ORIGINAL ARTICLE Evolution of the ischio-iliac lordosis during natural growth and its relation with the pelvic incidence Tom P. C. Schlo ¨sser • Michiel M. A. Janssen • Tomaz ˇ Vrtovec • Franjo Pernus ˇ • F. Cumhur O ¨ ner • Max A. Viergever • Koen L. Vincken • Rene ´ M. Castelein Received: 30 October 2013 / Revised: 27 April 2014 / Accepted: 27 April 2014 Ó Springer-Verlag Berlin Heidelberg 2014 Abstract Purpose Human fully upright ambulation, with fully extended hips and knees, and the body’s center of gravity directly above the hips, is unique in nature, and distin- guishes humans from all other mammalians. This biped- alism is made possible by the development of a lordosis between the ischium and ilium; it allows to ambulate in this unique bipedal manner, without sacrificing forceful extension of the legs. This configuration in space intro- duces unique biomechanical forces with relevance for a number of spinal conditions. The aim of this study was to quantify the development of this lordosis between ischium and ilium in the normal growing and adult spine and to evaluate its correlation with the well-known clinical parameter, pelvic incidence. Methods Consecutive series of three-dimensional com- puted tomography scans of the abdomen of 189 children and 310 adults without spino-pelvic pathologies were used. Scan indications were trauma screening or acute abdominal pathology. Using previously validated image processing techniques, femoral heads, center of the sacral endplate and the axes of the ischial bones were semi-automatically identified. A true sagittal view of the pelvis was automat- ically reconstructed, on which ischio-iliac angulation and pelvic incidence were calculated. The ischio-iliac angle was defined as the angle between the axes of the ischial bones and the line from the midpoint of the sacral endplate to the center of the femoral heads. Results A wide natural variation of the ischio-iliac angle (3°–46°) and pelvic incidence (14°–77°) was observed. Pearson’s analysis demonstrated a significant correlation between the ischio-iliac angle and pelvic incidence (r = 0.558, P \ 0.001). Linear regression analysis revealed that ischio-iliac angle, as well as pelvic incidence, Electronic supplementary material The online version of this article (doi:10.1007/s00586-014-3358-z) contains supplementary material, which is available to authorized users. T. P. C. Schlo ¨sser (&) Á M. M. A. Janssen Á F. C. O ¨ ner Á R. M. Castelein Department of Orthopaedic Surgery, G05.228, University Medical Center Utrecht, P.O. Box 85500, 3508 GA Utrecht, The Netherlands e-mail: t.p.c.schlosser@umcutrecht.nl M. M. A. Janssen e-mail: m.m.a.janssen@umcutrecht.nl F. C. O ¨ ner e-mail: f.c.oner@umcutrecht.nl R. M. Castelein e-mail: r.m.castelein@umcutrecht.nl T. Vrtovec Á F. Pernus ˇ Faculty of Electrical Engineering, University of Ljubljana, Ljubljana, Slovenia e-mail: tomaz.vrtovec@fe.uni-lj.si F. Pernus ˇ e-mail: franjo.pernus@fe.uni-lj.si M. A. Viergever Á K. L. Vincken Image Sciences Institute, University Medical Center Utrecht, Utrecht, The Netherlands e-mail: max@isi.uu.nl K. L. Vincken e-mail: k.l.vincken@umcutrecht.nl 123 Eur Spine J DOI 10.1007/s00586-014-3358-z