Micro-structure and mechanical properties of annulus fibrous of the L4-5 and L5-S1 intervertebral discs.

BACKGROUND Nowadays, the study on the structure of the annulus fibrous of intervertebral discs centers on the overall and microscopic structure. There are, however, few investigations about the structures and mechanical properties of the annulus fibrous of intervertebral discs in each point and layer. METHODS We observed the tiny anatomical structures and mechanical properties of the adult annulus fibrous of intervertebral discs (L4-5, L5-S1) at the fibrous layer level. Each annulus fibrous of intervertebral disc was delaminated through microsurgical technique. Eight testing points were evenly taken at every layer, and the angles between the fabric direction and the horizontal plane were measured. Meanwhile, five testing specimens were taken from each testing point on every layer along the fabric direction angle, with length (l)=15-20mm, width (b)=1-1.5mm, and thickness (t)=0.1-0.5mm. FINDINGS AND INTERPRETATION Through tension tests, we first measured the stress/strain curves to obtain the fitting curves and equations. Thus, mechanical property parameters such as the elastic moduli, damage strain, and damage stress of the testing points were obtained along with relevant equations. The results are as follows: first, there was no obvious difference between the micro-structures and mechanical properties of the annulus fibrous of intervertebral discs L4-5 and L5-S1. Second, the fiber orientation angle at each measurement point gradually increases with the increase of the fibrous layer from the outside to the inside along the radial direction in the horizontal plane. The minimum fiber orientation angle was 25-30 degrees . The fiber orientation angle at the same layer gradually increases from front to back. Furthermore, the fiber orientation angle was 70-90 degrees right in the middle of the back of the annulus fibrous of the lumbar intervertebral disc. The fiber orientation is basically consistent with the posterior longitudinal ligament going. Through the normalized equation and normalized line, the fiber orientation angle alpha at any point in any layer can be easily obtained. Lastly, the elastic moduli of each testing point decreased with the layers increasing along the radius from the outside to the inside. The damage stress decreased linearly from the outside to the inside.