Mechanical crosstalk between the intervertebral disc, facet joints, and vertebral endplate following acute disc injury in a rabbit model

Abstract

Background: Vertebral endplate sclerosis and facet osteoarthritis have been documented in animals and humans. However, it is unclear how these adjacent pathologies engage in crosstalk with the intervertebral disc. This study sought to elucidate this crosstalk by assessing each compartment individually in response to acute disc injury.

Methods: Eleven New Zealand White rabbits underwent annular disc puncture using a 16G or 21G needle. At 4 and 10 weeks, individual compartments of the motion segment were analyzed. Discs underwent T ₁ relaxation mapping with MRI contrast agent gadodiamide as well T ₂ mapping. Both discs and facets underwent mechanical testing via vertebra-disc-vertebra tension-compression creep testing and indentation testing, respectively. Endplate bone density was quantified via μCT. Discs and facets were sectioned and stained for histology scoring.

Results: Intervertebral discs became more degenerative with increasing needle diameter and time post-puncture. Bone density also increased in endplates adjacent to both 21G and 16G punctured discs leading to reduced gadodiamide transport at 10 weeks. The facet joints, however, did not follow this same trend. Facets adjacent to 16G punctured discs were less degenerative than facets adjacent to 21G punctured discs at 10 weeks. 16G facets were more degenerative at 4 weeks than at 10, suggesting the cartilage had recovered. The formation of severe disc osteophytes in 16G punctured discs between 4 and 10 weeks likely offloaded the facet cartilage, leading to the recovery observed.

Conclusions: Overall, this study supports that degeneration spans the whole spinal motion segment following disc injury. Vertebral endplate thickening occurred in response to disc injury, which limited the diffusion of small molecules into the disc. This work also suggests that altered disc mechanics can induce facet degeneration, and that extreme bony remodeling adjacent to the disc may promote facet cartilage recovery through offloading of the articular cartilage.

Keywords: annular puncture model; biomechanics; intervertebral disc degeneration; osteoarthritis; vertebral endplate sclerosis; zygaphophyseal joint.

FIGURE 1

Study overview. Control discs were punctured ex vivo using either a 16G or 21G needle and then subject to mechanical testing. Disc and facet degeneration in response to acute annular injury with either a 16G or 21G needle in New Zealand White rabbits was assessed at 4 and 10 weeks post‐puncture. 16G punctured discs and facets were assessed at both 4 and 10 weeks, while 21G punctured discs and facets were assessed at 10 weeks.

FIGURE 2

Disc mechanics. (A) Total range of motion, (B) neutral zone modulus, (C) compressive modulus, and (D) creep displacement of discs. (E) Composite T ₂ maps of disc NP and AF regions (n = 5–10), and (F) NP T ₂ relaxation time. (# indicates p < 0.05 vs. 16G acute discs; ## indicates p < 0.01 vs. 16G acute discs; ### indicates p < 0.001 vs. 16G acute discs; and #### indicates p < 0.0001 vs. 16G acute discs).

FIGURE 3

Disc tissue changes. (A) Safranin‐O/Fast Green histology of the entire disc (scale = 4 mm) for all experimental groups, as well as representative Hematoxylin and Eosin histology of disc NP, AF, and EP regions (scale = 200 μm). (B) Total ORS Spine section intervertebral disc score stratified by NP, AF, and EP regions (n = 4–6). (C) Osteophyte μCT bone volume quantification and (D) representative μCT reconstructions of 10‐weeks 16G and 21G punctured discs, with osteophyte volumes contoured in white (scale = 3 mm). (* indicates p < 0.05 vs. control discs; **** indicates p < 0000.1 vs. control discs).

FIGURE 4

Bony endplate changes. μCT quantification of (A) bone volume fraction, (B) trabecular number, (C) trabecular thickness, and (D) trabecular spacing, as well as (E) representative μCT cross‐sections (scale = 2 mm) of bony endplates adjacent to control and 10‐weeks 16G and 21G punctured discs. (F) Percent reduction in T ₁ relaxation time of the NP following gadodiamide administration.

FIGURE 5

Facet cartilage changes. (A) Tensile modulus, (B) compressive modulus, (C) strain‐dependent flow‐limited constant, and (D) hydraulic permeability of the facet articular cartilage measured by indentation testing. (E) Cartilage thickness. (F) OARSI score of facet Safranin‐O/Fast Green histology as well as histology scores stratified by individual (G) Safranin‐O/Fast Green score and (H) chondrocyte density score. (I) Least, mid, and most degenerative Safranin‐O/Fast Green histology of facet articular cartilage for all experimental groups (scale = 200 μm).