Biomechanical analysis of cranial settling after transoral odontoidectomy.

The authors conducted a biomechancial study to determine whether C-1 ring integrity is important in maintaining normal occiput-C-2 separation, specifically when the anterior arch is transected to provide access to the dens during an odontoidectomy procedure. Six human cadaveric occiput-C3 specimens were loaded under axial compression, and the bilateral horizontal separation of the C-1 lateral masses and the vertical compression of the occiput relative to C-2 were recorded. Specimens were first studied after odontoidectomy without C-1 ring transection, then after C-1 anterior arch transection, and finally after C-1 lamina transection. With applied compressive load corresponding to three times the weight of the head, the C-1 ring spread horizontally 1.57 +/- 0.30 mm more when the anterior arch of C-1 was transected than when left intact, resulting in 0.74 +/- 0.44 mm collapse in the occiput-C-2 vertical separation. After laminar transection, the C-1 ring spread 6.55 +/- 2.29 mm more than when it was intact. The resultant vertical separation was a 3.37 +/- 1.89-mm collapse in the occiput-C-2. All changes in C-1 spreading and the occiput-C-2 collapse were statistically significant (p < 0.05, paired Student's t-tests). The C-1 ring continuity prevents horizontal spreading caused by the wedging of C-1 between the occiput and C-2 and thus prevents cranial settling. Therefore, to prevent the subsequent development of disease related to cranial settling, the authors recommend that the surgeon resect part of C-1 only if necessary during odontoidectomy.