title: Is Cervical Sagittal Imbalance a Risk Factor for Adjacent Segment Pathomechanics After Multilevel Fusion?
abstract: METHODS
  A biomechanical study using human spine specimens.
  
  
  OBJECTIVE
  The aim of this study was to assess whether the presence of cervical sagittal imbalance is an independent risk factor for increasing the mechanical burden on discs adjacent to cervical multilevel fusions.
  
  
  BACKGROUND
  The horizontal offset distance between the C2 plumbline and C7 vertebral body (C2-C7 Sagittal Vertical Axis (SVA)) or the angle made with vertical by a line connecting the C2 and C7 vertebral bodies (C2-C7 tilt angle) are used as radiographic measures to assess cervical sagittal balance. There is level III clinical evidence that sagittal imbalance caused by kyphotic fusions or global spinal sagittal malalignment may increase the risk of adjacent segment pathology.
  
  
  METHODS
  Thirteen human cadaveric cervical spines (Occiput-T1; age: 50.6 years; range: 21-67) were tested first in the native intact state and then after instrumentation across C4-C6 to simulate in situ two-level fusion. Specimens were tested using a previously validated experimental model that allowed measurement of spinal response to prescribed imbalance. The effects of fusion on segmental angular alignments and intradiscal pressures in the C3-C4 and C6-C7 discs, above and below the fusion, were evaluated at different magnitudes of C2-C7 tilt angle (or C2-C7 SVA).
  
  
  RESULTS
  When compared with the pre-fusion state, in situ fusion across C4-C6 segments required increased flexion angulation and resulted in increased intradiscal pressure at the C6-C7 disc below the fusion in order to accommodate the same increase in C2-C7 tilt angle or C2-C7 SVA (P < 0.05). The adjacent segment mechanical burden due to fusion became greater with increasing C2-C7 tilt angle or SVA.
  
  
  CONCLUSIONS
  Cervical sagittal imbalance arising from regional and/or global spinal sagittal malalignment may play a role in exacerbating adjacent segment pathomechanics after multilevel fusion and should be considered during surgical planning.
  
  
  METHODS
  N/A.
authors.affiliation: *Edward Hines Jr. VA Hospital, Hines, IL †Loyola University Stritch School of Medicine, Maywood, IL ‡University of Illinois at Chicago, Chicago, IL §Northwestern Feinberg School of Medicine, Chicago, IL ||Wentworth Spine Clinic, Sydney, New South Wales, Australia.
authors.forename: Avinash G
authors.initials: AG
authors.lastname: Patwardhan
authors.forename: Saeed
authors.initials: S
authors.lastname: Khayatzadeh
authors.forename: Ngoc-Lam
authors.initials: NL
authors.lastname: Nguyen
authors.forename: Robert M
authors.initials: RM
authors.lastname: Havey
authors.forename: Leonard I
authors.initials: LI
authors.lastname: Voronov
authors.forename: Muturi G
authors.initials: MG
authors.lastname: Muriuki
authors.forename: Gerard
authors.initials: G
authors.lastname: Carandang
authors.forename: Zachary A
authors.initials: ZA
authors.lastname: Smith
authors.forename: William
authors.initials: W
authors.lastname: Sears
authors.forename: Laurie M
authors.initials: LM
authors.lastname: Lomasney
authors.forename: Alexander J
authors.initials: AJ
authors.lastname: Ghanayem
mesh_terms: D000328:Adult; D000368:Aged; D001696:Biomechanical Phenomena / Q000502:physiology; D002574:Cervical Vertebrae / Q000000981:diagnostic imaging* / Q000502:physiology / Q000601:surgery*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D004856:Postural Balance / Q000502:physiology; D016059:Range of Motion, Articular / Q000502:physiology; D012307:Risk Factors; D013123:Spinal Fusion / Q000009:adverse effects*; D055815:Young Adult