In Vivo Functional Assessment of Rat Masseter Muscle Following Surgical Creation of a Volumetric Muscle Loss (VML) Injury

Volumetric Muscle Loss (VML) is prevalent in civilian and military populations and represents a debilitating skeletal muscle injury surpassing the body's natural regenerative capacity. These injuries disrupt not only muscle fibers but also nerves, blood vessels, and the extracellular matrix, overwhelming the regenerative capacity of skeletal muscle and leading to severe fibrosis and permanent deficiencies in muscle structure and function. Current clinical management has many limitations, and thus, research is ongoing to develop more effective therapeutic approaches. Notably, however, much of the preclinical emphasis on VML injuries has focused on limb and trunk muscles, with limited investigation into craniofacial muscles. Differences in developmental biology and regenerative capacity between craniofacial and limb/trunk muscles may provide crucial insights that drive more injury-specific VML treatment options. Moreover, evaluation of functional recovery is critical to establishing therapeutic efficacy. In this regard, in vivo testing of muscle contraction with percutaneous nerve stimulation is a minimally invasive method that allows for repeated functional assessment over the course of a study - in the same animal. In light of these considerations, this paper describes a method for the in vivo assessment of muscle function in the rat masseter muscle before and after a VML injury. This protocol is the first published instance to detail the creation and functional evaluation of a biologically relevant craniofacial VML injury in the rat.