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.

Finite element stress analysis and fatigue behavior of cast circumferential clasps.

STATEMENT OF PROBLEM: Deformation and fracture of cast circumferential clasps may be a result of stresses induced during mastication. Most biomechanical clasp studies have been performed only under static conditions. There is little information regarding behavior of clasps over time. PURPOSE: The purpose of this study was to evaluate stress distribution on cast circumferential clasps, and the displacements or deformations, depending on the load placement and range. Fatigue analysis was then conducted to evaluate the behavior of clasps over time. MATERIAL AND METHODS: Static stress values and distribution induced in cast circumferential clasps were calculated and studied using 3-dimensional finite element experimental models for Co-Cr cast circumferential clasps. Average loads between 20 and 35 N were applied vertically along the clasp components to simulate static stress distribution during translation and rotation of the denture. After determination of stress concentration areas, the fatigue behavior of clasps was studied using finite element analysis during simulated cyclic masticatory loads (loads between 0 and 20 N included in 4500 masticatory cycles over 24 hours). RESULTS: For the translation simulation, the maximum stress was 310.27 MPa, located near the lower margin of the retentive arm, and for the rotation simulation, the maximum stress was 310.31 MPa, located near the upper margin of the same arm. Under simulated static load, the magnitude of stresses found in the clasps was under the reported yield strength (640 MPa) of the Co-Cr alloy. The maximum stresses indicated the area of highest fracture risk, but fractures occurred only under a simulated cyclic mastication load representing 5.5 years of service. CONCLUSION: Within the limitations of the simulation study, static stress analysis of cast circumferential clasps indicated the location of greatest fracture risk to be at the junction of the clasp arm with the body, for all situations. In addition, fatigue analysis estimated clasp degradation over time and the survival rate of the same clasps, which was found to be 5.5 years, on average.