Measurement of the Cobb angle on radiographs of patients who have scoliosis. Evaluation of intrinsic error.

Abstract

To quantitate the intrinsic error in measurement, fifty anteroposterior radiographs of patients who had scoliosis were each measured on six separate occasions by four orthopaedic surgeons using the Cobb method. For the first two measurements (Set I), each observer selected the end-vertebrae of the curve; for the next two measurements (Set II), the end-vertebrae were pre-selected and constant. The last two measurements (Set Ill) were obtained in the same manner as Set II, except that each examiner used the same protractor rather than the one that he carried with him. The pooled results of all four observers suggested that the 95 per cent confidence limit for intraobserver variability was 4.9 degrees for Set I, 3.8 degrees for Set II, and 2.8 degrees for Set III. The interobserver variability was 7.2 degrees for Set I and 6.3 degrees for Sets II and III. The mean angles differed significantly between observers, but the difference was smaller when the observers used the same protractor.

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

“…These findings are in contrast to those reported by Shea et al [11], who found a statistically better correlation in digital, compared to manual measurement of curves. However the intra-and interobserver reliability results of this study are comparable with those from previous studies [2,6,8] (Table 4). Analysing these previous studies it can be shown that most authors choose scoliotic curves smaller than 40°which may not reflect the true every day clinical situation [4] ( Table 1).…”

Reliability analysis for manual measurement of coronal plane deformity in adolescent scoliosis. Are 30 × 90 cm plain films better than digitized small films?

“…First, the mean angular difference was 2.81°±2.61°. The reproducibility of the Cobb angle measures obtained here appears equal to or better than previously reported for intra-observer or inter-observer studies using manual or computer-assisted techniques [4,5,15,17,18,20]. However, direct comparisons cannot be made with the previously mentioned studies because different radiographs were evaluated and differing statistical methods used in those studies.…”

“…Studies of inter-observer and intra-observer variability in measurement of this angle [3,7,9,15,16,20] have revealed that errors in radiographic measurements are typically ±5°and are comparable with thresholds of change that can influence treatment decisions [19]. Recent studies [4,5,8,14,18] demonstrate computer-assisted methods to reduce technical errors and the need for memorization of measurement and classification procedures.…”

“…No relevant literature data were available concerning the effect of image size on Cobb angle measurement reliability. Sources of errors may include incorrect selection of the upper and/or lower vertebral levels, random errors in drawing lines across the endplates, and systematic errors caused by goniometers [1,15]. Choosing the inappropriate end vertebrae in a scoliotic spine is known to be a major contributor to error [19], so we decided to define the end vertebrae in the current study to really focus solely on angular variations due to films' size.…”

“…Using both radiographs sizes, mean Cobb angle variations in inter-observer and intra-observer determinations were statistically significant between 1.55°± 1.15°and 3.58°± 3.47°. The precision of measure was better using 30 · 90 cm films for six out of the eight observers but, nonetheless, such minimal variations (less than 3°) between 30 · 90 cm and small films Cobb measurements could have therapeutic implications [1,3,15,16].…”