2006 Volume 69 Issue 3_4 Pages 151-159
We used three-dimensional finite element analysis to investigate how the shape of the screw type temporary anchorage device (TAD) and the direction of traction affected stress distribution in the screw during loading with orthodontic force. Four different shapes of screws, all made of titanium, were tested. The simulated alveolar bone had 1mm of cortical bone, with the remainder being cancellous bone.
Finite element models were prepared using a three-dimensional solid modeler. Stress was measured during loading of the TAD in five directions: directed upward 45 and 22.5 degrees relative to horizontal, 90 degrees to the long axis of the screw (perpendicular), and downward 22.5 and 45 degrees relative to horizontal. The data were subjected to linear static analysis using a universal structure analysis program.
In all types of TAD tested, the maximum principal stress was found to be directed to the cortical bone on the side opposite the load. We found that internal stress was extremely high with the slender cylinder type TAD, and lowest with the conical type. There was no significant difference between the cylinder and short cylinder types. We also found that internal stress was lowest at a traction angle of 45 degrees upwards for each type of TAD.
Our results suggest that the internal stress of the screw type TAD is affected by screw diameter, and that internal stress is lower when traction is applied upward on the screw.
