Ultrasound evaluation procedure
Upper airway ultrasound was performed with the patient in the supine position. No special device was required; a standard ultrasound device (SNiBLE; Konica Minolta, Tokyo, Japan) and high-frequency linear probe (L11-3; Konica Minolta) provided sufficient visualization. We identified the desired cervical spine level and tissue by examining the palpable laryngeal prominence of the thyroid cartilage in the cervical region and the palpable hard and smooth cricoid cartilage on the caudal side, which indicate the height of the 5th and 6th cervical vertebral body, respectively (Fig. 1).
Thyroid cartilage and cricoid cartilage in the transverse plane
The thyroid cartilage, laryngeal prominence, and cricoid cartilage were palpated in the anterior neck region. The probe was horizontally placed above the thyroid cartilage to visualize the anterior surface in the transverse plane; in the resultant ultrasound image, the thyroid cartilage had a triangular shape (Fig. 2A). The probe was then moved to the caudal side, and an arcuate area of the cricoid cartilage was observed on the caudal side of the cricothyroid ligament (Fig. 2B). By sliding the probe slightly outward, the anterior surface of the vertebral body was confirmed, and the thickness of the PST was evaluated (Fig. 2B). Sliding the probe cranio-caudally allowed detection of the cranio-caudal expansion of the PST.
Cricoid cartilage and tracheal cartilage in the longitudinal plane
The probe was placed longitudinally in the anterior neck region to visualize the thyroid cartilage and oval cricoid cartilage with acoustic shadows. Sliding the probe caudally allowed for observation of the tracheal cartilage and tracheal surface (Fig. 2C). The boundary between the air in the trachea and the anterior wall of the trachea was linearly hyperechoic.
Patient population of the pilot study
Eleven patients who underwent ACDF involving 1 or 2 segments (C5/6, C6/7, or both) for the treatment of radiculopathy or myelopathy were prospectively enrolled in this study between January 1 and June 30, 2020. The exclusion criteria included fusions involving >3 segments; previous revision surgery or corpectomy; surgically treated trauma, infections, or tumors; and presence of general metabolic diseases, such as rheumatoid arthritis and diabetes, and chronic heart and renal diseases.
The study protocol was approved by the Research Ethics Committee of Wakayama Medical University. All patient-related procedures performed in this study were in accordance with the ethical standards of the Research Ethics Committee of Wakayama Medical University and the 1964 Declaration of Helsinki and its later amendments. Informed consent was obtained from all participants.
The condition of the PST and upper airway was evaluated consecutively from day 0 (one day before operation) to postoperative day 14. The thickness of the PST at the C6 vertebral body was first measured on a lateral radiograph of the cervical spine and then in a cross section of the cricoid cartilage on the ultrasound image (Fig. 2B). The thickness of the upper airway was assessed in the longitudinal plane of the cricoid cartilage and tracheal cartilage via ultrasonography (Fig. 2C). To compensate for differences in the physique of the patients, changes in the thickness of the PST and upper airway over time were normalized to the preoperative thickness, referenced as value “1.”
The Bland–Altman method was used to evaluate the degree of agreement between the PST values obtained using radiography versus ultrasonography.  The 95% confidence intervals (CI) between radiography and ultrasonography were calculated; the clinically acceptable limit was set at ±1.96 times the standard deviation, which is the upper limit for interscan measurement variability and the threshold for a clinically significant change. [8,15-18] The Pearson correlation coefficient was used to measure the relationship between the PST values according to the method (radiography versus ultrasonography).
All statistical analyses were performed using JMP version 14 software (SAS Inc., Cary, NC, USA). P values were categorized as follows: <0.05, significant; ≥0.2, not significant, and 0.05 to ≤0.2, trending toward significance.