Screw placement in transoral atlantoaxial plate systems: an anatomical study.

OBJECT: The placement of an anterior atlantoaxial plate after transoral odontoid resection has been described by Harms. Recently, the authors of biomechanical and clinical studies have shown that this procedure, especially in combination with posterior wiring, is a good alternative to established, isolated posterior atlantoaxial fixation techniques. Reports on the anatomy of the atlas and axis primarily focus on the posterior surgical approach. Scarce research regarding the quantitative anatomy of the anterior aspect of C-1 and C-2 has been reported. This study was undertaken to measure relevant dimensions of C-1 and C-2 and their relation to the anterior transoral approach. The aim of the study was to determine "safe zones" for screw placement in anterior atlantoaxial plate fixation.
METHOD: Fifty human dry C-1 and C-2 vertebrae were obtained for direct anatomical, radiographic, and computerized tomography (CT) measurements. Thirty-two linear and four angular parameters were evaluated. All measurements were made using a digital caliper, ruler, or goniometer. Anatomical measurements were correlated with radiographic (anteroposterior, lateral, and craniocaudal) and CT (0.5-mm-slice thickness) measurements of the corresponding vertebrae. Additionally, bone mineral density (BMD) measurements of C-1 and C-2 were obtained in 20 patients. A safe zone for anterior screw placement in an atlas of bilateral trapezoid shape could be characterized. The average medial and lateral height of the trapezoid was 4.1 +/- 1.01 mm (range 1.4-6.7 mm) and 12.9 +/- 1.73 mm (range 8.7-17.4 mm), respectively. The distance between the sagittal plane and the medial and lateral walls of the trapezoid was 10.2 +/- 1.42 mm (range 8.9-12.8 mm) and 23.5 +/- 2.98 mm (range 21.7-30.7 mm), respectively. The average depth of the lateral masses was 22.3 +/- 2.04 mm (range 17.0-26.7 mm) in the sagittal plane. The average BMD in the safe zone of C-1 was 0.89 +/- 0.11 g/cm3 (range 0.75-1.01 g/cm3). Bone mineral density measurements at C-2 revealed a spheroid zone of low density at the basis of the dens (0.68 +/- 0.09 g/cm3). In contrast, high zones of BMD were found near the articular surfaces (C1-2: 0.97 +/- 0.11 g/cm3; C2-3: 0.94 +/- 0.12 g/cm3). The safe zone for anterior axis screw placement was V-shaped, limited cranially by a zone of low bone density and laterally by the vertebral artery groove. Correlations between radiographic and anatomical measurements were generally good (r2 = 0.78-0.95), but they were higher between CT and anatomical measurements (r2 = 0.86-0.99).
CONCLUSIONS: A quantitative understanding of the anterior anatomy of C-1 and C-2 is necessary when considering anterior atlantoaxial plate fixation after transoral odontoid resection. In this study the authors defined safe zones for anterior atlas and axis screw placement. The anterior atlantoaxial plate, as originally described by Harms, only partially respects these safe zones.