CAS is clinically characterized by progressive cartilage ossification combined with bone loss, resulting in a tubular spine that becomes stiff, fragile, and poorly resistant to stress and prone to fracture at low energy. Neck hyperextension is the most common mechanism of injury in CAS [22]. In the literature collected in this paper, there are also statistical data assessing the causes of injury. However, this paper did not analyze the causes of injury because different criteria for evaluating low-energy injuries and high-energy injuries are defined in the literature, hindering their comparison. Even though, it is easy to see that most of the literature still documents the etiology of low-energy injuries, indicating that low-energy trauma is a cause of fractures in patients with AS that cannot be ignored [23].

The incidence of cervical spine fractures in AS patients is three times higher than in the general population [24]. Despite the high incidence of fractures in patients with ankylosing spondylitis, delayed diagnosis is common. Chronic pain may mask patient's symptoms [24]. Simple DR films of the spine may not reveal fractures due to distorted anatomy, ligamentous ossification, and artifacts. On the other hand, early diagnosis and high suspicion of spinal cord injury may prevent possible long-term neurological damage. Therefore, due to the high susceptibility of patients with AS to spinal fractures and spinal cord injury even with only minor low-energy trauma, initial CT or MR imaging of the entire spine should be performed even if the patient is minimally symptomatic.

The change in neurological function was chosen as the main outcome indicator in this paper because it is an important indicator for evaluating the value of surgery. First, almost all the literature includes it as one of its outcome indicators for easy collection and statistics. Second, neurological function is also the purpose of our surgical treatment, and optimal neurological recovery can be achieved by an appropriate choice of surgical procedures to be applied. In the comparison presented in Table 1, we included the number of events, with no significant difference between the combined approach and the anterior and posterior approaches in terms of absolute values. We included inconsistent scales for the evaluation of neurological improvement in the literature (The B in The Fig 3 and 4), but two main scales owner applied: Frankel and ASIA grade scores. To enable the inclusion of a larger amount of data in this meta-analysis and to more objectively evaluate the outcome of the procedure, we employed the mean and variance values of the neurological improvement grade difference for comparison. However, during our statistical collection we found that better neurological recovery was obtained after the application of the combined approach than after that of the anterior and posterior approaches, but with no significant difference. This is also in line with the study of our current research, based on our analysis results and related literature show that the surgical approach is not directly related to the prognosis of patients [17]. Due to the particularity of the physical condition of patients with ankylosing spondylitis, the surgical methods should be individualized to avoid serious postoperative complications. Therefore, the indications of anterior, posterior and combined surgical approaches should depend on the nature of the injury, the mechanics at admission, the type of fracture, the degree of fracture displacement, the stability of fracture, the degree of nerve damage and so on. In the literature included here, the most appropriate procedure was performed, and some neurological recovery was achieved. Since the mechanics, severity and neurological outcome are quite different in the upper cervical spine (C0-C4) than lower (C5-C8) injuries, and the upper and lower cervical fractures are only partially divided in the included literature, this important detail is reflected in Table 1. In this article, there was no significant difference between the nerve damage of the upper and lower cervical injuries and the surgical approach. We are unable to collect more detailed conditions from the included literature, such as the type of fracture, spinal cord injury, fracture stability, fracture displacement and so on. However, the degree of postoperative neurological recovery is strongly correlated with the degree of cervical medullary injury damage, the presence or absence of diagnostic delay, and the adequacy of surgical decompression. Therefore, we recommend the implementation of stabilization surgery for CAS patients to achieve spinal balance and prevent further neurological impairment.

Metz-Stavenhagen et al. [25] recommended a combined anterior–posterior surgery approach in cases of severe combined posterior convexity, even though a higher complication rate exists after the use of this procedure. Michael et al. [26] found respiratory infections and urinary tract infections to be the least common complications in patients with CAS. In the present study, we also clearly established a higher complication rate in the combined approach, but with no statistically significant difference from those in the posterior and anterior approaches.

