Biomechanical evaluation of a novel biomimetic artificial intervertebral disc in canine cervical cadaveric spines

Abstract

Background context: Cervical disc replacement (CDR) aims to restore motion of the treated level to reduce the risk of adjacent segment disease (ASD) compared with spinal fusion. However, first-generation articulating devices are unable to mimic the complex deformation kinematics of a natural disc. Thus, a biomimetic artificial intervertebral CDR (bioAID), containing a hydroxyethylmethacrylate (HEMA)-sodium methacrylate (NaMA) hydrogel core representing the nucleus pulposus, an ultra-high-molecular-weight-polyethylene fiber jacket as annulus fibrosus, and titanium endplates with pins for primary mechanical fixation, was developed.

Purpose: To assess the initial biomechanical effect of the bioAID on the kinematic behavior of the canine spine, an ex vivo biomechanical study in 6-degrees-of-freedom was performed.

Study design: A canine cadaveric biomechanical study.

Methods: Six cadaveric canine specimens (C3-C6) were tested in flexion-extension (FE), lateral bending (LB) axial rotation (AR) using a spine tester in three conditions: intact, after C4-C5 disc replacement with bioAID, and after C4-C5 interbody fusion. A hybrid protocol was used where first the intact spines were subjected to a pure moment of ±1 Nm, whereafter the treated spines were subjected to the full range of motion (ROM) of the intact condition. 3D segmental motions at all levels were measured while recording the reaction torsion. Biomechanical parameters studied included ROM, neutral zone (NZ), and intradiscal pressure (IDP) at the adjacent cranial level (C3-C4).

Results: The bioAID retained the sigmoid shape of the moment-rotation curves with a NZ similar to the intact condition in LB and FE. Additionally, the normalized ROMs at the bioAID-treated level were statistically equivalent to intact during FE and AR while slightly decreased in LB. At the two adjacent levels, ROMs showed similar values for the intact compared to the bioAID for FE and AR and an increase in LB. In contrast, levels adjacent to the fused segment showed an increased motion in FE and LB as compensation for the loss of motion at the treated level. The IDP at the adjacent C3-C4 level after implantation of bioAID was close to intact values. After fusion, increased IDP was found compared with intact but did not reach statistical significance.

Conclusion: This study indicates that the bioAID can mimic the kinematic behavior of the replaced intervertebral disc and preserves that for the adjacent levels better than fusion. As a result, CDR using the novel bioAID is a promising alternative treatment for replacing severely degenerated intervertebral discs.

Keywords: biomechanical; biomimetic artificial disc; cervical spine; kinematics; total disc replacement.

FIGURE 1

(A) biomimetic artificial intervertebral disc (bioAID). (B) schematic representation of the design of the bioAID and its biomimicry compared to a natural disc.

FIGURE 2

(A) ventral(anterior in humans) view of intact cervical cadaveric canine spine embedded in polymethylmethacrylate resin including insertion of the triplet markers subjected to ±1 Nm pure moment using a 6‐DOF spine tester. (B) schematic representation of the hybrid test protocol in the following three conditions: intact, after replacement of C4‐C5 disc with bioAID, and after C4‐C5 fusion using an anchored cage (C‐LOX). ROM = range of motion. The Figure was partly generated using Servier Medical Art, provided by Servier, licensed under a Creative Commons Attribution 3.0 unported license.

FIGURE 3

Normalized segmental range of motion ± standard deviation for spinal levels between C3‐C6 in flexion/extension (FE), lateral bending (LB), and axial rotation (AR) for intact C4‐C5 disc, after replacement with bioAID, and after C4‐C5 fusion in intact specimens, with bioAID and fusion at C4‐C5. (Repeated measures ANOVA, Tukey post hoc: *p < 0.05; **p < 0.01; ***p < 0.001).

FIGURE 4

Representative moment‐rotation curves (A) and mean normalized neutral zone ± standard deviation (B) for intact C4‐C5 disc, after replacement with bioAID at C4‐C5 and after C4‐C5 fusion.(Repeated measures ANOVA, Tukey post hoc:**p < 0.01; ***p < 0.001).

FIGURE 5

Intradiscal pressure (MPa, mean ± standard deviation) at C3‐C4 for intact, after replacement of C4‐C5 with bioAID and after C4‐C5 fusion. (Mixed effect analysis, Tukey post hoc: *p < 0.05, **p < 0.01).