Changing Hormone Levels During the Menstrual Cycle Affect Knee Laxity and Stiffness in Healthy Female Subjects

Background: Whether knee laxity varies throughout the menstrual cycle remains controversial. As increased laxity may be a risk factor for anterior cruciate ligament (ACL) injury, further research is warranted. Hypothesis: Variation in estradiol and progesterone levels during the menstrual cycle influences knee laxity and stiffness. Study Design: Case control study; Level of evidence, 3. Methods: The serum estradiol and progesterone levels of 26 healthy female subjects were recorded in the follicular phase, ovulation, and the luteal phase. Knee joint laxity was assessed using a standard knee arthrometer at the same intervals. Stiffness changes in the load-displacement curve were determined. Hormone levels across the cycle were compared between responders and nonresponders, defined by whether changes in knee laxity at 89 N occurred. Results: Greater laxity at 89 N during ovulation was observed (ovulation: 5.13 +/- 1.70 mm vs luteal: 4.55 +/- 1.54 mm, P = .012). In knee laxity testing at manual maximum load, greater laxity was noticed during ovulation (14.43 +/- 2.60 mm, P = .018), as compared with the follicular phase (13.35 +/- 2.53 mm). A reduction in knee stiffness of approximately 17% (ovulation: 12.48 +/- 5.46 N/mm vs luteal: 15.02 +/- 7.71 N/mm, P = .042) during ovulation was observed. However, there were no differences in hormone levels between responders and nonresponders at 89 N. Conclusion: Female hormone levels are related to increased knee joint laxity and decreased stiffness at ovulation. To understand subject variations in knee joint laxity during the menstrual cycle in female athletes, further investigation is warranted.