Analysis of seating and fracturing torque of bicortical screws

The purpose of this study was to analyze and compare the seating and fracturing torque of 2.0-mm- versus 2.7-mm-diameter titanium screws. Six cadaver mandibles were sectioned at the symphysis and 20 drill hole locations were chosen on each half. Half the holes were designated for the 2.0-mm screws, tapped and nontapped, and the remaining half for 2.7-mm screws, tapped and nontapped. The maximum torque leading to the screw seating or fracturing was recorded using an electric torque tester (Artech Reliance, Chantilly, VA). The mean maximum seating torque for the 2.7-mm screws, tapped and nontapped, was 30% of that measured for the 2.0-mm screws. The mean maximum seating torque for all tapped screws was 35% less than for all nontapped screws. The region of the mandible requiring the greatest seating torque was the symphysis, followed by the body, condyle, ramus, and angle. None of the 2.7-mm screws fractured; however, 10% of the 2.0-mm screws fractured. Three were located in the symphysis and one in the ramus. With the 2.0-mm screws that did not fracture, the seating torque in the nontapped holes was 59% greater than in the tapped holes. With the 2.0-mm screws that fractured, the mean maximum fracturing torque was 64% greater than for those screws that did not fracture. This study shows that small screws placed in the mandible require greater seating torque, regardless of whether the holes are tapped, and that there is an increased chance of small screws fracturing in certain regions.