**4.2 Extrinsic esophageal compression**

The presence of anterior cervical osteophyte, diffuse idiopathic skeletal hyperostosis can cause dysphagia secondary to mechanical impingement of esophagus, as well as inflammation causing adhesion and fibrosis.

In ACDF with plating, the presence of a plate can contributes to the same pathophysiology cause of dysphagia. Although the concept is not yet fully established, it has been proven that a thicker cervical plate is associated with a higher incidence of dysphagia [22].

Several studies confirmed that standalone cages are associated with less incidence of dysphagia when compared with ACDF with plating. However, this conclusion is not universally accepted [29].

#### **4.3 Esophageal retraction**

Retraction of esophagus during surgery to expose the anterior cervical spine is one of the possible cause of postoperative dysphagia while some studies concluded that esophageal retraction may cause ischemia of the esophageal wall which in turn compromise the motility [12], one study failed to confirm the association between the intraoperative pressure of esophageal retraction and postoperative dysphagia [30].

#### **4.4 Neural traction**

Intraoperative nerve traction or injury is another possible cause of postoperative dysphagia. Different nerves traction will cause different esophageal segment dysphagia. For example damage or traction to Hypoglossal nerve will impact the oral phase of swallowing, while injury or traction to the connection between the pharyngeal plexus and pharyngeal muscle will impact the pharyngeal phase of swallowing. Injury of the recurrent laryngeal nerve (RLN) and superior laryngeal nerve (SLN) are both operative in the development of postoperative dysphagia, reason why a sound knowledge of their anatomy and meticulous surgical technique are essential to decrease the postoperative dysphagia [11].

Anderson et al. [11] summarized the causes of oropharyngeal dysphagia as seen in **Table 1**.

Regarding the surgical techniques, the causes of dysphagia are described in **Table 2**.

### **5. Risk factors**

#### **5.1 Patient-related**

#### *5.1.1 Age*

Smith-Hammond et al. [20] found that older patients have an higher risk of dysphagia following ACDF, while Lee et al. [19] and Bazaz et al. [19] found the no correlation between the age and the risk of dysphagia.
