title: Effects of short anterior lumbar interbody fusion on biomechanics of neighboring unfused segments.
abstract: METHODS
  Segmental mobility and intradiscal pressure were measured and the data compared in six cadaveric lumbar spine specimens before and after in vitro simulated single level L4-5 and double level L4-5-S1 anterior interbody fusions.
  
  
  OBJECTIVE
  The experimental objective was to study the biomechanical effects of single level L4-5 and double level L4-5-S1 anterior interbody fusions on the neighboring unfused segments.
  
  
  BACKGROUND
  The relationship between the local rigidity created by fusion mass and accelerated degeneration reported at the neighboring unfused intervertebral discs is not clear.
  
  
  METHODS
  Six cadaveric lumbar spine specimens were biomechanically tested in flexion and extension. Segmental mobility and intradiscal pressure of the specimens were measured before and after in vitro simulation of single level L4-5 and double level L4-5-S1 anterior interbody fusions.
  
  
  RESULTS
  The mobility of the motion segments immediately above and below an L4-5 fusion was increased in flexion. When the L5-S1 segment was also fused, the loss of segmental motion in both flexion and extension at the L4-5 and L5-S1 were compensated for by increased motion in all levels above the fusion. In both flexion and extension, the intradiscal pressures of all unfused intervertebral discs were increased after a single level L4-5 fusion and this increase was even more marked after a double level L4-5-S1 fusion.
  
  
  CONCLUSIONS
  There is no evidence that the neighboring unfused segments are loaded beyond their physiological limits due to the fusion. However, the neighboring unfused segments have to work more frequently toward the extremes of their functional ranges of motion after fusion and these effects will be more marked after a double level L4-5-S1 fusion.
authors.affiliation: Jockey Club Rehabilitation Engineering Centre, Hong Kong Polytechnic University.
authors.forename: D H
authors.initials: DH
authors.lastname: Chow
authors.forename: K D
authors.initials: KD
authors.lastname: Luk
authors.forename: J H
authors.initials: JH
authors.lastname: Evans
authors.forename: J C
authors.initials: JC
authors.lastname: Leong
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D002102:Cadaver; D005260:Female; D006801:Humans; D007403:Intervertebral Disc / Q000502:physiology; D008159:Lumbar Vertebrae / Q000378:metabolism*; D008161:Lumbosacral Region / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D009068:Movement; D011312:Pressure; D013123:Spinal Fusion / Q000009:adverse effects*; D013314:Stress, Mechanical