Impact of EODL extended to C1 and C2 on sagittal parameters of the upper cervical spine
Over the past few years, sagittal parameters of the cervical spine have received increasing attention, especially with regard for evaluating surgical efficacy and quality of life. Among these parameters, sagittal imbalance is considered the leading cause of poor prognosis and functional loss in patients. Among sagittal parameters, the C0–2 Cobb angle is usually used to evaluate the curvature of the upper cervical spine. Ota et al. [6] found that changes in the C0–2 Cobb angle have a strong linear correlation with changes in the narrowest oropharyngeal airway space (nPAS). In addition, some studies [7,8,9] have reported that postoperative dyspnoea and/or dysphagia are closely related to a decrease in the C0–2 Cobb angle.
The C1–2 Cobb angle is regarded as a pivotal factor for determining the curvature of the lower cervical spine since excessive C1–2 protrusion can potentially give rise to sagittal kyphosis of the lower cervical spine after surgery [10]. During the follow-up of 119 patients with atlantoaxial instability caused by the separation of the odontoid process, Yudoyono and his team [11] discovered a linear correlation between changes in C1–2 lordosis and C2–7 kyphosis both before and after the operation. Furthermore, Guo and colleagues [12] found that the C1-C2 Cobb angle is negatively correlated with the C2-C7 Cobb angle and that the former index is the crucial factor that affects the curvature of the lower cervical spine in operations involving atlantoaxial joint fixation. He also believed that the optimum C1–2 Cobb angle should be between 25° and 30°. Similarly, Wang et al. [13] confirmed that when the sequence of the upper cervical spine changes, the lower cervical spine will compensate for excessive lordosis or kyphosis to maintain body balance and horizontal vision. We believe an increase in the C1–2 Cobb angle may further accelerate the degeneration of the anterior longitudinal ligament, the intervertebral disc and facet joints, and these changes may, in turn, lead to the sagittal imbalance of the cervical spine.
In this study, none of the patients developed postoperative dyspnoea and/or dysphagia. We observed an increase in the C0–2 Cobb and C1–2 Cobb angles, and there was a positive correlation between the C0–2 Cobb angle and C2–7 SVA (Pearson = 0.287, P = 0.004). We propose that this may be due to a compensatory mechanism by which the body maintains visual or body balance. However, there were slight, non-significant increases in the C0–2 Cobb (P = 0.190) and C1–2 Cobb (P = 0.081) angles, indicating that extending EODL to C1 and C2 did not damage the stability of the upper cervical spine.
The impact of extending EODL to C1 and C2 on sagittal parameters of the lower cervical spine
The C2–7 Cobb angle, C2–7 SVA, and T1-Slope are important parameters that are used to measure the sagittal balance of the cervical spine. In this study, the C2–7 Cobb angle significantly decreased, while the C2–7 SVA and T1-Slope obviously increased postoperatively, indicating a trend toward forward inclination of the cervical spine. Ames et al. [14] found that all spine segments interact with each other. Consequently, a series of changes in the sagittal sequence of the spine can be regarded as a compensatory mechanism to adjust the sagittal balance of the spine. Previous studies have recommended C2–7 SVA as one of the most important parameters for predicting the outcome of surgery. Tang and his team [15] reported that C2–7 SVA was positively correlated with the NDI index and negatively correlated with SF-36 scores in 113 patients who underwent open-door surgery of the cervical spine. The researchers suggested that the increase in SVA observed in patients with an abnormal sagittal sequence in the cervical spine is one of the causes of their poor scores for health-related quality of life. Tang assumed that when the C2–7 SVA exceeds a threshold of 40 mm, quality of life will markedly decline. Furthermore, Mohanty et al. [16] suggested that the cervical spine is in the anteversion position when C2–7 SVA is increase and that a decrease in spinal canal volume and the available space in the spinal cord on MRI might be the cause of poor outcomes after the operation. In our study, although the C2–7 SVA increased to an average of 22.86 ± 6.49 mm after cervical surgery, this was much lower than the threshold of 40 mm proposed by Tang et al. Although an anteversion trend was observed after cervical surgery, no decrease in clinical efficacy was found during follow-up (Fig. 3).
As a predictor of the sagittal balance of the cervical spine, the T1-Slope is strongly correlated with cervical curvature. Knott [17] suggested that there might be a positive and negative sagittal imbalance when the T1-Slope is greater than 25° or lower than 13° on a lateral X-ray of the cervical spine, and the sagittal sequence should be comprehensively evaluated in these patients. Our data indicate that there is a significant positive correlation between the T1-Slope and C2–7 SVA, consistent with the report by Knott and his team. In addition, our experimental results showed that T1-Slope can be used as an important indicator for evaluating sagittal balance, predicting clinical effects after surgery and directing orthopaedic programs.
Causes of imbalance after cervical surgery
Some studies have reported on the treatment of upper cervical spinal stenosis. Matsuzaki et al. [18] used dome-like expansive laminoplasty for the second cervical vertebra. Kim [19] introduced a new surgical technique for C1–C2 fusion combined with C1 double-door laminoplasty to treat patients with C1–C2 instability, canal stenosis, and cervical spondylotic myelopathy. Zhang et al. [20] reported on the results of tension-band laminoplasty (TBL) performed with/without simultaneous C1 laminectomy and found that performing laminoplasty with simultaneous C1 laminectomy results in a greater posterior spinal cord shift than was observed for laminoplasty alone. In addition, Kong reported that 17 patients who underwent C3–7 or C2–7 open-door laminoplasty without alleviation or aggravation of spinal cord injury symptoms underwent reoperation with decompression upward to the C1 level. Due to the extended range of decompression, the spinal cord was fully decompressed, achieving satisfactory clinical effects. However, loss of cervical curvature occurred in 17 patients, 2 of whom had cervical lordosis that straightened and 1 straight case that developed kyphosis [21].
The spine is the central axis that supports the skull and trunk, and its stability and mobility are its major functional characteristics. According to the three-column theory of the cervical spine proposed by Louis [22], the anterior column, which is composed of the centrum and intervertebral disc, accounts for 36% of the load distribution generated by head weight, while the two posterior columns, which are composed of facet joints, account for a combined 64% of the load. Nolan [23] and Sherkl [24] stated that the cervical ligament complex, which is composed of the spinous process, interspinous ligament and supraspinal ligament and attached muscles, is the predominant factor that maintains the static stability of the cervical spine.
As the EODL requires the dissection of the bilateral paravertebral muscles and the cutting of the extensive posterior ligament structure, the resulting invasion will accelerate muscle fatigue, which could potentially bring about sagittal imbalance of the cervical spine, instability of the cervical vertebra and a decline in quality of life [25]. Accordingly, in this experiment, the suture method was modified by suturing the fascia layer only on the back to ensure the muscle’s central axis movement and retain the facet joint on the lateral side as far as possible. On the second day after the operation, the patients could stand and walk by wearing a collar, and they were encouraged to undergo progressive tolerance training during flexion, extension, rotation and lateral bending. It was recommended that the neck collar be worn within 2 weeks after the operation. Moreover, a self-designed titanium mini-plate was applied to reconstruct the lamina structure and achieve “rigid fixation,” increase cervical stability and prevent the “re-closing” of the lamina structure. During the follow-up, patients in our study achieved good clinical outcomes (Figs. 4 and 5).
Vertebral artery injury is a severe complication of upper cervical spine surgery. Although its incidence is low, affected patients may be at risk due to significant blood loss. Therefore, to avoid damaging the blood vessels and nerves in the vertebral artery sulcus, surgeons should allow an appropriate distance and be cautious when exposing or resecting the posterior arch of the atlas [26]. The author’s experience in imaging analysis and surgical summary of the posterior arch of the atlas suggests the following. (1) The posterior arch of the atlas has different resections and exposure ranges and should be treated differently. Because the posterior arch is fan-shaped and has a certain degree of curvature, the median medial distance should be used as the reference value when removing the posterior arch and should not exceed 12 mm. If it exceeds 12 mm, care should be taken to avoid damaging the blood vessels and nerves in the vertebral artery groove. (2) The posterior arch exposure should be controlled at 12 mm, and the posterior arch resection should be limited to 8 mm. (3) The posterior arch of the atlas should be stripped under the periosteum, with the posterior and lower part regarded as the safe area. After the vertebral artery is exposed, the posterior arch can be removed directly. (4) There are large individual differences in the safe area of the posterior arch of the atlas. Adults are larger than children, and males are larger than females. Therefore, posterior arch resection should be accurately individualized to each patient.