Assessment of changes in the micro-nano environment of intervertebral disc degeneration based on Pfirrmann grade.

Background Context: Pfirrmann grading can be used to assess intervertebral disc degeneration (IVDD). There is growing evidence that IVDD is not simply a structural disorder but also involves changes to the substructural characteristics of the disc. Whether Pfirrmann grade can accurately represent these micro-nano environmental changes remains unclear.

Purpose: We aimed to assess the micro-nano structural characteristics of the degenerative disc to provide more specific biomechanical information than the Pfirrmann score.

Study Design: A micro- and nano-level structural analysis of degenerative discs of rat tails.

Methods: In this study, 12-week-old adult male Sprague-Dawley rats were divided randomly into five groups: control (no intervention to the intervertebral disc of the tail) and four intervention groups that all had caudal vertebrae immobilized using a custom-made external device to fix four caudal vertebrae (Co7-Co10) but with variable subsequent compression of Co8 and Co9 for 2, 4, 6, or 8 weeks. Magnetic resonance imaging detection of rat coccygeal vertebrae was conducted at each time node of the experiment, and the T2 signal intensity and disc space were evaluated. Animals were euthanized and the caudal vertebrae were harvested for further analysis. Histopathology, glycosaminoglycan (GAG) content, histologic score, end plate structure, and elastic modulus of the intervertebral discs were evaluated.

Results: IVDD was observed at an earlier Pfirrmann grade (Pfirrmann II) under the microscope. With an increase in Pfirrmann grade to III-V, the pore structure of the bony end plate changed significantly and the number of pores decreased gradually. Furthermore, the total GAG content of the nucleus pulposus decreased from an average of 640.33 μg GAG/ng DNA in Pfirrmann grade I to 271.33 μg GAG/ng DNA in Pfirrmann grade V (p < .0001). At the early stage of clinical degeneration of intervertebral discs (Pfirrmann grades II and III), there were significant changes in mechanical properties of the outer annulus fibrosus compared with the inner layer (p < .05). Further, the fibril diameters exhibited significant changes compared with the control group (p < .05).

Conclusions: Our study found that the Pfirrmann grading system combined with intervertebral disc micro-nano structural changes more comprehensively reflected the extent of disc degeneration. These data may help improve our understanding of the pathogenesis and process of clinical disc degeneration.