4 Quantitative Anatomy of the Lateral Masses of the Atlas and Axis Vertebrae

10.1055/b-0034-81381

4 Quantitative Anatomy of the Lateral Masses of the Atlas and Axis Vertebrae

Shah, Abhidha, Goel, Atul

Both lateral mass fixation of the C1 and C2 vertebrae using the plate-and-screw method and transarticular screw fixation provide biomechanically stable unions in patients with atlantoaxial dislocation.1–3 Various groups, however, have indicated their concern about the safety of these techniques, considering the danger to the vertebral artery.4,5 To use these techniques optimally and safely, it is necessary for the surgeon to have exact anatomical information about the region in a three-dimensional perspective. Only a few reports in the literature describe the quantitative measurements of the craniovertebral region and the parameters appropriate for screw implantation. Most of these reports are based on radiological measurements6–8 and deal with the dens and canal diameters.9–12 There are only isolated reports detailing quantitative measurements of the lateral masses of C1 and C2.4,13,14

This chapter considers the various dimensions of the lateral masses of the atlas and axis vertebrae and analyzes their relation with the vertebral artery foramen. It also outlines the safe sites of entry and trajectories for screw implantation. These findings are based on a cadaveric study by Gupta and Goel of 50 dried C1 and C2 vertebrae.14

Anatomy

Atlas

In the study by Gupta and Goel, the superior facet of the atlas was oval and had a groove on either side in the center of the facet in 76% of specimens; it was kidney-shaped in 24% of specimens, and in those cases, the groove was on only one side of the facet ( Fig. 4.1 ).14 The anteroposterior dimension (mean 19.73 mm) was more than the transverse dimension (mean 11.12 mm) in all superior facets. In no specimen was the facet found to be exactly symmetrical to that on the contralateral side. Both the superior and inferior facets of the atlas face medially toward the spinal canal.14

**Table 4.1** Analysis of atlas vertebrae
Definition of Parameters	Range (mm)	Mean (mm)
Maximum anteroposterior dimension of the articular surface of the inferior facet	13–21	15.76
Maximum transverse dimension of the articular surface of the inferior facet	13–20	15.22
Maximum anteroposterior dimension of the articular surface of the superior facet	15–26	19.73
Maximum transverse dimension of the articular surface of the superior facet	8–16	11.12
Thickness of the medial surface of the lateral mass	4–12	8.57
Thickness of the lateral surface of the lateral mass	15–25	19.11
Screwable thickness of the posterior surface of the inferior facet in the vertical plane	3–7	4.16
Screwable thickness of the posterior surface of the inferior facet in the transverse plane	8.5–15.0	10.65
Thickness of the posterior arch of the atlas near the facet	3.0–7.5	5.05
Best angle for screw implantation in relation to the sagittal plane	15–20°	15.07°

The inferior facet of the atlas was circular in most of the vertebrae, and the mean anteroposterior (15.76 mm) and transverse (15.22 mm) dimensions of this facet were similar. The vertebral artery foramen lies in the transverse process lateral to the lateral mass. The thickness of the facet under the lateral aspect of the posterior arch of the atlas was 3 to 7 mm (mean 4.16 mm) in the vertical plane and 8.5 to 15 mm (mean 10.65 mm) in the transverse plane. The thickness of the posterior arch of the atlas in relation to the facet of the atlas varied from 3.0 to 7.5 mm (mean 5.05 mm).

The safe angle for screw implantation in the inferior facet of the atlas was determined to be 10 to 20° (mean 15.7°) medial to the sagittal plane and 10 to 20° (mean 15°) superior to the axial plane. In the atlas, the safe site for entry of the screw is in the middle of the posterior surface of the inferior facet. Table 4.1 shows the results of measurements of C1 vertebrae.

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