Computed Tomography – Based Morphometric Analysis of The Subaxial Cervical Spine Pedicles in a Filipino Population

Background:The use of transpedicular screw fixation has been seen as particularly effective in treating subaxial cervical spine (C3-C7) instability. However, despite its biomechanical superiority, its acceptance among surgeons is limited. Perceived as technically demanding because of the complex anatomy of the subaxial cervical spine and its inherently narrow pedicles allowing for a very limited space during transpedicular screw insertion; the proximity of vulnerable neural and vascular structures also lead to an increased risk of complications from pedicle violation. Characterizing the morphology of the pedicles not only enables accurate identification of the screw axis but also aids to avoid neurovascular complications, as a quantitative three-dimensional understanding of the morphology of these pedicles can assist the surgeon in making informed decisions whenever transpedicular screw fixation of the subaxial cervical spine is being considered. 

Objective:The objective of this study was to perform a morphometry of the subaxial cervical spine pedicles in a Filipino population. Measurements of the linear dimensions and angular projections of each pedicle were acquired, then analyzed to explore the level-, side-, and sex-specific morphologic trends and differences in the study population. These measurements were intended to contribute to the better understanding of the Filipino subaxial cervical spine pedicles in the application of surgical methods such as transpedicular screw fixation. 

Methods:This is a retrospective review of cervical computed tomography (CT) scans showing normal cervical vertebrae of Filipino patients 18 years of age and above. The following subaxial cervical spine pedicle parameters were measured; linear parameters: outer pedicle width (OPW), inner pedicle width (IPW), outer pedicle height (OPH), inner pedicle height (IPH), pedicle length (PL), pedicle axis length (PAL); angular projections: pedicle transverse angle (PTA), and pedicle sagittal angle (PSA). The mean and standard deviations for each parameter were calculated. The t test was used to determine if there were any significant differences (p < 0.05) in the pedicle morphologic parameters according to side and sex.

Results: A total of 1,470 pedicles were measured from 735 subaxial cervical vertebrae in 147 subjects. For all the linear parameters measured, a gradual increase was noted from C3 to C7; for the angular projections, the PTA variations from C3 to C7 demonstrated that they were wider in the upper subaxial cervical spine, C3 to C5, and narrower in the lower cervical region at C6 and C7. The PSA demonstrated a gradual change from upward inclination at the upper subaxial cervical spine, C3 to C5, to the downward inclination at the lower cervical region, C6 and C7. The respective differences between the left and right sides were not statistically significant (p > 0.05), except for: a. IPW at C4 (p = 0.008) and C7 (p = 0.047); b. PAL at C6 (p = 0.014) and PSA at C5 (p = 0.031). For the linear parameters measured, male subaxial cervical spine pedicles were significantly larger than females (p < 0.05) except for PL at C7 (p = 0.0786). The differences in the PTA for sex were not statistically significant (p > 0.05) except for C4 (p = 0.001); the PSA in males were significantly different (p < 0.05) than females except for C3 (p = 0.922) and C7 (p = 0.763).

Conclusion:A thorough understanding of the morphology of the subaxial cervical spine pedicles is the key to safe and successful transpedicular screw fixation. Hence, the importance of pre-operative planning with the use of CT and conventional radiography in conjunction with morphometric data cannot be overemphasized. Our data demonstrated that pedicle morphology is, and therefore screw placement and orientation is significantly different at each subaxial cervical spinal level and between men and women. It is therefore necessary to account for these differences during surgical planning to reduce the risks of pedicle perforation and neurovascular injury. 

Keywords: subaxial cervical spine, pedicle, morphometry, transpedicular screw fixation, computed tomography