Morphometric and volumetric analysis of the lateral masses of the lower cervical spine

STUDY DESIGN. Morphometric and volumetric analysis. OBJECTIVE. To define the morphometric and volumetric variance of the lateral masses of C5, C6, and C7. SUMMARY OF BACKGROUND DATA. Lateral mass screws are routinely placed throughout the subaxial cervical spine but the morphology of the C7 lateral mass is associated with greater difficulty in obtaining adequate purchase. Previous studies have suggested that modified techniques may be associated with better screw purchase in the C7 lateral mass and subsequent avoidance of complications such as foraminal intrusion and C7-T1 facet joint violation. It is necessary to systematically examine the morphometric and volumetric differences among lower cervical lateral masses in order to define the morphologic differences as this has implication with C7 lateral mass instrumentation. METHODS. Morphometric and volumetric analysis was performed on computed tomography scans of the of the lateral masses of C5, C6, and C7 in 25 male and 25 female patients in the axial, coronal, and sagittal planes. Volumetric analysis was conducted by tracing the lateral mass at progressive 1 mm axial slices and images were linked for complex volume calculation. RESULTS. At all levels and for all measurements, men and women differed significantly (P < 0.05). Between C5 and C6, sagittal height differed in both men and women, whereas axial articular process (AP) diameter differed only in males (9.2 mm at C5 vs. 10.5 mm at C6). C7 differed statistically from C5 and C6 as measurements increased in sagittal height, sagittal diagonal height, coronal height (women only), and axial AP diameter (women only). C7 measurements decreased relative to the other levels in medial-to-lateral measurements in the axial plane (12.8 vs. 11.5 mm in men, 11.4 vs. 10.5 mm in women), sagittal thickness and axial AP diameter only differed in women. Importantly, the angulation of the lower cervical spine decreases by approximately 6° moving toward C7. There was no statistical difference in lateral mass volume between any of the vertebrae. CONCLUSION. The degree of cervical angulation represents a consistent and significant anatomic relationship in the lower cervical spine and may necessitate a modified surgical approach.