

Abstract online: www.cjsportmed.com