Influence of coating thickness on retentive forces of FRC clasps

Objectives: The aim of this study was to investigate the influence of the coating layer thickness on retentive force and stress distribution of glass fiber-reinforced composite (FRC) clasps coated with veneering composite by nonlinear finite element analysis. Methods: The geometry of an abutment tooth, a lower second premolar surrounded by a circumferential clasp arm, was set as a barrel-shape. The basic uncoated clasp arm shape was without taper, and was 2.60 mm wide and 1.30 mm thick. FRC was treated as an elastic orthotropic material. All nodes at the 1/3 lower apical side of the abutment tooth were restrained, and forced displacements of 5 mm were applied to the nodes at the base of the clasp arm in the removal direction. The retentive force and compressive stresses of the basic uncoated FRC clasp were calculated. Similarly, those of FRC clasps coated with veneering composite were calculated. The veneering composite, thicknesses of which were set at 0.1 mm, 0.3 mm and 0.5 mm, was treated as an elastic isotropic material. Results: The retentive force of the basic uncoated FRC clasp arm was 6.36 N. The retentive forces of coated FRC clasp arms were 8.16 N (0.1 mm coating), 11.40 N (0.3 mm coating) and 13.40 N (0.5 mm coating). High compressive stresses were located at the outer surface of the coated clasp shoulder. The maximum compressive stresses were 69.1 MPa (0.1 mm coating), 69.6 MPa (0.3 mm coating) and 71.8 MPa (0.5 mm coating). Conclusions: Within the limitations of this analysis, the coating layer thickness had a great influence on the retentive forces but no apparent influence on the stress values of coated FRC clasps. The results show that the thinner coating layer could contribute to control the retentive forces of FRC clasps coated with veneering composite in clinical practice.