title: Biomechanical study on the effect of cervical spine fusion on adjacent-level intradiscal pressure and segmental motion.
abstract: METHODS
  A biomechanical study was performed using cadaveric cervical spine specimens.
  
  
  OBJECTIVE
  To determine the effect of cervical spine fusion on adjacent-level intradiscal pressure.
  
  
  BACKGROUND
  Clinical studies have reported that patients with spinal fusion are at greater risk of pathology and early disc degeneration at adjacent levels. It is hypothesized that eliminating motion at one level leads to hypermobility and increased forces at adjacent levels, thus increasing the rate of disc degeneration.
  
  
  METHODS
  Six cadaveric cervical spine specimens were tested. Specimens were stabilized at T1 and loaded at C3 to 20 degrees of flexion and 15 degrees of extension. Intradiscal pressures and segmental motion at C4-C5 and C6-C7 were recorded first on intact specimens, and then after anterior cervical plating at C5-C6. Changes in intradiscal pressure and segmental motion were calculated and statistically analyzed using a paired Student t test.
  
  
  RESULTS
  Intradiscal pressures were significantly increased during flexion at both adjacent levels. The pressure increased by 73.2% at C4-C5 (P = 0.002), and by 45.3% at C6-C7 (P = 0.006). Intradiscal pressures increased at both adjacent levels during extension, but not significantly. During flexion, segmental motion increased at both adjacent levels, with greater increases at C4-C5. During extension, segmental motion increased at both adjacent levels, with greater increases at C6-C7.
  
  
  CONCLUSIONS
  Clinical studies have reported increased rates of disc degeneration at levels adjacent to fusion. It is believed that eliminating motion through fusion shifts the load to the adjacent levels, causing earlier disc degeneration. This study has shown that significant increases in intradiscal pressure and segmental motion occur at levels adjacent to fusion during normal range of motion. These results may partially explain the mechanism of early disc degeneration at levels adjacent to cervical spine fusion.
authors.affiliation: Center for Sports Medicine and Orthopaedics Foundation for Research, Chattanooga, Tennessee, USA. jason-laurie@worldnet.att.net
authors.forename: Jason C
authors.initials: JC
authors.lastname: Eck
authors.forename: S Craig
authors.initials: SC
authors.lastname: Humphreys
authors.forename: Tae-Hong
authors.initials: TH
authors.lastname: Lim
authors.forename: Soon Tack
authors.initials: ST
authors.lastname: Jeong
authors.forename: Jesse G
authors.initials: JG
authors.lastname: Kim
authors.forename: Scott D
authors.initials: SD
authors.lastname: Hodges
authors.forename: Howard S
authors.initials: HS
authors.lastname: An
mesh_terms: D001696:Biomechanical Phenomena; D002102:Cadaver; D002574:Cervical Vertebrae / Q000502:physiology* / Q000601:surgery; D006801:Humans; D066298:In Vitro Techniques; D007403:Intervertebral Disc / Q000502:physiology*; D008954:Models, Biological; D009333:Neck; D011312:Pressure; D016059:Range of Motion, Articular / Q000502:physiology*; D015203:Reproducibility of Results; D013123:Spinal Fusion; D013314:Stress, Mechanical