title: Atlantoaxial rotatory subluxation with ligamentous disruption: a biomechanical comparison of current fusion methods.
abstract: OBJECTIVE
  We evaluated the biomechanical effects of 4 instrumented configurations after induced atlantoaxial rotatory subluxation: transarticular screw fixation (T/A) and polyaxial C1 lateral mass and C2 pedicle screw and rod fixation (LC1-PC2) for atlantoaxial arthrodesis with unilateral and bilateral instrumentation.
  
  
  METHODS
  Three-dimensional intervertebral motion was tracked stereophotogrammetrically while 14 human cadaveric spine specimens underwent nonconstraining pure moment loading. Nondestructive loads were applied quasi-statistically in 0.25-Nm increments to a maximum load of 1.5 Nm during flexion-extension, right and left axial rotation, and right and left lateral bending. Hyperrotation injuries were created using torsional loads applied during left axial rotation until visible failure occurred.
  
  
  RESULTS
  In the normal condition, the values for angular range of motion, lax zone (zone of ligamentous laxity), and stiff zone (zone of ligamentous stretching) were similar in both groups in all directions of loading, with no significant differences (P > 0.05) between groups at C0-C1 or C1-C2. Both instrumentation systems (bilateral configurations) substantially stabilized angular motion at C1-C2 (P < 0.05) during all loading modes for the T/A group, and during all but right lateral bending (P = 0.072) for the LC1-PC2 group. The mean failure load for both intact and instrumented specimens was slightly greater, but not significant for the LC1-PC2 group compared with the T/A group (P > 0.14).
  
  
  CONCLUSIONS
  Both methods fixated atlantoaxial subluxation equally well. Compared with unilateral instrumentation, a bilateral configuration with the LC1-PC2 technique significantly increased stability during extension (P < 0.05). During axial rotation, bilateral T/A screws significantly increased stability compared with unilateral fixation (P < 0.02).
authors.affiliation: Spinal Biomechanics Laboratory, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona, USA.
authors.forename: Rogério
authors.initials: R
authors.lastname: Rocha
authors.forename: Anna G U
authors.initials: AG
authors.lastname: Sawa
authors.forename: Seungwon
authors.initials: S
authors.lastname: Baek
authors.forename: Sam
authors.initials: S
authors.lastname: Safavi-Abbasi
authors.forename: Fadimatou
authors.initials: F
authors.lastname: Hattendorf
authors.forename: Volker K H
authors.initials: VK
authors.lastname: Sonntag
authors.forename: Neil R
authors.initials: NR
authors.lastname: Crawford
mesh_terms: D001268:Atlanto-Axial Joint / Q000293:injuries* / Q000473:pathology*; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002102:Cadaver; D003627:Data Interpretation, Statistical; D005593:Fracture Fixation, Internal; D050723:Fractures, Bone / Q000473:pathology* / Q000601:surgery*; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D009068:Movement / Q000502:physiology; D013123:Spinal Fusion / Q000379:methods*