What is causing so many women to have anterior cruciate ligament (ACL) injuries? Female athletes are at especially high risk. In fact, they are up to eight times more likely to have an ACL injury than men in the same sport. Soccer, basketball, and volleyball players injure the ACL most often.
The most obvious place to look is hormone levels. Women have far more estrogen and progesterone than men. Studies have shown estrogen changes the structure of the ACL. Most injuries occur when estrogen levels are the highest. This is usually during ovulation (days 10 to 14 of the menstrual cycle).
In this study researchers looked at the role of another hormone called relaxin. Relaxin is present throughout the body. It’s found most often in the breasts, uterus, and prostate. In pregnant women it loosens the ligaments around the groin and pelvic areas. It’s unclear what (if anything) it does to the ACL.
The level of relaxin was measured in ACL specimens. These were taken during total knee replacement surgery or ACL reconstruction. The ACL specimens came from four men and eight women. The level of relaxin was also measured in tissue samples collected from the uterus and from the knee meniscus. Uterine samples came from women having hysterectomies. Meniscal tissue came from patients having a total knee joint replacement.
The results of these three groups were compared. Relaxin was found in all uterine and ACL samples. Very little to no relaxin was found in the meniscal tissue. Young women had the highest amounts of relaxin. This shows that relaxin does bind to the ACL. How relaxin interacts with or changes the ACL remains unknown. This will be the subject of future studies.