title: Zero-profile hybrid fusion construct versus 2-level plate fixation to treat adjacent-level disease in the cervical spine.
abstract: OBJECTIVE
  Single-level anterior cervical discectomy and fusion (ACDF) is an established surgical treatment for cervical myelopathy. Within 10 years of undergoing ACDF, 19.2% of patients develop symptomatic adjacent-level degeneration. Performing ACDF adjacent to prior fusion requires exposure and removal of previously placed hardware, which may increase the risk of adverse outcomes. Zero-profile cervical implants combine an interbody spacer with an anterior plate into a single device that does not extend beyond the intervertebral disc space, potentially obviating the need to remove prior hardware. This study compared the biomechanical stability and adjacent-level range of motion (ROM) following placement of a zero-profile device (ZPD) adjacent to a single-level ACDF against a standard 2-level ACDF.
  
  
  METHODS
  In this in vitro biomechanical cadaveric study, multidirectional flexibility testing was performed by a robotic spine system that simulates flexion-extension, lateral bending, and axial rotation by applying a continuous pure moment load. Testing conditions were as follows: 1) intact, 2) C5-6 ACDF, 3) C4-5 ZPD supraadjacent to simulated fusion at C5-6, and 4) 2-level ACDF (C4-6). The sequence of the latter 2 test conditions was randomized. An unconstrained pure moment of 1.5 Nm with a 40-N simulated head weight load was applied to the intact condition first in all 3 planes of motion and then using the hybrid test protocol, overall intact kinematics were replicated subsequently for each surgical test condition. Intersegmental rotations were measured optoelectronically. Mean segmental ROM for operated levels and adjacent levels was recorded and normalized to the intact condition and expressed as a percent change from intact. A repeated-measures ANOVA was used to analyze the ROM between test conditions with a 95% level of significance.
  
  
  RESULTS
  No statistically significant differences in immediate construct stability were found between construct Patterns 3 and 4, in all planes of motion (p > 0.05). At the operated level, C4-5, the zero-profile construct showed greater decreases in axial rotation (-45% vs -36%) and lateral bending (-55% vs -38%), whereas the 2-level ACDF showed greater decreases in flexion-extension (-40% vs -34%). These differences were marginal and not statistically significant. Adjacent-level motion was nearly equivalent, with minor differences in flexion-extension.
  
  
  CONCLUSIONS
  When treating degeneration adjacent to a single-level ACDF, a zero-profile implant showed stabilizing potential at the operated level statistically similar to that of the standard revision with a 2-level plate. Revision for adjacent-level disease is common, and using a ZPD in this setting should be investigated clinically because it may be a faster, safer alternative.
authors.affiliation: Department of Neurosurgery, Neurological Institute, Cleveland Clinic, Cleveland, Ohio;
authors.forename: Andrew T
authors.initials: AT
authors.lastname: Healy
authors.forename: Swetha J
authors.initials: SJ
authors.lastname: Sundar
authors.forename: Raul J
authors.initials: RJ
authors.lastname: Cardenas
authors.forename: Prasath
authors.initials: P
authors.lastname: Mageswaran
authors.forename: Edward C
authors.initials: EC
authors.lastname: Benzel
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Mroz
authors.forename: Todd B
authors.initials: TB
authors.lastname: Francis
mesh_terms: D000328:Adult; D000368:Aged; D001860:Bone Plates; D002102:Cadaver; D002574:Cervical Vertebrae / Q000601:surgery*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*
keywords: ACDF = anterior cervical discectomy and fusion; ACP = anterior cervical plate; AR = axial rotation; FE = flexion-extension; LB = lateral bending; PMMA = polymethylmethacrylate; ROM = range of motion; ZPD = zeroprofile device; adjacent-level disease; anterior cervical discectomy and fusion; biomechanics; zero-profile fusion device