Effect of material characteristics of PCL/HA interbody fusion cage on its degradation and mechanical properties

Interbody fusion cages are key implants in spinal interbody fusion surgeries, and for the novel biodegradable interbody fusion cages that can be gradually absorbed and replaced in the body, the formulation system of the materials plays a crucial role in their long-term biocompatibility and mechanical stability. In this study, PCL/HA composites with high mass fractions (25%, 30%, 35%, 40%) hydroxyapatite (HA) and different molecular weights (50kDa, 80kDa) polycaprolactone (PCL) were fabricated. PCL/HA biodegradable interbody fusion cages of different material systems were printed by polymer melt differential three-dimensional (3D) printing technology. In vitro degradation tests were conducted in a simulated in vivo environment to quantitatively analyze the stability and degradation behavior of PCL/HA interbody fusion cages with different material compositions and molecular weights in an artificially simulated body fluid environment, as well as their mechanical properties under loads. The cytotoxicity assays of the material were also conducted. This study revealed that as the HA content increased, the degradation rate of the interbody fusion cage accelerated. The strength and stiffness of each material system exceed the minimum standard of human cancellous bone. When the HA content was the same, the degradation rate of the high molecular weight (80 kDa) interbody fusion cage was faster compared to that of the 50 kDa molecular weight cage. When the HA mass fraction was below 30wt%, the compressive strength and compressive modulus of the fusion cage increased with increasing HA content. However, when the HA mass fraction exceeded 30wt%, the mechanical properties of the fusion cage worsened with higher HA content. A comparative analysis of material properties indicates that when the HA content was 30wt%, the fusion cage exhibited a moderate degradation rate, high compressive strength and compressive modulus, and excellent 3D printing performance, making it the formulation system with the best overall performance. In addition, the PCL material which was also used in most literature exhibited different levels of cytotoxicity for different methods of cytotoxicity testing. Further animal implantation experiments of spinal fusion cages require the selection of medical-grade-implant materials.