Biological Performance of a Bioabsorbable Magnesium–Magnesium Phosphate Cement Interbody Fusion Cage in a Porcine Lumbar Interbody Fusion Model: a Feasibility Study

Background: Magnesium phosphate cement (MPC) is a novel biodegradable bone adhesive. The major drawback of the Magnesium phosphate cement (MPC) cage for implantation is its high brittleness. The magnesium was presumed to compensate for the lack of toughness in the MPC cage. The aim of the study was to evaluate the feasibility of a bioabsorbable cage consisting of magnesium and magnesium phosphate cement (MPC) in a porcine lumbar interbody fusion model. Methods: Twelve male Ba-Ma mini pigs underwent lumbar discectomy and fusion with an Mg-MPC cage or a PEEK cage at the L3/L4 and L4/L5 level. Computed tomography (CT) scans were made to evaluate the distractive property by comparing average disc space height (DSH) before and at 6, 12, and 24 weeks after the operation. After the lumbar spines were harvested at 6 or 24 weeks after the operation, micro-CT examination was conducted to analyze the fusion rate, and stiffness of motion segments was investigated through mechanical tests. A histological study was performed to evaluate the tissue type in the intervertebral space. Results: CT scans showed no significant difference between the two groups in average DSH at each time point. Micro-CT scans revealed an equal fusion rate in both groups (0% at 6 weeks, 83.3% at 24 weeks). Both groups showed time-dependent increases in stability, the Mg-MPC cages achieved an inferior stiffness at 6 weeks and a comparable stiffness at 24 weeks. Histologic evaluation showed the presence of newly formed bone in both groups. However, empty spaces were observed at the interface or around the Mg-MPC cages. Conclusion: Compared with the PEEK cages, the Mg-MPC cages achieved comparable distraction, fusion rate, and spinal stability at 24 weeks after the operation. However, due to inferior stiffness at the early stage and fast degradation, further modification of material composition and design are necessary.