title: Influence of PLIF cage size on lumbar spine stability.
abstract: METHODS
  In vitro biomechanical testing on functional spine units with posterior lumbar interbody fusion cage implants of progressively larger sizes.
  
  
  OBJECTIVE
  To determine the influence of increasing cage size on the restoration of spine stability after total facetectomy.
  
  
  BACKGROUND
  Bilateral insertion of cages in posterior lumbar interbody fusion commonly involves facetectomy. To restore stability with no additional instrumentation, the cages must provide sufficient distraction of the vertebrae and adequate tension in the anulus. The size of cages is therefore an important consideration in posterior lumbar interbody fusion.
  
  
  METHODS
  Eight human lumbar functional spine units were obtained and divided into two equal groups; one group underwent bending tests and the other twisting. The functional spinal units were tested intact, after total bilateral facetectomy and with three sets of cages that were progressively larger in size.
  
  
  RESULTS
  After facetectomy, the functional spine unit's stiffness reduced significantly from that of the intact spine in extension (48% of intact), lateral bending (25%), and torsion (39%). With the posterior insertion of small cages into the facetectomized functional spine units, only extension stiffness was restored to the intact level, whereas flexion stiffness reduced significantly (41% of intact). The medium cages restored the lateral bending stiffness of the facetectomized functional spine units; only the large cages managed to restore the torsional stiffness. Flexion stiffness of the facetectomized functional spine units with cages remained significantly less than that of the intact spine, regardless of cage size.
  
  
  CONCLUSIONS
  In the facetectomized lumbar spine unit, cage size influences lateral bending and torsional stability.
authors.affiliation: Department of Orthopaedic Surgery, National University of Singapore, Singapore. dosgohj@nus.edu.sg
authors.forename: J C
authors.initials: JC
authors.lastname: Goh
authors.forename: H K
authors.initials: HK
authors.lastname: Wong
authors.forename: A
authors.initials: A
authors.lastname: Thambyah
authors.forename: C S
authors.initials: CS
authors.lastname: Yu
mesh_terms: D001696:Biomechanical Phenomena; D002102:Cadaver; D004867:Equipment Design; D006801:Humans; D016268:Internal Fixators; D008159:Lumbar Vertebrae / Q000601:surgery*; D008297:Male; D009984:Orthopedic Fixation Devices; D013123:Spinal Fusion / Q000295:instrumentation*; D018709:Statistics, Nonparametric; D013314:Stress, Mechanical; D019415:Torque