Abstract: BACKGROUND: Cervical spondylotic myelopathy is a progressive disease leading to dysfunction in the middle-aged and elderly, and early diagnosis is difficult. In recent years, some clinical scholars have found that dynamic magnetic resonance imaging technology can detect spinal cord compression in a dynamic position earlier, but its specific biomechanical mechanism needs to be clarified. 
OBJECTIVE: To investigate the biomechanical compression characteristics of early cervical spondylotic myelopathy in hyperextension and flexion position, and to verify the effectiveness of dynamic magnetic resonance imaging in the diagnosis of early cervical spondylotic myelopathy. 
METHODS: A retrospective analysis was made on the patients who underwent cervical dynamic magnetic resonance imaging in the Department of Orthopedics of First Affiliated Hospital of Guangxi University of Chinese Medicine from January to June 2022. 16 subjects were selected and divided into two groups. The pathological group included 8 patients with early cervical spondylotic myelopathy with hypertrophy of ligamentum flavum as the main sign, with 5 male patients and 3 female patients. The normal group included 8 normal degenerative people, with 4 male patients and 4 female patients. All patients were photographed with cervical CT plain scan, magnetic resonance imaging plain scan, and dynamic magnetic resonance imaging plain scan. This study was divided into the following three parts: (1) collect the dynamic magnetic resonance imaging image DCOM data of two groups of subjects, and collect the cervical vertebra CT and neutral magnetic resonance imaging image DCOM data to understand the bone and soft tissue of the two groups of subjects in the neutral position. (2) Based on the DCOM data of magnetic resonance imaging and CT plain scan, the three-dimensional finite element models of lower cervical vertebra (C3-7) of normal degenerative population and early cervical spondylotic myelopathy patients were established by reverse engineering software. The equivalent stress and equivalent elastic strain of the spinal cord and posterior dura were analyzed, and the distribution of stress and strain was observed. (3) After obtaining the stress and strain data, the data between groups were compared to analyze the mechanical characteristics of spinal cord compression caused by early cervical spondylotic myelopathy in a dynamic position and to verify the effectiveness of dynamic magnetic resonance imaging in the diagnosis of early cervical spondylotic myelopathy.
RESULTS AND CONCLUSION: (1) When simulating the posterior extension, flexion and neutral position of the lower cervical vertebrae (C3-7) in the two groups, the values of stress and strain in the posterior part of the spinal cord were in the following order: extension > flexion > neutral (P < 0.05). The strain values from large to small were as follows: extension > flexion > neutral (P < 0.05). (2) Compared with the normal degenerative population model, the equivalent stress and strain of the spinal cord in the pathological group were higher than those in the normal group under two degrees of freedom of flexion and extension (P < 0.05). The distribution area of stress and strain in the posterior part of the spinal cord was irregular. (3) In the neutral position, there was no significant difference in the strain value of the spinal cord between the two groups (P > 0.05), and the strain distribution was uniform and regular. (4) It is indicated that in the cervical extension position, the dural sac and the posterior part of the spinal cord were compressed and deformed in the early cervical spondylotic myelopathy patients with the hypertrophy of ligamentum flavum as the main sign, and the degree of compression deformation of the spinal cord was significantly higher than that in the anterior flexion position and neutral position. In the neutral position, there were no obvious signs of spinal cord deformation in patients with early cervical spondylotic myelopathy. This study verified the role of dynamic magnetic resonance imaging in the diagnosis of early cervical spondylotic myelopathy from the point of view of biomechanics.

Key words: cervical spondylotic myelopathy, dynamic magnetic resonance imaging, dynamic position, early diagnosis, biomechanics