The effects of estrogen on material and mechanical properties of the intra- and extra-articular knee structures

Background: Female athletes have a higher incidence of anterior cruciate ligament injuries than do male athletes in similar sports. Hormones, particularly estrogen, have been indicated by some studies to be correlated with the disproportionate number of anterior cruciate ligament injuries in female athletes. Hypothesis: Estrogen does not affect the mechanical or material properties of primate anterior cruciate ligaments or patellar tendons. Study Design: Controlled laboratory study. Methods: Twenty-six cynomolgus monkeys (Macaca fascicularis) were divided into 2 groups: sham operated (estrogen maintained, n = 12) and ovariectomized (estrogen deficient, n = 14). After 2 years, the animals were sacrificed, and various properties of ligaments were tested. Two different ligaments were studied, the anterior cruciate ligament and patellar tendon. Results: No differences were found between the sham-operated (sham) and ovariectomized group for failure load (anterior cruciate ligament: sham, 394 +/- 67 N; ovariectomized, 392 +/- 96 N; P >.74), stiffness, elongation at failure, ultimate stress (anterior cruciate ligament: sham, 115 +/- 30 MPa; ovariectornized, 128 +/- 36 MPa; P >.58), ultimate strain, modulus, failure sites, or energy to failure. Conclusion: Endogenous estrogen does not directly affect the mechanical or material properties of the anterior cruciate ligament or the patellar tendon of cynomolgus macaques. Clinical Relevance: This study supports the idea that knee ligament injury in female athletes is not a direct result of estrogen's effect on ligament properties.