Effect of PCL/HA material characteristics on the degradation and mechanical properties of interbody fusion cages

Abstract

Interbody fusion cages are key implants in spinal interbody fusion surgeries. 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 hydroxyapatite (HA) mass fractions (25, 30, 35, 40%) and different polycaprolactone (PCL) molecular weights (50 and 80 kDa) were fabricated. Biodegradable PCL/HA 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, and their mechanical properties under loads. The cytotoxicity assays of the material were also conducted. The findings revealed that the degradation rate of the interbody fusion cage accelerated as the HA content increased. The strength and stiffness of each material system exceeded the minimum requirement for 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 indicated that when the HA content was 30wt%, the fusion cage exhibited a moderate degradation rate, high compressive strength and 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 varying levels of cytotoxicity depending on the cytotoxicity testing method. Further animal implantation experiments of spinal fusion cages require the selection of medical-grade implant materials.

Keywords

Interbody fusion cage / Polycaprolactone / Hydroxyapatite / Biocompatibility / Biodegradable / Mechanical property

Funding

This work is supported by the Beijing Natural Science Foundation (L212047) and the National Natural Science Foundation of China (52171149), under the “Research on the Mechanism of 3D Printing Degradable Spinal Interbody Fusion Cage’s Osteogenic Activity” and “Basic research on amorphous nanocrystalline magnetic powder 3D direct printing technology based on polymer bonding and its application” projects, respectively.

Conflict of interest

All authors declare that they have no competing interests.

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