title: Acute and long-term stability of atlantoaxial fixation methods: a biomechanical comparison of pars, pedicle, and intralaminar fixation in an intact and odontoid fracture model.
abstract: METHODS
  In vitro human cadaveric biomechanical study.
  
  
  OBJECTIVE
  The aims of this project were to evaluate the acute segmental fixation and long-term screw stability afforded by 3 C2 fixation techniques: intralaminar, pars, and pedicle.
  
  
  BACKGROUND
  C2 intralaminar screws offer the advantages of avoiding the vertebral artery; however, direct biomechanical comparison of this technique to the other methods of instrumenting C2 has not been performed.
  
  
  METHODS
  Fourteen cadaveric specimens were dual energy radiograph absorptiometry scanned and segregated into 2 groups (n = 7/group) matching the C2 bone mineral density. All specimens were instrumented with C1 lateral mass and C2 intralaminar screws while measuring the insertional torque (IT). In group 1 C2 pars screws were inserted while in group 2 pedicle screws were placed. Nondestructive testing was performed in axial rotation, flexion/extension (FE), and lateral bending. The odontoid was then resected and loading repeated. Subsequently, specimens were disarticulated about C2 and individually loaded for 2000 cycles in the cephalocaudad plane. The screws were then failed by a tensile load directed in the parasagittal plane. Full range of motion over C1-C2 and peak screw pull-out force was quantified. RESULTS.: Transpedicular technique generated significantly higher IT than the pars screws and marginally greater IT than intralaminar screws. With the intact atlantoaxial ligamentous complex, intralaminar fixation was superior to pars and similar to pedicle instrumentation at limiting axial torsion. After odontoid destabilization, however, this technique was less effective at reducing the lateral bending range of motion. Destructive loading revealed the highest pull-out forces with the pedicle screws, followed by intralaminar and pars screws.
  
  
  CONCLUSIONS
  Our results suggest that C2 intralaminar fixation provides a viable alternative to pedicle screws and is superior to pars instrumentation in cases with preserved atlantoaxial ligamentous attachments. In the presence of a traumatic dens fracture, however, intralaminar fixation may not be the optimal choice.
authors.affiliation: Department of Orthopaedics and Rehabilitation, Walter Reed Army Medical Center, Washington, DC 20012, USA. aedortho@gmail.com
authors.forename: Anton E
authors.initials: AE
authors.lastname: Dmitriev
authors.forename: Ronald A
authors.initials: RA
authors.lastname: Lehman
authors.forename: Melvin D
authors.initials: MD
authors.lastname: Helgeson
authors.forename: Rick C
authors.initials: RC
authors.lastname: Sasso
authors.forename: Craig
authors.initials: C
authors.lastname: Kuhns
authors.forename: Daniel K
authors.initials: DK
authors.lastname: Riew
mesh_terms: D015502:Absorptiometry, Photon; D001268:Atlanto-Axial Joint / Q000000981:diagnostic imaging / Q000601:surgery*; D001696:Biomechanical Phenomena; D015519:Bone Density; D001863:Bone Screws; D002102:Cadaver; D002574:Cervical Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D005593:Fracture Fixation, Internal / Q000009:adverse effects / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D066298:In Vitro Techniques; D007593:Joint Instability / Q000000981:diagnostic imaging / Q000601:surgery*; D009809:Odontoid Process / Q000000981:diagnostic imaging / Q000293:injuries / Q000601:surgery*; D011475:Prosthesis Failure; D016059:Range of Motion, Articular; D016103:Spinal Fractures / Q000000981:diagnostic imaging / Q000601:surgery*; D013718:Tensile Strength; D013997:Time Factors; D019415:Torque