Long-Term Retention of Circumferential Clasps Manufactured by Selective Laser Melting

Objectives: This in vitro study aimed to compare the long-term retention of cobalt-chromium (Co-Cr), commercially pure titanium (CP Ti), and titanium alloy (Ti-6Al-4V) circumferential clasps manufactured by selective laser melting (SLM) and to investigate the effects of the clasp arm size and undercut engagement.
Methods: Circumferential clasps for premolars were designed, with clasp arm size A (1.9-mm width/1.1-mm thickness at the body and 0.8-taper) or B (1.2 times A) and 0.25-mm (A1, B1) or 0.50-mm (A2, B2) undercut engagement. SLM-built Co-Cr, CP Ti, and Ti-6Al-4V clasps with designs A1, A2, B1, B2 were fabricated, taking the cast Co-Cr clasp with design A1 as the control group (n=6). The stress and displacement of clasps during dislodging were predicted through finite element analyses (FEA). An insertion/removal test of clasps for 15,000 cycles was conducted to simulate over 10 years of clinical use. The permanent deformation of retentive arms was measured through 3D deviation analyses. Repeated measures analysis of variance (ANOVA) was performed to analyze the retention change of SLM-built clasps. One-way ANOVAs with the initial or final retention or permanent deformation as the dependent variable, respectively, were performed (α=.05).
Results: The retention decreased after 1500 insertion/removal cycles, then increased and showed no significant difference with the initial one until after 7500 cycles, and after that, decreased gradually. The FEA predicted that SLM-built Co-Cr clasps with design A1 and CP Ti and Ti-6Al-4V clasps with design A2 had similar initial retention with the control group, which was in accord with the insertion/removal test. Compared with the control group, SLM-built Co-Cr clasps with design A1 and Ti-6Al-4V clasps with design A2 had similar final retention and lower permanent deformation; CP Ti clasps with design A2 had lower finial retention and higher permanent deformation.
Conclusions: Considering the long-term retention, SLM-built Co-Cr clasps with design A1 and Ti-6Al-4V clasps with design A2 are recommended in clinical practice.