title: Adjacent-level biomechanics after single versus multilevel cervical spine fusion.
abstract: OBJECTIVE
  Previous studies have demonstrated that patients with spinal fusion are at greater risk for adjacent-segment disease and require additional surgery. It has been postulated that excessive motion of a given motion segment unit (MSU) leads to an increased risk of disc degeneration. It is the authors' hypothesis that a greater increase in adjacent-segment motion will be observed following a 2-level versus a single-level anterior cervical discectomy and instrumented fusion (ACDF). Therefore, they undertook this study to determine the effect of single-level versus 2-level ACDF on the biomechanics of adjacent MSUs.
  
  
  METHODS
  Ten fresh-frozen human cervical spines were used in this study. The specimens were potted at C-4 and T-1 and tested in flexion and extension. Range of motion (ROM) was 30° of flexion and 15° of extension at a maximum load of 50 N. The specimens were tested intact and then were randomized into 2 groups of 5 specimens each. Group 1 underwent a single-level ACDF at the C5-6 level first, and Group 2 underwent the procedure at the C6-7 level. After testing, both groups had the fusion extended to include the C5-7 levels, and the testing was repeated. Changes in overall ROM, stiffness, and segmental motion were calculated and statistically analyzed using a paired Student t-test.
  
  
  RESULTS
  An increase in sagittal ROM of 31.30% above (p = 0.012) and 33.88% below (p = 0.066) the fused MSU was found comparing a 2-level with a 1-level ACDF. The overall stiffness of the entire spinal construct increased 37.34% (p = 0.051) in extension and 30.59% (p = 0.013) in flexion as the second fusion level was added. As expected, the overall sagittal ROM of the entire spinal construct decreased by 13.68% (p = 0.0014) with a 2-level compared with a 1-level fusion.
  
  
  CONCLUSIONS
  This study has shown that the biomechanics at adjacent levels to a cervical spine fusion are altered and that there is increased adjacent-segment motion at the levels above and below, after a 2-level compared with a 1-level ACDF.
authors.affiliation: Department of Orthopaedics, University of Miami, Miami, Florida, USA. markprasarn@yahoo.com
authors.forename: Mark L
authors.initials: ML
authors.lastname: Prasarn
authors.forename: Dinah
authors.initials: D
authors.lastname: Baria
authors.forename: Edward
authors.initials: E
authors.lastname: Milne
authors.forename: Loren
authors.initials: L
authors.lastname: Latta
authors.forename: William
authors.initials: W
authors.lastname: Sukovich
mesh_terms: D000328:Adult; D000368:Aged; D001696:Biomechanical Phenomena / Q000502:physiology; D002102:Cadaver; D002574:Cervical Vertebrae / Q000502:physiology* / Q000601:surgery; D005260:Female; D006801:Humans; D015994:Incidence; D055959:Intervertebral Disc Degeneration / Q000453:epidemiology / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D011183:Postoperative Complications / Q000453:epidemiology / Q000503:physiopathology*; D016059:Range of Motion, Articular / Q000502:physiology*; D012307:Risk Factors; D013123:Spinal Fusion / Q000009:adverse effects* / Q000379:methods / Q000706:statistics & numerical data; D055009:Spondylosis / Q000453:epidemiology / Q000503:physiopathology / Q000601:surgery*; D013314:Stress, Mechanical