Dysphagia, or swallowing dysfunction, can possibly reduce patient satisfaction with surgery, lead to various degrees of discomfort, and increase the risk of various complications such as bronchospasm, aspiration pneumonia, dehydration, asphyxia and malnutrition [22, 24]. Dysphagia is a postoperative result triggered by multiple factors and is reported to be the most common early complication after ACDF [22–24]. Rihn et al. [25] concluded that dysphagia appeared in 70% of patients who underwent anterior cervical surgery. With a mean of 7.2 years of follow-up, Yue et al. found varying degrees of dysphagia after ACDF with an anterior cervical plate [26], with an incidence of 35.1% at the final follow-up.
The mechanism underlying the occurrence of postoperative dysphagia is still controversial. Dysphagia is possibly associated with damage to the swallowing center in the central nervous system or cortical areas, dysfunction of efferent nerves or the muscular drive or decreased pharyngolaryngeal sensitivity. Disorders of esophageal motility caused by intraoperative traction or mechanical stimulation during surgery have often been mentioned in previous studies [27]. Swelling of the soft tissue around the esophagus and anterior cervical plate after ACDF are also commonly recognized as causes of postoperative dysphagia [28–29]. Attempts have been made in recent years to overcome the limitations of the traditional anterior plate. Lee et al. [30] proved that a low-profile plate design can minimize soft tissue irritation, thus decreasing the incidence of dysphagia after traditional ACDF. New devices for ACDF have been developed in recent years, such as the Zero-P Implant System. In theory, the Zero-P Implant System reduces the incidence of dysphagia after surgery due to a lack of posterior irritation and constriction by anterior plates.
Several retrospective studies have reported that the change in lordosis plays an important role in the development of dysphagia in both anterior and posterior cervical spine surgery [13–15]. Tian et al. [15] found that the dC2 − 7A for dysphagia patients was significantly higher than that for non-dysphagia patients after ACDF with anterior plate. In general, the dC2 − 7A can cause changes in the anatomical relationship between the cervical spine and the anterior esophagus, which may cause dysphagia. However, the traditional anterior cervical plate can reportedly better reconstruct the overall curvature of the cervical spine and the curvature of the surgical segment than Zero-P Implant System [31–32]. Studies specifically focusing on the effect of the dC2 − 7A on the development of dysphagia after ACDF with the Zero-P are rare. Therefore, we designed this retrospective study.
In this study, comparison of the dC2 − 7A between the dysphagia group and the non-dysphagia group showed that the difference was statistically significant. Meanwhile, the incidence of dysphagia when the dC2 − 7A was <-1° was obviously less than that when the postoperative C2 − 7A was ≥-1°. A greater dC2 − 7A was found to be significantly associated with a higher incidence of dysphagia. Therefore, properly adjusting the C2 − 7A in ACDF with the Zero-P Implant System may reduce the incidence of dysphagia. Some surgeons exert more powerful traction on the prevertebral tissue and intervertebral space to restore the physiological curvature of the cervical spine, decrease postoperative cervical degeneration and create a larger space for the implant, while others insert the screws more smoothly during the operation; ignoring this step may lead to an angle that is too large, resulting in protrusion of the pharyngolaryngeal wall and ultimately postoperative dysphagia [33]. Notably, Spearman’s correlation coefficient between the degree of dysphagia and the dC2 − 7A showed no significant correlation in the dysphagia group. We can therefore assume that dysphagia is a kind of subjective feeling, which might be different with similar anatomical changes after surgery.
We noticed that the risk factors for dysphagia after ACDF with Zero-P varied substantially in different studies. Miyata et al. [13] and Meng et al. [14]. reported that the difference between postoperative and preoperative O-C2As is a key factor in the development of postoperative dysphagia after occipitocervical fusion. Kalb et al. reported that possible risk factors included multilevel surgeries, the involvement of C4 − 5 and C5 − 6, and age but not operating time in their study [34]. Jang et al. concluded that age and sex were not related to postoperative dysphagia [35]. Elderly age, female sex and multilevel surgery were found to be possible risk factors for postoperative dysphagia in the study by Zeng et al. [36]. Some of the factors, such as the change in the O-C2A, sex, age, the levels of surgical segments and the highest segment of surgery, were not associated with a higher incidence of dysphagia in this study. Several reasons may account for this finding. First, the Zero-P Implant System can reduce the incidence of dysphagia after surgery compared with traditional ACDF because of the absence of an anterior cervical plate, possibly resulting in the lack of a significant difference between the two groups with the highest segments involved in the surgery and surgical levels. Second, routine application of the anterior cervical soft tissue spreader reduces intraoperative traction on the esophagus. Moreover, injury of the recurrent laryngeal nerve (RLN) and superior laryngeal nerve (SLN) is also avoided due to improvements in anterior cervical surgery in recent years. In addition, cultural factors in different regions may affect the risk factors for dysphagia. Notably, PSTS had an important effect on the occurrence of dysphagia in this study, which is consistent with previous findings [37–38]. This result might be related to intraoperative traction of the esophagus and trachea, splitting of the prevertebral fascia, stimulation of implants or even the change in the cervical physiological curvature, which can cause PSTS leading to dysphagia. Minimizing intraoperative traction and routine use of corticosteroids after surgery are necessary according to the results. In addition, smoking was significantly associated with a higher incidence of dysphagia in the ordinal logistic regression model in this study, which has rarely been reported before. Possible reasons are that smoking can cause pharyngolaryngitis and increase the sensitivity of the pharynx and larynx. Studies with larger sample sizes are possibly needed to validate the authenticity of this result.
Limitation
The limitations of the study are as follows. First, adoption of the Bazaz scale is the primary limitation of this study. As an unvalidated grading scale used to evaluate dysphagia, this scale is based on qualitative information collected by an investigator to assess a patient’s subjective sensation of difficulty when swallowing liquids and solids; this scale has been commonly applied in previous studies [39]. As the scale is based on subjective feelings, possible sensory disruptions causing postoperative dysphagia may be challenging to explain and may not reflect accurate clinical outcomes. The gold standard for dysphagia assessment, which is fiberoptic endoscopic evaluation or video fluoroscopy, can be used to ensure the veracity of these findings [40–41]. Second, the limitations of the retrospective design are obvious; therefore, future randomized controlled studies are also needed to verify our conclusions. Third, not all potential risk factors were considered in the statistical analysis, such as the changes in the cervical curvature. In addition, the mechanisms by which the dC2 − 7A affects the development of dysphagia after ACDF with the Zero-P Implant System are not completely clear. Therefore, multi-center and randomized controlled studies are needed to verify our conclusions in the future.