Aim: The current understanding of the relationship between the structure and function of the transition zone (TZ), a region at the interface on the nucleus (NP) and annulus (AF) of the intervertebral disc (IVD, is little. The current study aims to understand the viscoelastic and failure mechanical properties of the TZ region under physiological loading and to present a structural model for the integration between the NP and AF at the fibre level.
Method: 36 NP-AF blocks of tissue (t = 1mm) were harvested from the anterior region of ovine IVDs. Samples from the NP, TZ and IAF (inner AF) were subjected to cyclic dynamic loading up to 70% of their initial length (n = 24) at three different strain rates (1, 3, and 5%s-1) and another 8 TZ samples were subjected to tensile failure tests. Tissue blocks (n = 4) were partially digested to identify the structural organization of elastic and collagen fibres at the NP, TZ, and IAF including the integration mechanisms between the NP and AF.
Results and conclusion: We observed different viscoelastic behaviours for different regions of interest and various strain rates. A Friedman test revealed a significant effect of region (NP, TZ, and IAF) and strain rates on stiffness, χ2 (11) = 80.694, p < 0.001, W = 0.917. The relevant mean rank was the highest for the IAF, followed by TZ and NP regions with a growing trend as strain rates increased from 1 to 5%s-1. From the failure mechanical test, we found that the overall effect of strain magnitude on TZ stiffness was significant (p < 0.001; one-way ANOVA). Our structural analysis revealed different organizations for both collagen and elastic network from the NP towards the AF and identified three mechanisms of adaptation, direct penetration and entanglement between the TZ and IAF fibres at the NP-AF interface.