Examination of the failure properties of the anterior cruciate ligament during the estrous cycle

The purpose of this study was to determine whether the mechanical properties of the rat anterior cruciate ligament (ACL) vary when tested in vitro at different stages of the estrous cycle. Sixty female rats were allocated to four groups according to their stage of the estrous cycle: diestrus (n=16), proestrus (n=17), estrus (n=13) and metestrus (n=14). Right hindlimbs were harvested for mechanical testing and left hindlimbs were harvested for immunohistochemical staining to confirm the presence of the estrogen receptor. Results from the first relaxation test showed a significant difference between the estrus and proestrus stage, which was not observed in a second subsequent relaxation test. Likewise, no significant differences were found when comparing failure load and stiffness between the different stages of the estrous cycle. These results suggest that normal physiological fluctuations in estrogen during the estrous cycle did not alter the failure properties of the rat ACL.