title: Atlas injury mechanisms during head-first impact.
abstract: METHODS
  An in vitro biomechanical study.
  
  
  OBJECTIVE
  To investigate atlas injury mechanisms due to horizontally aligned head-first impacts of a cadaveric neck model and to document atlas fracture patterns and associated injuries.
  
  
  BACKGROUND
  Experimental atlas injuries have been created by applying compression or radial forces to isolated C1 vertebrae, dropping weight or applying sagittal moments to the upper cervical spine segments, or vertical drop testing of head-neck specimens or whole cadavers. Atlas injuries that commonly occur due to horizontally aligned head-first impacts have not been previously investigated.
  
  
  METHODS
  Horizontally aligned head-first impacts into a padded barrier were simulated at 4.1 m/s, using a human cadaver neck model mounted horizontally to a torso-equivalent mass on a sled and carrying a surrogate head. Atlantal radial force was computed using head and neck load cell data. Postimpact dissection documented atlas and associated injuries. Average atlantal radial force peaks and their occurrence times were statistically compared (P < 0.05) among the first local and global peaks using paired t tests.
  
  
  RESULTS
  The first average local peak in radial atlantal force was significantly smaller (1240 vs. 2747 N) and occurred significantly earlier (24 ms vs. 46 ms) than the global force peak. Atlas injuries consisted of either 3- or 4-part burst fractures or incomplete lateral mass fracture unilaterally. Associated injuries included bony avulsion of the transverse ligament unilaterally and fractures of the occipital condyles, superior facets of the axis, or odontoid.
  
  
  CONCLUSIONS
  The results indicated that the varied atlas fracture patterns were due primarily to radial forces causing outward lateral expansion of its lateral masses. Anterior and posterior arch fracture locations are dependent, in part, upon the cross-sectional arch dimensions. Transverse ligament rupture or bony avulsion is likely associated with real-life atlantal burst fractures.
authors.affiliation: Biomechanics Research Laboratory, Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, New Haven, CT 06520, USA. paul.ivancic@yale.edu
authors.forename: Paul C
authors.initials: PC
authors.lastname: Ivancic
mesh_terms: D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D002102:Cadaver; D001270:Cervical Atlas / Q000293:injuries*; D002574:Cervical Vertebrae / Q000293:injuries; D006259:Craniocerebral Trauma / Q000150:complications*; D005260:Female; D050723:Fractures, Bone / Q000209:etiology* / Q000473:pathology; D006801:Humans; D008297:Male; D014949:Wounds, Nonpenetrating / Q000209:etiology* / Q000473:pathology