The options of C2 fixation for os odontoideum: a radiographic study for the C2 pedicle and lamina anatomy

Introduction Patients with os odontoideum always present instability in atlantoaxial joint and need atlantoaxial fixation. C2 pedicle or laminar screws fixation has proven to be efficient and reliable for atlantoaxial instability. However, os odontoideum is a congenital or developmental disease, featured with anomalous bony anatomies. The anatomic measurements and guidelines for C2 pedicle screw placement in general population tends to differ with those of os odontoideum patients, for whom C2 pedicle screws are often needed. The option and techniques of C2 fixation are still challenging and yet to be fully explored. Material and methods We recruited 29 adult patients with os odontoideum and measured the dimension of C2 pedicle and lamina for each patient to examine how well do they match with the screws anatomically. In order to access the intra-observer reliability and inter-observer repeatability of the measurements, the intraclass correlation coefficient (ICC) was also calculated. Results The results for reliability of the CT measurements showed excellent intraobserver (ICC = 0.95 and 0.96) and interobserver correlation coefficient (ICC = 0.93). The diameter and length of C2 pedicle were found to be 6.06 ± 1.37 and 24.05 ± 2.54 mm, while the corresponding figures of C2 laminar were 6.95 ± 0.82 and 25.60 ± 2.18 mm, respectively. In the measurements, all 29 cases had suitable diameter (larger than 5.5 mm) for C2 laminar screw (the laminar diameters ranged from 5.52 to 8.82 mm). In C2 pedicle measurements, the diameters of the 29 cases were from 3.50 to 9.86 mm, while 20 pedicles (34.5%) in 14 cases were less than 5.5 mm in diameter. Six had bilateral small pedicles where the diameter was less than 5.5 mm. Conclusion Anatomically, we found laminar screw is a better match in comparison with pedicle screw for C2 fixation in os odontoideum. The options for C2 fixation should be made based on careful preoperative imaging and thorough consideration. Preoperative reconstructive CT scan can offer great assistance for the choice of fixation in os odontoideum by revealing the anatomy of the C2 pedicles in detail.