Special Poster Session 50th International Society for the Study of the Lumbar Spine Annual Meeting 2024

Introduction

Intervertebral disc herniation is a primary cause of spinal surgeries¹ and a significant source of disability. Discs contain two adhering matrices: the interlamellar matrix between lamellae, and the intralamellar between collagen fibres within each lamella. The strength of adhesion within, between, and across lamellae functions to provide structure to the annulus fibrosus.

In a previous study, 1 ng/mL concentration of IL-1β, or cyclic tensile strain (CTS) alone were insufficient to alter mechanical properties of cultured bovine AF rings – while the combination of IL-1β and CTS led to a 40% reduction in annular adhesion strength². Conversely, human annulus cells showed no differences in inflammatory biomarkers between a 10 ng/mL concentration of IL-1β and IL-1β with CTS³; showing that degenerative phenotypic changes can be observed with an inflammatory stimulus alone. However, this study was conducted with isolated cell cultures, and did not evaluate the mechanical changes associated with higher concentration IL-1β. The goal of this investigation was thus to examine the interaction of high-dose IL-1β exposure and CTS of bovine caudal annular rings and determine if a stronger inflammatory environment can induce mechanical changes.

Methods

Twenty bovine caudal discs (Ca2/3 to Ca6/7) were randomized to receive CTS or no loading, with or without 10 ng/mL IL-1β supplemented media (n = 5 per group). CTS tissues were mounted on hooks in a Mechanoculture J1 system (CellScale Biomaterials Testing, Waterloo, Canada) and subjected to 1.5-2.5 mm CTS for 3h at 1 Hz. Media exchanges and cell viability analyses were performed to ensure cell survival (Figure 1). Media collections were conducted for future analysis of inflammatory factors. Novel force-displacement data was sampled throughout the culturing period to examine changes in tensile properties. After 144 hours, 180° peel tests, single layer, and bilayer tensile tests quantified the properties of the inter-and-intralamellar matrices of the annulus.

Results

Single layer mechanics: IL-1β exposure interacted with CTS loading by decreasing intralamellar initial failure stress and maximum stress in CTS samples but not in unloaded samples (p=0.016 and p=0.055 respectively). CTS loading had a main effect of increasing toe region stress (p=0.031).

Bilayer mechanics: IL-1β exposure also interacted with CTS loading where IL-1β increased Young’s Modulus and toe region stress in CTS samples but not unloaded samples (p=0.043 and p=0.007 respectively).

Peel and cyclic loading mechanics: No significant differences in interlamellar peel properties or tensile properties throughout the culturing period were found (p>0.05).

Discussion

Interaction effects present for single-and-bilayer tensile tests indicate that inflammation interacts with loading to alter the adhesive properties of the inter-and-intralamellar matrices. This analysis characterizes tensile properties previously unexamined, and further emphasizes the importance of including biological stimuli to better simulate tissue responses to loading.