title: Kinematics of cervical total disc replacement adjacent to a two-level, straight versus lordotic fusion.
abstract: METHODS
  In vitro biomechanical study.
  
  
  OBJECTIVE
  To characterize cervical total disc replacement (TDR) kinematics above two-level fusion, and to determine the effect of fusion alignment on TDR response.
  
  
  BACKGROUND
  Cervical TDR may be a promising alternative for a symptomatic adjacent level after prior multilevel cervical fusion. However, little is known about the TDR kinematics in this setting.
  
  
  METHODS
  Eight human cadaveric cervical spines (C2-T1, age: 59 ± 8.6 years) were tested intact, after simulated two-level fusion (C4-C6) in lordotic alignment and then in straight alignment, and after C3-C4 TDR above the C4-C6 fusion in lordotic and straight alignments. Fusion was simulated using an external fixator apparatus, allowing easy adjustment of C4-C6 fusion alignment, and restoration to intact state upon disassembly. Specimens were tested in flexion-extension using hybrid testing protocols.
  
  
  RESULTS
  The external fixator device significantly reduced range of motion (ROM) at C4-C6 to 2.0 ± 0.6°, a reduction of 89 ± 3.0% (P < 0.05). Removal of the fusion construct restored the motion response of the spinal segments to their intact state. The C3-C4 TDR resulted in less motion as compared to the intact segment when the disc prosthesis was implanted either as a stand-alone procedure or above a two-level fusion. The decrease in motion of C3-C4 TDR was significant for both lordotic and straight fusions across C4-C6 (P < 0.05). Flexion and extension moments needed to bring the cervical spine to similar C2 motion endpoints significantly increased for the TDR above a two-level fusion compared to TDR alone (P < 0.05). Lordotic fusion required significantly greater flexion moment, whereas straight fusion required significantly greater extension moment (P < 0.05).
  
  
  CONCLUSIONS
  TDR placed adjacent to a two-level fusion is subjected to a more challenging biomechanical environment as compared to a stand-alone TDR. An artificial disc used in such a clinical scenario should be able to accommodate the increased moment loads without causing impingement of its endplates or undue wear during the expected life of the prosthesis.
authors.affiliation: Loyola University Stritch School of Medicine, Maywood, IL, USA.
authors.forename: Shelden
authors.initials: S
authors.lastname: Martin
authors.forename: Alexander J
authors.initials: AJ
authors.lastname: Ghanayem
authors.forename: Michael N
authors.initials: MN
authors.lastname: Tzermiadianos
authors.forename: Leonard I
authors.initials: LI
authors.lastname: Voronov
authors.forename: Robert M
authors.initials: RM
authors.lastname: Havey
authors.forename: Susan M
authors.initials: SM
authors.lastname: Renner
authors.forename: Gerard
authors.initials: G
authors.lastname: Carandang
authors.forename: Celeste
authors.initials: C
authors.lastname: Abjornson
authors.forename: Avinash G
authors.initials: AG
authors.lastname: Patwardhan
mesh_terms: D000368:Aged; D001696:Biomechanical Phenomena / Q000502:physiology; D002574:Cervical Vertebrae / Q000502:physiology* / Q000601:surgery*; D005260:Female; D006801:Humans; D008141:Lordosis / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D016059:Range of Motion, Articular / Q000502:physiology*; D013123:Spinal Fusion / Q000379:methods*; D059145:Total Disc Replacement / Q000379:methods*