Annulus fibrosus (AF) damage commonly occurs due to intervertebral disc (IVD) degeneration/herniation. The dynamic mechanical role of the AF is essential for proper IVD function and thus it is imperative that biomaterials developed to repair the AF withstand the mechanical rigors of the native tissue. Furthermore, these biomaterials must resist accelerated degradation within the proteolytic environment of degenerate IVDs while supporting integration with host tissue. We have previously reported a novel approach for developing collagen-based, multi-laminate AF repair patches (AFRPs) that mimic the angle-ply architecture and basic tensile properties of the human AF. Herein, we further evaluate AFRPs for their: tensile fatigue and impact burst strength, IVD attachment strength, and contribution to functional spinal unit (FSU) kinematics following IVD repair. Additionally, AFRP resistance to collagenase degradation and cytocompatibility were assessed following chemical crosslinking. In summary, AFRPs demonstrated enhanced durability at high applied stress amplitudes compared to human AF and withstood radially-directed biaxial stresses commonly borne by the native tissue prior to failure/detachment from IVDs. Moreover, FSUs repaired with AFRPs and nucleus pulposus (NP) surrogates had their axial kinematic parameters restored to intact levels. Finally, carbodiimide crosslinked AFRPs resisted accelerated collagenase digestion without detrimentally effecting AFRP tensile properties or cytocompatibility. Taken together, AFRPs demonstrate the mechanical robustness and enzymatic stability required for implantation into the damaged/degenerate IVD while supporting AF cell infiltration and viability.

Intervertebral disc degeneration (IVDD) is a progressive condition marked by tissue destruction and inflammation. The therapeutic effector functions of mesenchymal stem cells (MSCs) makes them an attractive therapy for patients with IVDD. While several sources of MSCs exist, the optimal choice for use in the inflamed IVD remains a significant question. Adipose (AD)‐ and amnion (AM)‐derived MSCs have several advantages compared with other sources, however, no study has directly compared the impact of IVDD inflammation on their effector functions. Human MSCs were cultured in media with or without supplementation of interleukin‐1β (IL‐1β) and tumor necrosis factor‐α at concentrations reportedly produced by IVDD cells. MSC proliferation and production of pro‐ and anti‐inflammatory cytokines were quantified following 24 and 48 h of culture. Additionally, the osteogenic and chondrogenic potential of AD‐ and AM‐MSCs was characterized via histology and biochemical analysis following 28 days of culture. In inflammatory culture, AM‐MSCs produced significantly more anti‐inflammatory IL‐10 (14.47 ± 2.39 pg/ml; p = 0.004) and larger chondrogenic pellets (5.67 ± 0.26 mm2; p = 0.04) with greater percent area staining positively for glycosaminoglycan (82.03 ± 3.26%; p < 0.001) compared with AD‐MSCs (0.00 ± 0.00 pg/ml; 2.76 ± 0.18 mm2; 34.75 ± 2.49%; respectively). Conversely, AD‐MSCs proliferated more resulting in higher cell numbers (221,000 ± 8,021 cells; p = 0.048) and produced higher concentrations of pro‐inflammatory cytokines prostaglandin E2 (1,118.30 ± 115.56 pg/ml; p = 0.030) and IL‐1β (185.40 ± 7.63 pg/ml; p = 0.010) compared with AM‐MSCs (109,667 ± 5,696 cells; 1,291.40 ± 78.47 pg/ml; 144.10 ± 4.57 pg/ml; respectively). AD‐MSCs produced more mineralized extracellular matrix (3.34 ± 0.05 relative absorbance units [RAU]; p < 0.001) compared with AM‐MSCs (1.08 ± 0.06 RAU). Under identical inflammatory conditions, a different effector response was observed with AM‐MSCs producing more anti‐inflammatories and demonstrating enhanced chondrogenesis compared with AD‐MSCs, which produced more pro‐inflammatory cytokines and demonstrated enhanced osteogenesis. These findings may begin to help inform researchers which MSC source may be optimal for IVD regeneration. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2445–2456, 2019

