Degenerative changes of the canine cervical spine after discectomy procedures, an in vivo study

Abstract

Background: Discectomies are a common surgical treatment for disc herniations in the canine spine. However, the effect of these procedures on intervertebral disc tissue is not fully understood. The objective of this study was to assess degenerative changes of cervical spinal segments undergoing discectomy procedures, in vivo.

Results: Discectomies led to a 60% drop in disc height and 24% drop in foraminal height. Segments did not fuse but showed osteophyte formation as well as endplate sclerosis. MR imaging revealed terminal degenerative changes with collapse of the disc space and loss of T2 signal intensity. The endplates showed degenerative type II Modic changes. Quantitative MR imaging revealed that over 95% of Nucleus Pulposus tissue was extracted and that the nuclear as well as overall disc hydration significantly decreased. Histology confirmed terminal degenerative changes with loss of NP tissue, loss of Annulus Fibrosus organization and loss of cartilage endplate tissue. The bony endplate displayed sclerotic changes.

Conclusion: Discectomies lead to terminal degenerative changes. Therefore, these procedures should be indicated with caution specifically when performed for prophylactic purposes.

Keywords: Annulus fibrosus; Canine spine; Degeneration; Disc fenestration; Disc herniation; Discectomy; Nucleus pulposus; Quantitative MRI.

Fig. 1

Radiograph of the lower cervical spine. a Lateral view, discectomy segment C5/C6 compared to adjacent healthy control C6/C7. Red arrow marks both disc spaces. The discectomy segment is almost completely collapsed. The neuroforamen of both segments are marked with a yellow ellipse. Loss in disc height subsequently lead to a loss of foraminal height. Both segments show no signs of kyphotic or lordotic deformity. b Ventro-dorsal view of the same specimen. Red arrow points to the disc space which is significantly narrowed at C5/C6. c Lateral view of different specimen. Bony endplates are marked with yellow ellipse. The healthy endplate at C6/C7 shows trabecular bone pattern. Sclerotic changes of the degenerated bony endplate at C5/C6. Endplate is more radio dense indicating increased cortical bone and decreased trabecular bone. Red arrow marks an osteophyte which formed ventrally at the C5 vertebral body

Fig. 2

Sagittal T2 weighted MR images of the lower cervical spine. Right side of each slide is dorsal (D) a Red arrows point to the intervertebral discs between the vertebral bodies (VB) which are ventral to the spinal canal (SC). Healthy control discs at C6/C7 demonstrate a bright hyperintense, homogenous NP. The hyperintensity indicates normal NP tissue hydration. There is a clear border to the hypointense AF without loss of disc height. The endplates are hypointense. In contrast, the discectomy segment C5/C6 shows a black disc sign. There is complete loss of nuclear hyperintensity. The disc space is collapsed. The endplates (yellow arrow) are hyperintense which is indicative of type II Modic degenerative changes. b Different specimen. Yellow arrows point to disc space. Red arrow points to extruded disc at the discectomy segment. There is no significant spinal cord compression present. c Lateral view to the neuroforamen (marked by yellow ellipse). The C5/C6 discectomy segment shows a narrower foramen compared to the healthy C6/C7 segment. However, the nerve root (hypointense structure within foramen) appears not to be compressed at the discectomy segment

Fig. 3

Display of quantitative MR imaging for nuclear size and hydration measurements. a Sagittal T2 weighted MR image of a healthy (top) and degenerated disc (bottom). b Corresponding T2 relaxation time of the NP displayed as a heat map. Red colors represent high T2 relaxation time (high tissue hydration), blue colors low T2 times (low tissue hydration). All MRI voxels not representing NP tissue according to their T2 relaxation time were subtracted leaving only MR voxels representing nuclear tissue in the slide. The size of the nucleus was measured by the amount of NP voxels it was composed of. The degenerated disc on the bottom shows no residual NP voxels indicating that there is no nuclear tissue present on that slide

Fig. 4

Histology sections of explanted cervical segments (×2 magnification). a Healthy discs. Top Picrosirius red stain (stains for collagen), bottom Safranin-O stain (stains for proteoglycans). Both discs show a well organized AF composed of lamellar aligned collagen fibers which stain intense for Picrosirius red. There is a clear border to the NP which appears as a homogenous structure. It stains with Safranin-O indicating proteoglycan rich matrix. The cartilaginous endplate (black arrow) forms a border between the NP to the endplate bone (EPB). It stains intensively with Safranin-O indicating high proteoglycan content. b Discectomy discs. Top Picrosirius red, bottom Safranin-O stain. The annulus is disrupted and disorganized on both sides. Annular Picrosirius red staining is less intense. There is no NP tissue left in the disc space (DS). The cartilaginous endplate completely disappeared. The Endplate bone is sclerotic indicated by dense cortical bone structure and decreased trabecular bone. The collapse of the disc space is visible on histological sections

Fig. 5

Endplate bone Alcian blue stain (stains for proteoglycans). a Healthy disc ×8 magnification. The endplate cartilage (EPC) consists of chondrocytes (red arrows) embedded in a proteoglycan rich matrix (blue stain on alcian blue). The cartilage endplate borders to the endplate bone (EPB). b Discectomy disc ×4 magnification. The bony endplate with its cortical bone structure and osteoclasts (red arrows) borders directly to the disc space. There is no cartilage endplate in between

Fig. 6

Safranin-O stain of the Annulus Fibrosus which is attached to the endplate bone (EPB). Chondrocytes (red arrows) are infiltrating the annulus replacing fibroblasts, a degenerative process called chondrocytic metaplasia

Fig. 7

Alcian Blue stained slides of the AF under polarized light (×4). a Healthy disc, the AF is organized in a multilamellar structure. The lamellae (black star) are composed of parallel aligned fibers and are therefore birefringent (bright) under polarized light. They alternate in their orientation resulting in alternating birefringence. b Dorsal annulus of a discectomy disc. Tissue lost its multilamellar organization. The Annular fibers lost their organization and parallel alignment indicated by lost in birefringent behavior under polarized light