Assessment of surface micro-roughness and de-torque of selective laser melting, cast, and machined implant abutments: An in vitro study

Purpose: The purpose of this in vitro study was to assess the surface micro-roughness and de-torque measurement of selective laser melting (SLM) implant abutments in comparison to cast and machined implant abutments. Materials and Methods: Forty abutment specimens were divided equally into four groups (n = 10) as follows: machined titanium (Ti) abutments (Control), cast cobalt-chromium (CoCr) abutments, SLM-CoCr abutments, and SLM-Ti abutments. Fabricated abutments were assessed for surface micro-roughness using a 3D optical noncontact surface microscope (Contour GT-K 3D Optical Microscope, Bruker, Tucson, Arizona, USA). In addition, abutment displacement was assessed using de-torque values. The abutments were torqued to the implant at 30 Ncm and retorqued after 15 min according to the manufacturer's recommendation using a Tohnichi BTGE digital torque gauge. After 2 min, preload reverse torque values (RTVs) measurements were performed. Before cyclic loading, the abutments were retorqued to the implant at 30 Ncm, and metal crowns were cemented to the abutments with a temporary cement (CharmTemp EU; DentKist Inc., Korea) using a 6 kg force applied for 10 min. After 24 h of cementation, the specimens were subjected to 5 kg of loads for 600,000 cycles at a frequency of 1 Hz using the chewing simulation machine. Immediately after the cyclic loading, the postload-RTVs were measured. This was performed to identify the comparative effect of fabrication techniques on the reverse torque difference (RTD). Data was assessed using Analysis of variance (ANOVA) and Tukey Post hoc comparison tests for all the variables. Pearson correlation was used to assess dependence between independent variables (surface roughness) and dependent variables (de-torque values). Results: SLM abutments were compared with the cast and machined abutments in the surface micro-roughness and de-torque values. SLM-Ti abutments showed a significantly rougher surface (p < 0.05) among the study groups with a mean Ra of 2.24 ± 0.36 µm. SLM-CoCr abutments were smoother than cast abutments with mean Ra of 1.30 ± 0.11 µm and 1.58 ± 0.17 µm, respectively (p < 0.05). For the de-torque test, mean preload RTVs of SLM-Ti (23.39 ± 3.74 Ncm) and SLM-CoCr (19.73 ± 1.97 Ncm) presented comparable (p > 0.05) results with cast CoCr (22.87 ± 2.21 Ncm). However, only the mean postload RTVs were significantly lower (p < 0.05) in SLM-CoCr (16.36 ± 3.90 Ncm) than machined Ti (20.52 ± 2.23 Ncm) abutments. There was no significant difference between the groups for RTDs (p > 0.05). Conclusion: SLM CoCr abutments showed a smoother surface than cast abutments, while SLM Ti abutments had the highest surface roughness in comparison with other groups. No significant difference was found between the groups for RTDs. No significant correlation was found between the surface roughness and postload RTVs. However, a negative correlation was found between surface roughness and RTDs.