[Classification of atlas pedicles and methodological study of pedicle screw fixation]

To investigate the classification of atlas pedicles and the methods of the pedicle screw fixation.

To study the classification of atlas pedicles, 48 dry adult atlas specimens were measured. By atlas 3D-CT reconstruction, two transverse sections were established by going through the one third of the lateral atlas pedicle and 2 mm below the vertebral artery sulcus. By setting 3.50 mm and 1.75 mm as the standardized diameter and radius for the screw and according to the thickness of bone substance of vertebral artery sulcus that went through the one third of the lateral atlas pedicle, the anatomical morphology of atlas pedicles were classified into three types: general type with 40 specimens (83%), light variation type with 6 specimens (13%), and severe variation type with 2 specimens (4%). The entry pathway was confirmed by the intersection line of the two transverse sections that went through the lateral one third of the atlas pedicle and 2 mm below the vertebral artery sulcus. The project-point of the entry pathway on the atlas posterior arch was considered to be the entry point. Forty-eight dry atlas specimens were used to measure the following relevant anatomic data with an electronic caliper: the distance between the entry point and the posterior margin of the lateral mass (L1), the height of atlas pedicle at the entry point (L2), the vertical distance between the entry point and the inferior articular facet of the lateral mass (L3), the mass height at the entry point (L4), the mass width at the entry point (L5), the width of the atlas pedicle at the entry point (L6), the thickness of the pedicle under the vertebral artery sulcus at the entry pathway (H1). To research the method of the pedicle screw fixation, 12 fresh-frozen adult atlas specimens were adopted to simulate the fixation of the pedicle screw. The thickness of the bone substance of vertebral artery sulcus on both the left and the right sides of the pathway was grinded into 3 types: 1.5 mm and 2.5 mm, 1.5 mm and 4.0 mm, 2.5 mm and 4.0 mm, and each type had four specimens. The entry pathway - was confirmed by the intersection line of two transverse sections that went through the lateral one third of atlas pedicle and 2 mm below the vertebral artery sulcus.

On the left side, L1 was (5.79 +/- 1.24) mm, L2 (4.55 +/- 1.29) mm, L3 (5.12 +/- 1.06) mm, L4 (12.43 +/- 1.01) mm, L5 (12.66 +/- 1.37) mm, L6 (7.86 +/- 0.77) mm, and H1 (4.11 +/- 1.25) mm. On the right side, L1 was (5.81 +/- 1.26) mm, L2 (4.49 +/- 1.22) mm, L3 (5.15 +/- 1.05) mm, L4 (12.49 +/- 0.98) mm, L5 (12.65 +/- 1.38) mm, L6 (7.84 +/- 0.78) mm, and H1 (4.13 +/- 1.29) mm. There was no significant difference between the two sides (P > 0.05). After simulation of inserting screws, no screw in the specimens was found to break the bone substance in the sulcus of vertebral artery. Conclusion For the pedicle screw fixation of those patients whose atlas posterior arches are not high enough, we might partly drill through or beyond the atlas posterior arch. The entry point should be ascertained by preoperative 3D-CT reconstruction and intra-operative exploration.