Biomechanical Changes in Kyphotic Cervical Spine After Anterior Cervical Discectomy and Fusion with Different Degrees of Correction.

Cervical kyphosis is a debilitating disease, and its surgical treatment involves correction to restore sagittal alignment. Few studies have explored the appropriate degree of correction, and the biomechanical impact of correction on the cervical spine is still unclear. This study aimed to compare the biomechanical changes in the cervical spine after different degrees of correction by two-level anterior cervical discectomy and fusion (ACDF). Three-dimensional finite element (FE) models of the intact cervical spine (C2-C7) with normal physiological lordosis and kyphosis were constructed. Based on the kyphotic model, three two-level ACDF in C4-6 surgical models were developed: (1) non-correction: only the intervertebral heights were restored; (2) partial correction: the cervical curvature was adjusted to straighten; (3) complete correction: the cervical curvature was adjusted to physiological lordosis. A pure moment of 1.0 Nm combined with a follower load of 73.6 N was applied to the C2 vertebra to simulate flexion, extension, lateral bending, and axial rotation. The stress of vertical bodies and facet joints, intradiscal pressure (IDP), and the overall ROMs of all models were computed. The peak von Mises stress on the upper (C4) and lower (C6) instrumented vertebral bodies in the kyphotic model was greater than that of the physiological lordosis model, with the exception of C6 under lateral bending. The maximum stress was observed in C4 during lateral bending after complete correction, which increased by 145% compared to preoperative von Mises stress. For the middle (C5) instrumented vertebral body, the peak von Mises stress increased after surgery. The maximum stress was observed in partial correction during flexion. Compared to physiological lordosis, the peak von Mises stress on the facet joints in kyphotic segments was lower; however, it was higher in the adjacent segments, except C4/5 in extension. The stress on the facet joints in kyphotic segments decreased, with the most significant decrease observed in partial correction. The IDPs in adjacent segments, except for C6/7 in flexion, showed no significant difference before and after surgery. Additionally, correction seemed to have little impact on IDPs in adjacent segments. In conclusion, for the treatment of cervical kyphosis with two-level ACDF, complete correction resulted in the highest peak von Mises stress on the upper instrumented vertebral body. Partial correction mitigated von Mises stress within the facet joints in kyphotic segments, albeit at the expense of high von Mises stress on the middle instrumented vertebral body.