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

TLDR

The study aimed to quantify the intrinsic error in Cobb angle measurement on scoliosis radiographs by having four orthopaedic surgeons measure 50 anteroposterior images six times each. Each radiograph was measured six times: in Set I observers chose end‑vertebrae, in Set II end‑vertebrae were pre‑selected, and in Set III the same protractor was used for all measurements. Intra‑observer 95 % limits of variability were 4.9°, 3.8°, and 2.8° for Sets I–III, inter‑observer variability was 7.2° for Set I and 6.3° for Sets II–III, and mean angles differed significantly between observers but less so when the same protractor was used.

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 III) 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.