Effect of Static Compression Loads on Intervertebral Disc: An in Vivo Bent Rat Tail Model.

OBJECTIVE: To evaluate how well different magnitudes of compression-induced degenerative changes using a bent rat tail model simulated human lumbar lordosis. It has been shown that compression plays an important role in intervertebral disc degeneration (IDD).
METHODS: Sprague-Dawley rats (n = 25) were instrumented with a special compressive apparatus that was used to bend the intervertebral disc between the 8th and the 10th caudal vertebral bodies using two Kirschner wires inserted percutaneously into the middle of two tail vertebrae. Then, rats were divided into five different static compression loads (control, sham, 1.8 N, 4.5 N, and 7.2 N). The degeneration of the discs was evaluated by magnetic resonance imaging (MRI), histology, gene expression of anabolism and catabolism after 2 weeks. We used the signal characteristics of the disc in T2-weighted MRI to reflect the changes caused by degeneration as this is the most relevant and clinically recognized way to assess IDD. Pfirrmann classification was used to classify disc images. The tail discs from C8-9 and C9-10 with their two adjacent half vertebrae were carefully cut out and decalcified. Then the sections were paraffin-embedded and cut into 5-μm sections by histotome. Finally, they were stained with Safranin O-Fast Green and hematoxylin, and hematoxylin and eosin, respectively. Images were taken using a microscope and staining and compression-induced changes were assessed by a Masuda's grading scale. The relative expression levels of mRNA encoding rat anabolic genes and catabolic genes were evaluated by real-time reverse transcription (RT)-polymerase chain reaction (PCR). The mRNA expression fold change of the target gene was calculated using the 2-ΔΔCt method in the loaded and unloaded disc.
RESULTS: As the loading magnitude increased, static compression produced a significantly progressive decrease in nucleus intensity on T2-weighted MRI, a decrease of aggrecan and Type II collagen, an increase in Matrix metallopeptidase-3 (MMP-3) and MMP-13 expressions, and a histomorphological degeneration. The sham group had a score of 1.4 ± 0.3, the 1.8 N group had a score of 2.4 ± 0.3, the 4.5 N group had a score of 3.2 ± 0.3, and the 7.2 N group had a score of 4.4 ± 0.3, which was based on the Pfirrmann classification score, in which the control group had a score of 1. These results demonstrated that the sham group was not significantly different from the control group. Histological analysis showed that in the loaded disc, the size of the nucleus was reduced and that the annular layer was disorganized. Based on the Masuda grading scale, scores were as follows: for the control group, 3.8 ± 0.35; sham, 4.2 ± 0.35; 1.8 N, 5.4 ± 0.35; 4.5 N, 7.6 ± 0.35; and 7.2 N, 10 ± 0.35. The gene expression was divided into the following: anabolic genes (aggrecan, collagen type1-α1, and collagen type2-α1) and catabolic genes (MMP-3 and MMP-13). Aggrecan and collagen type 2 were, respectively, downregulated from 0.42 ± 0.04 to 0.21 ± 0.04 and from 0.93 ± 0.06 to 0.17 ± 0.06 as the magnitude of compression increased, whereas collagen type 1 was significantly upregulated, from 2.49 ± 0.19 to 4.40 ± 0.19, when compared with the control group (from 1.8 to 7.2 N, P < 0.05). Catabolic genes MMP-3 and MMP-13 were significantly upregulated in all experimental groups (P < 0.05, MMP-3: from 1.46 ± 0.18 to 3.44 ± 0.18; MMP-13: from 1.19 ± 0.12 to 2.82 ± 0.13); however, MMP-13 exhibited no significant changes but tended to be upregulated when compared with the 1.8 N group with the 4.5 N group.
CONCLUSIONS: Different stresses led to different processes of degenerative changes, the concave disc degenerating more severely as stress gradually increased.