title: Biomechanics of halo-vest and dens screw fixation for type II odontoid fracture.
abstract: METHODS
  An in vitro biomechanical study of halo-vest and odontoid screw fixation of Type II dens fracture.
  
  
  OBJECTIVE
  The objective were to determine upper cervical spine instability due to simulated dens fracture and investigate stability provided by the halo-vest and odontoid screw, applied individually and combined.
  
  
  BACKGROUND
  Previous studies have evaluated posterior fixation techniques for stabilizing dens fracture. No previous biomechanical study has investigated the halo-vest and odontoid screw for stabilizing dens fracture.
  
  
  METHODS
  A biofidelic skull-neck-thorax model was used with 5 osteoligamentous whole cervical spine specimens. Three-dimensional flexibility tests were performed on the specimens while intact, following simulated dens fracture, and following application of the halo-vest alone, odontoid screw alone, and halo-vest and screw combined. Average total neutral zone and total ranges of motion at C0/1 and C1/2 were computed for each experimental condition and statistically compared with physiologic motion limits, obtained from the intact flexibility test. Significance was set at P < 0.05 with a trend toward significance at P < 0.1.
  
  
  RESULTS
  Type II dens fracture caused trends toward increased sagittal neutral zone and lateral bending range of motion at C1/2. Spinal motions with the dens screw alone could not be differentiated from physiologic limits. Significant reductions in motion were observed at C0/1 and C1/2 in flexion-extension and axial rotation due to the halo-vest, applied individually or combined with the dens screw. At C1/2, the halo-vest combined with the dens screw generally allowed the smallest average percentages of intact motion: 3% in axial rotation, 17% in flexion-extension, and 18% in lateral bending.
  
  
  CONCLUSIONS
  The present reduction in C1/2 motion observed, due to the halo-vest and dens screw combined is similar to previously reported immobilization provided by the polyaxial screw/rod system and transarticular screw fixation combined with wiring. The present biomechanical data may be useful to clinicians when choosing an appropriate treatment for those with Type II dens fracture.
authors.affiliation: Biomechanics Research Laboratory, Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, New Haven, CT 06520-8071, USA. paul.ivancic@yale.edu
authors.forename: Paul C
authors.initials: PC
authors.lastname: Ivancic
authors.forename: Naseem N
authors.initials: NN
authors.lastname: Beauchman
authors.forename: Fred
authors.initials: F
authors.lastname: Mo
authors.forename: Brandon D
authors.initials: BD
authors.lastname: Lawrence
mesh_terms: D001696:Biomechanical Phenomena; D001863:Bone Screws; D016267:External Fixators; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology / Q000601:surgery / Q000628:therapy; D008953:Models, Anatomic; D009333:Neck; D009809:Odontoid Process / Q000293:injuries* / Q000502:physiology / Q000601:surgery*; D016059:Range of Motion, Articular; D012886:Skull; D016103:Spinal Fractures / Q000503:physiopathology / Q000601:surgery* / Q000628:therapy*; D013123:Spinal Fusion; D013909:Thorax