Interobserver and Intraobserver Reliability of Cobb Angle Measurement: Endplate Versus Pedicle as Bony Landmarks for Measurement: A Statistical Analysis

Background: The Cobb-Lippman technique measures the curve severity by measuring the angle between the upper and lower endplates of the upper-end and lower-end vertebrae, respectively, using pedicles to measure the angle when they are better visualized than the endplates. Vertebral endplates in younger children provide less distinct bony landmarks and pedicles may be more easily identifiable in these children. The purpose of this study is to evaluate the reliability of the pedicle method of the Cobb angle measurement and compare it with the conventional endplate method of measurement. Methods: Three hundred and eighteen whole spine preoperative anteroposterior radiograms of children with varying degrees of idiopathic scoliosis involving the thoracic spine were evaluated. These radiograms were grouped based on the child's age (< 7 y, 7 to 10 y and > 10 y), the position of the upper-end vertebra (upper-end vertebra at or above T5 and upper-end vertebra caudal to T5), and based on curve severity (mild, < 20 degrees; moderate 20 to 40 degrees, and severe > 40 degrees). Three observers independently examined the radiograms using the endplate method and the pedicle method 3 times each using the digitized computer system. Both intraobserver and interobserver agreements were accessed by calculating the intraclass correlation coefficient (ICCC). In addition, a Bland-Altman plot was made in which the strength of the relationship between the score differences and their mean was indicated by the slope of a regression line. Results: The single ICCC values were better for all observers for < 7-year age group using the pedicle method, indicating lesser intraobserver variability. The average ICCC values, indicating interobserver variability, were similar for all age groups. All the ICCC values lay in the excellent or substantial group. Tests for significance showed no significant difference between the 2 methods of measurement. Conclusions: Differences in measurement values with the pedicle method and end-plate method of measurement of the Cobb angle are not statistically significant. Either method can be used for measurement when using a computer-digitized system, which may have helped to minimize measurement discrepancies between these 2 methods.