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Introduction
  •  A fracture of the dens of the axis (C2)
  • Epidemiology
    • incidence
      • most common fracture of the axis
      • account for 10-15% of all cervical fractures
    • demographics
      • occur in bimodal fashion in elderly and young patients
        • elderly
          • commonoften missed, and caused by simple falls
          • associated with increased morbidity and mortality compared to younger patients with this injury 
        • young patients
          • result from blunt trauma to head leading to cervical hyperflexion or hyperextension
  • Pathophysiology
    • mechanism
      • displacement may be anterior (hyperflexion) or posterior (hyperextension)
        • anterior displacement
          • is associated with transverse ligament failure and atlanto-axial instability
        • posterior displacement
          • caused by direct impact from the anterior arch of atlas during hyperextension
    • biomechanics
      • a fracture through the base of the odontoid process severely compromises the stability of the upper cervical spine.
  • Associated conditions
    • Os odontoideum  
      • appears like a type II odontoid fx on xray
      • previously thought to be due to failure of fusion at the base of the odontoid
      • evidence now suggests it may represent the residuals of an old traumatic process
      • treatment is observation
Anatomy
  • Axis Osteology
    • axis has odontoid process (dens) and body
    • embryology
      • develops from five ossification centers  
      • subdental (basilar) synchondrosis is an initial cartilagenous junction between the dens and vertebral body that does not fuse until ~6 years of age  
      • the secondary ossification center appears at ~ age 3 and fuses to the dens at ~ age 12 
  • Axis Kinematics
    • CI-C2 (atlantoaxial) articulation
      • is a diarthrodal joint that provides 
        • 50 (of 100) degrees of cervical rotation 
        • 10 (of 110) degrees of flexion/extension 
        • 0 (of 68) degrees of lateral bend 
    • C2-3 joint
      • participates in subaxial (C2-C7) cervical motion which provides
        • 50 (of 100) degrees of rotation
        • 50 (of 110) degrees of flexion/extension
        • 60 (of 68) degrees of lateral bend
  • Occipital-C1-C2 ligamentous stability 
    • provided by the odontoid process and its supporting ligaments
      • transverse ligament  
        • limits anterior translation of the atlas
      • apical ligaments
        •  limit rotation of the upper cervical spine
      • alar ligaments
        • limit rotation of the upper cervical spine
  • Blood Supply
    • a vascular watershed exists between the apex and the base of the odontoid   
      • apex is supplied by branches of internal carotid artery
      • base is supplied from branches of vertebral artery
      • the limited blood supply in this watershed area is thought to affect healing of type II odontoid fractures.
Classification
  •  Anderson and D'Alonzo Classification  
Anderson and D'Alonzo Classification
Type I  Oblique avulsion fx of tip of odontoid. Due to avulsion of alar ligament. Although rare, atlantooccipital instability should be ruled out with flexion and extension films.
  
Type II  Fx through waist (high nonunion rate due to interruption of blood supply).
  
 
Type III  Fx extends into cancellous body of C2 and involves a variable portion of the C1-C2 joint.
  
 
Grauer Classification of Type II Odontoid fractures
Type IIA Nondisplaced/minimally displaced with no comminution. Treatment is external immobilization  
Type IIB Displaced fracture with fracture line from anterosuperior to posteroinferior. Treatment is with anterior odontoid screw (if adequate bone density).  
Type IIC  Fracture from anteroinferior to posterosuperior, or with significant comminution. Treatment is with posterior stabilization.  
 
Presentation
  • Symptoms
    • neck pain worse with motion
    • dysphagia may be present when associated with a large retropharyngeal hematoma
  • Physical exam
    • myelopathy
      • very rare due to large cross section area of spinal canal at this level
Imaging
  • Radiographs
    • required views
      • AP, lateral, open-mouth odontoid view of cervical spine
    • optional views
      • flexion-extension radiographs are important to diagnose occipitocervical instability in Type I fractures and Os odontoideum
        • instability defined as
          • atlanto-dens-interval (ADI) > 10mm 
          • < 13mm space available for cord (SAC)
    • findings
      • fx pattern best seen on open-mouth odontoid  
  • CT
    • study of choice for fracture delineation and to assess stability of fracture pattern
  • CT angiogram
    • required to determine location of vertebral artery prior to posterior instrumentation procedures 
  • MRI
    • indicated if neurologic symptoms present  
Treatment

Treatment Overview
Os Odontoideum Observation
Type I  Cervical Orthosis
Type II Young Halo if no risk factors for nonunion
Surgery if risk factors for nonunion
Type II Elderly Cervical Orthosis if not surgical candidates
Surgery if surgical candidates
Type III  Cervical Orthosis
  • Nonoperative
    • observation alone
      • indications
        • Os odontoideum
          • assuming no neurologic symptoms or instability
    • hard cervical orthosis for 6-12 weeks   
      • indications
        • Type I 
        • Type II in elderly who are not surgical candidates 
          • union is unlikely, however a fibrous union should provide sufficient stability except in the case of major trauma
        • Type III fractures   
          • no evidence to support Halo over hard collar
    • halo vest immobilization for 6-12 weeks 
      • indications
        • Type II young patient with no risk factors for nonunion
      • contraindications
        • elderly patients
          • do not tolerate halo (may lead to aspiration, pneumonia, and death)
  • Operative
    • posterior C1-C2 fusion 
      • indications
        • Type II fractures with risk factors for nonunion
        • Type II/III fracture nonunions 
        • Os odontoideum with neurologic deficits or instability
    • anterior odontoid osteosynthesis
      • indications
        • Type II fractures with risk factors for nonunion AND
          • acceptable alignment and minimal displacement
          • oblique fracture pattern perpendicular to screw trajectory
          • patient body habitus must allow proper screw trajectory
      • outcomes
        • associated with higher failure rates than posterior C1-2 fusion
    • transoral odontoidectomy
      • indications
        • severe posterior displacement of dens with spinal cord compression and neurologic deficits
Surgical Techniques
  • Halo immobilization 
    • in children and adults 
  • C1-C2 posterior fusion techniques
    • approach
      • posterior midline cervical approach
    • stabilization technique
      • sublaminar wiring techniques (Gallie or Brooks)
        • require postoperative halo immobilization and rarely used
      • posterior C1-C2 transarticular screws construct  
        • contraindicated in patients with an aberrant vertebral artery 
      • posterior C1 lateral mass screw and C2 pedicle screw construct  
        • modern screw constructs do not require postoperative halo immobilization
    • outcomes
      • C1-C2 fusion will lead to 50% loss of neck motion
  • Anterior odontoid screw osteosynthesis  
    • approach
      • anterior approach to cervical spine 
    • technique
      • single screw adequate
    • pros & cons
      • associated with higher failure rate than posterior C1-2 fusion
      • advantage is preservation of atlantoaxial motion
  • Transoral odontoidectomy post
    • technique
      • usually combined with posterior stabilization procedure
Complications
  • Nonunion
    • increased risk in Type II fractures due to poor blood supply
      • average nonunion rate 33% (up to as high as 88%)
    • risk factors for nonunion include  
      • ≥ 6 mm displacement (>50% nonunion rate)
        • strongest reason to opt for surgery
      • age > 50 years
      • fx comminution
      • angulations > 10°
      • delay in treatment
      • smoker
 

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