title: Less invasive posterior fixation method following transforaminal lumbar interbody fusion: a biomechanical analysis.
abstract: BACKGROUND
  Current surgical trends increasingly emphasize the minimization of surgical exposure and tissue morbidity. Previous research questioned the ability of unilateral pedicle screw instrumentation to adequately stabilize posterior fusion constructs. No study to date has addressed the effects of reduced posterior instrumentation mass on interbody construct techniques. Unilateral surgical exposure for transforaminal lumbar interbody fusion (TLIF) allows ipsilateral pedicle screw placement. Theoretically, percutanous contralateral facet screw placement could provide supplemental construct support without additional surgical exposure.
  
  
  OBJECTIVE
  Identify the biomechanical effects of reduced spinal fusion instrumentation mass on interbody construct stability.
  
  
  METHODS
  An in vitro biomechanical study using human lumbar spines comparing stability of TLIF constructs augmented by: (1) bilateral pedicle screw fixation, (2) unilateral pedicle screw fixation, or (3) a novel unilateral pedicle screw fixation supplemented with contralateral facet screw construct.
  
  
  METHODS
  Seven fresh frozen human cadaveric specimens were tested in random construct order in flexion/extension, lateral bending, and axial rotation using +/-5.0 Nm torques and 50 N axial compressive loads. Analysis of torque rotation curves determined construct stability. Using paired statistical methods, comparison of construct stiffness and total range of motion within each specimen were performed using the Wilcoxon signed ranks test with a Holm-Sidák multiple comparison procedure (alpha=0.05).
  
  
  RESULTS
  In flexion/extension, lateral bending, and axial rotation, there were no measurable differences in either stiffness or range of motion between the standard bilateral pedicle screw and the novel construct after TLIF. After TLIF, the unilateral pedicle screw construct provided only half of the improvement in stiffness compared with bilateral or novel constructs and allows for significant off-axis rotational motions, which could be detrimental to stability and the promotion for fusion.
  
  
  CONCLUSIONS
  All tested TLIF constructs with posterior instrumentation decreased segmental range of motion and increased segmental stiffness. While placing unilateral posterior instrumentation decreases overall implant bulk and dissection, it allows for significantly increased segmental range of motion, less stiffness, and produces off-axis movement. The technique of contralateral facet screw placement provides the surgical advantages of unilateral pedicle screw placement with stability comparable to TLIF with bilateral pedicle screws.
authors.affiliation: The Permanente Medical Group, Inc., Regional Spine Surgery, 280 West MacArthur Blvd., Oakland, CA 94611, USA.
authors.forename: Andrew V
authors.initials: AV
authors.lastname: Slucky
authors.forename: Darrel S
authors.initials: DS
authors.lastname: Brodke
authors.forename: Kent N
authors.initials: KN
authors.lastname: Bachus
authors.forename: John A
authors.initials: JA
authors.lastname: Droge
authors.forename: John T
authors.initials: JT
authors.lastname: Braun
mesh_terms: D000328:Adult; D000368:Aged; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002102:Cadaver; D019245:Compressive Strength; D005260:Female; D006801:Humans; D016268:Internal Fixators; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D019060:Minimally Invasive Surgical Procedures / Q000295:instrumentation / Q000379:methods; D011859:Radiography; D016059:Range of Motion, Articular / Q000502:physiology*; D018570:Risk Assessment; D012680:Sensitivity and Specificity; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*; D013314:Stress, Mechanical