title: Biomechanical investigation of a novel integrated device for intra-articular stabilization of the C1-C2 (atlantoaxial) joint.
abstract: BACKGROUND
  The anatomy of the atlantoaxial joint makes stabilization at this level challenging. Current techniques that use transarticular screw fixation (Magerl) or segmental screw fixation (Harms) give dramatically improved stability but risk damage to the vertebral artery. A novel integrated device was designed and developed to obtain intra-articular stabilization via primary interference fixation within the C1-C2 lateral mass articulation.
  
  
  OBJECTIVE
  To assess the atlantoaxial stability achieved with a novel integrated device when compared with the intact, destabilized, and stabilized state using the Harms technique.
  
  
  METHODS
  A biomechanical study of implants in human cadaveric cervical spines.
  
  
  METHODS
  Six human cadaveric specimens were used. Biomechanical testing was performed with moment control in flexion-extension, lateral bending, and axial rotation. Range of motion (ROM) was measured in the intact state, after both destabilization by creation of a Type II odontoid peg fracture and sequential stabilization using the integrated device and the Harms technique.
  
  
  RESULTS
  Mean flexion-extension ROM of the intact specimens at C1-C2 was 14.1°±2.9°. Destabilization increased the ROM to 31.6°±4.6°. Instrumentation with the Harms technique reduced flexion-extension motion to 4.0°±1.4° (p<.01). The integrated device reduced flexion-extension motion to 3.6°±1.8° (p<.01). In lateral bending, the respective mean angular motions were 1.8°±1.1°, 14.1°±5.8°, 1.4°±0.7°, and 0.4°±0.3° for the intact destabilized Harms technique and integrated device. For axial rotation, the respective mean values were 67.3°±13.8°, 74.2°±16.1°, 1.4°±0.7° and 0.9°±0.7°. Both the Harms technique and integrated device significantly reduced motion compared with the destabilized spine in flexion-extension, lateral bending, and axial rotation (p<.05). Direct comparison of the Harms technique and the integrated device revealed no significant difference (p>.10).
  
  
  CONCLUSIONS
  The integrated device resulted in interference fixation at the C1-C2 lateral mass joints with comparable stability to the Harms technique. Perceived advantages with the integrated device include avoidance of fixation below the C2 lateral mass where the vertebral artery is susceptible to injury, and access to the C1 screw entry point through the blade of the integrated device avoiding extended dissection superior to the C2 nerve root and its surrounding venous plexus.
authors.affiliation: The Orthopaedic Clinic, Mercy Specialist Centre, 100 Mountain Rd, Epsom, Auckland 1023, New Zealand. p.a.robertson@xtra.co.nz
authors.forename: Peter A
authors.initials: PA
authors.lastname: Robertson
authors.forename: Parmenion P
authors.initials: PP
authors.lastname: Tsitsopoulos
authors.forename: Leonard I
authors.initials: LI
authors.lastname: Voronov
authors.forename: Robert M
authors.initials: RM
authors.lastname: Havey
authors.forename: Avinash G
authors.initials: AG
authors.lastname: Patwardhan
mesh_terms: D000368:Aged; D001268:Atlanto-Axial Joint / Q000502:physiology* / Q000601:surgery; D001696:Biomechanical Phenomena / Q000502:physiology*; D001863:Bone Screws; D002574:Cervical Vertebrae / Q000502:physiology / Q000601:surgery; D006801:Humans; D007593:Joint Instability / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D016059:Range of Motion, Articular / Q000502:physiology*; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods