Accuracy And Fitness Of Removable Partial Denture Metal Framework Fabricated By Selective Laser Melting

[Objective]The present study aimed to analyze the accuracy of Co-Cr and commercial pure Ti(CP-Ti)removable partial denture(RPD)frameworks fabricated by selective laser melting(SLM),addressing the trueness and reproducibility.To compare the fitness of SLM Co-Cr and SLM CP-Ti frameworks with Co-Cr frameworks by conventional loxwax technique(CA Co-Cr)at targeted locations.[Materials and methods]A Kennedy Class I maxillary arch model was selected and prepared for the present study.The preparation was scanned in vitro by using the 3Shape Trios 3 intraoral scanner and then the file was sent to the software program(Dental System 2019,3Shape A/S Denmark)to design the RPD framework.RPI clasps were set on the left and right first premolar and the palatal strap were drawn.After adding the support structures,the CAD file was sent to the specific SLM printer to fabricate cobalt-chromium alloy(CoCr)and commercial pure titanium(CP-Ti)RPD frameworks(n = 7).After the heat treatment,wire-electrode cutting from the platform,manual removal of support structures,rough grinding,sandblasting,ultrasonic cleaning and drying,the frameworks were completed.Each SLM framework was scanned by using the 3Shape D2000 desktop scanner and saved as STL file The data were input to Geomagic software.To analyze the trueness,the data obtained from each SLM RPD framework were superimposed onto the CAD file via "best fitness alignment" after selecting the majority part of the major connector.Then "3D comparison" was performed to generate 3D deviation map and analysis report of RPD CAD data and each SLM framework data.Root mean square(RMS)was used for quantitative.Eight points were selected for each SLM RPD framework,including the central point of teeth #14 and #24 occlusal rests,the tip point of teeth #14 and #24 I bars,the central point in the middle of the teeth #14 and #24minor connectors connected to the occlusal rests and the central point of teeth #24 and#14 proximal plates.The absolute value of deviation of the targeted points was taken as the trueness of the point.To analyze the reproducibility,six frameworks were selected respectively from SLM CP-Ti group and SLM Co-Cr group.Each group’s data were compared in pairs.Taking the overall RMS value of pairwise comparison as the measurement standard.In the study of fitness,the cast cobalt chromium alloy(CA Co-Cr)frameworks were set as control group,The CAD file was input to the DLP 3D printer to fabricate resin patterns.Following the traditional lost wax casting procedure,six CA Co-Cr frameworks were fabricated.Each RPD framework and the maxillary model were scanned respectively using the desktop scanner,and then the framework was completely located in the maxillary model for the scanning.Align the framework and maxillary model with the corresponding part of located model file respectively.The aligned framework and maxillary model were input into 3D viewer software to observe the gap between the RPD tissue surface and the model.Seven locations were selected including central points of teeth #14 and #24 occlusal rests,the tip points of teeth #14and 24 I bars,the central points of teeth #14 and #24 proximal plates and the central point of palatal strap.The average value of seven positions represents the overall fit of the framework.The data of accuracy and fitness were input into SPSS statistical software(SPSS18.0 IBM Corp USA)respectively.An independent 2-sample t test was used to analyze the differences between the trueness and reproducibility of SLM CP-Ti and SLM CoCr(α=.05).One-way ANOVA was used to analyze the data of internal discrepancy under specific points and overall frameworks during CA Co-Cr,SLM CP-Ti and SLM Co-Cr frameworks.[Results]1.The accuracy of SLM RPD frameworksThe RMS of SLM CP-Ti frameworks was 121.3±17.0μm,and that of SLM CoCr frameworks was 174.0±18.3μm.There was a significant difference in the overall trueness between SLM Co-Cr and SLM CP-Ti group(P < 0.05).The overall trueness of SLM CP-Ti group was better.There was no significant difference between SLM CPTi and SLM Co-Cr at the selected 8 points(P > 0.05).In the reproducibility results,the overall RMS of SLM CP-Ti group was 89.3±16.3μm,and that of SLM Co-Cr group was 91.9±12.6μm.There was no significant difference(P > 0.05).The two groups revealed good reproducibility.1.The fitness of SLM RPD frameworksThe gap size of CA Co-Cr group was 242±23μm,SLM CP-Ti was 219±33μm and SLM Co-Cr was 255±27μm.There was no significant difference in the overall gap size among CA Co-Cr group,SLM CP-Ti group and SLM Co-Cr group(P > 0.05).In the central point of palatal strap,the gap size of SLM CP-Ti(383±55μm)was significantly better than that of CA Co-Cr(464±44μm)and SLM Co-Cr(489±91μm).At the #24 occlusal rest,the gap value of CA Co-Cr group(474±67 μ m)was significantly larger than that of SLM group(344±126μm for SLM Co-Cr and 314±52μm for SLM CP-Ti).However,at the #24 proximal plate,the CA Co-Cr group showed the best fitness(118±68μm).There were significant differences among the three groups at the specific locations including the central point of palatal strap,the #24occlusal rest and the #24 proximal plate(P < 0.05Conclusion]1.SLM process could produce accurate dental prosthesis.Under the limitation of the study,the overall trueness of SLM CP-Ti RPD frameworks indicated better trueness than SLM Co-Cr frameworks.2.Both SLM CP-Ti frameworks and SLM Co-Cr frameworks revealed good reproducibility,which indicated that the technique had good stability and was easy to control the product quality.3.SLM revealed similar fitness as the traditional lost-wax technique when fabricating RPD frameworks,indicating that SLM technique could meet the requirements of clinical application,and was expected to be applied in the routine clinical treatment of RPD frameworks.However,the fitness of specific sites such as rests need to be further improved.