Screw placement in transoral atlantoaxial plating systems: an anatomical study

Abstract

<jats:p content-type="fine-print"><jats:italic>Object.</jats:italic> 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.</jats:p> <jats:p content-type="fine-print"><jats:italic>Method.</jats:italic> 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.</jats:p> <jats:p content-type="fine-print">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/cm<jats:sup>3</jats:sup> (range 0.75–1.01 g/cm<jats:sup>3</jats:sup>). 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/cm<jats:sup>3</jats:sup>). In contrast, high zones of BMD were found near the articular surfaces (C1–2: 0.97 ± 0.11 g/cm<jats:sup>3</jats:sup>; C2–3: 0.94 ± 0.12 g/cm<jats:sup>3</jats:sup>). 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 (r<jats:sup>2</jats:sup> = 0.78–0.95), but they were higher between CT and anatomical measurements (r<jats:sup>2</jats:sup> = 0.86–0.99).</jats:p> <jats:p content-type="fine-print"><jats:italic>Conclusions.</jats:italic> 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.</jats:p>

Anatomical study of axis for odontoid screw thickness, length, and angle

“…For 2-screw method, the critical transverse outer diameter of odontoid is recommended; at least 9 mm for the placement of two 3.5 mm cortical screws [16]. In our measurements, the outer diameter was found to be 11.3 ± 0.9 mm and well correlated with Schaffer et al's study as 11.6 mm [19], with Kandziora et al's study as 10.9 mm [12]. Chang et al reported that type II odontoid fractures stabilized with a single 4.5 mm cannulated Herbert screw have increased torsional stiffness as compared with a 2-screw technique using 3.5 mm anterior odontoid screws [4].…”

C2-fractures: part I. Quantitative morphology of the C2 vertebra is a prerequisite for the radiographic assessment of posttraumatic C2-alignment and the investigation of clinical outcomes

“…In an anatomical study of Kandiziora et al [34], based on 50 axis vertebrae, the radiologic average of posterior body height was 18.2 mm, though this measurement was 24.6 mm in the current study using a different measuring technique. Similarly, the C2 superior body depth was 12.5 mm [34], but was 14.9 mm in our study using the Harris-Ring-C2 as the reference landmark.…”

“…Calculating our data according to their measurement technique gave similar results (65.9°left sides/68.6°right sides). In the study of Kandiziora et al [34], the angle between the horizontal plane and the superior facet of C2, which was 23.4°in the current study, was 23.9°on average. Zapletal et al [71] evaluated the incidence of AAOA radiologically in 355 patients and considered the C1-2 joints degenerative when severe narrowing or obliteration of the joint space, subchondral sclerosis, and/or osteophytes were present.…”

“…Similarly, the C2 superior body depth was 12.5 mm [34], but was 14.9 mm in our study using the Harris-Ring-C2 as the reference landmark. Monu et al [47] also measured the odontoid tilt angle with a technique identical to ours in 175 patients with a mean age of 32.2 years.…”

“…In a study of Kandiziora et al [34], only slight differences were observed between measurements performed using direct caliper technique, radiographs and CT-scans. To simplify the application of our radiographic results to further studies, including CT-and MRI-scans, we calculated anatomical ratios and side-related differences scaled in 'millimeters'.…”