Heterotopic ossification (HO) is a double‐edged sword. Pathological HO presents as an undesired clinical complication, whereas controlled heterotopic bone formation by synthetic osteoinductive materials shows promising therapeutic potentials for bone regeneration. However, the mechanism of material‐induced heterotopic bone formation remains largely unknown. Early acquired HO being usually accompanied by severe tissue hypoxia prompts the hypothesis that hypoxia caused by the implantation coordinates serial cellular events and ultimately induces heterotopic bone formation in osteoinductive materials. The data presented herein shows a link between hypoxia, macrophage polarization to M2, osteoclastogenesis, and material‐induced bone formation. Hypoxia inducible factor‐1α (HIF‐1α), a crucial mediator of cellular responses to hypoxia, is highly expressed in an osteoinductive calcium phosphate ceramic (CaP) during the early phase of implantation, while pharmacological inhibition of HIF‐1α significantly inhibits M2 macrophage, subsequent osteoclast, and material‐induced bone formation. Similarly, in vitro, hypoxia enhances M2 macrophage and osteoclast formation. Osteoclast‐conditioned medium enhances osteogenic differentiation of mesenchymal stem cells, such enhancement disappears with the presence of HIF‐1α inhibitor. Furthermore, metabolomics analysis reveals that hypoxia enhances osteoclastogenesis via the axis of M2/lipid‐loaded macrophages. The current findings shed new light on the mechanism of HO and favor the design of more potent osteoinductive materials for bone regeneration.

Critical-size bone defects (CSDs), which are those that do not self-repair in a given period, are essential for evaluating bone-regeneration strategies. We established CSDs models in the rabbit cranium and ulna, and the bone-regeneration capacities of porous calcium phosphate (CaP) ceramics were assessed. A 12.6-mm cranial defect was confirmed as a CSDs after 12 weeks, with submicron surface-structured biphasic calcium-phosphate (BCP) implants [consisting of 20% hydroxyapatite and 80% tricalcium phosphate (TCP)] demonstrating significantly higher bone formation (32.2% ± 10.6%) than micron surface-structured TCP (TCP-B) implants (17.8% ± 4.6%, p = 0.0121). Ulna defects (15.0 mm in length) failed to heal spontaneously within 24 weeks when the periosteum was removed from both the ulna and radius, and the radius was covered with an expanded polytetrafluoroethylene (ePTFE) membrane. No bone bridging (i.e., union) was observed in the BCP implants at 12 weeks, whereas 80% of BCP implants (four out of five) achieved union by 24 weeks. Furthermore, the bone area within the available space of BCP implants increased significantly from 19.3% ± 7.3% at 12 weeks to 37.7% ± 8.5% at 24 weeks (p = 0.0063), accompanied by significant BCP resorption (14.8% at 12 weeks and 30.2% at 24 weeks). This study offers two rabbit CSDs models for evaluating bone-regeneration strategies (including bone substitution), and the overall data obtained in the current study indicate the possibility of repairing CSDs with CaP ceramics demonstrating improved bone-forming ability given adequate implantation time.

Osteoinductive calcium phosphate ceramics (CaPs) hold great promise for bone repair, yet the metabolic principles governing their efficacy are poorly defined. Here, we identify mitochondrial oxidative phosphorylation (OXPHOS) as an early and indispensable pathway activated specifically by an osteoinductive tricalcium phosphate (TCPS) in material-induced bone formation. Our findings demonstrate that OXPHOS critically governs the M2 macrophage-osteoclast axis for bone formation by regulating both cellular energy supply and reactive oxygen species (ROS) homeostasis. Disrupting either function-by inhibiting OXPHOS (reducing ATP production) or by disrupting ROS balance (including ROS scavenging or accumulation)-uncouples this metabolic-immunological cascade and abrogates bone formation. Our work establishes mitochondrial OXPHOS as a central metabolic hub that integrates bioenergetics with ROS balance to orchestrate material-induced bone formation via M2 macrophage polarization-osteoclastogenesis axis. Consequently, these insights provide a rational basis for therapeutic modulation of the immunometabolic cascade in bone regeneration, via strategies designed to co-activate OXPHOS and fine-tune ROS dynamics.