The incidence of Achilles tendon rupture has been rising over the past few decades in both men and women, with about 84 percent of cases occurring in men. Some studies have suggested that female hormones like estrogen reduce the risk of rupture in women, but the hormones’ precise role has been unclear. In addition, some scientists have argued that the typically larger, stronger calf muscles in men would exert greater forces on the tendon and increase the risk of rupture.
To gain a better understanding of the factors influencing sex-specific differences in vulnerability to damage, a team of investigators led by Louis J. Soslowsky, Ph.D., of the University of Pennsylvania, compared the material properties of the Achilles tendon/muscle unit in male and female rats. To specifically test for effects of female sex hormones, they also studied female rats that had been made estrogen-deficient by having their ovaries removed.
Their measurements showed that while Achilles tendons from males are larger, those from females are stronger and remain more elastic during movement. They also noted that muscle fibers were larger in male rats compared to females, as expected. These findings suggest that inferior properties of the tendon coupled with greater muscle size could explain men’s increased susceptibility to Achilles tendon ruptures.
Female sex hormone deficiency did not affect tendon strength, however it did lead to an expansion of calf muscle fibers compared to normal females. The researchers concluded that both hormonal and other sex-specific factors influence the structure of the Achilles-tendon/muscle unit. Additional research is needed to uncover the molecular mechanisms that underlie these effects.
Achilles tendon ruptures are one of the few musculoskeletal injuries that are more common in men, with anterior cruciate ligament (ACL) ruptures, for example, being more common in women. This study contributes to the growing body of data suggesting that a tendon’s local environment influences its biology, and highlights the importance of evaluating each tendon individually for factors that affect its risk of tears or ruptures.
"Rupture of the Achilles tendon is a devastating injury that results in significant pain, disability and healthcare costs," said Dr. Soslowsky. "A better understanding of the tissue-level properties of the tendon may enable us to develop new treatment strategies that would speed patients’ return to normal activities and improve long-term outcomes."
This work was supported by the NIH’s NIAMS (grants R01-AR064216S1, P30-AR050950 and T32-AR007132) and National Center for Advancing Translational Sciences (TL1-TR000138). The National Science Foundation also contributed support.
Kirstie Saltsman, Ph.D.