The ACDF technique is a standard procedure with good clinical effect for treating patients presenting with CSM. Ylinen et al. reported that although more than half (57%) of patients recover well after surgery, many patients still suffer from chronic neck pain with reduced neck mobility . Although ACDF may reduce the incidence of postoperative axial neck pain by preserving the posterior muscles, Matsumoto et al. reported that axial neck pain was observed in 25.8% of patients after ACDF during follow-up . The exact mechanism of postoperative axial neck pain has not been fully elucidated. Many risk factors can contribute to postoperative axial neck, such as muscle spasm, diseases of the joints, intervertebral discs and ligaments, and vertebral instability. Facet joints are implicated as the origin of a good percentage of cervical pain. Previous studies support that over-stretch of cervical facet-joint capsules may lead to cervical pain through releasing inflammatory cytokines [19,20]. Moreover, some studies believed that excessive distraction of intervertebral space by insertion of too large cage causes postoperative neck pain due to distraction of the facet joint or spasm of the posterior neck muscles[21,22]. However, there have been no studies regarding the cage placement with or without releasing the Caspar cervical retractor after decompression is associated with postoperative axial neck pain. In the present study, in none-spread group, due to the release of Caspar cervical retractor after decompression, the height of intervertebral space was restored to its “natural height”, cervical physiological lordosis was restored, and the height and cross-sectional area of intervertebral foramen were moderately increased. An increase in intervertebral foramen cross-sectional area indicates the increased outlet space of nerve root, which may improve blood supply and reduce compression of damaged nerves. Barley et al. found that the height of intervertebral space finally stabilized, slightly higher than the preoperative height, indicating that the ideal height of cage may be associated with preoperative baseline parameters. Excessive distraction of Caspar cervical retractor can result in damage to joint capsule and ligament, dislocation or subluxation of the joint, mechanical irritation or damage to the intervertebral ligaments, and ultimately axial pain. If the cage is too high without releasing the Caspar cervical retractor, the pressure between the upper and lower vertebral plates will increase, which may lead to osteonecrosis, disc collapse, subsidence, and herniation, as a result of higher incidence of axial pain.
According to some previous studies, the incidence of subsidence is about 13.2%-62.5% [25-28]. Although surgical outcomes depend primarily on adequate decompression of the spinal cord and nerve roots, postoperative radiologic changes do not directly affect the quality of life of the patients . However, the loss of height of intervertebral space leads to the loss of segmental cervical curvature, affecting the cervical alignment and is thus associated with the occurrence of adjacent segmental disease. The aggravation of neck pain caused by subsidence is a major cause of poor clinical outcomes. Subsidence can result in reduced fusion rates, kyphotic deformity of the cervical spine, and reduced height of intervertebral space, eventually leading to radiculopathy [26-28]. There were similar changes in cervical curvature and height of intervertebral space in two groups. It has been reported that postoperative cervical lordosis and segmental lordosis both affect long-term clinical outcomes .Although there was no statistical difference in JOA score between the two groups in our study, it may be related to the short follow-up time. Wu et al. reported that cage subsidence and cervical lordosis improvement does affect the long-term clinical outcomes. In our study, changes in cage subsidence and kyphosis were higher in the spread group than in the none-spread group. Moreover, although the curvature and height of intervertebral space could be better restored after the operation without releasing the Caspar cervical retractor, JOA score in the last follow-up was not affected, but VAS score was worse. This indicates that the recovery of nerve function in the short term mainly depends on the decompression of spinal cord and nerve root, and the severity of preoperative nerve injury. The height of postoperative intervertebral space is not significantly correlated with the recovery of nerve function, which is consistent with the results reported in previous literatures [31,32]. Besides, the greater the cage height, the greater the risk of cage subsidence. Cervical curvature is associated with degenerative changes, and studies have shown that with the improvement of cervical lordosis, height of intervertebral space increases.The disc degeneration scores of the two groups are significantly different, which may be the result of a combination of natural degeneration and loss of curvature. Therefore, future studies on postoperative adjacent segment degeneration in the two groups are of more important clinical significance.
Although the cervical curvature and height of intervertebral space were better recovered immediately after the operation in spread group, it was found that the height of intervertebral space decreased too fast and finally stabilized during the follow-up, and there was no significant difference between the two groups. Subsidence is a common phenomenon and mild subsidence helps maintain internal stability. However, large subsidence must be avoided, which often results in postoperative cervical foramen stenosis and displacement. Whether subsidence affects clinical outcomes is still debatable [34,35]. Parks et al. reported that subsidence was associated with short intervertebral foramen and poor cervical lordosis.In contrast, Lee and colleagues conclude that cage subsidence does not, but segmental cervical kyphosis does affect the long-term results. Truumees et al. reported that the higher the height of the cage, the higher the distractive forces and compression forces would be, leading to the greater the risk of subsidence and displacement. In our cases, many subsidence occurred in the first month after surgery, and if when the placement of cage without releasing Caspar cervical retractor, axial pain occurs within a few days after the operation, and it was more likely to cause cage subsidence during long-term follow-up. In addition, the cage subsidence is often accompanied by the loss of local curvature of the cervical spine.Park et al. believed that cage subsidence could lead to aggravation of local cervical curvature, but ultimately did not affect C2-C7 Cobb angle. In this study, the C2-C7 Cobb angle showed a decreasing trend, indicating that the curvature of the cervical spine was straightened (two groups 7.29 ± 8.66 degrees VS 6.24 ± 6.63 degrees). However, subsidence does not necessarily affect clinical JOA scores. Similarly, in our study, no significant difference was observed in JOA in the last follow-up between the two groups, but there were significant differences in VAS scores after surgery and during follow-up, which may be due to large cage and cage subsidence.