Morphometric and radiological assessments of dimensions of Axis in dry vertebrae: A study in Indian population

BACKGROUND: The technique of intralaminar screw placement for achieving axis (C2) fixation has been recently described. The purpose of the study was to provide the morphometric and radiological measurements in Indian population and to determine the feasibility of safe translaminar screw placement in this population. To the best of our knowledge there is no study (cadaveric or radiological) done in Indian population to detect suitability of axis bone for laminar screw fixation. MATERIAL AND METHODS: 38 dry axis vertebrae from adult South Indian population were subjected to morphometric measurement and CT scan analysis. Height of posterior arch, midlaminar width(bilateral) in upper 1/3(rd), middle 1/3(rd) and lower 1/3(rd) were measured using high precision Vernier Calipers. Each vertebra was subjected to a spiral CT scan (Philips brilliance 16 slice) thin 0.5 mm slices were taken and reconstruction was done in coronal and sagittal plane. Analysis was done on a CT work station. Using axial slices, sagittal cuts were reconstructed in plane perpendicular to the lamina at the mid laminar point and upper-middle and lower 1/3(rd) width of the lamina measured. Height of the posterior arch was measured in the sagittal plane. Intralaminar angle was measured bilaterally. RESULTS: Middle 1/3(rd) lamina was the thickest portion (mean 5.17 mm +/- 1.42 mm). A total of 32 (84.2%) specimen were having midlaminar width in both lamina greater than 4 mm, however only 27 (71%) out of them had spinous process more than 9 mm. CT scan measurement in middle and lower 1/3(rd) lamina was found to be strongly correlated with the direct measurement. CONCLUSION: There is high variability in the thickness of the C2 lamina. As compared to western population, the axis bones used in the present study had smaller profiles. Hence the safety margin for translaminar screw insertion is low.