How annulus defects can act as initiation sites for herniation.

PURPOSE: Both posture and loading rate are key factors in the herniation process and can determine the mechanism of disc failure. The aim of this study was to test the hypothesis that disruption visible with HR-MRI post-testing corresponds with microstructural features and further elucidate the mechanism by which this disruption weakens the disc. This will enable us to gain new insights into the herniation process. METHODS: Thirty ovine lumbar spinal segments were subjected to combinations of four loading conditions (0-12° flexion, 0-9° lateral bending, 0-4° axial rotation, 0-1500 N axial compression) for 1000 loading cycles at 2 Hz in a dynamic disc loading simulator. The discs were scanned in an ultra-high field MRI (11.7 T) then examined using brightfield microscopy to examine their microstructure. RESULTS: Four discs herniated and seven discs suffered nucleus displacement. These discs contained pre-existing defects in the central posterior annulus. Generally, following testing discs contained more posterior annulus disruption, Microstructural investigation revealed there was clear correspondence between HR-MRI and microstructural observations, and that the mid-outer annular-endplate junction had failed in all discs examined in this study. CONCLUSIONS: While all discs suffered outer annulus damage, only the discs containing pre-existing defects herniated. These pre-existing defects weakened the inner and mid annulus, allowing herniation to occur once the mid and outer annular wall was compromised. We propose this can occur during the degenerative cascade.

Disc wall structural abnormalities can act as initiation sites for herniation.

Both posture and loading rate are key factors in the herniation process and can determine the failure mechanism of the disc. The influence of disc structure on the herniation process has yet to be directly observed, thus the aim of this study was to test the hypothesis that discs containing greater levels of pre-existing disruption would be more vulnerable to herniation when subjected to severe levels of posture and loading. 30 ovine lumbar motion segments were subjected to combinations of 4 loading conditions (0 - 12° flexion,0 - 9° lateral bending, 0 - 4° axial rotation, 0-1500 N axial compression) for 1000 loading cycles at 2 Hz in a dynamic disc loading simulator. The discs were scanned in an ultra-high field MRI (magnetic resonance imaging, 11.7 T) prior to and following testing. 4 discs herniated and 7 discs suffered nucleus displacement. These discs contained pre-existing defects in the central dorsal annulus. Generally, following testing, discs contained more dorsal annulus disruption, including 7 discs which developed similar characteristic defects although these did not herniate. Overall, more severe complex postures produced more disruption. While more severe postures such as twisting and bending increased disc damage, these results are probably the first directly showing that naturally occurring defects in the disc can act as initiation sites for herniation. The clinical significance of these findings is that, in principle at least, MRI based techniques could be capable of identifying vulnerable discs, with the obvious caveat that further correlation with clinical techniques is required.