Schmid Samuel L, Buck F M, Böni T, Farshad M
Department of Orthopaedics, Balgrist University Hospital, University of Zürich, Forchstrasse 340, 8008, Zurich, Switzerland.
Eur Spine J. 2016 Feb;25(2):379-84. doi: 10.1007/s00586-015-4259-5. Epub 2015 Sep 30.
The Cobb angle measurement is well established for the measurement of coronal deformity aspect of scoliotic curves. The effect of positional differences in relation to the apex side of the scoliosis is not yet fully quantified. While theoretically plausible that positioning error with rotation toward the apex of the scoliosis would decrease the Cobb angle, the relations are not investigated yet and were object of this study.
Multiple measurements of the Cobb angle were performed, while turning a spine-pelvic cadaveric specimen with a right-sided thoracic scoliosis of 47° (in neutral position) from 45° to -45° in steps of 5° using biplanar radiography. Statistical methods were applied to find the critical position, in which measurement errors potentially become clinically relevant (Cobb angle deviation >5°).
Turning the specimen to the right (toward the apex of the scoliosis) produced during the first -15° of rotation, a Cobb angle ranging from 47° to 45°. At -20°, the Cobb angle was 42°, at -25° rotation 37° and at -30° rotation 36°. Above -30° rotation, the measured Cobb angle decreased to 36° (77 % of the original Cobb angle). No relevant differences were found by rotating the specimen to the left (away from the apex) (47° at neutral rotation and 44° at maximal error rotation of +45°).
The influence of rotational misplacement of the patient at the time of image acquisition on Cobb angle measurements is negligible for a rotational misplacement of ±20° of rotation for a idiopathic right-sided thoracic scoliosis of 47°. Over 20° of rotational misplacement of the patient toward the apex of the scoliosis falsely decreases the Cobb angle.
Cobb角测量法在测量脊柱侧弯冠状面畸形方面已得到广泛应用。脊柱侧弯顶点侧位置差异的影响尚未完全量化。理论上，向脊柱侧弯顶点旋转时的定位误差会减小Cobb角，但这种关系尚未得到研究，本研究以此为对象。
使用双平面放射摄影，对一个右侧胸段脊柱侧弯47°（中立位）的脊柱 - 骨盆尸体标本，以5°步长从45°旋转至 - 45°，同时多次测量Cobb角。应用统计方法找出临界位置，即测量误差可能在临床上具有相关性的位置（Cobb角偏差>5°）。
将标本向右侧（朝向脊柱侧弯顶点）旋转，在最初的 - 15°旋转过程中，Cobb角从47°变化到45°。在 - 20°时，Cobb角为42°， - 25°旋转时为37°， - 30°旋转时为36°。在超过 - 30°旋转后，测量的Cobb角降至36°（原始Cobb角的77%）。将标本向左旋转（远离顶点）未发现相关差异（中立旋转时为47°，最大误差旋转+45°时为44°）。
对于47°的特发性右侧胸段脊柱侧弯，在图像采集时患者旋转错位±20°对Cobb角测量的影响可忽略不计。患者向脊柱侧弯顶点旋转超过20°会错误地减小Cobb角。
