Abstract: BACKGROUND: Cervical muscles play an important role in maintaining the mechanical balance of the vertebral body and the pathogenesis of cervical spondylosis at each stage and link. They are also the leading cause of various clinical cervical symptoms, and mechanical imbalance of the cervical spine and degeneration of the cervical discs can be regarded as the results of soft tissue lesions dominated by cervical muscles.
OBJECTIVE: To observe the pathological changes of endplate chondrocytes in cervical intervertebral discs in a rabbit model of dynamic disequilibrium due to anterior cervical muscle spasm and to study the rules of apoptosis in intervertebral disc cells under abnormal stress.
METHODS: Twenty-eight healthy adult New Zealand rabbits were divided randomly into model group and sham operation group (n=14 per group). The bilateral sternocleidomastoid muscles of the rabbits in the model group were shortened by a medical pipe approximately 1 cm below the outer and lower thyroid cartilage to establish an animal model of cervical dynamic disequilibrium. In the sham operation group, bilateral sternocleidomastoid muscles were only exposed followed by layer-by-layer suture. Endplate cartilage tissue of the cervical intervertebral discs was simultaneously taken from each group at 2 months after modeling, and morphological changes were observed using hematoxylin-eosin staining. The migration of endplate chondrocytes in the cervical intervertebral discs were observed under light microscope. TUNEL method was used to detect the apoptosis of cells in the endplate cartilage and annulus fibrosus. The study protocol was approved by the Animal Experimental Ethics Committee of the Third Affiliated Hospital of Beijing University of Chinese Medicine. 
RESULTS AND CONCLUSION: Obvious pathological changes occurred in the cervical intervertebral disc in the model group, but not in the sham operation group. There was a clear boundary between the articular cartilage area and annulus fibrosus area of the endplate in the model group. No obvious migration was found in the model group, but the cells in the growth cartilage area of the endplate obviously migrated into the articular cartilage area. The model group showed excessive apoptosis in the articular cartilage area and growth cartilage area, and the apoptotic index was significantly higher than that of the sham operation group (P < 0.01). Apoptosis was also found in the annulus fibrosus area, and the apoptotic index was extremely significantly higher than that in the sham operation group (P < 0.05). To conclude, when a dynamic disequilibrium is caused by anterior cervical muscle spasm (abnormal stress caused by cervical muscular spasm), no obvious migration of cells between endplate cartilage and annulus fibrosus occurs in the short term. However, the cells in the growth cartilage area can migrate into the articular cartilage area at an early stage, and significant apoptosis occurs in the endplate cartilage and annulus fibrosus area of the intervertebral disc, which can provide experimental evidence for studying the etiological mechanism of early degeneration of the intervertebral disc.

Key words: muscle spasm, intervertebral disc degeneration, cervical intervertebral disc, endplate, chondrocyte, apoptosis, rabbit