ORIGINAL ARTICLE Analysis of the reliability of surgeons’ ability to differentiate between idiopathic and degenerative spinal deformity in adults radiologically. What descriptive parameters help them decide? Umit Ozgur Guler 1 • Selcen Yuksel 2 • Sule Yakici 1 • Montserrat Domingo-Sabat 3 • Ferran Pellise 3 • Francisco J. S. Pe ´rez-Grueso 4 • Ibrahim Obeid 5 • Ahmet Alanay 6 • Frank Kleinstu ¨ck 7 • Emre Acaroglu 1 • European Spine Study Group 8 Received: 13 April 2015 / Revised: 26 December 2015 / Accepted: 28 December 2015 Ó Springer-Verlag Berlin Heidelberg 2016 Abstract Purpose Adult spinal deformity (ASD) may be classified as idiopathic (ID) or degenerative (DD) (or other) based on classifier’s perception, the reliability of and factors inherent to which remain unknown. The aim of this study is to evaluate the inter- and intra-observer reliability of sur- geons’ perception in differentiating ID from DD and to identify the determinants of this differentiation. Methods From a multicentric prospective database of ASD, 179 patients were identified with the diagnosis of ID (n = 103) or DD (n = 76); without previous surgery; and a lumbar coronal curve larger than 20°. Standing antero-pos- terior and lateral X-rays of these patients were sent to five experienced spine surgeons to be identified as DD or ID (or other); followed by a second round after reshuffling. Weighted kappa statistics were used, the strength of agreement for the kappa coefficient was considered as; 0.81–1 = almost perfect, 0.61–0.8 = substantial, 0.41–0.60 = moderate, 0.21–0.40 = fair, 0.01–0.20 = slight, and B0 = poor. Patients were then stratified based on the number of agreements on a total of 10 rounds as excellent (10 out of 10), good (more than 7 out of 10) and fair/poor (7 and less). These excellent and good agreements were further compared for additional radiological parameters. Results Agreement levels were moderate to substantial for intra but mostly fair for inter-observer comparisons. For ID patients, there were 42 cases with excellent and 38 with very good agreement whereas for DD, there were no excellent and only 17 cases with very good agreement. Upon comparison of these (ID vs DD for at least very good cases), it was seen that they were different for some coronal parameters such as lumbar Cobb angle (larger in ID, p \ 0.001), central sacral vertical line (CSVL) modifier (C more common in ID, p = 0.007) and presence of rotatory subluxation (less common in DD, p = 0.017), but very different for sagittal parameters (lumbar lordosis, sagittal vertical axis, T2 sagittal tilt, pelvic tilt, sacral slope, and global tilt; increased sagittal imbalance in DD, all p B 0.001). Conclusion Surgeons in this study demonstrated reason- able (moderate to substantial) intra-observer agreement, but only fair agreement amongst them. Alarming as it may appear, we should be cautious in interpreting these results based on only radiology and no clinical information. In patients with good agreement, the most consistent radio- logic determinant of degenerative ASD appeared to be the presence of sagittal imbalance. Keywords Spine Á Deformity Á Adult Á Idiopathic Á Degenerative & Emre Acaroglu acaroglue@gmail.com 1 Ankara Spine Center, Iran Caddesi, Kavaklidere, Cankaya, 45/2, 06700 Ankara, Turkey 2 Department of Biostatistics, Yildirim Beyazit University, Ankara, Turkey 3 Spine Unit, Hospital Universitari Vall d’ Hebron, Barcelona, Spain 4 Spine Unit, Hospital Universitari La Paz, Madrid, Spain 5 Spine Unit, Bordeaux University Hospital, Bordeaux, France 6 Acibadem Maslak Hospital, Istanbul, Turkey 7 Schultess Clinic, Zurich, Switzerland 8 Fundacio ´ Institut Vall d’ Hebron, Barcelona, Spain 123 Eur Spine J DOI 10.1007/s00586-015-4366-3