The "horizon grey band" represents normal nucleus pulposus cells condense rather than intervertebral disc degeneration signal.

Abstract

Background: The Pfirrmann classification is a commonly utilized system for assessing lumbar disc degeneration, primarily concentrating on the hydration of the nucleus pulposus (NP). However, it overlooks the significance of annulus fibrosus (AF) tears. This study aims to challenge the conventional perspective by reinterpreting the MRI-detected "horizon grey band" as a normal anatomical characteristic of condensed NP cells instead of a sign of degeneration. To fill this gap and improve clinical applicability, we propose a new grading system that emphasizes the integrity of the AF.

Methods and materials: We carefully assessed IVD structures in different species, including human, with MRI examination and histological analysis. The "horizon grey band" and surrounding "high signal" area were isolated from goat and evaluated by proteomic analysis to identify their composition respectively. We examined the volume of each compartment of intervertebral discs (IVDs) with PACS software in 224 Chinese patients (144 healthy individuals and 80 with IVDD) using T2 MRI and factors influencing IVD volume were analyzed. A new grading system for lumbar disc degeneration, incorporating AF tears, was developed. The reliability of this grading system was tested on MRI scans of 500 lumbar intervertebral discs from 100 low back pain patients, with three independent observers. Intra-and interobserver reliabilities were assessed using kappa statistics. The clinical symptoms and prognosis of 100 patients with this grading system were analyzed through 1 year's follow-up.

Results: The result of T2 mapping MRI that each NP consists of central "horizon grey band" and surrounding "high signal" area. The histology and proteomic revealed the tissues from "horizon band" area represent NP cell and "high signal" represents water-rich NP cell ECM tissues. The NP cell volumes across various lumbar segments (L1-L2 to L4-L5) were 2488.2 ± 1114.09 mm3, 3238.44 ± 1327.15 mm3, 3638.34 ± 1439.94 mm3, 3752.83 ± 1514.61 mm3, and 2997.02 ± 1319.43 mm3, respectively. The ratio of NP cell volume to total IVD volume was consistent across segments (0.167 to 0.184), with no significant variation. Further analysis found that NP cell volume, NP volume (NP cell and surrounding water-rich ECM), and IVD volume were correlated with segmental position, as well as with height, weight, and age. Using the new grading system, we identified 62 Grade I discs (62%), 6 Grade IIA discs (6%), 5 Grade IIB discs (5%), 18 Grade III discs (18%), and 9 Grade IV discs (9%) when examined the patients with low back pain. Intra-observer and inter-observer agreements were substantial to excellent, with complete agreement in 91.8% to 93.2% of the 500 discs. The new grading system are closely related with patient' symptom and prognosis.

Conclusion: T2-weighted imaging identified the "horizon grey band" as normal NP cell condense, not degeneration signal. Our findings redefine the anatomy of the intervertebral disc, statistically analyse its component volumes, and introduce a classification system that can be used for surgical purposes. This study is in line with recent translational innovations in spine care, effectively bridging the gap between MRI biomarkers and clinical practice.