Additive-manufactured Ti-6Al-4 V/Polyetheretherketone composite porous cage for Interbody fusion: bone growth and biocompatibility evaluation in a porcine model

Abstract

Background: We developed a porous Ti alloy/PEEK composite interbody cage by utilizing the advantages of polyetheretherketone (PEEK) and titanium alloy (Ti alloy) in combination with additive manufacturing technology.

Methods: Porous Ti alloy/PEEK composite cages were manufactured using various controlled porosities. Anterior intervertebral lumbar fusion and posterior augmentation were performed at three vertebral levels on 20 female pigs. Each level was randomly implanted with one of the five cages that were tested: a commercialized pure PEEK cage, a Ti alloy/PEEK composite cage with nonporous Ti alloy endplates, and three composite cages with porosities of 40, 60, and 80%, respectively. Micro-computed tomography (CT), backscattered-electron SEM (BSE-SEM), and histological analyses were performed.

Results: Micro-CT and histological analyses revealed improved bone growth in high-porosity groups. Micro-CT and BSE-SEM demonstrated that structures with high porosities, especially 60 and 80%, facilitated more bone formation inside the implant but not outside the implant. Histological analysis also showed that bone formation was higher in Ti alloy groups than in the PEEK group.

Conclusion: The composite cage presents the biological advantages of Ti alloy porous endplates and the mechanical and radiographic advantages of the PEEK central core, which makes it suitable for use as a single implant for intervertebral fusion.

Keywords: 4 V (Ti alloy)/polyetheretherketone (PEEK) composite porous cage, porcine study; 6Al; Additive manufacturing (3D printing), Ti.

Fig. 1

Fabrication of the additive-manufactured Ti-6Al-4 V (Ti alloy)/polyetheretherketone (PEEK) composite porous cage. a Surface modification through laser grooving and plasma spraying makes the interfacial layer thicker than 300 μm. b Finished product of the Ti alloy/PEEK composite porous cage. c Shear strength of the bonding interface between the metallic layer and PEEK substrate exceeded 30 MPa. d Schematic of the porous structure with inset. e Schematic of the interporous distance. f Results of the mechanical compression test on the composite porous cage. g Results of the mechanical torsion test on the composite porous cage

Fig. 2

The ROIs (region of interest) of bone ongrowth analysis and bone ingrowth analysis were shown. ROI of bone ongrowth was defined as 0–500 μm around metallic implant border. ROI of bone ingrowth was defined as the area inside metallic implant border. The border was calculated by computer using shrink-warp algorithm

Fig. 3

Scanning electron microscopy images of Ti-tested blocks with different porosities

Fig. 4

Quantitative analyses using micro-CT. a Total percent bone volume to total volume (total BV/TV) (%). b Outer percent bone volume to total volume (outer BV/TV) (%). c Inner percent bone volume to total volume (inner BV/TV) (%). d Total percent bone surface to total volume (total BS/TV) (1/mm). e Outer percent bone surface to total volume (outer BS/TV) (1/mm). f Inner percent bone surface to total volume (inner BS/TV) (1/mm). The groups were as follows: commercialized PEEK cage group (Anterior Cervical Interbody Fusion Cage®, BAUI biotech, New Taipei City, Taiwan) and composite Ti alloy/PEEK cage groups including nonporous Comp_NonP composites with 40%-, 60%-, and 80%-porosity endplates (Comp_40%P, Comp_60%P, and Comp_80%P, respectively) [* p < 0.05 between PEEK and other groups; # p < 0.05 between Comp_NonP group and other porous groups; % p < 0.05 between Comp_40%P and porous groups (Comp_60%P and Comp_80%P); and p < 0.05 between Comp_60%P and Comp_80%P groups]

Fig. 5

Representative images of micro-CT. Comp_NonP (a and d), Comp_60%P (b and e), and Comp_80%P (c and f). The black arrows indicate bone ongrowth and white arrows indicate bone ingrowth. Scale bar = 1 mm

Fig. 6

Representative backscattered-electron scanning electron microscopy images of Comp_NonP (a), Comp_40%P (b), Comp_60%P (c), and Comp_80%P (d)

Fig. 7

Histological analysis and fluorescence microscopy. Sections a–d were stained with Sanderson’s Rapid Bone Stain and counterstained with acid fuchsin (RBS). Sections E–H were examined with fluorescence microscopy to identify new bone formation labeled with tetracycline. A and E: PEEK cage; B and F: Ti alloy nonporous cage; C and G: Ti alloy/PEEK composite cage with 60% porosity; and D and H: Ti alloy/PEEK composite cage with 80% porosity. A and E: white arrows indicate a gap between the bone and implant. B and F: white arrows denote close contact between the bone and implant, with new bone formation on the interface. C, D, G, and H: white arrows represent bone growth into the porous structure of the implant

Fig. 8

Histological analysis of bone sections by using tetracycline through fluorescence and bright-field microscopy. New bone formation was easily identified inside the porous structure of the implant (Pig No. 14, L1/2, Ti alloy/PEEK composite cage with 60% porosity). Areas are picked-up by observed obvious bone ingrowth