High Strength and Shape Memory Spinal Fusion Device for Minimally Invasive Interbody Fusions

Abstract

Introduction: Lumbar interbody fusion is widely employed for both acute and chronic spinal diseases interventions. However, large incision created during interbody cage implantation may adversely impair spinal tissue and influence postoperative recovery. The aim of this study was to design a shape memory interbody fusion device suitable for small incision implantation.

Methods: In this study, we designed and fabricated an intervertebral fusion cage that utilizes near-infrared (NIR) light-responsive shape memory characteristics. This cage was composed of bisphenol A diglycidyl ether, polyether amine D-230, decylamine and iron oxide nanoparticles. A self-hardening calcium phosphate-starch cement (CSC) was injected internally through the injection channel of the cage for healing outcome improvement.

Results: The size of the interbody cage is reduced from 22 mm to 8.8 mm to minimize the incision size. Subsequent NIR light irradiation prompted a swift recovery of the cage shape within 5 min at the lesion site. The biocompatibility of the shape memory composite was validated through in vitro MC3T3-E1 cell (osteoblast-like cells) adhesion and proliferation assays and subcutaneous implantation experiments in rats. CSC was injected into the cage, and the relevant results revealed that CSC is uniformly dispersed within the internal space, along with the cage compressive strength increasing from 12 to 20 MPa.

Conclusion: The results from this study thus demonstrated that this integrated approach of using a minimally invasive NIR shape memory spinal fusion cage with CSC has potential for lumbar interbody fusion.

Keywords: NIR responsive; calcium phosphate cement; interbody fusion cage; minimally invasive; shape memory.

Figure 1

Schematic illustration of molecular structure and shape memory. (a) Chemical formula of bisphenol A diglycidyl ether; polyetheramine D-230; decylamine; (b) chemical formula of the shape memory polymer (SMPs), and (c) transition between the permanent and temporary shape of the SMPs.

Figure 2

Temperature-sensitive and NIR-sensitive shape memory. (a) heating curve under near infrared laser irradiation (1W, 20 cm); (b) 20 times deformation recovery ratio, shape fixation and shape recovery behavior in hot water: (c) SMPs; (d) LSMPC-0.5; (e) deformation behavior of SMPs and LSMPC-0.5 under near infrared laser irradiation (pink shows the NIR light reflection on the table), (f) LSMPC-0.5 programming process: shape recovery-fixed-recovery.

Figure 3

Thermodynamic properties. (a) DSC curve; (b) TGA curve; (c) force–displacement curve; (d) compressive modulus (ns, no significant difference); (e) storage modulus; and (f) tan δ curve of SMPs and LSMPC-0.5.

Figure 4

Cytocompatibility. MC3T3 cells cultured on the SMPs materials for 1 and 3 days: (a) cell density (ns, no significant difference; ***p<0.001); (b–g) Merged fluorescent images.

Figure 5

In vivo histocompatibility of SMPs in rats. (a) Schematic illustration of subcutaneous implantation experiment in rats, (b, c) images of wound appearance at day 0 and 14 of subcutaneous implantation in rats, histological analysis of LSMPC-0.5 after subcutaneous implantation for 14 days: (d) HE staining; and (e) Masson trichrome staining.

Figure 6

Design and fabrication of Cage-LSMPC. (a) Preparation and application process of Cage-LSMPC, (b) schematic of the three-dimensional structure of Cage-LSMPC, and (c) size of Cage-LSMPC before and after deformation.

Figure 7

Feasibility of spinal fusion device. (a) Shape recovery process of Cage-LSMPC under NIR light irradiation (pink reflection shows the NIR light), (b) demonstration of the Cage-LSPMC shape change in the model before and after shape change, (c) the process of injecting bone cement into the Cage-LSMPC, (d) stress–strain curve of Cage-LSMPC and Cage-LSMPC-cement, cyclical fatigue test of the (e) Cage-LSMPC and (f) Cage-LSMPC-cement.