| Background Usually,conventional dewaxing casting continues to be used for the manufacturing of removable partial denture(RPD)framework,which includes wax molding,embedding,casting and other steps.In that case,the tolerance of framework depends on various factors including deformation and shrinkage of wax mold,selection of embedding material,design of sprue,etc.Besides,the process features triviality and long lead time,let alone the high demands for the proficiency of dental technician.As a kind of rapid prototyping technology,three-dimensional(3D)printing is available for quick output of 3D solid through layered manufacturing with materials in different states and following superimposition of layers on the basis of 3D digital model.Widely used for 3D printing,selected laser melting(SLM)is able to offer the finished printed parts the compactness of almost 100% with its fully melted powder material being heated and the absence of indirect bond adhesive during manufacturing.SLM gets popular in the field of stomatology due to its simple manufacturing and short lead time.Sufficient support to the suspension structure of the framework is necessary when SLM is employed for the manufacturing of RPD,in this manner to facilitate forming and heat transmission as well as the prevention of structural collapse occurred at the part of undercut and crinkled deformation of the suspension part caused by thermal concentration.We are still under further investigations,i.e.,whether the angle selection between the support and RPD framework makes difference to its adaptability,accuracy,mechanical performance,etc.Objective The purpose of this study is to evaluate the fitness of removable partial denture framework and the mechanical properties of standard parts fabricated by selective laser melting technique with different support angles.Methods 1)Make the model of bilateral mandibular posterior teeth loss as original model,build 3-D digit model through DS-EX 3-D scanner scan the original model and input the model data into exocad Dental CAD to design removable partial denture frameworks.Set the original model without frameworks as contrast group to overcome the system error during scanning.2)SLM produce three groups different support angle RPD supports(6 Pcs./group)after Materialise Magics 20 software added 0°(horizontal),45° and 90°(vertical)supports for 3-D digit model of removable partial denture frameworks and use Ti-6Al-4V titanium alloy powder in Digi Maker 2250 metal 3-D printer.Annealing the frameworks which finish printed,remove support,sand blasting,acid washing and few polishing.3)Plaster cast which duplicated the structure of tissue surface of the RPD framework was obtained through copy cast method.A three-dimensional scanner was used to scan original and duplicated plaster casts.The gaps between framework and the model in different parts were analyzed by Geomagic Qualify software to evaluate the fitness of the framework with visual method.4)Use Rhinoceros for 3D model design of standard printed part with ISO 22674:2016 standard;build standard printed parts of titanium alloy(6 Pcs./group)with supporting angles of 0°(horizontality group),45°and 90°(verticality group)respectively using titanium alloy powder and SLM;test the mechanical performance of the printed parts with electronic universal tester.Results 1)The framework fits on the plaster model completely,and its tissue surface fitted on the plaster model well.The deviation between frameworks and plaster casts: the mean deviations of the horizontal,45° and vertical group were(0.146±0.017),(0.182±0.015)and(0.185±0.022)mm.The mean deviation of the horizontal group was significantly less than 45° group as well as vertical group(P<0.05).Moreover,there was no significant difference in the mean deviation between the 45° group and the vertical group(P>0.05).2)The mean deviation of occlusal rest of the horizontal group was significantly less than 45° group(P<0.05);however,the mean deviation of occlusal rest in vertical group showed no significant difference than that in horizontal group,neither did that in 45° group(P>0.05);3)There was no significant difference in the mean deviation of occlusal rest between the vertical group and the horizontal group,the 45° group(P>0.05).The mean deviation of clasp of the horizontal group was significantly less than 45° group and the vertical group(P<0.05);whereas,there was no significant difference in the mean deviation of clasp between the 45° group and the 90° group(P>0.05).Moreover,no significant difference was found in the mean deviation of lingual bar above 3 groups(P>0.05).4)The yield strength,tensile strength,reduction of area and Young modulus of the horizontality group,45° group and verticality group are(1007.3±2.6)N/mm2,(1029.0±3.4)N/mm2,(12.3±0.3)%,[(84.3±0.3)x103]N/mm2;(990.3±4.3)N/mm2,(1019.5±4.2)N/mm2,(12.6±0.2)%,[(86.9±0.5)x103]N/mm2;(982.5±10.2)N/mm2,(1010.5±9.3)N/mm2,(12.8±0.3)%,[(90.8±1.2)x103]N/mm2 respectively.5)It shows that the yield strength of horizontality group is far more than those of 45°group and verticality group(P<0.05),on the other hand,the difference in terms of yield strength between 45° group and verticality group has no statistical significance(P>0.05);It also shows the tensile strength of horizontality group is far more than verticality group(P<0.05)and the differences in terms of tensile strength among 45°group,horizontality group and verticality group have no statistical significance(P>0.05);The Young modulus of horizontality group is far less than those of 45° group and verticality group(P<0.05)and in the meantime,Young modulus of 45° group is far less than that of verticality group(P<0.05).The differences in terms of reduction of area among the standard printed parts of titanium alloy in 3 groups have no statistical significance(P>0.05).6)The mechanical properties of the three groups of titanium alloy standard printings are higher than the requirements of "technical requirements for medical devices" for removable partial dentures with metal framework.Conclusions 1)Among the three kinds of bilateral free-end dentition defect removable partial denture framework fabricated by SLM in different support angles,horizontal printing is proved to reach a minimal deviation,even though the fitness of these three kinds of frameworks all basically meets the clinical requirements according to previous studies.2)All standard printed parts of titanium alloy prepared by SLM in 3 supporting angles satisfy the clinical demands in terms of mechanical performance.In addition,the supporting angle is available to be selected according to the design requirements of clinical framework. |
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