Annulus fibrosus micro-damage in Mild intervertebral disc degeneration

Abstract

BACKGROUND CONTEXT: Degeneration of the intervertebral discs (IVDs) causes significant biomechanical changes and structural defects, often leading to pain or disability. These defects impair both the biomechanics and physiology of the IVD, contributing to further structural failure of the IVD. Although the function and structure of the annulus fibrosus (AF) affect overall disc function, its structural change during degeneration is not fully characterized. PURPOSE: The objective of this study was to investigate AF microdamage and mechanical function in mildly degenerated discs. STUDY DESIGN: Basic laboratory study using human cadaveric intervertebral discs. METHODS: Microdamage and its functional implications in the AF of human IVDs with mild degeneration were assessed through a comprehensive approach focusing on biomechanical, and structural analyses. Eighteen fresh-frozen human cadaveric discs (T10-L5, n=3 spines, 43-58 years old) were analyzed from anterior, posterior, and lateral regions of the inner and outer AF. Collagen microdamage was assessed using Collagen Hybridizing Peptide (CHP), and Second Harmonic Generation (SHG) imaging to provide high-resolution insights into collagen fiber structure. Mechanical testing was performed on the same regions. RESULTS: Damage analyses showed no significant differences between the degenerative grades except in the posterior outer AF, where Grade 3 discs showed higher CHP and SHG signal intensity compared to Grade 2. Plus, mechanical properties did not differ significantly between the grades, although large variances were observed, particularly in the posterior outer AF. CONCLUSIONS: Despite limited mechanical differences, early structural damage was found in the posterior outer AF. This indicates that this part is the initial site of AF damage and biological response during IVD degeneration. CLINICAL SIGNIFICANCE: These findings highlight the posterior outer AF as a potential target for early diagnostic and therapeutic interventions to mitigate further degeneration and preserve disc function.