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 bi-cortical 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 C). 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.
Triangulation of two anterior vertebral screws without penetration of the cortex achieve
pullout strengths similar to that of two-parallel double-cortical screws. This is
an attractive alternative in anterior spinal instrumentation that avoids the potential
risks of cortical penetration.