title: Postinjury biomechanics of Achilles tendon vary by sex and hormone status.
abstract: Achilles tendon ruptures are common injuries. Sex differences are present in mechanical properties of uninjured Achilles tendon, but it remains unknown if these differences extend to tendon healing. We hypothesized that ovariectomized females (OVX) and males would exhibit inferior postinjury tendon properties compared with females. Male, female, and OVX Sprague-Dawley rats (n = 32/group) underwent acclimation and treadmill training before blunt transection of the Achilles tendon midsubstance. Injured hindlimbs were immobilized for 1 wk, followed by gradual return to activity and assessment of active and passive hindlimb function. Animals were euthanized at 3 or 6 wk postinjury to assess tendon structure, mechanics, and composition. Passive ankle stiffness and range of motion were superior in females at 3 wk; however, by 6 wk, passive and active function were similar in males and females but remained inferior in OVX. At 6 wk, female tendons had greater normalized secant modulus, viscoelastic behavior, and laxity compared with males. Normalized secant modulus, cross-sectional area and tendon glycosaminoglycan composition were inferior in OVX compared with females at 6 wk. Total fatigue cycles until tendon failure were similar among groups. Postinjury muscle fiber size was better preserved in females compared with males, and females had greater collagen III at the tendon injury site compared with males at 6 wk. Despite male and female Achilles tendons withstanding similar durations of fatigue loading, early passive hindlimb function and tendon mechanical properties, including secant modulus, suggest superior healing in females. Ovarian hormone loss was associated with inferior Achilles tendon healing.
authors.affiliation: McKay Orthopaedic Laboratory, Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: George W
authors.initials: GW
authors.lastname: Fryhofer
authors.affiliation: McKay Orthopaedic Laboratory, Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Benjamin R
authors.initials: BR
authors.lastname: Freedman
authors.affiliation: McKay Orthopaedic Laboratory, Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Cody D
authors.initials: CD
authors.lastname: Hillin
authors.affiliation: McKay Orthopaedic Laboratory, Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Nabeel S
authors.initials: NS
authors.lastname: Salka
authors.affiliation: McKay Orthopaedic Laboratory, Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Adam M
authors.initials: AM
authors.lastname: Pardes
authors.affiliation: McKay Orthopaedic Laboratory, Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Stephanie N
authors.initials: SN
authors.lastname: Weiss
authors.affiliation: McKay Orthopaedic Laboratory, Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Farber
authors.affiliation: McKay Orthopaedic Laboratory, Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, Pennsylvania soslowsk@upenn.edu.
authors.forename: Louis J
authors.initials: LJ
authors.lastname: Soslowsky
mesh_terms: D000125:Achilles Tendon / Q000378:metabolism / Q000503:physiopathology*; D000818:Animals; D001696:Biomechanical Phenomena / Q000502:physiology*; D024061:Collagen Type III / Q000378:metabolism; D005221:Fatigue / Q000378:metabolism / Q000503:physiopathology; D005260:Female; D006614:Hindlimb / Q000378:metabolism / Q000503:physiopathology; D006728:Hormones / Q000378:metabolism*; D008297:Male; D016059:Range of Motion, Articular / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012727:Sex Characteristics; D013708:Tendon Injuries / Q000378:metabolism / Q000503:physiopathology*; D014945:Wound Healing / Q000502:physiology*
chemical_list: D024061:Collagen Type III; D006728:Hormones
keywords: Achilles tendon; biomechanics; female athlete; ovariectomy; sex differences; tendon healing