The reoperation rate is one of the indicators we utilized to evaluate the choice of the appropriate procedure. In this meta-analysis, we found a significant difference in the reoperation rate between the anterior and the combined approaches (p = 0.05), but not between the posterior and the combined approaches.

No significant difference was observed in mortality between the combined approach and the anterior and posterior approaches. Thus, we concluded that limited association existed between mortality in CAS patients and the procedure. Our statistical analysis showed that concurrent pulmonary infections and thrombosis were important causes of death in patients with CAS. Providing patients with strong immobilization and early lowering to the floor can effectively prevent such complications and reduce mortality.

Anterior fixation alone is the weakest and prone to complications method, leading to complications such as screw loosening and titanium plate displacement. Conversely, anterior surgery has the advantages of less trauma, complete decompression, and higher fusion rate, and is suitable for patients with good bone quality, less severe posterior column displacement and not accompanied by severe retroconvex deformity. Many reports of successful outcomes have been previously published. For example, Kouyoumdjian et al. [15] concluded that the anterior application of lengthened internal fixation plates can effectively counteract abnormalities. However, if the fracture is not completely repositioned by the anterior approach, it is in an unstable state due to the displacement of the posterior fracture end, and the increased anterior stress can easily cause internal fixation failure. Hence, the combined approach is more effective.

The posterior approach alone is stronger than the anterior, with adequate exposure and exact repositioning, and is suitable for most CAS patients, especially when the application of the anterior approach is difficult due to the presence of a posterior convexity deformity in the cervical spine. Its shortcomings are limited stability and the need for sufficiently long fixed segments to extend the surgical range [27]. It is also not suitable for CAS patients with fractures in the anterior column. Liu et al. [16] established no statistical difference in the degree of neurological recovery between the posterior-only approach and the combined approach, but the former had the advantages of shorter operative time, less surgical bleeding, and fewer surgery-related complications, and therefore the posterior-only approach is more favorable. In this article, there was no statistically significant difference between the combined anterior–posterior and posterior approaches. However, evidence suggests that patients with combined severe kyphosis are not suitable for simple posterior surgery because (1) it requires extensive dissection of the neck muscles, increases the risk of wound infection, and does not relieve the anterior spinal cord compression; (2) it is not suitable for patients with bone defects in the anterior column [28]; and (3) the stiffness of the posterior component and the vertebral body osteophytes, which may hinder the determination of the anatomical location of the instrument insertion point. As a result, it may lead to pedicle fractures, screw misalignment, nerve defects, and vertebral artery injury. Okten et al. [10] documented intraoperative complications of nerve root damage and vertebral artery injury due to improper screw placement in three patients with posterior approaches.

The combined anterior–posterior approach provides the best firmness compared to that achieved by the anterior and posterior approaches. We consider that the combined anterior–posterior approach may be the best option for treating cervical fractures in patients with AS. The use of this approach facilitates the achievement of the balance of the cervical spine, strong fixation of the anterior, middle, and posterior columns, and adequate and rapid nerve decompression. In addition, this combined approach can overcome the disadvantages of anterior surgery and posterior surgery alone. The rate of early three-column fusion increases after strict stabilization of the cervical fracture; and the incidence of loosening, dislodgement, fracture, and sinking of the implanted instrumentation is also lower. Due to firm fixation, patients are able to maintain better strength of their cervical motion and are able to get out of bed earlier, which helps to reduce complications, such as hemorrhagic pneumonia, deep vein thrombosis and decubitus ulcers. Payer et al. [29] reported in their study the treatment of four CAS cases with the combined approach. After a mean follow-up of 11 months, all patients showed good alignment with no deformities. Einsiedel et al. [12] suggested a combined approach with an anterior approach followed by a posterior approach in the first stage of surgery, whereas Ji et al. [30] supported that a sequential approach followed by an anterior approach was more appropriate. Olerud et al. [11] concluded that although a combined approach is associated with more surgical bleeding, more trauma, and longer operative time, a combined anterior and posterior approach is still recommended. However, due to the long operative time and high bleeding volume, it is not suitable for CAS patients with a poor physical condition.