Maladaptive immune-fibrotic axis drives impaired long bone regeneration under mechanical instability

Delayed and non-healing fractures, affecting 5-10% of cases, are associated with prolonged disability and diminished quality of life. Although acute inflammation is required to initiate repair, persistent mechanical instability can sustain maladaptive immune and fibrotic responses that impede regeneration. Existing animal models do not adequately recapitulate mechanical instability, the principal driver of hypertrophic nonunion in clinical settings, thereby limiting translational relevance. In this study, we developed a murine model of delayed fracture healing using tunable intramedullary fixation to impose controlled interfragmentary strain. High-strain conditions (low-stiffness nail, 15-30% strain) produced enlarged calluses characterized by delayed ossification, increased fibrotic tissue (2.9-fold, p = 0.0099), and reduced biomechanical integrity (1.6-fold decrease in stiffness, p = 0.024) relative to low-strain controls (high-stiffness nail, <5% strain). Spatial transcriptomic analysis identified persistent fibrotic niches in high-strain calluses enriched with fibroblast-associated genes (e.g., Pdgfrb, Lgals3) and dysregulated macrophage-fibroblast signaling (Spp1, Mmp9). Systemically, high-strain fractures were associated with distinct immune signatures in which CD206 macrophages and CD25 regulatory T cells predicted healing outcomes (R = 0.72, p = 0.004), suggesting early immune polarization as a determinant of repair trajectory. Elevated CD8 T cell responses were also observed, consistent with a sustained inflammatory state associated with impaired healing. These findings identify mechanical instability as a driver of pathological immune-stromal interactions and establish a preclinical platform for investigating mechanobiology-informed therapeutic strategies. This work supports a conceptual framework in which hypertrophic nonunion is understood as a disorder arising from dysregulated interactions between mechanical cues and immune responses. TeaserMechanical strain hijacks immune signaling to induce fibrosis and block bone regeneration in unstable fractures.