Experimental investigation of failure load and fracture patterns of C2 (axis)

The injury mechanism and magnitude of failure load of C2 fractures are impo rtant in the clinical treatment of its fixation. The magnitudes of the fail ure load of C2 and the mechanism of injury in vivo are uncertain. According ly, nine C2 vertebrae obtained from cadaver spines, ranging in age from 51 to 80 years, were used for the study. Special restraint conditions were app lied to yield specific fracture of C2. With the posterior element potted po stero-anteriorly up to one-quarter of the inferior facet, posterior shear f orce ranging from 840 to 1220N was required to cause fracture across the pa rs interarticularis. For odontoid fracture study, a special rig was fabrica ted to encapsulate the body of C2 in a cell using ISOPON, and a thin layer of ISOPON sandwiched between the inferior facets and two lateral plates. Th e assembled rig permits slight sagittal movement of C2 about the cup latera l pivot supports. Failure load of between 900 and 1500 N was recorded for o dontoid fracture. These values are in agreement with published data. The ex periment carried out under these two different restraint conditions had spe cifically resulted in different fractures of C2. In reality, depending on f actors such as the inclination of this force vector applied to the head, th e precise posture at the time of trauma, the spinal geometry, and the physi cal properties, different types of fracture patterns of C2 may be produced. This additional data will be useful in the biomechanical study of C2 verte bra using analytical approaches, and in surgical anterior/posterior fixatio n using screws. (C) 2001 Elsevier Science Ltd. All rights reserved.