Anatomical Brushite-Coated Mg–Nd–Zn–Zr Alloy Cage Promotes Cervical Fusion: One-Year Results in Goats

In this study, an anatomical brushite-coated Mg–Nd–Zn–Zr alloy cage was fabricated for cervical fusion in goats. The purpose of this study was to investigate the cervical fusion effect and degradation characteristics of this cage in goats. The Mg–Nd–Zn–Zr alloy cage was fabricated based on anatomical studies, and brushite coating was prepared. Forty-five goats were divided into three groups, 15 in each group, and subjected to C2/3 anterior cervical decompression and fusion with tricortical bone graft, Mg–Nd–Zn–Zr alloy cage, or brushite-coated Mg–Nd–Zn–Zr alloy cage, respectively. Cervical radiographs and computed tomography (CT) were performed 3, 6, and 12 months postoperatively. Blood was collected for biocompatibility analysis and Mg 2+ concentration tests. The cervical spine specimens were obtained at 3, 6, and 12 months postoperatively for biomechanical, micro-CT, scanning electron microscopy coupled with energy dispersive spectroscopy, laser ablation-inductively coupled plasma-time-of-flight mass spectrometry, and histological analysis. The liver and kidney tissues were obtained for hematoxylin and eosin staining 12 months after surgery for biosafety analysis. Imaging and histological analysis showed a gradual improvement in interbody fusion over time; the fusion effect of the brushite-coated Mg–Nd–Zn–Zr alloy cage was comparable to that of the tricortical bone graft, and both were superior to that of the Mg–Nd–Zn–Zr alloy cage. Biomechanical testing showed that the brushite-coated Mg–Nd–Zn–Zr alloy cage achieved better stability than the tricortical bone graft at 12 months postoperatively. Micro-CT showed that the brushite coating significantly decreases the corrosion rate of the Mg–Nd–Zn–Zr alloy cage. In vivo degradation analysis showed higher Ca and P deposition in the degradation products of the brushite-coated Mg–Nd–Zn–Zr alloy cage, and no hyperconcentration of Mg was detected. Biocompatibility analysis showed that both cages were safe for cervical fusion surgery in goats. To conclude, the anatomical brushite-coated Mg–Nd–Zn–Zr alloy cage can promote cervical fusion in goats, and the brushite-coated Mg–Nd–Zn–Zr alloy is a potential material for developing absorbable fusion cages.