Fig. 1

Surgical procedures, radiological methods, cutting level of LF specimen, and measurements of thickness. a Removal of L5-L6 paraspinal muscle. b Complete resection of L5 and L6 spinous process, semi-grinding of the bilateral L5/6 facet joints. c A Grinder and round drill bit used to semi-grind the facet joints. d Over extension and flexion position of rats using a retractable holder. e Measurement indicators: ①Disc height ratios (DHR) in flexion and extension were calculated as follows: anterior DHR = 2×A2/(A1 + A3), middle DHR = 2×M2/(M1 + M3), posterior DHR = 2×P2/(P1 + P3). ②Flexion ratio (FR) and extension ratio (ER) were calculated as follows: flexion ratio = A2/P2 in flexion, extension ratio = A2/P2 in extension. A anterior, M middle, P posterior. f The cutting level of LF after decalcification: the L5/6 posterior lumbar spinal was sliced along the cross plane at the midpoint of upper and lower edges of LF. g Measurements of LF thickness and width: blue line “a” represents width, blue line “b” represents thickness