title: Quantification of intradiscal pressures below thoracolumbar spinal fusion constructs: is there evidence to support "saving a level"?
abstract: METHODS
  In vitro cadaveric study.
  
  
  OBJECTIVE
  The purpose of this study was to quantify the relative biomechanical protection resulting from "saving a level" in long spinal fusions.
  
  
  BACKGROUND
  "Saving levels" in spinal deformity surgery is desirable. Constructs with lowest instrumented vertebra (LIV) in the lumbar spine may increase loads on unfused lumbar intervertebral discs, leading to accelerated disc degeneration. No study to date has quantified the relative pressure changes that occur in the unfused caudal discs with progressively longer fusions.
  
  
  METHODS
  We used a validated in vitro cadaveric long fusion model to assess intradiscal pressures (IDPs) below simulated fusions. Eight fresh frozen T8-S1 specimens were instrumented from T8 to L5. A follower-type loading system and 7.5-N·m moments were applied in flexion and extension. IDP profiles were assessed with a pressure transducer. After acquiring IDP measurements at a given construct length, the rod was cut 1 level higher until LIV = T12. IDP data from each unfused disc were averaged and normalized to the mean value of the disc when immediately subjacent to the LIV.
  
  
  RESULTS
  In both flexion and extension, the mean normalized IDP of the unfused discs below the LIV increased with increasing fusion length. For each 1-level increase in construct length, pressure increased by 3.2% ± 4.8% in flexion and 4.3% ± 4.5% in extension for each unfused disc. Although the differences in pressure for a given unfused disc with differing LIV were not significant, there were significant differences between unfused discs at a given LIV. With shorter fusion lengths, pressure in the disc immediately subjacent to the fusion was consistently greater than for the caudal-most discs.
  
  
  CONCLUSIONS
  Unfused caudal lumbar discs experienced increased IDPs with increasing length of instrumentation, most notably at the subjacent discs closest to the LIV.
authors.affiliation: Department of Orthopaedics, Bronx-Lebanon Hospital Center, Albert Einstein College of Medicine, Bronx, NY 10457, USA. auerspine@gmail.com
authors.forename: Joshua D
authors.initials: JD
authors.lastname: Auerbach
authors.forename: Baron S
authors.initials: BS
authors.lastname: Lonner
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Errico
authors.forename: Andrew
authors.initials: A
authors.lastname: Freeman
authors.forename: Derek
authors.initials: D
authors.lastname: Goerke
authors.forename: Brian P
authors.initials: BP
authors.lastname: Beaubien
mesh_terms: D002102:Cadaver; D005260:Female; D006801:Humans; D007403:Intervertebral Disc / Q000033:anatomy & histology / Q000502:physiology / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000033:anatomy & histology / Q000502:physiology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D015203:Reproducibility of Results; D013121:Spinal Curvatures / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D013123:Spinal Fusion / Q000009:adverse effects / Q000379:methods / Q000592:standards*; D013904:Thoracic Vertebrae / Q000033:anatomy & histology / Q000502:physiology / Q000601:surgery*; D016474:Weight-Bearing / Q000502:physiology*