Validity and reliability of a new iPhone method for rapidly measuring cervical sagittal parameters

All methods were performed in accordance with relevant guidelines and regulations. This study was approved, and the requirement for informed consent was waived by the Ethics Committee of the First Affiliated Hospital of Chongqing Medical University.

Patient selection

One hundred consecutive patients admitted to our hospital with a diagnosis of cervical spondylopathy or cervical spondylopathy were reviewed retrospectively. Inclusion criteria were clear lateral lumbar vertebrae of the cervical spine. Exclusion criteria were kyphosis of the cervical spine.

measurement methods

Four angles, including C0-2 Cobb angle, C2-7 Cobb angle, T1S and neck tilt (NT), were chosen to represent the sagittal parameters of the cervical spine in the present research. The C0-2 Cobb angle was defined as the angle included between the McGregor line and the line parallel to the lower endplate C2 (or the included angle perpendicular to the McGregor line and the lower endplate C2); The C2-7 Cobb angle was defined as the angle included between the line parallel to the lower end plate C2 and the line parallel to the lower end plate C7 (or the included angle perpendicular to the lower end plate C2 and the lower end plate C7); T1S was defined as the angle included between the T1 upper end plate and the horizontal line; NT was defined as the angle included between the vertical line and the line joining the upper end of the sternum and the center of the upper terminal plate T13,8,11,12 (Fig. 1).

Figure 1
Figure 1

Definition of C0-2 Cobb angle, C2-7 Cobb angle, T1S and NT.

To reduce measurement deviation, the same computer screen and the same iPhone were used in this study. The PACS intrinsic system was used to manually measure the sagittal parameters of the cervix by marking and automatically reading the lines.

The iPhone’s built-in camera was used to take measurement photos. To reduce parallax errors, the iPhone screen plane must remain parallel to the surface of the computer screen when the photo is taken. A built-in image editing function was used to rotate the X-ray film images, allowing grid lines and the angle scale of rotation to be clearly and simultaneously determined (Fig. 2A–E).

Figure 2
Figure 2

Measurement of C0-2 Cobb angle, C2-7 Cobb angle, T1S and NT by iPhone’s intrinsic photo editing function. (a) The target image to be edited has been selected. (BThe image was rotated until the McGregor line overlapped or was parallel to the horizontal grid lines, and then that angle was recorded. (c) the image was rotated until the lower endplate of C2 overlapped or was parallel to the horizontal grid lines, and then this angle was recorded. (Dr) the image was rotated until the lower endplate of C7 overlapped or was parallel to the horizontal grid lines, and then this angle was recorded. (e) the image was rotated until the upper endplate of T1 overlapped or was parallel to the horizontal grid lines, and this angle was recorded. (FThe image was rotated until the line connecting the upper end of the sternum and the center of the upper terminal plate T1 overlapped or was parallel to the horizontal grid lines, and this angle was recorded.

To measure the angle of C0-2 Cobb, first, the image was rotated until the McGregor line overlapped or was parallel to the horizontal grid lines, and then this angle was recorded; Second, the image was rotated until the lower end plate of C2 was staggered or parallel to the horizontal grid lines, and then this angle was recorded; Finally, the difference between these two angles representing the C0-2 Cobb angle was calculated and recorded (Fig. 2b,c).

To measure the angle of C2-7 Cobb, first, the image was rotated until the lower end plate of C2 overlapped or paralleled the horizontal grid lines, and then this angle was recorded; Second, the image was rotated until the lower endplate of C7 overlapped or was parallel to the horizontal grid lines, and then this angle was recorded; Finally, the difference between these two angles representing the C2-7 cup angle was calculated and recorded (Fig. 2C,D).

To measure T1S, the image was rotated until the upper end plate of T1 was overlapping or parallel to the horizontal grid lines, and this angle was recorded as T1S (because the horizontal line always represents 0°) (Fig. 2E).

For NT measurement, the image was rotated until the line connecting the upper end of the sternum and the center of the T1 terminal plate overlapped or was parallel to the horizontal grid lines, and this angle was recorded. NT was the angle supplementary to this angle (if NT was less than 45°, the image was rotated until the line connecting the upper end of the sternum and the center of the upper terminal plate T1 overlapped or parallel to the vertical grid lines, and this angle was recorded as NT (Fig. 2F).

One of the spine surgeons present as an experienced observer (observer A) and one orthopedic resident in training as an inexperienced observer (observer B) independently reviewed the radiographs. Observers were blinded to patient information. Observer A and Observer B took photos from iPhone and measured angles independently. To assess the reliability of the internal server, sagittal parameters of the cervix were measured using the intrinsic image editing function of the iPhone by two observers. Observer A measured angles with the iPhone again to assess intraobserver reliability and with PACS to assess the validity of the new iPhone method13. The interval between each round of measurements was more than two weeks. The order of the radiographs was randomized to reduce potential recall. A stopwatch was used to record the time of each measurement. To simulate the actual measurement process, the measurement time by PACS started by entering the radiograph number into PACS and ended with angle recording. iPhone measurement time started with the camera function on and ended with angle recording. Excel 2016 was used to record the angle data.

statistical analysis

All data were analyzed by SPSS 21.0 exempt. Intraclass correlation coefficients (ICCs) were used to evaluate the validity and reliability of the intraobserver and interobserver. The validity of this new iPhone method was evaluated by a PACS measurement taken by observer A and the mean results of two iPhone measurements taken by observer A. The intraobserver reliability was assessed by comparing the results of two measurements by observer A with the iPhone. Internal server reliability was assessed by comparing results between the mean results of two iPhone measurements taken by observer A and a single measurement taken by observer B13.

A good ICC is defined as more than 0.75, a very good ICC is defined as more than 0.85, and an excellent ICC is defined as more than 0.913. A Bland-Altman plot, with mean difference and 95% confidence interval (CI) visualized by MedCalc software, was used to graphically assess the validity and reliability of this new method. The time difference between the PACS and iPhone methods was evaluated by the times captured by PACS and the average time of two iPhone measurements taken by observer A, which was performed by the T-test of independent samples.

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