Mechanical restoration and failure analyses of a hydrogel and scaffold composite strategy for annulus fibrosus repair

Abstract

Unrepaired defects in the annulus fibrosus of intervertebral disks are associated with degeneration and persistent back pain. A clinical need exists for a disk repair strategy that can seal annular defects, be easily delivered during surgical procedures, and restore biomechanics with low risk of herniation. Multiple annulus repair strategies were developed using poly(trimethylene carbonate) scaffolds optimized for cell delivery, polyurethane membranes designed to prevent herniation, and fibrin-genipin adhesive tuned to annulus fibrosus shear properties. This three-part study evaluated repair strategies for biomechanical restoration, herniation risk and failure mode in torsion, bending and compression at physiological and hyper-physiological loads using a bovine injury model. Fibrin-genipin hydrogel restored some torsional stiffness, bending ROM and disk height loss, with negligible herniation risk and failure was observed histologically at the fibrin-genipin mid-substance following rigorous loading. Scaffold-based repairs partially restored biomechanics, but had high herniation risk even when stabilized with sutured membranes and failure was observed histologically at the interface between scaffold and fibrin-genipin adhesive. Fibrin-genipin was the simplest annulus fibrosus repair solution evaluated that involved an easily deliverable adhesive that filled irregularly-shaped annular defects and partially restored disk biomechanics with low herniation risk, suggesting further evaluation for disk repair may be warranted.

Statement of significance: Lower back pain is the leading cause of global disability and commonly caused by defects and failure of intervertebral disk tissues resulting in herniation and compression of adjacent nerves. Annulus fibrosus repair materials and techniques have not been successful due to the challenging mechanical and chemical microenvironment and the needs to restore biomechanical behaviors and promote healing with negligible herniation risk while being delivered during surgical procedures. This work addressed this challenging biomaterial and clinical problem using novel materials including an adhesive hydrogel, a scaffold capable of cell delivery, and a membrane to prevent herniation. Composite repair strategies were evaluated and optimized in quantitative three-part study that rigorously evaluated disk repair and provided a framework for evaluating alternate repair techniques.

Keywords: Annular closure; Annulus fibrosus repair; Intervertebral disk degeneration; Intervertebral disk herniation; Spine biomechanics.

Figure 1. Part 1 Study Design

Torsion tests to determine biomechanical behaviors and herniation risk of 3 repair strategies. (A) Samples were distributed to five groups: Intact, Injured with a cylindrical biopsy punch defect, FibGen, Cylindrical scaffold and Conical (truncated cone) scaffold. Both scaffold groups also had FibGen adhesive and polyurethane membrane. (B) Individual components of the repair strategies. (C) Torsion stiffness test protocol increased torsion angle for each ‘round’ of 20 loading cycles. (D) Torsion angle magnitudes increased to super-physiological levels.

Figure 2. Part 2 Study Design

Moderate and rigorous bending stiffness tests to determine biomechanical behaviors and herniation risk of 2 repair strategies. (A) Samples were tested Intact, Injured with a cylindrical biopsy punch defect, and after being Repaired with either FibGen or a composite repair with Conical Scaffold. (B) Specimens were subjected to a bending stiffness test protocol. (C) Bending angle was applied on-axis or off-axis with the injury.

Figure 3. Part 3 Study Design

Multi-axial flexibility tests were applied to determine biomechanical behaviors of FibGen repair. (A) Samples were tested Intact, Injured with a cruciate-style defect and Repaired with FibGen. (B) Flexibility testing was performed using a constant angular velocity to a moment controlled protocol limit.

Figure 4. Part 1 Results. Torsional biomechanics

All repair strategies partially restored torsional stiffness and torque range to intact levels. (A) Torque-Rotation curves were used to calculate (B) torsional stiffness. No significant differences in torsional stiffness between groups were detected. Data presented as mean ± standard deviation. Torsional stiffness and torque range were calculated for each sample until point of extrusion so that results are only presented up to 5° for Cylindrical and Conical groups.

Figure 5. Part 1 Results. Herniation biomechanics

FibGen herniated at larger rotations and torques than the scaffold based repairs and all herniated at super-physiological levels. (A) Nominal axial stress recorded during torsional loading with abrupt decrease of axial force concurrent with herniation (indicated by arrow). Maximum levels of (B) nominal axial stress, (C) rotation and (D) torque were calculated at the cycle just prior to herniation. Data presented as mean ± standard deviation. *p<0.05 compared to FibGen. ^# Means for FibGen were calculated using values at the maximum test rotation angle for the 4 FibGen samples that did not herniate so that these data under-represent failure properties for FibGen.

Figure 6. Part 2 Results. Herniation characterization, disc height & biomechanics

FibGen did not herniate and restored disc height while the Conical scaffold group herniated and did not restore disc height. (A) Photos and histological sections of sagittal bovine IVDs sections following repair and moderate loading for Conical samples and moderate and cyclic failure loading for FibGen. Scale bars are 1000 μm in mosaic image and 50 μm in magnified areas. For Conical group, failure occurred between Conical scaffold and FibGen (arrows, top). For FibGen group, FibGen remained adhered to the AF tissue with failure occurring by cracking of the FibGen mid-substance (black arrow) and fissuring of native disc tissue (white arrow) (bottom) (B) Axial stress and bending angle during moderate testing for characteristic sample with times of herniation for 5 of the conical scaffolds indicated (∨). (C) Disc height loss was greatest for the specimens in Defect condition and was restored for FibGen. (D) Axial and bending biomechanical results for Injured Control, FibGen and Conical scaffold groups. FibGen had the largest stiffness ratio values for all testing modes, but no significant biomechanical differences were observed with group for any stiffness ratio value. Data presented as mean ± standard deviation. *p<0.05.

Figure 7. Part 3 Results. Multi-axial range of motion

ROM was significantly increased in the Injured group compared to Intact for all degrees of freedom. FibGen had ROM values lower than for Injured in all degress of freedom but this was only significant in flexion-extension (A) Moment and angle curves were used to calculate the ROM as the total angular motion between ±1.5Nm moment for each of the 3 rotational degrees of freedom for (B) flexion-extension, (C) lateral bending and (D) torsion. Data presented as mean ± standard deviation. *p<0.05, **p<0.01.