Sub-failure Stretch Injury Response in Rat Achilles Tendon

Sub-failure ligament and tendon injuries are of great concern to athletes and the general population because they account for a significant proportion of all musculoskeletal injuries per year. The goals of this manuscript were to design and evaluate the performance of a reproducible small animal model for tendon sub-failure injury and investigate the tissue response to a singular, controlled sub-failure stretch damage in rat Achilles tendon matrices using biochemical, biomechanical, and image analyses. Utilizing a new tendon stretch injury device, we showed that sub-failure stretch lead to significant changes in the elastic moduli of the toe region of the stress-strain curves of tendons. Scanning electron microscopy was used to examine the ultrastructural damage and disorganization of the normally aligned collagen matrix of the tendon. Additionally, ex vivo biomechanical testing confirmed the persistence of compromised biomechanical properties for up to 7 days post-injury in the healing Achilles tendon. Furthermore, in vivo magnetic resonance imaging and histological analysis showed strong evidence of the injury during repair. Lastly, immunohistochemical staining revealed increased expression of myofibroblast markers elastin and smooth muscle actin, indicative of matrix remodeling facilitated by migrating tissue fibroblasts. These results support the clinical utility of this new in vivo model for the study of sub-failure injury of tendon. Taken together, these findings provide clinically relevant information about the most common type of ligament and tendon injury. Sub-failure sprain and strain injuries to ligaments and tendons are common injuries of the musculoskeletal system. We set out to design a device that could accurately model the response of a sub-failure tendon stretch injury in rat. Using 3D-printed parts, we developed a new injury device and tested the biological and biomechanical response of stretch injury in rat cadavers and live rats for up to 1 week. Our results showed sub-failure stretch of the Achilles tendon led to changes in the biomechanics, most notably the toe region modulus under tension. We saw persistence of compromised biomechanics for up to 1 week, accompanied by a cellular response led by migrating tendon fibroblasts. Future studies will evaluate the long-term response of the stretch injury in rat, in addition to adapting the device for larger animal models.