1Intervertebral disc degeneration (IVDD) is a progressive condition marked by 2 inflammation and tissue destruction. The effector functions of mesenchymal stem cells (MSCs) 3 make them an attractive therapy for patients with IVDD. While several sources of MSCs exist, the 4 optimal choice for use in the inflamed IVD remains a significant question. Adipose (AD)-and 5 amnion (AM)-derived MSCs have several advantages compared to other sources, however, no 6 study has directly compared the impact of IVDD inflammation on their effector functions. Human 7 MSCs were cultured in media with or without supplementation of interleukin-1β and tumor 8 necrosis factor-α at concentrations produced by IVDD cells. MSC proliferation and production of 9 pro-and anti-inflammatory cytokines were quantified following 24-and 48-hours of culture. 10Additionally, the osteogenic and chondrogenic potential of AD-and AM-MSCs was characterized 11 via histology and biochemical analysis following 28 days of culture. In inflammatory culture, AM- 12MSCs produced significantly more anti-inflammatory IL-10 (p=0.004) and larger chondrogenic 13 pellets (p=0.04) with greater percent area staining positively for glycosaminoglycan (p<0.001) 14 compared to AD-MSCs. Conversely, AD-MSCs proliferated more resulting in higher cell numbers 15 (p=0.048) and produced higher concentrations of pro-inflammatory cytokines PGE2 (p=0.030) and 16 IL-1β (p=0.010) compared to AM-MSCs. Additionally, AD-MSCs produced more mineralized 17 matrix (p<0.001) compared to AM-MSCs. These findings begin to inform researchers and 18 clinicians as to which MSC source may be optimal for different IVD therapies including those that 19 may promote regeneration or fusion. Further study is warranted evaluating these cells in systems 20 which recapitulate the nutrient-and oxygen-deprived environment of the degenerate IVD. 21

Intervertebral disc (IVD) degeneration (IVDD) leads to structural and functional changes.Biomaterials for restoring IVD function and promoting regeneration are currently being investigated; however, such approaches require validation using animal models that recapitulate clinical, biochemical, and biomechanical hallmarks of the human pathology. Herein, we comprehensively characterized a sheep model of chondroitinase-ABC (C-ABC) induced IVDD.Briefly, C-ABC (1U) was injected into the L1/2, L2/3, and L3/4 IVDs. Degeneration was assessed via longitudinal magnetic resonance (MR) and radiographic imaging. Additionally, kinematic, Statement of SignificanceSelecting the appropriate model for assessing biomaterials to repair and/or support regeneration of the intervertebral disc (IVD) has been controversial, leading to the use of many methods of simulating IVD degeneration (IVDD) in multiple species. Many of these models lack thorough characterization of their fidelity to human IVDD, which could hinder the translation of novel biomaterials and therapies due to unknown confounding factors. Herein, further investigation of one such model was performed using the matrix-degrading enzyme chondroitinase-ABC to induce degeneration in sheep lumbar IVDs. Degenerative changes were quantified using outcome measures relevant to human IVDD, and this dosage and method induces an aggressive degeneration environment that could be used to assess biomaterials that mimic the structure and function of the entire composite IVD. These findings may aid investigators in their selection of an appropriate animal model for preclinical testing of biomaterials and other therapeutics.

Focal defects in the annulus fibrosus (AF) of the intervertebral disc (IVD) arising from herniation have detrimental impacts on the IVD's mechanical function. Thus, biomimetic-based repair strategies must restore the mechanical integrity of the AF to help support and restore native spinal loading and motion. Accordingly, an annulus fibrosus repair patch (AFRP); a collagen-based multi-laminate scaffold with an angle-ply architecture has been previously developed, which demonstrates similar mechanical properties to native outer AF (oAF). To further enhance the mimetic nature of the AFRP, interlamellar (ILM) glycosaminoglycan (GAG) was incorporated into the scaffolds. The ability of the scaffolds to withstand simulated impact loading and resist herniation of native IVD tissue while contributing to the restoration of spinal kinematics were assessed separately. The results demonstrate that incorporation of a GAG-based ILM significantly increased (p<0.001) the impact strength of the AFRP (2.57 ± 0.04 MPa) compared to scaffolds without (1.51 ± 0.13 MPa). Additionally, repair of injured functional spinal units (FSUs) with an AFRP in combination with sequestering native NP tissue and a full-thickness AF tissue plug enabled the restoration of creep displacement (p=0.134), short-term viscous damping coefficient (p=0.538), the long-term viscous (p=0.058) and elastic (p=0.751) damping coefficients, axial neutral zone (p=0.908), and axial range of motion (p=0.476) to an intact state. Lastly, the AFRP scaffolds were able to prevent native IVD tissue herniation upon application of supraphysiologic loads (5.28 ± 1.24 MPa). Together, these results suggest that the AFRP has the strength to sequester native NP and AF tissue and/or implants, and thus, can be used in a composite repair strategy for IVDs with focal annular defects thereby assisting in the restoration of spinal kinematics.

