To investigate the feasibility of establishing a canine model of lumbar intervertebral disc degeneration through the application of cumulative axial load and a six-phase combined motion on the vertical sitting dog's lumbar spine.

Twenty adult female grass dogs, each weighing 10.0±0.5 kg, were randomly divided into two groups, with 10 dogs in each group. In the model group, dogs were secured to an exercise machine in a vertical position, and six phases of lumbar spine movement (flexion and extension, left and right lateral flexion, left and right rotation, 45° each) were combined with a specific number of cycles under continuous axial load (245 N). In the control group, dogs were secured to the exercise machine in a vertical position without any intervention. Radiographic examinations were performed before and after 20,000, 50,000, 100,000, and 150,000 compound exercises in the model group. The disc height index (DHI) was measured through lateral X-ray, and MRI T2-mapping was used for quantitative analysis of intervertebral disc degeneration. When intervertebral disc degeneration was evident on MRI T2-weighted imaging (modified Pfirrmann system > Grade V), the combined motion was halted. Micro-CT quantitative analysis of bone mineral density (BMD) in the upper and lower endplates, trabecular bone structure, and histological staining (HE staining, "O" staining, Sirius red staining) were employed to verify and assess the degree of intervertebral disc degeneration.

After 50,000 compound exercises, mild degeneration of the intervertebral discs at L_6-7 and L₇S₁ was observed on T2-weighted imaging. With the accumulation of exercise load, the degree of degeneration progressively increased, reaching a moderate degree of degeneration after 100,000 composite exercises, and DHI began to decrease. Mild degeneration was also observed in the upper L_5-6 intervertebral disc. When the cumulative exercise volume reached 150,000 repetitions, the height of intervertebral spaces in the L_5-6, L_6-7, and L₇S₁ segments further decreased, and the intervertebral discs exhibited severe degeneration (improved Pfirrmann grading system Grades IV-VI). The upper L_4-5 intervertebral discs also displayed mild degeneration. Histological scores were as follows: L_5-6 (8.2±0.8), L_6-7 (9.5±0.7), and L₇S₁ (10.3±0.5), indicating a degree of degeneration in the order of L_5-6<L_6-7<L₇S₁. HE and safranine "O" staining confirmed the significant collapse of the intervertebral spaces in the L_5-6, L_6-7, and L₇S₁ segments, characterized by severe shrinkage of the nucleus pulposus tissue, a disordered internal structure, and nearly absent vacuolar-like nucleus pulposus cells. Sirius red staining revealed pronounced folds, disordered arrangement, and multiple fractures in the fibers of the anterior and posterior rings of the intervertebral disc. The posterior ring of the disc exhibited more pronounced changes than the anterior ring, and the thickness of the bone endplate and bone trabecular density became thinner and less dense. Micro-CT quantitative analysis further confirmed that the BMD and number of trabeculae in the upper and lower endplates of the L_5-6, L_6-7, and L₇S₁ segments were significantly lower than those in the control group and other segments of the model group, while the trabecular separation was significantly higher than that in the control group and other segments of the model group.

The utilization of the "Lumbar Composite Exercise Machine" can effectively replicate the biomechanical and kinematic characteristics of human lumbar intervertebral discs. Cumulative axial load and six-phase composite exercise can induce varying degrees of chronic degeneration in canine lumbar intervertebral discs, which is related to the exercise load, particularly in the L_5-6, L_6-7, and L₇S₁ segments.

Intervertebral disc degeneration; Biomechanical phenomena; Models, animal; noninvasive