Effect of Thread Design Parameters on Central Screw Loosening: A 3D Finite Element Analysis

The objective of this study was to evaluate the effect of thread shape outline, thread pitch and preload on the central screw loosening or fracturing by using 3D finite element (FE) method.

By using a commercial implant system as prototypes, nine central screw models and matching implant components were created by combinations of two parameters with different values: three screw shapes (triangle, buttress and reverse buttress) and three screw pitches (0.2 mm, 0.3 mm and 0.4 mm). These models were inserted into a bone block. After applying pre-tightening torque to each central screw, a 100 N load tilted at 45 degrees was applied to the abutment to simulate an occlusal load. The stability performance of the central screws was evaluated through FE software.

For the central screw shape, the highest stress was always located at the level of the first thread, while the highest stress of the reverse buttress group was always located at the level of the second thread. Triangle (average 650 MPa) and reverse buttress (average 729 MPa) threads were more conducive to reducing central screw stress than buttress thread (average 920 MPa). Moreover, it is necessary to avoid thread parameters design with buttress shape/0.3 mm pitch and buttress shape/0.4 mm pitch.

Thread shape outline and thread pitch significantly influenced central screw stability performance. The central screw with a triangle thread shape and a pitch of 0.2 mm presented the best mechanical properties.