COMPUTED TOMOGRAPHIC EVALUATION OF THE INTERNAL STRUCTURE OF THE LATERAL MASS OF THE LOWER CERVICAL-SPINE

Axial computed tomography scans with a slice thickness of 2 mm taken f rom 12 cervical spines were used to study the internal structure of th e lateral mass. Images representing the middle of zone I (Heller's cla ssification) and the top of zone III were analyzed. The measurement of zone I involved anterior cortex thickness (ACT) while measurements of zone III included ACT, lateral cortex thickness (LCT), posterior cort ex thickness (PCT), lateral mass thickness (LMT), and lateral mass wid th (LMW). The percentage of the ACT and PCT with respect to the LMT (A CT/LMT and PCT/LMT) were calculated. Results showed the average ACT in zone I ranged from 1.6 to 1.8 mm. In zone III, the average LMT and LM W ranged from 8 to 9 mm and 13 to 15 mm, respectively. The smallest LM T was found at C7. The average ACT and PCT for all levels ranged from 1.8 to 2 mm. The ACT with respect to the LMT (ACT/LMT) was approximate ly 17% to 19% for C3 to C5 and C7, and 15% for C6 separately. The PCT with respect to the LMT (PCT/LMT) was approximately 16% to 18% for C3 to C6, and 20% for C7. These results show the ventral cortex of the la teral mass is relatively thicker and support the concept of bicortical screw purchase.