Applied Anatomy of Transoral Atlantoaxial Reduction Plate Internal Fixation

Study Design. A C1–C2 operation by the transoral approach was simulated to study the anatomic stratification, various structures, and adjacent relationships. The anatomic parameters in relation to transoral atlantoaxial reduction plate (TARP) internal fixation were measured. Objectives. To study relevant anatomic features of the structures involved in TARP internal fixation through transoral approach for treating irreducible atlantoaxial dislocation, so as to provide anatomic basis for the clinical application of TARP. Summary and Background Data. Irreducible anterior atlantoaxial dislocation (IAAD) with ventral spinal cord compression is difficult for surgical correction. Despite previous description of direct plate internal fixations through the transoral approach, the problem has not been fully resolved: the Harms' plate lacked a locking mechanism while the other plates unable to achieve immediate reduction of the atlantoaxial joint. We therefore designed the TARP system with which the decompression, reduction, internal fixation, and fusion procedures could be completed in the same transoral approach. The anatomic structures and stratification involved in the transoral approach, which were seldom addressed in previous anatomic studies, need to be clarified for internal fixation with TARP system. Methods. Twenty fresh craniocervical specimens were microsurgically dissected layer by layer according to a transoral approach. Stratification of the posterior pharyngeal wall, course of the vertebral artery, anatomic relationships of the adjacent structures of the atlas and axis, and closely relevant anatomic parameters for TARP internal fixation were measured. Results. The posterior pharyngeal wall consisted of two layers and two interspaces: the mucosa, prevertebral fascia, retropharyngeal space, and prevertebral space. The range from the anterior edge of the foramen magnum to C3 could be exposed by this approach. The thickness of the posterior pharyngeal wall was 3.6 ± 0.3 mm (range, 2.9–4.3 mm) at the anterior tubercle of C1, 6.1 ± 0.4 mm (range, 5.2–7.1 mm) at lateral mass of C1 and 5.5 ± 0.4 (range, 4.3–6.5 mm) at the central part of C2, respectively. The distance from the incisor tooth to the anterior tubercle of C1, C1 screw entry point, and C2screw entry point was 82.5 ± 7.8 mm (range, 71.4–96.2 mm), 90.1 ± 3.8 mm (range, 82.2–96.3 mm), and 89.0 ± 4.1 mm (range, 81.3–95.3 mm), respectively. The distance between the vertebral artery at atlas and the midline was 25.2 ± 2.3 mm (range, 20.4–29.7 mm) and that between the vertebral artery at the axis and the midline was 18.4 ± 2.6 mm (range, 13.1–23.0 mm). The allowed width of the atlas and axis for exposure was 39.4 ± 2.2 mm (range, 36.2–42.7 mm) and 39.0 ± 2.1 mm (range, 35.8–42.3 mm), respectively. The distance (a) between the two atlas screw insertion points (center of anterior aspect of C1 lateral mass) was 31.4 ± 3.3 mm (range, 25.4–36.6 mm). The vertical distance (b) between the line connecting the two C1 screw entry points and that connecting the two C2 screw entry points (at the central part of the vertebrae, namely, 3 to 4 mm lateral to the midline of C2 vertebrae) was 21.3 ± 2.7 mm (range, 19.4–24.3 mm), with an a/b ratio of 1.3 to 1.5. The screws of TARP had a lateral tilt of 12.2° ± 0.4° (range, 10.2°–14.6°) at C1 and a medial tilt of 7.3° ± 0.3° (range, 5.1°–9.4°) at C2 relative to the coronal plane. Conclusion. An atlantoaxial surgery through transoral approach is safe and feasible. This approach is suitable for an anterior TARP internal fixation, and the design of the internal fixation system should be based on the above anatomic data.