Anatomy of Lamina in the Subaxial Cervical Spine with the Special Reference to Translaminar Screws: CT and Cadaveric Analysis with Screw Trajectory Simulation.

STUDY DESIGN: Cadaveric Study.

SUMMARY OF BACKGROUND DATA: Translaminar screws were initially developed for C2 fixation. Since then, their usage has expanded to include the subaxial cervical spine, and thoracic and lumbar spine. To our knowledge, special anatomy for inserting translaminar screws in the subaxial cervical spine have not been studied.

OBJECTIVE: To report the special anatomy for inserting translaminar screws in the subaxial cervical spine.

METHODS: 18 cadaveric spines were harvested from C3 to C7 and 1-mm CT scans and 3D reconstructions were obtained. Bilateral translaminar screw entry points and trajectories were simulated at each level from C3 to C7 utilizing Kodak Carestream/Pacs Ver 10.2. Constructs were selected to achieve maximal bony purchase with one screw, designated the "primary screw". The contralateral screw, designated the "secondary screw", was selected to achieve the optimal allowable diameter possible while avoiding a simulated cortical breach, which was not always necessarily the "best purchase" diameter. Initial screw diameters selected were 3.5 mm; however, in the event that a narrower portion was encountered, then a 3.0 mm diameter screw was utilized instead. The crossing area of both screws were calculated geometrically. Maximal thickness of the lamina was considered in determining the diameter of screws. Whenever possible 3.5 mm screws were selected in both lamina (3.5/3.5-mm), however if a 3.5 mm screw was utilized as the primary screw, but the permissible range (P) for the secondary screw was less than 3.5 mm, then a hybrid construct was utilized (3.5/3.0-mm). In cases where P was less than 3 mm, then both screws were studied at 3 mm (3.0/3.0-mm). Screw diameters that optimized trajectory and bony purchase, while remaining within the permissible range, were analyzed, tabulated, and recorded. On CT, along the trajectory of the screws, the image was cut and measured in terms of screw length, the narrowest portion of the lamina, vertical angle, and horizontal angle in both primary and secondary screws. On the individually separated cervical spine segments in cadavers (11 out of 18), we performed caliper measurements on the same portions which were measured on CT. It could not be exactly the same portions, however, due to the three dimensional characteristics of the specimens.

RESULTS: For C3, only one specimen allowed two screws (3/3 mm), while the remaining specimens permitted a unilateral primary screw (3.5 or 3 mm) only. For C4, 37% of specimens allowed two screws (3.5/3 mm or 3/3 mm), but the rest allowed only a unilateral primary screw(3.5 or 3 mm). For C5, 58% allowed two screws (3.5/3.5 mm, 3.5/3 mm, or 3/3 mm). For C6, 89% of specimen allowed two screws (3.5/3.5 mm, 3.5/3 mm, or 3/3 mm). For C7, all levels allowed two screws (3.5/3.5 mm, 3.5/3 mm, 4/4 mm, 4/3 mm, 4.5/3 mm, 4.5/3.5 mm, or 4/3.5 mm). On CT, the average lengths of the 1° and the 2° screws were 26.14 mm and 24.01 mm, respectively. The average vertical and horizontal angles were 22.26° and 40.66° for the 1° screw, and 3.45° and 45.59° for the 2° screw. On cadavers, the average lengths of the 1° and the 2° screws were 22.58 mm and 23.44 mm, respectively. The average vertical and horizontal angles were 23.67° and 54.44° for the 1° screw, and 2.28° and 54.89° for the 2° screw.

CONCLUSION: This is a report of the anatomy of the lamina in the subaxial cervical spine with the special reference to translaminar screws. It was analyzed with CT and cadaveric spines along with simulated screw trajectories. For the 1° translaminar screw, the entry point is the distance of the diameter of desired screw superior to the inferior margin of lamina-spinous process junction. The trajectory should be targeted towards the most superomedial corner of lateral mass. For the 2° translaminar screw, the entry point is the distance of the diameter of desired screw below the superior margin of lamina-spinous process junction, and the target is the most superolateral corner of lateral mass, which is typically horizontal. Further studies are needed to assess the feasibility of translaminar screw insertion in the actual subaxial cervical spine.