The effects of estradiol and epidermal growth factor on the material properties of the skeletally immature rabbit anterior cruciate ligament

There exists a troubling discrepancy between the number of anterior cruciate ligament (ACL) injuries between males and females. Indeed, the root causes of this difference are multifactorial in nature. One possible factor is the presence of female sex hormones in the ACL and their effects on collagen production. Also, cross-talk is known to exist between the estrogen receptor and epidermal growth factor (EGF) in reproductive tissues affecting collagen production. Twenty-four pre-pubescent New Zealand White rabbits were separated into three groups consisting of a control group receiving no hormones, a group receiving only estradiol, and a group receiving both estradiol and EGF. The rabbits were euthanized after the two-week period and had their hind limbs removed and frozen with soft tissue intact. All rabbit handling, hormone injections, and cataloging was performed by a separate laboratory creating a blind study environment. Before undergoing a standard tensile test, each ACL was analyzed using a 3D scanning technique capable of finding the minimal cross-sectional area and volume of the ligament. The test data was analyzed to assess significant differences in such values as the failure strength, strain at failure, modulus of elasticity, toughness, minimal cross-sectional area, length, and volume. Since all of the rabbits were pre-pubescent, no previous exposure to estradiol influenced the test results beyond the influence of the sex hormones which were administered. No differences were found between any groups for strain, length, minimal area, and volume. Estradiol was shown to lower the maximum stress by 39% while EGF in addition to estradiol had no effect on that property. EGF in combination with estradiol significantly lowered the modulus of elasticity while significance could not be proven for the effect of estradiol alone on the modulus. Toughness behaved similarly to maximum stress in its response to estradiol and EGF. Estradiol induced collagen degradation is a viable explanation for these differences in maximum stress and toughness. Further testing is necessary to establish a relationship between estradiol, EGF, and the modulus of elasticity.