title: Hilus pulmonis as the center of gravity for AS thoracolumbar kyphosis.
abstract: BACKGROUND
  The sagittal vertical axis (SVA) is a meaningful measurement and widely used for evaluating sagittal balance, and is considered a design standard for surgery, including most ankylosing spondylitis (AS) kyphotic deformity planning. However, recent research indicates that the C7 plumb line is actually not the center of gravity (CG) line. Therefore, whether there is a better radiological marker as the CG of the trunk for AS thoracolumbar kyphosis remains unknown. This research is to investigate a radiological marker for the CG of the trunk in lateral radiographs for AS thoracolumbar kyphosis.
  
  
  METHODS
  The center of gravity of an irregular object can be obtained by hanging or supporting it in different points and directions, and the CG will be on the point of intersection. According to this principle of mechanics, we could use the pre- and post-operative hip axis vertical lines to locate the CG of the trunk. We evaluated 38 AS-fixed thoracolumbar kyphotic patients with pedicle subtraction osteotomies. Full-length, free-standing lateral radiographs, including the spine and pelvis, were available for all patients. Pre- and post-operative radiological parameters were measured, including SVA, horizontal distance between hip axis and C7 (HDHC), horizontal distance between hip axis and T5 (HDHT5), horizontal distance between hip axis and T9 (HDHT9), and horizontal distance between hip axis and hilus pulmonis (HDHH). Pre- and post-operative radiological parameter changes were compared by paired samples t tests. The intraclass correlation coefficient (ICC) was used to determine the intra- and interobserver reliabilities of HDHH.
  
  
  RESULTS
  Pre-operative SVA, HDHC, HDHT5, HDHT9, and HDHH values were, respectively, 21.1, 12.7, 3.5, -3.8, and 2.7 cm, and their post-operative values were, respectively, 9.1, 4.2, -2.1, -5.6, and 0.9 cm. Changes in SVA, HDHC, HDHT5, and HDHT9 were significant (p < 0.05), while the change in HDHH was not (p > 0.05). The ICC for overall interobserver reliability was 0.958 (p < 0.001), and it was 0.963 (p < 0.001) for overall intraobserver reliability.
  
  
  CONCLUSIONS
  The hilus pulmonis fell approximately on the hip axis both pre- and post-operatively. It was a better marker as the center of gravity of the trunk for deformity planning for AS thoracolumbar kyphosis.
authors.affiliation: Department of Orthopaedics, Chinese PLA General Hospital (301 Hospital), 28 Fuxing Rd., Beijing, 100853, People's Republic of China.
authors.forename: Kai
authors.initials: K
authors.lastname: Song
authors.forename: Guoquan
authors.initials: G
authors.lastname: Zheng
authors.forename: Yonggang
authors.initials: Y
authors.lastname: Zhang
authors.forename: Geng
authors.initials: G
authors.lastname: Cui
authors.forename: Xuesong
authors.initials: X
authors.lastname: Zhang
authors.forename: Keya
authors.initials: K
authors.lastname: Mao
authors.forename: Yan
authors.initials: Y
authors.lastname: Wang
mesh_terms: D000328:Adult; D005260:Female; D006112:Gravitation; D006801:Humans; D007738:Kyphosis / Q000000981:diagnostic imaging* / Q000209:etiology / Q000601:surgery; D008159:Lumbar Vertebrae / Q000601:surgery; D008168:Lung / Q000000981:diagnostic imaging*; D008297:Male; D010027:Osteotomy; D010384:Pelvic Bones / Q000000981:diagnostic imaging*; D011187:Posture; D011859:Radiography; D015203:Reproducibility of Results; D013131:Spine / Q000000981:diagnostic imaging*; D013167:Spondylitis, Ankylosing / Q000150:complications / Q000000981:diagnostic imaging* / Q000601:surgery; D013904:Thoracic Vertebrae / Q000601:surgery