Unilateral, daily bouts of muscle loading leads to mineral and extracellular adaptation of the immature Achilles enthesis in mice, Elahe Ganji

The tendon-to-bone attachment (i.e., enthesis) translates force from tendon to bone. The enthesis matures during postnatal growth and is susceptible to overuse injuries. Increased activity during periods of rapid growth, such as in children and during adolescence, can lead to micromotion at the enthesis and apophysis, resulting in inflammation and the clinical presentation of Sever Disease. However, the temporal effect of training on the growth and remodeling of the enthesis remains unclear.
Previously, muscle unloading has been shown to disrupt the structural and mechanical properties of the growing postnatal enthesis. In the present study, we aimed to determine the effect of repeated daily bouts of increased muscle on the growth and adaptation of the immature and mature enthesis. We hypothesized that the mechanoadaptation differs between the immature and mature enthesis, and we compared the mineralization and structural changes of the enthesis in response to in vivo loading-induced remodeling. To do this, we used non-invasive optogenetic control of muscle contraction, which relies on the expression of light-responsive microbial opsins in activatable cells to translocate ions across the cell membrane. We and others have previously demonstrated the feasibility of eliciting sustained muscle contractions in the triceps surae muscle group by using optogenetics to control muscle contraction in vivo in mice. Here, we used optogenetics to induce isometric muscle contraction for non-invasive loading of the Achilles tendon enthesis daily, for three weeks, in Immature and Mature mice.
Using high-resolution imaging and histology, we found irregular apophyseal bone formation and increased vascularization, with smaller fibrocartilage and decreased bone quality, following repetitive loading in the Immature but not Mature enthesis. These findings model the structural and mechanical changes similar to those occurring during overuse activities in the young athlete and are generally associated with pediatric apophyseal pathologies.

Department Name: Mechanical Engineering
Appears In: GSG presents 10th Annual Graduate Students’ Forum

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