title: Would an anatomically shaped lumbar interbody cage provide better stability? An in vitro cadaveric biomechanical evaluation.
abstract: METHODS
  A biomechanical cadaveric study of lumbar spine segments.
  
  
  OBJECTIVE
  To compare the immediate stability provided by parallel-shaped and anatomically shaped carbon fiber interbody fusion I/F cages in posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) constructs with posterior pedicle screw instrumentation.
  
  
  BACKGROUND
  Few biomechanical data are available on the anatomically shaped cages in PLIF and TLIF constructs.
  
  
  METHODS
  Twenty human lumbar segments were tested in flexion-extension (FE) (8 N m flexion, 6 N m extension), lateral bending (LB) (± 6 N m), and torsional loading (± 5 N m). Each segment was tested in the intact state and after insertion of interbody cages in one of 3 constructs: PLIF with 2 parallel-shaped or anatomically shaped cages and TLIF with 1 anatomically shaped cage. All cages received supplementary pedicle screw fixation. The range-of-motion (ROM) values after cage insertion and posterior fixation were compared with the intact specimen values using analysis of variance and multiple comparisons with Bonferroni correction.
  
  
  RESULTS
  All constructs significantly reduced segmental motion relative to intact (P < 0.001). The motion reductions in FE, LB, and axial rotation were 85 ± 15%, 83 ± 18%, and 67 ± 6.8% for the PLIF construct using parallel cages, 79 ± 5.5%, 87 ± 10%, and 66 ± 20% for PLIF using anatomically shaped cages, and 90 ± 6.8%, 87 ± 12%, and 77 ± 22% for TLIF with an anatomically shaped cage. In FE and LB, the reductions in the ROM caused between the 3 constructs were equivalent (P > 0.05). In axial rotation, the TLIF cage provided significantly greater limitation in the ROM compared with the parallel-shaped PLIF cage (P = 0.01).
  
  
  CONCLUSIONS
  The parallel-shaped and anatomically shaped I/F cages provided similar stability in a PLIF construct. The greater stability of the TLIF construct was likely due to a more anterior placement of the TLIF cage and preservation of the contralateral facet joint.
authors.affiliation: Musculoskeletal Biomechanics Laboratory, Department of Veterans Affairs, Edward Hines Jr. VA Hospital, Hines, IL, USA.
authors.forename: Parmenion P
authors.initials: PP
authors.lastname: Tsitsopoulos
authors.forename: Hassan
authors.initials: H
authors.lastname: Serhan
authors.forename: Leonard I
authors.initials: LI
authors.lastname: Voronov
authors.forename: Gerard
authors.initials: G
authors.lastname: Carandang
authors.forename: Robert M
authors.initials: RM
authors.lastname: Havey
authors.forename: Alexander J
authors.initials: AJ
authors.lastname: Ghanayem
authors.forename: Avinash G
authors.initials: AG
authors.lastname: Patwardhan
mesh_terms: D000368:Aged; D001696:Biomechanical Phenomena; D002102:Cadaver; D002244:Carbon; D000077482:Carbon Fiber; D004867:Equipment Design; D006801:Humans; D066298:In Vitro Techniques; D016268:Internal Fixators; D008159:Lumbar Vertebrae / Q000601:surgery*; D008875:Middle Aged; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000295:instrumentation*; D021801:Zygapophyseal Joint / Q000601:surgery
chemical_list: D000077482:Carbon Fiber; D002244:Carbon