Effect of screw insertion torque on push-out and cantilever bending properties of five different angle-stable systems.

OBJECTIVE:
To compare the screw push-out strength and resistance to cantilever bending of 5 different angle-stable systems using 4 different insertion torque values to tighten locking screws.
STUDY DESIGN:
In vitro mechanical testing of 5 screw-plate constructs.
SAMPLE POPULATION:
Screw plate constructs (n = 60) were tested; 12 of each design, 3 for each torque value.
METHODS:
To compare push-out strength, screws were loaded in axial direction on the screw tip until loosening of the locking mechanism was recorded. For cantilever bending test, screws were loaded perpendicularly to their longitudinal axis at 2 mm of distance from the under surface of the plate. Load was applied in displacement control at 1 mm/min.
RESULTS:
There was a significant difference between the 5 different angle-stable systems regarding both push-out and cantilever bending strength. There was an influence of insertion torque value on push-out strength for 2 systems and insertion torque value influenced cantilever bending behavior only in 1 locking system.
CONCLUSIONS:
Locking mechanisms using "thread in thread" principle provided a stronger screw push-out behavior. Screws materials and core diameter of the different screws were directly related to cantilever bending strength.