The angle‐ply multilaminate structure of the annulus fibrosus is not reestablished following discectomy which leads to reherniation of the intervertebral disc (IVD). Biomimetic scaffolds developed to repair these defects should be evaluated for their ability to support tissue regeneration by endogenous and exogenous cells. Herein a collagen‐based, angle‐ply multilaminate patch designed to repair the outer annulus fibrosus was assessed for its ability to support mesenchymal stromal and annulus fibrosus cell viability, elongation, alignment, extracellular matrix gene expression, and scaffold remodeling. Results demonstrated that the cells remained viable, elongated, and aligned along the collagen fiber preferred direction of the scaffold, upregulated genes associated with annulus fibrosus matrix and produced collagen on the scaffold yielding biaxial mechanical properties that resembled native annulus fibrosus tissue. In conclusion, these scaffolds have demonstrated their potential to promote a living repair of defects in the annulus fibrosus and thus may be used to prevent recurrent IVD herniations.

Focal defects in the annulus fibrosus (AF) of the intervertebral disc (IVD) from herniation or surgical injury have detrimental impacts on IVD mechanical function. Thus, biomaterial-based repair strategies, which can restore the mechanical integrity of the AF and support long-term tissue regeneration are needed. Accordingly, a collagen-based multi-laminate scaffold with an underlying "angle-ply" architecture has been previously reported demonstrating similar mechanical properties to native AF tissue. The objectives of this work were to: 1) enhance the biomaterials impact strength, 2) define its contribution to spinal kinematics, and 3) assess its ability to prevent IVD herniation. First, AFRP's were enriched with a glycosaminoglycan-based (GAG) interlamellar matrix (ILM), and then tested for its radially-directed impact resistance under physiological stresses. Subsequent kinematic testing was conducted using a characterized GAGenriched AFRP as an AF focal defect closure device. In summary, AFRPs demonstrated 1) incorporation of a GAG-based ILM significantly increased radial impact strength, 2) restoration of axial FSU kinematics and 3) ability to prevent herniation of native IVD tissues. Together, these results suggest that the AFRP demonstrates the mechanical robustness and material properties to restore an IVD's physiological mechanical function through the adequate closure of an AF focal defect.

Intervertebral disc (IVD) degeneration (IVDD) leads to structural and functional changes. Biomaterials for restoring IVD function and promoting regeneration are currently being investigated; however, such approaches require validation using animal models that recapitulate clinical, biochemical, and biomechanical hallmarks of the human pathology. Herein, we comprehensively characterized a sheep model of chondroitinase-ABC (C-ABC) induced IVDD. Briefly, C-ABC (1U) was injected into the L1/2, L2/3, and L3/4 IVDs. Degeneration was assessed via longitudinal magnetic resonance (MR) and radiographic imaging. Additionally, kinematic, biochemical, and histological analyses were performed on explanted functional spinal units (FSUs). At 17-weeks, C-ABC treated IVDs demonstrated significant reductions in MR index (p=0.030) and disc height (p=0.009) compared to pre-operative values. Additionally, C-ABC treated IVDs exhibited significantly increased creep displacement (p=0.004) and axial range of motion (p=0.007) concomitant with significant decreases in tensile (p=0.034) and torsional (p=0.021) stiffnesses and long-term viscoelastic properties (p=0.016). C-ABC treated IVDs also exhibited a significant decrease in NP glycosaminoglycan: hydroxyproline ratio (p=0.002) and changes in microarchitecture, particularly in the NP and endplates, compared to uninjured IVDs. Taken together, this study demonstrated that intradiscal injection of C-ABC induces significant degeneration in sheep lumbar IVDs and its potential for use in evaluating biomaterials for IVD repair.Statement of SignificanceSelecting the appropriate model for assessing biomaterials to repair and/or support regeneration of the intervertebral disc (IVD) has been controversial, leading to the use of many methods of simulating IVD degeneration (IVDD) in multiple species. Many of these models lack thorough characterization of their fidelity to human IVDD, which could hinder the translation of novel biomaterials and therapies due to unknown confounding factors. Herein, further investigation of one such model was performed using the matrix-degrading enzyme chondroitinase-ABC to induce degeneration in sheep lumbar IVDs. Degenerative changes were quantified using outcome measures relevant to human IVDD, and this dosage and method induces an aggressive degeneration environment that could be used to assess biomaterials that mimic the structure and function of the entire composite IVD. These findings may aid investigators in their selection of an appropriate animal model for preclinical testing of biomaterials and other therapeutics.