Poster Presentation 50th International Society for the Study of the Lumbar Spine Annual Meeting 2024

INTRODUCTION: An improved understanding of intervertebral disc (IVD) structure and function is required for treatment development. Loading induces micro-fractures at the interface between the nucleus pulposus (NP) and the annulus fibrosus (AF), which is hypothesized to induce a cascade of cellular changes leading to degeneration. However, there is limited understanding of the structural relationship between the NP and AF at this interface and particularly response to load. This study utilised X-ray scattering to provide hierarchical morphometric information of collagen structure across the IVD, especially the interface region under load.

METHODS: Bovine IVDs were imaged using the I22 SAXS/WAXS beamline at Diamond Light Source. Peaks associated with the D-banded structure of collagen fibrils were fitted to quantify their azimuthal distribution, as well the magnitude and direction of internal strains. WAXS imaging with the new high-throughput VMXi beamline was performed of non-degenerate and degenerate bovine IVDs under 0-20% applied strains to investigate the hierarchical response to load. Total collagen distribution and tropocollagen molecule orientation determined together with Azimuthal distribution of intermolecular peaks during load.

RESULTS: SAXS imaging provided structural “AF-like” and “NP-like” fingerprints, with clear bidirectionality within the AF, whilst the NP showed multidirectional strains. D-band peak intensities between the AF and NP also alluded to variations in charge distribution along tropocollagen molecules. With highest D period seen within the NP demonstrating high internal strains on collagen fibers, likely a result of internal tissue swelling due to high proteoglycan content. The interface region exhibited a linear rearrangement from AF-like to NP-like structure over a region approximately 500µm wide with no other distinguishing features. WAXS imaging of non-degenerate and degenerate IVDs demonstrated clear changes in collagen content and orientation within degenerate discs (Figure 1). Increased strain in healthy discs appeared to increase collagen distribution within the interface zone although no clear changes in orientation were observed (Figure 1). Degenerate discs showed higher levels of collagen within a fibrotic region which disrupted the collagen orientation particularly in the NP and decreased orientation in the AF. With hot spots seen during loading and potential fissure propagation.

DISCUSSION & CONCLUSIONS: SAXS and WAXS have been utilised to develop an improved understanding of collagen structure across the intervertebral disc. Demonstrating high internal strains on collagen fibers particularly within the NP region of the disc. AF and NP regions showed distinct collagen orientation and internal strains with an apparent lack of bracing structure seen at the interface between the differential mechanical tissues (NP and AF). This could lead to the generation of shear forces under load, leading to micro-fractures and failure seen during degeneration at the interface region. WAXS imaging of normal and degenerate discs under strain demonstrated clear differences in collagen content and orientation between normal and degenerate discs, with potential increased collagen organization within the interface zone under load in normal but not degenerate discs. Current work is investigating the D period internal strains on collagen fibers on these data to determine changes in collagen strains during external loading.