Battlefield trauma primarily occurs to the limbs and involves fracture of bones and a significant loss of surrounding muscle tissue. Advanced orthopaedic surgical procedures that include the use of therapies that cause bone to regenerate are effective in treating the majority of these complex musculoskeletal injuries. However, depending on injury and patient characteristics, 5%-50% of the fractured bones do not heal appropriately. Additionally, the lost muscle tissue will not regrow, and there are no regenerative therapies currently available to the surgeon. As a result, these patients have the greatest loss of function, likelihood of chronic disability, and incur the greatest medical care costs. At this point, the persistence of these complicated healing outcomes indicates that the current paradigm of musculoskeletal treatments needs revision. That is, there exists opportunity to improve muscle and bone healing by treating the surrounding injured musculature.

The idea that improvements in fracture healing may be gained through definitive treatment of the surrounding traumatized musculature is borne from current clinical practice and preclinical animal studies. The most recognized classification of tibia open fractures established by Gustilo and Anderson is a primary example. In this classification scheme, more severe types of open fractures that associate with increasingly poor healing outcomes are primarily stratified by the severity of surrounding soft tissue comorbidities and not the characteristics of fracture itself. That is, soft tissue comorbidities such as contamination, vascular damage, and loss of surrounding muscle tissue are primary determinants of fracture repair. Preclinical studies have further established the importance of surrounding muscle tissue on fracture healing through controlled experimentation. While muscle can provide numerous elements of support to the fracture healing process, our laboratory has recently observed in rodents that the inflammatory response to surrounding muscle tissue trauma is a primary mediator of fracture healing. One of the primary goals in the proposed work is to further interrogate these findings in a more clinically relevant and stringent large animal model of open fracture.

The loss of muscle tissue, resulting from volumetric muscle loss (VML) injury, causes disability after open fracture, even if the bone heals. Our group has performed extensive work investigating regenerative therapies for skeletal muscle, specific to the VML injury condition. To date, an autologous minced muscle graft approach has promoted the greatest amount of muscle tissue regeneration and functional recovery in our observations and is currently under testing in a small clinical study for chronic VML injury. Herein, we will also investigate the capacity of autologous minced muscle grafts to improve skeletal muscle regeneration and strength following open fracture that includes VML injury.

This work directly addresses the “Surgical Care Focus Area,” contributing towards improvements in Volumetric Muscle Loss, Extremity Fractures, and Soft Tissue Trauma. Our overarching hypothesis is that the repair of severely traumatized skeletal muscle using autologous minced muscle grafts will “modulate” the local wound environment, resulting in rescued fracture healing and restoration of skeletal muscle form and function. If successful, the results of this study may support rapid translation to the clinic, as autologous minced grafts are a minimally manipulated tissue. The knowledge gained from this novel large animal study will establish a foundation for innovative solutions towards advancing the standard of care for musculoskeletal trauma that involves VML injury.

To test this hypothesis, we are proposing two specific aims:

Specific Aim 1: Determine the impact of concomitant muscle trauma on a non-critical size tibia defect. We will investigate the impact of concomitant VML injury on tibia healing in a porcine model of open fracture. We have already established isolated peroneous tertius (PT) VML injury and tibia fracture models in the pig. This aim will integrate these existing models, wherein the respective injuries will be juxtaposed. Indices of inflammation will be surveyed from the tibia defect site intermittently during the acute phase of healing and correlated with prolonged bone healing outcomes out to 12 weeks post-injury. We hypothesize that concomitant VML injury will promote heightened and prolonged fracture site inflammation and reduce growth factor concentrations and, ultimately, impair endogenous bone regeneration.

Specific Aim 2: Interrogate a muscle tissue augmentation therapy for improved musculoskeletal healing outcomes. In this aim, we will determine if augmentation of the VML defect using autologous minced muscle grafts rescues impaired fracture healing and partially restores neuromuscular strength. We hypothesize that direct treatment of VML injured muscle with autologous minced muscle grafts will promote de novo regeneration of muscle fibers, partially restore muscle strength, improve endogenous fracture healing, and attenuate fracture site inflammation.