Impact on the Intervertebral Foramen After Posterior Cervical Fixation Surgery Using Cervical Pedicle Screws

Purpose We aimed to compare postoperative changes in intervertebral foraminal areas and the mechanisms of foraminal stenosis following fixation surgery for cervical spondylotic myelopathy (CSM) with local kyphosis and instability, using three-dimensional computed tomography (3DCT). Methods We retrospectively analyzed 55 patients who underwent posterior cervical spinal fixation using pedicle screws. A total of 71 spinal levels (C2/3 to C7/T1) and 144 intervertebral foramina with anchor screws inserted in the upper and lower vertebrae were examined. The C2/7 angle and segmental local angle (SA) were measured, and the change in SA (postoperative minus preoperative, defined as the α angle) was calculated. 3DCT was used to measure the craniocaudal (CC) and anteroposterior (AP) diameters and the area of the intervertebral foramina pre- and postoperatively. The percentage change in area was calculated as the difference between postoperative and preoperative areas, divided by the preoperative area. Correlations between the α angle and percentage change in foraminal area were analyzed. Results The primary mechanism of foraminal stenosis was a "lag effect" in which the upper fixed vertebra was pulled backward due to posterior slippage. The foraminal area tended to increase when the α angle was corrected to less than 0°. However, large corrections with an α angle exceeding 10° significantly reduced the foraminal area compared to corrections to less than 0° (p < 0.01). The greatest risk of foraminal narrowing was observed when the α angle was ≥ 10°, resulting in a 16.4% (5.4 mm²) reduction in foraminal area. This phenomenon was also observed at the C4/5 level, where the intervertebral foraminal area decreased by 26.9% (8.6 mm²) and the α angle was approximately 10° in patients with C5 palsy. Conclusion These findings enhance our understanding of foraminal area changes after posterior fusion using cervical pedicle screws and may help prevent postoperative nerve root palsy, particularly C5 palsy.