title: Biomechanical assessment of anchored cervical interbody cages: comparison of 2-screw and 4-screw designs.
abstract: BACKGROUND
  A new anchored cervical interbody polyetheretherketone spacer was devised that uses only 2 integrated variable-angle screws diagonally into the adjacent vertebral bodies instead of the established device that uses 4 diagonal fixed-angle screws.
  
  
  OBJECTIVE
  To compare in vitro the stability provided by the new 2-screw interbody spacer with that of the 4-screw spacer and a 4-screw anterior plate plus independent polyetheretherketone spacer.
  
  
  METHODS
  Three groups of cadaveric specimens were tested with 2-screw anchored cage (n = 8), 4-screw anchored cage (n = 8), and standard plate/cage (n = 16). Pure moments (1.5 Nm) were applied to induce flexion, extension, lateral bending, and axial rotation while measuring 3-D motion optoelectronically.
  
  
  RESULTS
  In all 3 groups, the mean range of motion (ROM) and lax zone were significantly reduced relative to the intact spine after discectomy and fixation. The 2-screw anchored cage allowed significantly greater ROM (P < .05) than the standard plate during flexion, extension, and axial rotation and allowed significantly greater ROM than the 4-screw cage during extension and axial rotation. The 4-screw anchored cage did not allow significantly different ROM or lax zone than the standard plate during any loading mode.
  
  
  CONCLUSIONS
  The 2-screw variable-angle anchored cage significantly reduces ROM relative to the intact spine. Greater stability can be achieved, especially during extension and axial rotation, by using the 4-screw cage or standard plate plus cage.
authors.affiliation: Spinal Biomechanics Laboratory, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona.
authors.forename: Marco T
authors.initials: MT
authors.lastname: Reis
authors.forename: Phillip M
authors.initials: PM
authors.lastname: Reyes
authors.forename: Neil R
authors.initials: NR
authors.lastname: Crawford
mesh_terms: D001577:Benzophenones; D001696:Biomechanical Phenomena; D001860:Bone Plates; D001863:Bone Screws; D002102:Cadaver; D002574:Cervical Vertebrae / Q000601:surgery*; D005260:Female; D006801:Humans; D007659:Ketones; D008875:Middle Aged; D011092:Polyethylene Glycols; D011108:Polymers; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000295:instrumentation*
chemical_list: D001577:Benzophenones; D007659:Ketones; D011108:Polymers; C063834:polyetheretherketone; D011092:Polyethylene Glycols