Cyclic loading induces anabolic gene expression in ACLs in a load‐dependent and sex‐specific manner

Abstract

Anterior cruciate ligament (ACL) injuries are historically thought to be a result of a single acute overload or traumatic event. However, recent studies suggest that ACL failure may be a consequence of fatigue damage. Additionally, the remodeling response of ACLs to fatigue loading is unknown. Therefore, the objective of this study was to investigate the remodeling response of ACLs to cyclic loading. Furthermore, given that women have an increased rate of ACL rupture, we investigated whether this remodeling response is sex specific. ACLs were harvested from male and female New Zealand white rabbits and cyclically loaded in a tensile bioreactor mimicking the full range of physiological loading (2, 4, and 8 MPa). Expression of markers for anabolic and catabolic tissue remodeling, as well as inflammatory cytokines, was quantified using RT‐qPCR. We found that the expression of markers for tissue remodeling of the ACL is dependent on the magnitude of loading and is sex specific. Male ACLs activated an anabolic response to cyclic loading at 4 MPa but turned off remodeling at 8 MPa. These data support the hypothesis that noncontact ACL injury may be a consequence of failed tissue remodeling and inadequate repair of microtrauma resulting from elevated loading. Compared to males, female ACLs failed to increase anabolic gene expression with loading and exhibited higher expression of catabolic genes at all loading levels, which may explain the increased rate of ACL tears in women. Together, these data provide insight into load‐induced ACL remodeling and potential causes of tissue rupture.This article is protected by copyright. All rights reserved.

Intermittent Cyclic Stretch of Engineered Ligaments Drives Hierarchical Collagen Fiber Maturation in a Dose- and Organizational-Dependent Manner

“…It is well established mechanical cues are critical for tissue development in vivo [1,5,21,22] and drive maturation of engineered tissues in vitro [18,[23][24][25][26][27][28][29][30][31]. In particular, intermittent cyclic loading at or below 5% strain is well established to improve fibril alignment, fibril diameter, and collagen production in engineered tendons and ligaments [1,5,17,[21][22][23][24][25][26][27][28][29][30][31]47]; however the effect at the fiber and fascicle length-scale is largely unknown [1,19,23,48,49]. In this study we found that intermittent cyclic loading accelerates and improves hierarchical collagen organization at the fibril, fiber, and fascicle length-scale (Fig 3-4).…”

Allograft and Autograft Anterior Cruciate Ligament Reconstructions Exhibit a Similar Biological Response to Cyclic Loading

“…Following harvest, 12 autografts and 12 allografts were prepared for mechanical testing under sterile conditions following a previously published protocol 36 . Briefly, all surrounding tissues were removed, and the femur and tibia were cut down into bone blocks to grip the sample during mechanical loading.…”

“…The 2 MPa stress level was chosen since it is representative of physiological in situ ACL loading and since preliminary testing found that it was the maximum stress that the reconstructions could sustain for 8 h of cyclic loading. 36…”

“…The bioreactor then cyclically loaded the samples to 2 MPa at 0.5 Hz for 8 h. Control samples were maintained under a 0.1 MPa static load for the same duration. The duration and frequency of loading were chosen based on prior studies of native ACL explants 36 . The 2 MPa stress level was chosen since it is representative of physiological in situ ACL loading and since preliminary testing found that it was the maximum stress that the reconstructions could sustain for 8 h of cyclic loading.…”

“…Additionally, we also hypothesized that allografts contain elevated levels of inflammatory immune cells (e.g., M1 macrophages), which may explain the impaired mechanobiological response of allografts. To test these hypotheses, we utilized a previously published explant model 36 to cyclically load autograft and allograft explants and quantify gene expression between autografts and allografts in response to mechanical loading.…”