Abstract
Objective The aim of this study was to evaluate the effect of screw insertion angle and insertion
torque on the mechanical properties of a 3.5 fixed-angle locking plate locking compression
plate (LCP) and 3.5 variable-angle locking plate polyaxial locking system (PLS).
Methods In the LCP group, screws were placed abaxially at 0, 5 and 10 degrees. In the PLS
group, screws were placed at 0, 5, 10, 15 and 20 degrees abaxially. The insertion
torque was set to 1.5 and 2.5 Nm in the LCP and PLS groups respectively. A load was
applied parallel to the screw axis, and the screw push-out force was measured until
the locking mechanism was loosened.
Results The 3.5 LCP showed higher push-out strength than the 3.5 PLS when the screws were
placed at 0 degree regardless of the insertion torque. The off-axis insertion of 3.5
LCP locking screws resulted in a significant decrease in push-out strength (p < 0.05). A higher insertion torque value significantly increased the screw holding
strength for the 3.5 LCP (p < 0.05). The 3.5 PLS system had a significantly higher push-out force when the screws
are at 0 degree than at 5, 10 and 15 degrees, and 20 degrees (p < 0.05) at any given insertion torque. An increase in the insertion torque did not
have a significant effect on the push-out strength of the 3.5 PLS locking system.
Conclusion The 3.5 PLS is more sensitive to the screw insertion angle than to the insertion
torque, whereas the 3.5 LCP is affected by both factors. Placing 3.5 LCP locking screws
off-axis significantly reduces the screw holding strength; therefore, this approach
has to be avoided. The findings of our research indicate that a 1.5 Nm torque can
be used for a 3.5 PLS.
Keywords polyaxial - locking screw - locking plate - insertion angle - insertion torque
