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Biomechanical comparison of conventional technique versus oblique screw placement in plate fixation.

BACKGROUND
The aim of this study was to evaluate whether plate fixation with screws in an oblique fashion would change the strength of the fixation to bending forces and whether any relationship existed between deformity and increased strength of fixation.


METHODS
Rear leg tibias from 2-year to 3-year-old sheep were affixed with the help of a custom-made guide to a six-hole dynamic compression plate and divided into three groups. In group 1, fixation was performed using screws perpendicular to the plate axis in two planes. In group 2, screws were placed at a 20-degree angle to the axis of the shaft and perpendicular to the transverse plane. In group 3, the screws were placed at a 20-degree angle to the shaft axis and 7 degrees on the transverse plane. Gap-close bending and side-bending tests were performed in a three-point bending fashion. The maximum moment of force and deformation at the time of the failure was analyzed.


RESULTS
In gap-close bending tests, mean maximum moment of forces for groups 1, 2, and 3 was 51.90 Nm, 67.47 Nm, 82.05 Nm, respectively (p<0.05 for group 1 vs. groups 2 and 3; p=0.053 for group 2 vs. group 3). In side-bending tests, the mean maximum moment of force was 34.63 Nm, 49.91 Nm, and 49.29 Nm, respectively (p<0.05 for group 1 vs. groups 2 and 3; no significant difference was observed between groups 2 and 3). When the two bending tests were evaluated together, only the mean moment of forces of groups 1 and 3 were statistically different (p=0.006). There was no difference between any groups in terms of the deformity at the time of the maximum moment of force.


CONCLUSION
Oblique screw placement in two planes increases the strength of the plate-screw fixation under bending forces; however, there is no relationship between deformity and increased strength of fixation.