Fit accuracy assessment of removable partial denture frameworks produced by direct metal laser sintering - a clinical trial

Abstract

Objectives: The purpose of this cross-over clinical study was to compare the number of framework repetitions, percentage of framework components adjusted, clinical acceptability, and fit accuracy of removable partial denture frameworks produced by the direct metal laser sintering technique (DMLS) and conventional technique.

Materials and methods: For each dental arch (n = 26), two cobalt-chromium frameworks were produced through two protocols: direct metal laser sintering (experimental group) and conventional lost-wax casting technique (control group). The number of framework repetitions, the percentage of components that had to be adjusted, and the clinical acceptability were registered. The fit accuracy of functional components was assessed by a qualitative method using endodontic files to identify maladjustments and compared to a quantitative method based on silicone specimens digitized by micro-computed tomography. The normality was checked (Shapiro-Wilk test), and data were analyzed with McNemar, Wilcoxon and paired-t tests (α = 0.05).

Results: No statistically significant differences were found between conventional and digital frameworks for most of the variables tested (p > 0.05) except the fewer laboratory repetitions (p = 0.046), higher percentage of components adjusted (p = 0.011), and better reciprocal arms fit (p = 0.044) in the frameworks produced by DMLS protocol. No statistically significant (p = 0.174) difference was found between the fit accuracy qualitative and quantitative assessment methods.

Conclusions: The DMLS and conventional protocols were similar. Despite the DMLS protocol exhibiting a higher percentage of components adjusted, it presented better reciprocal arms fit accuracy with no framework repetition.

Clinical relevance: Metal frameworks can be produced using DMLS eliminating casting problems.

Keywords: Accuracy; Computer-Aided design; Computer-Aided manufacturing; Direct metal laser sintering; Removable partial dentures.

Fig. 1

Fit accuracy assessment by direct qualitative method, inspecting the gap between teeth and framework component surfaces using an endodontic plugger size 35. (A) Absence of a gap thickness equal to or higher than 350 μm. (B) Presence of a gap thickness equal to or higher than 350 μm

Fig. 2

Procedures to assess silicone specimens after micro-CT digitization. (A) 3D Registration and analysis of two corresponding silicone molds, by overlapping of the datasets using the same VOI in the DataViewer, accordingly the three dimensional space planes (COR, coronal plane; TRA, transverse plane, SAG, sagittal plane). (B) Mean thickness calculation in the CT-Analyzer, after segmentation with a confirmation by a 2D measurement of the mean thickness of a silicone mold transverse segment

Fig. 3

Box plots for silicone thickness values (in micrometers) in occlusal rests of the groups under study (significance using a paired t-test: p = 0.471). No difference (p = 0.174) was found between the occlusal rest maladjustments identified by the quantitative (fifty-six) and qualitative methods (forty-six) (Table 2)