Abstract
Research to develop improved methods for regeneration of functional muscle tissue following volumetric muscle loss (VML) injuries remains an active area of preclinical investigation. Defined as a traumatic or surgical loss of skeletal muscle resulting in permanent cosmetic and functional impairments, VML is a common feature of battlefield injuries to service members as well as civilians who experience high-impact trauma. The critical importance of the unmet medical need, and the lack of availability of therapeutics that can restore form and function after VML, continues to drive innovation in this area of military medicine.
Traditionally, metrics for evaluating therapeutics for VML injuries focus on tissue histology, volume reconstitution, and muscle force generation post-treatment. However, studies in humans have repeatedly demonstrated that there is not a direct relationship between improvements in muscle volume or muscle strength and improvements in functional movement ability. In this work, the primary goal was to move beyond these historically relied upon metrics for VML injury/treatment evaluation and advance into sophisticated gait assessment. Overall, the ability to measure changes in 3D gait parameters, such as joint angles (kinematics) and joint moments (kinetics), provides information on how study animals are functionally utilizing muscle and mechanistic insights into strength, motion planning, and control strategies. These mechanisms define the true operational and functional significance of VML injury and VML repair for lower limb trauma—as suboptimal gait restoration will result in additional post-repair physiological and functional deficits.
With this in mind, in order to better quantify the effects of VML injury and repair on movement function we initially developed a model and methodology to measure the 3D kinematics of rat gait during treadmill walking. We investigated the tibialis anterior (TA) and employed our motion capture approach to evaluate a 20% VML injury. This initial publication identified significant differences between injured and healthy animals at all post-surgical timepoints. These results were especially relevant considering the low gait impact of the TA, as it is solely responsible for ankle dorsiflexion and toe clearance during swing. We then advanced into a more sophisticated motion capture arena to observe over-ground walking and capture concurrent ground reaction force (GRF) data for 3D inverse dynamic calculation. The TA and the 20% VML injury continued to be the model system, but a treatment was added in the form of the Tissue Engineered Muscle Repair (TEMR) cell-seed porcine bladder construct. Differences were again seen across the board in the VML group, with definitive improvements shown in the TEMR treated animals.
In the second half of this dissertation, this initial work was leveraged into the investigation of increasingly severe injury models. These consisted of two separate VML injuries in different animal groups: the traditional injury to the TA, and a new injury to a major gait contributor in the lateral gastrocnemius (LG). As a two-joint muscle in the posterior compartment, and the primary muscle for energy transfer in the lower limb during gait, the functionality of the LG is critical for movement. These injury models were extended to groups of animals receiving partial lacerations of the tibial nerve and peroneal nerve, as well as animals receiving muscle-nerve polytraumas consisting of VML injury combined with laceration to the upstream nerve associated with the injured muscle. It was here that the ultimate value of biomechanical analysis truly shined, providing insight into injury response and compensation patterns that would otherwise be hidden to investigators.
Altogether, this work represents the culmination of an idea to dig deeper into injury compensation and recovery metrics in order to achieve a deeper understanding of the biomechanical impacts of VML injury and repair. In short, the ability to perform kinetic analysis allows for novel mechanistic insight into the inner workings of muscle compartments in response to traumatic injuries. This analysis will improve the design/evaluation of regenerative therapeutics for VML injuries by identifying the contributions and compensations in the complex muscle compartments that are the target of battlefield relevant injuries. The results of these studies demonstrate that it is possible to extract an incredible amount of information about injury response and recovery using 3D gait analysis, thereby allowing investigators to improve timelines and regenerative technologies to maximize functional returns and limit pathological compensation.
