Bony ingrowth potential of 3D-printed porous titanium alloy: a direct comparison of interbody cage materials in an in vivo ovine lumbar fusion model

Kirk C McGilvray, Jeremiah Easley, Howard B Seim, Daniel Regan, Sigurd H Berven, Wellington K Hsu, Thomas E Mroz, Christian M Puttlitz, Kirk C McGilvray, Jeremiah Easley, Howard B Seim, Daniel Regan, Sigurd H Berven, Wellington K Hsu, Thomas E Mroz, Christian M Puttlitz

Abstract

Background context: There is significant variability in the materials commonly used for interbody cages in spine surgery. It is theorized that three-dimensional (3D)-printed interbody cages using porous titanium material can provide more consistent bone ingrowth and biological fixation.

Purpose: The purpose of this study was to provide an evidence-based approach to decision-making regarding interbody materials for spinal fusion.

Study design: A comparative animal study was performed.

Methods: A skeletally mature ovine lumbar fusion model was used for this study. Interbody fusions were performed at L2-L3 and L4-L5 in 27 mature sheep using three different interbody cages (ie, polyetheretherketone [PEEK], plasma sprayed porous titanium-coated PEEK [PSP], and 3D-printed porous titanium alloy cage [PTA]). Non-destructive kinematic testing was performed in the three primary directions of motion. The specimens were then analyzed using micro-computed tomography (µ-CT); quantitative measures of the bony fusion were performed. Histomorphometric analyses were also performed in the sagittal plane through the interbody device. Outcome parameters were compared between cage designs and time points.

Results: Flexion-extension range of motion (ROM) was statistically reduced for the PTA group compared with the PEEK cages at 16 weeks (p-value=.02). Only the PTA cages demonstrated a statistically significant decrease in ROM and increase in stiffness across all three loading directions between the 8-week and 16-week sacrifice time points (p-value≤.01). Micro-CT data demonstrated significantly greater total bone volume within the graft window for the PTA cages at both 8 weeks and 16 weeks compared with the PEEK cages (p-value<.01).

Conclusions: A direct comparison of interbody implants demonstrates significant and measurable differences in biomechanical, µ-CT, and histologic performance in an ovine model. The 3D-printed porous titanium interbody cage resulted in statistically significant reductions in ROM, increases in the bone ingrowth profile, as well as average construct stiffness compared with PEEK and PSP.

Keywords: 3D porous titanium; Interbody cage; Ovine; PEEK; Spine; Spine fusion.

Copyright © 2018 The Authors. Published by Elsevier Inc. All rights reserved.

Figures

Fig. 1.
Fig. 1.
Digital image showing typical histologic sections taken in the sagittal plane through the cranial and caudal vertebral bodies including the surgically treated disc space for each treatment variant. These sample images are from the 16-week sacrifice time point. The ROI analysis (outlined with yellow dots) encompasses the entire implant. Measured parameters included the amount (%) of bone, fibrous tissue, implant, and “background” within each ROI. These images are coded as follows: bone stained, red; fibrous tissue, gray; implant, black (metal) or tan (plastic); and background, white. PEEK, polyetheretherketone; PSP, plasma sprayed porous titanium-coated PEEK; PTA, porous titanium alloy; ROI, region of interest.
Fig. 2.
Fig. 2.
Range of motion (Top) and stiffness (Bottom) data collected during non-destructive pure moment loading. (A) Significant decreases in ROM under axial rotation were observed for the PEEK, PSP, and PTA groups at the 16-week time point compared with the PTA 8-week group (A, C: p=.03; B: p=.02). (B) Significant decreases in ROM under flexion-extension were observed for the PTA group at 16 weeks compared with all treatments at the 8-week sacrifice time point (D: p=.04; F: p=.01; G: p<.01). The PSP group demonstrated significant temporal decreases from 8 weeks to 16 weeks in flexion-extension ROM (E: p=.05). With the 16-week time point, the PTA group also had significantly less flexion-extension ROM compared with the PEEK treatment (H: p=.04). (C) Significant decreases in ROM under lateral bending were observed for the PTA group at 16 weeks compared with all treatments at the 8-week sacrifice time point (I: p=.01; N, L: p<.01). The PEEK group demonstrated significant temporal decreases from 8 weeks to 16 weeks in lateral bending ROM compared with the PEEK and PTA treatments (J: p=.02; M: p=.03). The PSP group demonstrated significant temporal decreases from 8 weeks to 16 weeks in lateral bending ROM (K: p=.05). (D) Significant increases in stiffness were observed under axial rotation for all 16-week groups compared with the 8-week PTA treatment (O: p=.05; P, Q: p<.01). (E) Significant increases in stiffness under flexion-extension were observed for the PTA group at the 16-week time point compared with the PSP and PTA 8-week groups (R, S: p<.01). (F) Significant increases in stiffness under lateral bending were observed for all 16- week groups compared with the 8-week PSP treatment (T: p=.02; U; p=.03; V: p<.01). The PTA group demonstrated significant temporal increase from 8 weeks to 16 weeks in lateral bending stiffness (W: p=.02). PEEK, polyetheretherketone; PSP, plasma sprayed porous titanium-coated PEEK; PTA, porous titanium alloy; ROM, range of motion.
Fig. 3.
Fig. 3.
Example of μ-CT 3D renderings in the coronal (A) and midsagittal (B) planes of the interbody device at the 8-week and 16-week sacrifice time points. (C) Significant increases were observed in BV/TV for the PTA group compared with the PEEK and PSP groups at both the 8-week and 16-week sacrifice time points (A, B, C, D: p<.01). The PTA treatment at 16 weeks also demonstrated significantly greater BV/TV compared with the PEEK and PSP treatments at 8 weeks (E: p=.01; F p<.01). The 16-week PTA treatment demonstrated significantly lowered BV/TV compared with the 8-week PTA samples (G: p=.02). (D) A significantly greater MDBV/MDTV ratio was observed for the PTA group compared with the PEEK and PSP groups at the 8-week sacrifice time points (H: p<.01; I: p=.02). The PTA treatment also indicated a higher MDBV/MDTV ratio compared with the PSP samples within the 16-week time point (J: p<.01). BV/TV, bone volume/total volume; CT, computed tomography; MDBV/MDTV, mean density of bone volume/mean density of total volume; PEEK, polyetheretherketone; PSP, plasma sprayed porous titanium-coated PEEK; PTA, porous titanium alloy; ROM, range of motion.
Fig. 4.
Fig. 4.
Histomorphometric parameters collected from midsagittal sections of the FSU. (A) A significant increase in the PTA group at 16 weeks compared with the PEEK, PSP, and PTA treatments at the 8-week sacrifice time point (A, B, C: p<.01). The PTA treatment at 16 weeks also demonstrated a significantly increased percent bone compared with the 16-week PEEK group (D: p=.04). (B) The PTA group at both the 8-week and 16-week time points had significantly less percent implant within the ROI compared with the PEEK and PSP treatments at both the 8-week and 16-week time points (E, F, G, H, I, J, K, L: p<.01). (C) No significant difference in the percent soft tissue were calculated across sacrifice time points or within treatment variants (p=.41). (D) No significant difference in the percent background were calculated across sacrifice time points or within treatment variants (p=.41). FSU, functional spinal unit; PEEK, polyetheretherketone; PSP, plasma sprayed porous titanium-coated PEEK; PTA, porous titanium alloy; ROI, region of interest.

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