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

International Orthopaedics (SICOT) DOI 10.1007/s00264-016-3359-0 ORIGINAL PAPER Comparison of two- and three-dimensional measurement of the Cobb angle in scoliosis Ricarda Lechner 1 & David Putzer 2 & Dietmar Dammerer 1 & Michael Liebensteiner 1 & Christian Bach 3 & Martin Thaler 1 Received: 5 September 2016 / Accepted: 21 November 2016 # The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Purpose The Cobb angle as an objective measure is used to determine the progression of deformity, and is the basis in the planning of conservative and surgical treatment. However, studies have shown that the Cobb angle has two limitations: an inter- and intraobserver variability of the measurement is approximately 3–5 degrees, and high variability regarding the definition of the end vertebra. Scoliosis is a three-dimensional (3D) pathology, and 3D pathologies cannot be completely assessed by two-dimensional (2D) methods, like 2D radiography. The objective of this study was to determine the intraobserver and interobserver reliability of end vertebra definition and Cobb angle measurement using X-rays and 3D computer tomography (CT) reconstructions in scoliotic spines. Methods To assess interoberver variation the Cobb angle and the end vertebra were assessed by five observers in 55 patients using X-rays and 3D CT reconstructions. Definition of end vertebra and measurement of the Cobb angle was repeated two times with a three-week interval. Intraclass correlation coefficients (ICC) were used to determine the interobserver and intraobserver reliabilities. 95% prediction limits were provided for measurement errors. Results Intraclass correlation coefficient (ICC) showed excellent reliability for both methods. The measured Cobb angle * Martin Thaler 1 Department of Orthopaedic Surgery, Medical University Innsbruck, Anichstr. 35, 6020 Innsbruck, Austria 2 Department of Orthopaedic Surgery – Experimental Orthopaedics, Medical University Innsbruck, Innrain 36, 6020 Innsbruck, Austria 3 Department of Orthopaedic Surgery, Landeskrankenhaus Feldkirch, Carinagasse 47, 6807 Feldkirch, Austria was on average 9.2 degrees larger in the 3D CT group (72.8°, range 30–144) than on 2D radiography (63.6°, range 24–152). Conclusions In scoliosis treatment it is very essential to determine the curve magnitude, which is larger in a 3D measurement compared to 2D radiography. Keywords Cobb angle . Scoliosis . Three-dimensional measurement . Interobserver, intraobserver reliability Introduction Accuracy and consistency are crucial in the radiographic assessment of scoliosis. The findings of these measurements have significant implications for the treatment and management of patients [1]. Decisions to observe, brace, or recommend surgical intervention for scoliosis are based in large part on radiographic criteria, especially the Cobb angle [2]. The Cobb method has become the standard parameter for quantifying scoliosis curve magnitude [3]. However, the Cobb method has some limitations. Studies of interobserver and intraobserver reliability in measurement of the Cobb angle have shown a measurement error of approximately 3–5° [3–6]. 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. However, scoliosis is a threedimensional (3D) deformity, whereas the Cobb angle measured in a plane X-ray is a two-dimensional (2D) value. 3D pathologies cannot be completely assessed by 2D methods, like radiography. The direction of the X-ray beam cannot be adjusted to display the largest extent of the curve. Thus, accurate measurement of the radiological extent of the deformity is International Orthopaedics (SICOT) hardly possible. New software technologies and the improvement in CT accuracy have made it possible to generate anatomically accurate and detailed 3D reconstructions of the vertebral column. The aim of our study was to investigate Cobb angle measurement in 3D reconstructed images of the spine and to compare them with conventional supine radiographs, as well as to determine the interobserver and intraobserver variability associated with both techniques. Materials and methods Anterior-posterior digital full-length spine supine radiographs and CT scans of the vertebral column were performed in 55 patients with idiopathic and neuropathic scoliosis. Therefore the patients’ feet were positioned in a sub-talar joint neutral position with each foot on the ground and standing in a relaxed manner. The chin was directed as if to avoid its shadow on the spinal X-ray radiographs. Furthermore, the arms while standing were maintained straight in relaxed position on both sides. A General Electrics Lightspeed 16© (GE Healthcare, Wisconsin, USA) was used with 100 kV, 100 mA source, rotation 0.8 sec, DFOV 15, noise index 20 and slice thickness of 0.625 mm. Scan coverage was vertebral levels C1 to S1. Reformatted three-dimensional (3D) reconstructions were produced from the axial CT slides using the Advantage imaging processing software (Advantage Windows V4.01, GE Medical Systems). The 3D reconstructions of the whole spine can then be rotated virtually in any direction (Fig. 1). Therefore, the largest extent of every single scoliotic curve can be assessed. Five observers, including two experienced spine surgeons, two spine fellows and one senior paediatric orthopaedic surgeon were instructed to measure the Cobb angle. They were all familiar with the measurement method of the Cobb angle and carried out the measurements independently twice in each setting (measurement on digital radiographs, measurement of the 3D reconstructions), with a three-week interval between each session. All observers were blinded to their prior measurements and to the other observers. In a first step end vertebrae were defined in the radiographs and the 3D reconstructions. In a second step the Cobb angles were measured with predefined end vertebrae. Therefore, the upper and lower end vertebrae of each curve were defined by the senior author by measuring the maximum inclination angle on the same radiographs and 3D images. Lines were drawn through the end plates of each vertebra, and the one with the largest angle to the horizontal was defined as an end vertebra. The radiographs as well as the 3D reconstructions were all blinded and numbered. The measurements were carried out using the Advantage Windows software. For the 3D reconstructions of the vertebral column the observers were asked to rotate the curve to the largest extent of curve magnitude. The Fig. 1 3D reconstruction of the whole spine rotated to the largest extension of the scoliotic curve. The Cobb angle was measured with the end plates perpendicular to the plane of the viewer. The Cobb an (...truncated)