This is the first study to confirm self-reported menstrual histories with salivary sex-hormone profiles at the time of ACL injury. We found that 26 of 37 athletes tore their ACLs during the follicular phase of the menstrual cycle. Among athletes who self-reported their menstrual histories, 10 of these 27 injuries occurred during the few days before and the 2 days after the onset of menses (Figure ). The levels of E2
and P are both low at this time (Figure ). This hormonal condition contrasts with the follicular phase during which P is low and E2
peaks sharply before ovulation, and it follows the midluteal phase during which E2
and P are both elevated for several days. These results are consistent with a report of no correlation between ACL injury and the general category of “luteal” phase,3
as well as 2 other reports indicating that injury is more likely during the late luteal and early follicular phases of the cycle.21,22
Figure 2 Changes in concentrations of estrogen and progesterone during the menstrual cycle. Estrogen concentration rises during the follicular phase, reaches a peak just before ovulation, and then drops sharply. It rises again and, along with progesterone, reaches (more ...)
We did not obtain information on the typical lengths of the athletes' menstrual cycles. Although the “normal” menstrual cycle lasts 28 days, 3 of our 37 athletes (8% of our sample) sustained injuries after day 28. The probability that ACL injuries occurred during the prolonged menstrual interval (>28 days) was no different from the probability determined by a uniform distribution. That is, we could not reject the null hypothesis that the injuries occurring during this time were due to random chance.
We chose to use computer-simulated subjects because there was no well-defined control group. To qualify to be in a legitimate experimental control group, subjects would have needed the same menstrual cycles as the injured females but different distributions of injury among the cycles' phases. Therefore, instead of performing an experimental-control group comparison, we tested a simpler null hypothesis that the ACL injuries occurred randomly in each day of the menstrual cycle. Each computer-generated subject had an equal chance (0.0357) of injury in each day of her menstrual cycle. That is, these subjects had the same menstrual cycles as the injured subjects, but the probability of injury was different. We rejected our null hypothesis and found that ACL injuries occurred most frequently during the early menstrual cycle.
Past studies of the correlation between salivary and serum sex-hormone measurement have yielded conflicting results.24–28
However, more recent supersensitive, double-antibody techniques for measuring sex hormones in saliva have shown good correlation between saliva and serum levels.23
One group has even recommended using saliva to obtain hormone profiles in patients with difficult venous access.24
Using saliva to obtain sex-hormone profiles of athletes fits well in the athletic arena because little planning or equipment is needed to obtain and store the saliva. A simple ziplock-type bag works well to hold the saliva and can be placed immediately on ice and transferred to a freezer soon thereafter or upon return from a road trip.
Our findings depend heavily on the accuracy with which we determined the day of the menstrual cycle at the time of injury. Such determinations are complicated by problems in obtaining blood from injured athletes in an athletic setting and the fact that a single measurement of one hormone cannot unequivocally define the day of the menstrual cycle. However, we successfully overcame these problems by measuring both E2 and P in saliva. This approach is effective because elevated (at or near the typical highest concentration) E2 is characteristic of the follicular phase (with an E2 spike occurring at ovulation), elevated P is characteristic of the luteal phase, and low E2 and P are characteristic of menses. Menstrual phases defined in this way correlated well (>95%) with the self-reported menstrual histories (see ), thereby confirming the accuracy of our athletes' recollections of their menstrual cycles. We, therefore, have confidence that all but 1 or 2 of our athletes accurately recalled the dates of their last menstrual periods before ACL injury.
Although we do not know why ACL injuries occur around the time of menses, our current research is focused on characterizing sex differences in ACL tissue remodeling. Cyclic changes in E2 and P may alter expression of genes encoding tissue-remodeling enzymes and proteins, which, in turn, could favor either net tissue degradation or repair at specific times during the menstrual cycle. If a molecular basis for sex differences in ACL injury is found, treatments may be instituted to decrease the injury rates in females.
In conclusion, ACL injuries occurred most frequently on days 1 and 2 of menses, suggesting that ACL injury is not random but occurs more often around the time of menses, when circulating sex-hormone levels are low and after a time when both E2 and P were elevated. Additionally, salivary sex-hormone profiling correlates well with serum profiling, and in this athletic population, adequately identified menstrual cycle phase at the time of injury. This is, therefore, an effective technique to identify correlations between injury and hormone patterns in athletes.