title: Effect of two-level total disc replacement on cervical spine kinematics.
abstract: METHODS
  Biomechanical study using human cadaver spines.
  
  
  OBJECTIVE
  To characterize kinematics of cervical spines implanted with total disc replacement (TDR) at 2-levels referencing the implanted and adjacent levels.
  
  
  BACKGROUND
  Cervical TDR is an appealing alternative to fusion particularly when treating multilevel disease, where the advantages of maintaining motion and reducing adjacent level stresses with TDR are compelling. To our knowledge there are no biomechanical studies evaluating multilevel cervical TDR.
  
  
  METHODS
  Six human cadaveric cervical spine specimens (C3-C7, age: 57 +/- 12 years) were tested (i) intact, (ii) after TDR (Discover, DePuy, Raynham, MA) at C5-C6, and (iii) after additional TDR at C6-C7. Specimens were subjected to flexion/extension, lateral bending and axial rotation (+/-1.5 Nm). Segmental range of motion (ROM) was measured using optoelectronic instrumentation and fluoroscopy.
  
  
  RESULTS
  Insertion of TDR at C5-C6 increased flexion/extension ROM of the implanted segment compared with intact (8.6 +/- 1.0 vs. 12.3 +/- 3.3 degrees , P < 0.025). The TDR maintained ROM to intact levels in lateral bending (7.4 +/- 2.6 vs 6.0 +/- 1.6, P > 0.025) and axial rotation (5.5 +/- 1.9 vs. 6.0 +/- 2.9, P > 0.025). The TDR at C5-C6 did not affect ROM at the adjacent levels. Implantation of a second TDR at C6-C7 maintained the ROM at that segment to intact values in flexion/extension (9.6 +/- 4.3 vs. 11.2 +/- 5.5, P > 0.025), lateral bending (6.1 +/- 4.0 vs. 4.1 +/- 2.1, P > 0.025), and axial rotation (6.7 +/- 3.6 vs. 5.5 +/- 3.7, P > 0.025). The second TDR at C6-C7 did not affect the ROM of the prosthesis implanted at C5-C6. Two-level TDR at C5-C6-C7 did not affect the ROM at C4-C5 in flexion/extension or axial rotation, however, in lateral bending a small increase occurred (8.9 +/- 3.6 vs. 10.1 +/- 4.5, P < 0.025).
  
  
  CONCLUSIONS
  Cervical TDR at 2 levels can provide near-normal mobility at both levels without destabilizing the implanted segments or affecting adjacent segment motions. These observations lend support to the notion that single or multilevel cervical TDR may be advantageous when compared to fusion.
authors.affiliation: Musculoskeletal Biomechanics Laboratory, Edward Hines Jr. VA Hospital, Hines, IL, USA.
authors.forename: Frank M
authors.initials: FM
authors.lastname: Phillips
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: Gerard
authors.initials: G
authors.lastname: Carandang
authors.forename: Andrew
authors.initials: A
authors.lastname: Dooris
authors.forename: Avinash G
authors.initials: AG
authors.lastname: Patwardhan
mesh_terms: D000368:Aged; D001696:Biomechanical Phenomena / Q000502:physiology; D002102:Cadaver; D002574:Cervical Vertebrae / Q000033:anatomy & histology / Q000502:physiology*; D019245:Compressive Strength / Q000502:physiology; D017586:Diskectomy / Q000009:adverse effects* / Q000379:methods; D005260:Female; D019416:Head Movements / Q000502:physiology; D006801:Humans; D007049:Iatrogenic Disease / Q000517:prevention & control; D007403:Intervertebral Disc / Q000033:anatomy & histology / Q000502:physiology*; D007405:Intervertebral Disc Displacement / Q000503:physiopathology* / Q000517:prevention & control / Q000601:surgery; D008297:Male; D008875:Middle Aged; D009068:Movement / Q000502:physiology; D011183:Postoperative Complications / Q000503:physiopathology / Q000517:prevention & control; D019736:Prostheses and Implants / Q000009:adverse effects; D016059:Range of Motion, Articular / Q000502:physiology*; D013123:Spinal Fusion / Q000009:adverse effects; D013314:Stress, Mechanical; D016474:Weight-Bearing / Q000502:physiology