title: Stress distribution on the screws in posterior lumbar fusion of isthmic spondylolisthesis with 2- or 3-vertebra fixation techniques: a biomechanical cadaveric study.
abstract: BACKGROUND
  Two- or three-vertebra fixation techniques are both used in the treatment of spondylolisthesis. However, the number of spinal segments that should be implanted in spondylolisthesis reduction and fixation is still controversial, and there are no published reports on stress distribution on the screws with 2- or 3-vertebra fixation techniques. Understanding stress distribution in screws would be of potential great clinical importance and supply more biomechanical evidence in surgery. The aim of this study was to compare and quantitatively analyze the stress distribution on the screws in 2- or 3-vertebra fixation techniques in cadaveric models of spondylolisthesis.
  
  
  METHODS
  Sixteen fresh specimens of human lumbar spines were used in this study. The spondylolisthesis model was generated by Panjabi method and fixed with the SINO universal spine system by 2- (group A) or 3-vertebra (group B) fixation technique. Rectangular electrical resistance strain gauges were fixed at upper and lower surface of the root of screws bilaterally. The samples were tested under flexion/extension, left/right lateral bending, and axial compression loading. Stress on the screws was measured by strain gauge monitor, respectively.
  
  
  RESULTS
  Under the five different loading conditions, the stress could be compressive stress or tensile stress. Under the compression, flexion, and bending loading condition, the stress in reduction screws in group A is higher than in group B (P < 0.01). However, under the extension loading condition, stress of lower surface in reduction screws in group A is 49% lower than in group B. With regard to the anchor screws, under flexion and lateral bending conditions, stress in group A is lower than in group B (P < 0.05). Under compression and extension loading conditions, stress in group A is slightly higher than in group B, but no significant difference is detected.
  
  
  CONCLUSIONS
  In most loading conditions, stress in reduction screws in 2-vertebra fixation technique was higher than in 3-vertebra fixation technique. The 3-vertebra fixation technique might effectively reduce stress on the reduction screws, and decrease the probability of fatigue fractures of the screws.
authors.affiliation: Institute of Orthopedics, Xijing Hospital, The Fourth Military Medical University, Xi'an, Shaanxi Province, PR China.
authors.forename: Zi-xiang
authors.initials: ZX
authors.lastname: Wu
authors.forename: Ce
authors.initials: C
authors.lastname: Zhan
authors.forename: Geng
authors.initials: G
authors.lastname: Cui
authors.forename: Da
authors.initials: D
authors.lastname: Liu
authors.forename: Shi-yong
authors.initials: SY
authors.lastname: Wan
authors.forename: Yang
authors.initials: Y
authors.lastname: Zhang
authors.forename: Xiong
authors.initials: X
authors.lastname: Zhao
authors.forename: Wei
authors.initials: W
authors.lastname: Lei
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002102:Cadaver; D005260:Female; D015775:Fractures, Stress / Q000453:epidemiology; D006801:Humans; D016268:Internal Fixators / Q000592:standards; D008159:Lumbar Vertebrae / Q000601:surgery*; D008297:Male; D008422:Materials Testing; D008875:Middle Aged; D008953:Models, Anatomic; D012307:Risk Factors; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods*; D013168:Spondylolisthesis / Q000601:surgery*; D013314:Stress, Mechanical