Considerations for the use of C7 crossing laminar screws in subaxial and cervicothoracic instrumentation.

Study Design: Radiographical and biomechanical analyses.

Objective: To determine the applicability of C7 laminar screw fixation using radiographical and biomechanical analysis.

Summary Of Background Data: The unique anatomy of C7 creates a challenge during instrumentation at the caudal aspect of the cervical spine and cervicothoracic junction. The C7 lateral mass is often smaller, resulting in increased difficulty for pedicle screw placement. The use of crossing laminar screw fixation is common in the upper cervical and thoracic spine; its use at the C7 level, however, has only recently appeared in the literature.

Methods: Radiographical: Computed tomographic scans from 72 patients were used to measure laminar thickness, spinolaminar angle, and length (i.e., from the spinolaminar junction to the contralateral lamina-lateral mass junction) for each C7 vertebrae. Biomechanical: The C2 and C7 vertebrae from 13 cadaveric cervical spines were obtained, scanned using pQCT (Stratec Electronics, Pforzheim, Germany) for bone mineral density, and then instrumented in the following manner: (1) bilateral crossing intralaminar screws in C2, (2) bilateral crossing intralaminar screws in C7, and (3) bilateral pedicle screws in each C7 specimen after completion of laminar screw biomechanical testing. Each specimen was cyclically loaded for 5000 cycles after which axial screw pullout tests were performed.

Results: Radiographical: Mean laminar thickness and length were 5.67 ± 1.00 mm and 25.49 ± 2.73 mm, respectively. Biomechanical: The mean load to failure was 610.3 ± 251 N for C7 laminar screws, 666.33 ± 373N for C7 pedicle screws, and 355 ± 250 N for C2 laminar screws. A student t test indicated no statistical difference in pullout strength between C7 laminar and C7 pedicle screws (P = 0.6).

Conclusion: The radiographical anatomy at C7 suggests that intralaminar screws can be placed in the majority of patients. The in vitro biomechanical analysis performed indicates that C7 laminar screws are as strong as C7 pedicle screws and significantly stronger than laminar screws at C2.

Level Of Evidence: N/A.