Multi-scale engineering of biodegradable Zn-Mg interbody cages for stronger, faster spinal fusion

Spinal interbody fusion cages must be able to bear heavy loads while integrating seamlessly with the surrounding bone. However, the cages currently used in spinal surgery often fall short on both fronts. To meet the multi-faceted requirements, here, we introduce, for the first time, additively manufactured, biodegradable Zn-Mg interbody fusion cages with multi-scale structural control, combining eutectic microstructure, heterogeneous grain architecture, and gyroid lattices reinforced by interpenetrating ribs. The resulting cages showed a compressive strength comparable to that of cortical bone, together with good ductility and low elastic modulus. In vitro, balanced release of Zn²⁺ and Mg²⁺ enhanced osteogenic differentiation while mitigating Zn²⁺ toxicity. Zn-Mg extracts effectively alleviated the negative effects of estrogen deficiency on osteoblasts and osteoclasts. In an anterior cervical discectomy and fusion (ACDF) sheep model, the Zn-Mg cages exhibited excellent biocompatibility, rapid osseointegration, and robust mechanical interlocking, and maintained intervertebral stability during in vivo degradation for 24 weeks. The AM Zn-Mg cages through dual biomechanical-biological optimization are demonstrated to be a transformative alternative to current permanent metallic and polymeric implants in spinal fusion.