Distinct stress-relaxation behaviour of the interlamellar matrix and transition zone in the intervertebral disc

The intervertebral disc (IVD) exhibits time-dependent mechanical behaviour that is central to load redistribution and tissue integrity, yet the regional contributions of distinct interfacial tissues remain poorly understood. In this study, we compared the stress-relaxation behaviour of two mechanically critical IVD interfaces, the interlamellar matrix (ILM) and the nucleus pulposus–annulus fibrosus interface (transition zone; TZ), to quantify differences in relaxation magnitude, kinetics, and cumulative mechanical exposure. Isolated ovine ILM and TZ specimens underwent controlled radial stress-relaxation testing under maintained hydration, with responses analysed using model-independent metrics and complementary Peleg and Kohlrausch–Williams–Watts modelling. The ILM exhibited significantly higher initial and apparent equilibrium stresses, prolonged relaxation timescales, and substantially greater cumulative stress exposure, consistent with sustained load retention. In contrast, the TZ showed rapid early stress decay, lower residual stress, and greater relative stress dissipation. Viscoelastic modelling indicated longer characteristic relaxation times in the ILM than the TZ, whereas no statistically significant regional difference in the stretch exponent β was detected within the experimental timeframe.