Objectives: Complicated fracture healing is often associated with the severity of surrounding muscle tissue trauma. Since inflammation is a primary determinant of musculoskeletal health and regeneration, it is plausible that delayed healing and non-unions are partly caused by compounding local inflammation in response to concomitant muscle trauma.
Methods and results: To investigate this possibility, a Lewis rat open fracture model [tibia osteotomy with adjacent tibialis anterior (TA) muscle volumetric muscle loss (VML) injury] was interrogated. We observed that VML injury impaired tibia healing, as indicated by diminished mechanical strength and decreased mineralized bone within the fracture callus, as well as continued presence of cartilage instead of woven bone 28 days post-injury. The VML injured muscle presented innate and adaptive immune responses that were atypical of canonical muscle injury healing. Additionally, the VML injury resulted in a perturbation of the inflammatory phase of fracture healing, as indicated by elevations of CD3(+) lymphocytes and CD68+ macrophages in the fracture callus at 3 and 14d post-injury, respectively.
Conclusions: These data indicate that heightened and sustained innate and adaptive immune responses to traumatized muscle are associated with impaired fracture healing and may be targeted for the prevention of delayed and non-union following musculoskeletal trauma.