Region-dependent properties of lamellae constituents: A microscopic insight into intervertebral disc herniation mechanisms

Joint bending is associated with intervertebral disc (IVD) herniations. To better understand herniation disorders, the mechanics of annulus layers have been studied extensively. However, the properties and potential contributions of independent constituents (i.e., collagen fibres and the intra-lamellar matrix) to these larger-scale responses remain poorly understood but this knowledge could uncover molecular insights into herniation pathways. This study characterized the tensile properties of isolated collagen fibres and the adhesion properties of the intra-lamellar matrix in the posterior and anterior IVD regions. IVDs were extracted from eight porcine cervical spines. Single annulus layers were dissected from the anterior and posterior regions. From each layer, two separate samples were harvested: i) isolated collagen fibre and ii) two adjacent collagen fibres together with the matrix that connects them, totalling 32 samples. Once mounted, isolated fibres were longitudinally stretched while double fibre specimens were displaced with respect to each other. All preloaded specimens were strained at 1 % per second of the initial specimen length until failure occurred. From the stress-strain relationships, the Young's modulus along with stress and strain at yield and ultimate failure were determined. Within-specimen differences were evaluated with paired-tests and non-parametric Wilcoxon tests. In isolated collagen fibres, the Young's modulus and ultimate stress were 45 % and 51 % greater in the posterior region compared to the anterior region (p ≤ 0.047). All properties of intra-lamellar matrix adhesion were similar between regions (p ≥ 0.345). Interestingly, these constituents experienced comparable strains at yield (30 %) and failure (40 %), but the tensile strength of isolated fibres was approximately 5 times greater than the intra-lamellar matrix adhesion. This study demonstrated unique mechanical properties of annulus layer constituents. When incorporated into future models, these data could help discern the sequence of molecular damage leading to IVD herniations.