Enhancing Bone Healing with a Priming Stimulus.

Bone healing is a complex regenerative process regulated by interactions between skeletal and host biologic responses, and failure of bone repair remains a major challenge in orthopedic surgery. Using a murine model, this study investigated whether a preemptive priming stimulus could enhance healing of a subsequent contralateral cortical bone defect and whether the type and timing of the stimulus influenced this response. Skeletally mature male mice were randomized into six groups (n = 6/group) receiving either no stimulus, a skin incision, skin and muscle incisions, or a unicortical femoral drill hole stimulus. A subcritical-sized 1 mm × 2 mm unicortical defect was subsequently created in the contralateral femur after intervals of 2, 6, or 12 weeks, depending on group allocation. Femora were harvested 8 weeks later for micro-computed tomography, histology, and immunofluorescence analyses. Mice undergoing muscle elevation 2 weeks prior to defect creation and mice receiving drill hole stimulus 12 weeks prior demonstrated the greatest degree of cortical regeneration and healing of the contralateral subcritical-sized defect, with normalized cortical thicknesses reaching 104% and 109% of adjacent native cortex, respectively. Histologic analysis confirmed restoration of mature cortical architecture in these groups. Immunofluorescence analysis demonstrated a relative shift toward an Arg1-associated reparative macrophage profile with reduced iNOS-associated inflammatory signaling, suggesting that modulation of the innate immune response contributed to the enhanced regenerative healing observed. These findings demonstrate that priming stimuli can enhance subsequent bone healing in a timing- and stimulus-dependent manner and may represent a novel strategy to optimize bone regeneration.