Long-Term Effects of ACL Transection on the Gastrocnemius Muscle in Rats

We investigated long-term changes in the morphological and contractile properties of the gastrocnemius muscle in a rat Anterior Cruciate Ligament (ACL) tear model. The experimental animals were 8-week-old male Wistar rats in which the right ACL was transected. The gastrocnemius muscle in the affected limb and the same muscle in the contralateral limb were harvested 4 and 48 weeks after ACL transection. We created serial frozen sections of the tissues and performed immunohistochemical staining. In results, at 4 weeks, the muscle wet weight-to-body weight ratio and the Cross-Sectional Area (CSA) of type I and IIb muscle fiber in the deep portion of the muscle were both significantly smaller on the transection-affected side than on the contralateral side (p < 0.05, respectively). In addition, at 48 weeks, the CSA of type I muscle fibers in the deep portion was significantly smaller on the affected side than on the contralateral side (p < 0.05). We surmise that the changes were due mainly to modulation of muscle activity as a result of physical joint instability and functional failure of proprioception, which both occurred as a result of the ACL transection. We showed that these morphological changes may persist over the long term, particularly in the case of type I fibers. Results of this study suggest that long-term rehabilitation for not only the thigh but also the leg is necessary when ACL injuries are encountered clinically.