Outstanding in vivo mechanical integrity of additively manufactured spinal cages with a novel “honeycomb tree structure” design via guiding bone matrix orientation

• Published in: The spine journal 2022-10, Vol.22 (10), p.1742-1757
• Main Authors: Ishimoto, Takuya, Kobayashi, Yoshiya, Takahata, Masahiko, Ito, Manabu, Matsugaki, Aira, Takahashi, Hiroyuki, Watanabe, Ryota, Inoue, Takayuki, Matsuzaka, Tadaaki, Ozasa, Ryosuke, Hanawa, Takao, Yokota, Katsuhiko, Nakashima, Yoshio, Nakano, Takayoshi
• Format: Article
• Language: English
• Subjects: anisotropy; spinal cage, trabecular architecture, bone matrix, collagen/apatite orientation, bone quality, pull-out strength
• ISSN: 1529-9430; 1878-1632
• DOI: 10.1016/j.spinee.2022.05.006

•A novel spinal cage configuration to induce high quality bone was designed.•Highly anisotropic bone mimicking sound bone was introduced inside the cage.•Newly formed bone showed preferentially oriented ECM (collagen and apatite).•Novel cage showed outstanding in vivo mechanical integrity.•Novel cage performed excellently without an autologous bone graft inside the cage. Therapeutic devices for spinal disorders, such as spinal fusion cages, must be able to facilitate the maintenance and rapid recovery of spinal function. Therefore, it would be advantageous that future spinal fusion cages facilitate rapid recovery of spinal function without secondary surgery to harvest autologous bone. This study investigated a novel spinal cage configuration that achieves in vivo mechanical integrity as a devise/bone complex by inducing bone that mimicked the sound trabecular bone, hierarchically and anisotropically structured trabeculae strengthened with a preferentially oriented extracellular matrix. In vivo animal study. A cage possessing an anisotropic through-pore with a grooved substrate, that we termed “honeycomb tree structure,” was designed for guiding bone matrix orientation; it was manufactured using a laser beam powder bed fusion method through an additive manufacturing processes. The newly designed cages were implanted into sheep vertebral bodies for eight and 16 weeks. An autologous bone was not installed in the newly designed cage. A pull-out test was performed to evaluate the mechanical integrity of the cage/bone interface. Additionally, the preferential orientation of bone matrix consisting of collagen and apatite was determined. The cage/host bone interface strength assessed by the maximum pull-out load for the novel cage without an autologous bone graft (3360 ± 411 N) was significantly higher than that for the conventional cage using autologous bone (903 ± 188 N) after only 8 weeks post-implantation. These results highlight the potential of this novel cage to achieve functional fusion between the cage and host bone. Our study provides insight into the design of highly functional spinal devices based on the anisotropic nature of bone. The sheep spine is similar to the human spine in its stress condition and trabecular bone architecture and is widely recognized as a useful model for the human spine. The present design may be useful as a new spinal device for humans.