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Stress distribution in retentive arms of combination clasps used on premolars.

INTRODUCTION
Stresses resulting from cast clasp arms during insertion and the removal of removable partial dentures are the main causes of deformations or fractures. Therefore, achieving clasp designs producing less stress is very important.


OBJECTIVE
Retentive clasp arms used for premolars were investigated through the reverse engineering approach. The aim was to determine stress distribution in oval and half-oval clasps cross-sections in order to analyse biomechanical behavior.


MATERIAL AND METHODS
Purposely designed experimental three-dimensional (3D) models of the clasp arms were constructed on the buccal surface of an upper first premolar, to be used for structural simulations. 3D teeth models obtained after laser scanning were used as a support for retentive clasp arms modeling. Parameters of the clasp arms like length, thickness and cross-section were considered for the simulation of stainless steel wires. A concentrated load of 5 N was applied at the inner tip of the clasp arm.


RESULTS
A precise model of the coronal buccal surface of an upper first premolar was generated. This model was a useful tool in designing stainless steel clasp arms of different thickness and cross-section. In all cases, high stress values were located on the inner surface of the clasp arm, in the part located above the height of the contour. A similar bending stiffness was observed between the half-round cross-section design with a diameter of 1 mm and the round cross-section design with a diameter between 0.6 and 0.7 mm.


CONCLUSIONS
This in vitro study demonstrated that the reverse engineering approach and structural analyses provide a powerful tool for designing clasps and visualizing fracture risk areas and for choosing the adequate cross-section for each case. Within the limitations of this study, it was suggested that, on premolars, the biomechanical performance of half-round cross-sections for the retentive arms may be higher than round sections of clasp arms showing similar mechanical stiffness.