A biomechanical analysis of triangulation of anterior vertebral double-screw fixation

Objective. This study tested the hypothesis that triangulation of two anterior vertebral screws without penetration of the cortex offers more resistance to pullout than two screws placed in parallel and penetrated. Design. The pullout strength for two parallel or two triangulated anterior vertebral screws fixation, with a uni-cortical or bicortical purchase, were tested and compared to the strength of a single-screw fixation with a bi-cortical purchase. Four porcine spines (six months old) were used for biomechanical test and bone mineral density was measured for each specimen before testing. Background. The potential hazards from penetration by anterior vertebral cortex screws including neurovascular and organs injuries are well documented. However, bi-cortical screw penetration is widely recognized as necessary for good anterior spinal stabilization. The authors are not aware of any biomechanical study on the anterior placement of triangulated vertebral screws without penetration and its effect on the fixation strength of anterior vertebral device remains unclear. Methods. In this study five modes of screw fixations in lateral vertebral bodies were performed: Group A, triangulated screws with one screw penetration; Group B, triangulated screws without penetration; Group C, parallel penetrating screws, Group D, parallel nonpenetrating screws; and Group E, a single-screw with bi-cortical purchase. Biomechanical analysis with a material testing system machine was performed to determine the pull out strength of each configuration. Results. The results showed that the pullout strength in the various double-screw fixation modes were statistically increased as compared to that of the single-screw with bi-cortical purchase mode. There existed statistical differences (P < 0.05) between Groups A and B, Groups C and D and Groups D and E, respectively. However, no significant difference was found between Groups B and C (P = 0.144). Conclusions. Based on the current data, triangulation of two anterior vertebral screws without penetration of the cortex (Group B) achieved pullout strengths similar to that of two-parallel double-cortical screws (Group Q. The authors believe that this is an attractive alternative in anterior spinal instrumentation avoiding the potential risks of cortical penetration. However, in the event of pullout failure, the triangulation configuration will produce a more disastrous consequence.