Inter- and intraobserver reliability assessment of the Cobb angle: manual versus digital measurement tools

Abstract

The objective of this study is to determine the intraobserver and interobserver reliability of end vertebra definition and Cobb angle measurement using printed and digital radiographs of 48 patients with scoliosis. The Cobb angle and the end vertebra were assessed by six observers in 48 patients with scoliosis using printed and digital radiographs. Definition of end vertebra and measurement of the Cobb angle was repeated three times with a 3 week interval. Intraclass correlation coefficients (ICC) were used to determine the interobserver and intraobserver reliabilities. 95% prediction limits for the errors in measurements are provided. For the Cobb angle a mean ICC of 0.97 was determined for intra-and interobserver reliability measurement of the printed radiographs. For the electronic radiographs a mean ICC value of 0.93 was determined for interobserver reliability and a mean ICC value of 0.96 for intraobserver reliability. Intraobserver ICC for definition of end vertebrae was 0.8 for both methods. Interobserver ICC was 0.83 for the manual and 0.74 in the digital method. One pitfall in angle measurement implies the Cobb method itself which measures in two dimensions. Until we develop a proper tri-dimensional measuring system an error is introduced. For the Cobb angle measurement the definition of end vertebrae introduces the main source of error. Digital radiography does not improve the measurement accuracy.

Comparison of two- and three-dimensional measurement of the Cobb angle in scoliosis

“…The intraobserver reliability for the Cobb angle found in this study was nearly the same for both methods (mean ICC radiographs, 0.978; mean ICC 3D CT images, 0.961). The variability and reliability of Cobb angle measurement in 3D CT images described in the present study are comparable to the results previous studies reported for manual or digital measurement of plain radiographs [7,22]. We measured the Cobb angle with predefined end vertebrae because it has been shown that the reliability is higher when the error of end plate selection is limited [7].…”

“…Despite this technical support, an inaccuracy in angle measurement persists due to a disadvantage of the Cobb method itself. Inter-and intraobserver variabilities of approximately 3-5°h ave been published [4,7]. Another systematic bias is that there is a high variability in end vertebra selection.…”

“…The end plate does not have a trajectory as a single clear line when the angle of its plane is other than perpendicular to the plane of the film. Even in the best conditions, an end plate that appears fusiform on the screen or on the film cannot be reduced to a single line [7]. Since each vertebra has a distinct rotational component, it is impossible to select a plane in X-rays in which the scoliotic region has a true anterior-posterior or posterior-anterior view of profile projection [11].…”

“…Studies of interobserver and intraobserver reliability in measurement of the Cobb angle have shown a measurement error of approximately 3-5° [3][4][5][6]. Sources of error may include a wrong definition of end vertebra, an incorrect drawing of the lines through end plates or through the pedicles, and drawing of perpendiculars or the measurement of the angle itself [7]. The Cobb angle is usually assessed from anterior-posterior or posterior-anterior standing radiographs.…”

Answer to the Letter to the Editor of A. Gardner concerning “Loss of apical vertebral derotation in adolescent idiopathic scoliosis: 2-year follow-up using multi-planar reconstruction computed tomography” (by G. Cui, K. Watanabe, Y. Nishiwaki, N. Hosogane, T. Tsuji, K. Ishii, M. Nakamura, Y. Toyama, K. Chiba, M. Matsumoto; Eur Spine J (2012) Jun;21(6):1111–20. doi:10.1007/s00586-012-2633-0)

“…However, the change had no clinical impact when evaluated by SRS-22 scores. Based on our previous study, the coefficient of variations for intra-observer measurement was 2.3 % for the MT curve [6] using a digital measurement software on appropriately adjusted X-ray films [7,8], and by selecting the same end vertebra in each patient at different time course [9]. In this study, we also used CT scans to demonstrate the correction loss in the axial plane and, in doing so, we tried to minimize measurement errors caused by patient positioning during CT examination using the relative AVR (rAVR), which was defined as the difference between the axial rotation angle of the upper instrumented vertebra and the apical vertebra.…”