Structure‐function relationships at the human spinal disc‐vertebra interface

Abstract

Damage at the intervertebral disc‐vertebra interface associates with back pain and disc herniation. However, the structural and biomechanical properties of the disc‐vertebra interface remain underexplored. We sought to measure mechanical properties and failure mechanisms, quantify architectural features, and assess structure‐function relationships at this vulnerable location. Vertebra‐disc‐vertebra specimens from human cadaver thoracic spines were scanned with micro‐computed tomography (μCT), surface speckle‐coated, and loaded to failure in uniaxial tension. Digital image correlation (DIC) was used to calculate local surface strains. Failure surfaces were scanned using scanning electron microscopy (SEM), and adjacent sagittal slices were analyzed with histology and SEM. Seventy‐one percent of specimens failed initially at the cartilage endplate‐bone interface of the inner annulus region. Histology and SEM both indicated a lack of structural integration between the cartilage endplate (CEP) and bone. The interface failure strength was increased in samples with higher trabecular bone volume fraction in the vertebral endplates. Furthermore, failure strength decreased with degeneration, and in discs with thicker CEPs. Our findings indicate that poor structural connectivity between the CEP and vertebra may explain the structural weakness at this region, and provide insight into structural features that may contribute to risk for disc‐vertebra interface injury. The disc‐vertebra interface is the site of failure in the majority of herniation injuries. Here we show new structure‐function relationships at this interface that may motivate the development of diagnostics, prevention strategies, and treatments to improve the prognosis for many low back pain patients with disc‐vertebra interface injuries. © 2017 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 36:192–201, 2018.

Multidimensional vertebral endplate defects are associated with disc degeneration, modic changes, facet joint abnormalities, and pain

“…All endplate defect dimensions were also in significant correlation with posterior bulging of the intervertebral discs supporting the previous observation linking lumbar disc herniation with endplate junction failure and endplate damage . There are number of studies who have reported, based from cartilaginous and bony endplates in the surgical samples of herniated discs, that the pathophysiology of lumbar disc herniation is more likely to be mechanical and the high bending movements can cause overstretching of the annular fibers which can pull off the vertebral endplate causing junctional failure and endplate damage . Another possible explanation of this phenomenon may be that dehydrated and fibrosed discs due to endplate damage are more prone to develop fissures and tears causing disc to bulge and prolapse…”

Contribution of the Endplates to Disc Degeneration

“…The collagen fibers in the lamellae of the inner annulus fibrosus are continuous with the collagen fibers in the endplate, whereas the integration between collagen fibers in the nucleus pulposus and the CEP is more convoluted [7, 21]. The CEP is not structurally anchored into the BEP, and consequently the interface is easily separated: the tensile failure strength of the CEP/BEP interface is approximately 0.4 MPa where it integrates with the annulus [22]. The relatively low separation strength may reflect the loading environment at this location: in a healthy disc, the CEP/BEP interface predominately experiences compression.…”

“…At the outer annulus, the vertebral interface is formed by an enthesis – a fibrocartilaginous composite where the annular fibers are embedded into a zone of calcified cartilage that is anchored to the subchondral bone via a complex, geometric interdigitation [22]. The complex morphology and graded material properties minimize stress concentrations during complex loading that includes tension, compression, and shear forces [23, 24].…”

“…When the disc is loaded rapidly, the AF fibers have little time to stretch, leading to annular displacements over the full disc height, including at the junction between the AF and CEP. This may strip or avulse the CEP from the underlying bone when there is poor structural integration between those two tissues [22] (Fig. 1H).…”

Ethanol‐mediated compaction and cross‐linking enhance mechanical properties and degradation resistance while maintaining cytocompatibility of a nucleus pulposus scaffold

“…Each IVD comprises an aggrecan‐rich core known as the nucleus pulposus (NP), 15–25 concentric sheets of aligned collagen type I encircling the NP known as the annulus fibrosus (AF), and thin layers of hyaline cartilage covering the cranial and caudal surfaces of the IVD known as the cartilage endplates (CEP) . Adjacent vertebrae interface with the IVD through the CEP, which sequesters the NP within the disc while allowing for nutrient exchange with vertebral blood vessels . The encapsulation of the NP and its anionic, dense matrix impart triphasic time‐dependence to its mechanical properties, allowing it to dissipate loads via fluid exudation and prevent harmful stress concentrations in AF and CEP through hydraulic pressurization …”