Vertical load distribution between the support structures and regional annulus fibrosus stress patterns in the cervical spine during flexion and extension: a finite element study

Background. Degenerative cervical spine disorders are one of leading causes of chronic pain and neurological deficit. Altered load distribution between the anterior and posterior support structures is considered an important biomechanical factor in their development. Quantifying load distribution in the intact spine is necessary to study degeneration and plan interventions with minimal impact on adjacent segments. The purpose was to assess load distribution between the discs, joints, and ligaments from C2-C3 to C6-C7 during flexion and extension, and to compare intradiscal pressure with the stress state of annulus fibrosus sectors. Materials and methods. A finite-element C0-Th1 cervical spine model was built from computed tomography data. A vertical 50 N force representing head weight was applied to the occiput while motion from +54.9° flexion to –44.5° extension was prescribed (176 states). Load distribution was determined using a free-body diagram, and the annulus fibrosus was analyzed by sectors. Results. In the loaded neutral position, the discs transmitted from 39 % of the external load at C4-C5 to 77 % at C6-C7, whereas the facet joints transmitted 28–57 %. The facet path dominated at C4-C5, while disc transmission dominated at C2-C3 throughout the range. Intradiscal pressure was 0.147–0.294 MPa. During flexion, the facets disengaged, and tension in the posterior ligamentous complex produced an oppo­sing reaction; consequently, the vertical component of disc-transmitted force reached approximately twice the external load: the pressure rose to 0.940 MPa at C5-C6. During extension, the facets carried most of the load at the mid- and lower-cervical levels. Peak contact pressure increased three- to fivefold and reached 10.62 MPa at C3-C4. The horizontal contact-force component was –124.2 N, directed posteriorly, and reflected substantial shear. At C5-C6, relatively low intradiscal pressure coincided with an increase in equivalent stress to 8.2 MPa and tensile maximum principal stress in the anterior annulus sector. The global model response was consistent with published data: C2-C7 range of motion was 59.1°, and the maximum reaction moment was 2.42 N · m. Conclusions. Load distribution between the support structures depends on head position and segmental level. These data may provide a reference for assessing the effects of degenerative changes and surgeries. Intradiscal pressure does not fully characterize the state of peripheral annulus and should be supplemented by equivalent and principal stress analysis.