Abstract: BACKGROUND: Lumbar disc herniation is the main clinical cause of low back pain. The pathogenesis of lumbar disc herniation is very complex, and the accumulation of long-term mechanical loads and sudden overload injuries are usually considered as the main etiological factors. Most herniated discs are affected by alterations in their mechanical state. Therefore, the stress/strain characteristics of the lumbar disc as a whole and internally need to be studied to suggest guidelines for the prevention and treatment of lumbar disc herniation.
OBJECTIVE: To evaluate the fatigue characteristics and rupture mechanism of the intervertebral disc from multiple angles by in vitro experiments.
METHODS: Lumbar spines of fresh sheep slaughtered within 4-6 hours were used in the experiment, and the intervertebral disc samples made up of them were used in different experiments. In the fatigue experiments, the fatigue characteristics of mild rupture and healthy intervertebral disc were explored. In the internal displacement and load distribution experiment, the optimized digital image correlation technique is used to test the internal displacement distribution of the intervertebral disc during the loading process. Fiber Bragg grating technology was used to measure the internal load distribution of the intervertebral disc during the loading process.
RESULTS AND CONCLUSION: (1) For normal and early ruptured lumbar discs, before and after fatigue, all rupture stress-strain curves showed multi-stage characteristics consisting of four main stages: toe stage, linear stage, yielding stage and damage stage. (2) The toe region of normal lumbar discs was very obvious, while the toe region of early ruptured lumbar discs almost disappeared. For both normal and early ruptured lumbar discs, the post-fatigue elastic modulus was slightly greater than the pre-fatigue elastic modulus. For normal lumbar discs, the proportional limit after fatigue was greater than that before fatigue. For early ruptured lumbar discs, the proportional limit after fatigue was less than that before fatigue. For normal lumbar discs, the yield stress after fatigue was greater than before fatigue. For early ruptured lumbar discs, the yield stress after fatigue was less than before fatigue. (3) Ventral annulus fibrosus stress was less than dorsal annulus fibrosus stress. On the dorsal side, the outer annulus fibrosus stress was less than the inner annulus fibrosus stress; while the ventral side showed the opposite pattern. (4) The introduction of damage factors to establish the damage constitutive model can fit the mechanical properties of disc injury well. The experimental results have the theoretical guidance for the prevention of lumbar disc herniation in daily life. 

Key words: lumbar disc rupture, constitutive model, stress, strain, fatigue injury