Laminectomy is a surgical procedure with more clinical applications. By removing the posterior laminae, the volume of the spinal canal was effectively enlarged and the spinal cord moves backwards under the action of the tension band, thus avoiding the lasting damage caused by the compressions. It is suitable for patients with multi-segment cervical disc herniation, continuous ossification of posterior longitudinal ligament, severe spinal canal encroachment and clamp type of compression [1, 3–6, 8]. Some literatures had reported that for the patients underwent laminectomy without fixation, the cervical curvature will have a lost of 4o in the short-term follow-up (1 year after operation) , and 21% (9/42) of patients developed postlaminectomy deformity in median follow-up (4 years after operation) . After intraoperative fixation with lateral mass screws, the trend of progressive cervical curvature loss and high frequency of kyphosis deformities were effectively controlled, but the cervical curvature became smaller in some patients still existence [3, 5]. From the analysis on anatomy and biomechanics, both open-door laminoplasty and laminectomy will destroy the spinous process, vertebral plate and posterior ligaments complex to varying degrees, resulting in loss of effective attachment points of the cervical posterior muscle group, weakening of the muscle tension band, loss or straightenness of curvature due to the failure to maintain the original cervical curvature [4, 6, 9, 11–12]. It has been reported that maintaining a good physiological curvature can make cervical spine more elastic, relieve shock and buffer stress during movement, and play a role in protecting the spinal cord .
Through imaging measurement and clinical data analysis, borden et al . found that the normal cervical curvature should be in the state of lordosis, and the cervical lordosis depth ranged from 7 mm to17mm. When the cervical lordosis depth is less than 7 mm, it means the cervical curvature decreases, and when the numerical value is greater than 17 mm, it means the cervical curvature increases. According to the above theory, we conducted a retrospective analysis of 78 CSM patients with normal cervical curvature treated with LCSF, and found that no patients had increased cervical curvature. Forty-two patients with reduced cervical curvature (group A) and 36 patients with normal cervical curvature (group A), the cervical lordosis depth was (5.1 ± 1.2) mm and (12.3 ± 2.4) mm, respectively. According to the statistical analysis, the patients with reduced postoperative cervical curvature accounted for 53.8% among all the included patients. What caused the result that more than half of the patients developed abnormal cervical curvature after operation? We consider the poor positioning of cervical vertebra during the operation as one of the key factors. In the prone position, the trunk is higher than the head and the cervical vertebra is in a forward flexion state. If the cervical lordosis is not restored by elevating the head before nut locking, the reduced cervical curvature will appear after operation. Secondly, the intraoerative curvature of the titanium rod is one of the key factors. By increasing the curvature of the titanium rod, the poor preoperative curvature of patients can be corrected [4, 13]. The normal cervical lordosis can be affected by the undersized curvature of the titanium rod as well. Thirdly, the destruction of cervical osseous structure, the loss of effective attachment points of extensor groups and the weakening of muscle tension are also key factors inducing the reduced postoperative cervical curvature [14–15].
Therefore, will the neurological function recovery and the occurrence of adverse events affected by the reduced postoperative cervical curvature? In this study, the widths of laminectomy in both groups were generally the same (21.5 mm vs 21.9 mm). However, there was significant difference in the postoperative drift distance, which was (1.9 ± 0.4) mm in group A and (2.6 ± 0.7) mm in group B. The post-operative neurological function of both groups recovered significantly and the recovery rate of both groups showed no significant statistical difference (61.5% vs 62.7%). Although it had been reported that the significant loss of postoperative cervical curvature or kyphosis was a risk factor for poor prognosis [3–4], we believe that the recovery effect of nerve function is mainly related to the sufficient or insufficient decompression of the spinal cord, and the reduced postoperative cervical curvature will not affect the recovery of nerve function. The transverse diameters of the cervical spinal cord differed significantly in different segments. Even at the widest C5 segment, the transverse diameter of the spinal cord was only 13 mm . Theoretically, during laminectomy, the laminectomy width is only required to be greater than the transverse diameter of the corresponding segment of the spinal cord. Zhao et al . found that the postoperative neurological function of the patients recovered significantly by setting the laminectomy width as 16.7 mm. Therefore, in this study, the laminectomy width of more than 21 mm could completely decompress the spinal cord. In addition, we believe that the spinal cord drift distance is related to cervical curvature, agreeing with the opinion of Baba et al . After laminectomy, the effective space of the spinal canal will be released, and the dural sac will drift backward under the action of the “bowstring principle” while expanding. The drift distance is an imaging manifestation of the tensile stress generated by the cervical curvature, rather than the evidence of the sufficient or insufficient decompression of the spinal cord.
We are still unknown about the specific mechanism of the formation of AS and C5 palsy. Some literatures had reported that the destruction of muscle-ligament complex and articular capsule, the atrophy of posterior cervical muscles and the change of cervical curvature and cervical instability are associated with the occurrence of AS [3–4, 6, 9–10, 12]. However, C5 palsy is associated with multiple mechanisms such as ischemia and hypoxia, segmental spinal cord disorders, and embolism caused by increased nerve root tension [5, 13, 18–19]. Zhao et al . and Du et al . found that the incidence of postoperative C5 palsy significantly decreased by reducing the nerve root tension with nerve root canal decompression. Lau et al . believed that the increase of postoperative cervical lordosis was negatively correlated with the severity of axial pain, especially for lordosis greater than 20°. While the increased cervical lordosis would promote the backward drift of the spinal cord, and further increase the tension of C5 nerves, leading to the occurrence of C5 palsy . For this contradiction, since the balance point of cervical curvature is difficult to select, it is difficult for spine surgeons to balance the occurrence of AS and C5 palsy by controlling the cervical curvature. In this study, the postoperative severity of AS in group A with reduced cervical curvature was significantly higher than that in group B with normal curvature. Although the incidence of C5 palsy in group B was higher than that in group A (11.1% and 7.1%, respectively), there was no statistical difference between both groups. Therefore, we can make a conclusion that, as long as the postoperative cervical curvature is maintained as the normal level as that before the operation, it will form a new balance between reducing AS and avoiding the occurrence of C5 palsy, neither increasing the severity of AS nor inducing the occurrence of C5 palsy.