| [Objective] The objective of this study is to investigate effect of multiple factors on fit of dental implant superstructure frameworks manufactured by selective laser melting(SLM).In this study,we employed optical scanner and 3D analysis to determine patterns and degrees of implant frameworks deformation under different conditions during manufacture including 1 materials,2 building direction,3 adding stanchions,4 shapes of support structure,5 heat treatment,6 holding temperature during heat treatment and 7 gaseous environment during heat treatment.[Materials and methods] Four implant replicas(20°Uniabutment DENTSPLY SIRONA Inc.USA)was installed on a standard mandible edentulous gypsum model at canine and first-molar regions.Then the model was scanned using a model scanner with scanbodies on it(D2000,3Shape A/S Denmark),and the STL files attained were input into CAD software(Dental System 18.0,3Shape A/S Denmark).An implant framework was then designed and exported as STL format.After that,the digital frameworks designed were laid on a base plate and supports were added,the files were then imported into SLM devices(Profeta Intelligent Technology Co.Ltd,China).Grouping criteria are listed as followed: frameworks parallel arranged,without stanchions,hollowed-cone shaped structure supported,with heat treated under 1150℃ for an hour and nitrogen protected were labelled as control group for cobalt chromium alloy(C0,C stands for cobalt chromium alloy,similarly hereinafter),frameworks with heat treated under 650℃ for an hour in a low vacuum(10-1Pa)environment and other parameters the same as C0 were labelled as control group for titanium(T0,T stands for titanium,similarly hereinafter).In experimental groups the following parameters were modified respectively,frameworks were arranged 50° to base palate(C1/T1),stanchions were added on to frameworks(C2/T2),hollowed-cone shaped support was employed instead(C3/T3),no heat treatment was conducted(C4/T4),heat treatment was conducted in highly vacuumed(10-3Pa for cobalt chromium alloy,10-5Pa for titanium)environment(C5/T5).Heat treatment was conducted right after the frameworks were built on plates.The specimens were heated under the rate of 8℃/min till temperature reaches the level set previously,and keep the temperature in furnace for an hour,then perform furnace cooling to 300℃ for cobalt chromium alloy and 250℃ for titanium frameworks.Frameworks were cut from the plate using wire electrical discharge machining(WEDM).Then the specimens were sand blasted under the pressure of 0.2MPa and cleaned using an ultrasonic cleaning machine.Each framework was scanned with a model scanner,and data ware analysed using Geomagic Control 2015(3D Systems Corp USA).First,fit intaglio of frameworks using best fitting algorism to create cones,then fit interfaces plane using three-point fitting algorism.After that the axes of cones were intersected with the planes to determine the central point of the interfaces.the cone at the position of left mandible first molar was labelled as cone1,and then cone 2,cone 3,cone 4 were labelled next to the previous one orderly,and point 1,point 2,point 3,point 4 were labelled accordingly.Based-on-feature command was applied to align the test models with the CAD file with priority of point 1,axis of cone 1,point 2 orderly to achieve complete constraint.The coordinates as well as bias in all direction of all central points were output.Those points were input into Origin Pro 9.1(Origin Lab Ltd USA),a plotting software,to show deformation patterns.Mann-Whitney U tests were performed to detect the bias between groups to determine deformation degrees which represent fit in this study.Data are displayed in the form of median(Q1,Q3),α=0.05.Multivariate linear regression were performed to determine contribution to deformation of factors with P<0.1.[Results] Fit of frameworks manufactured by selective laser melting was analyzed through based-on-feature alignment and 3D analysis.Using this innovative method,virtual analysis of implant frameworks could be similar to one-screw-test,which is a commonly used approach of fit testing for implant frameworks.This could help to avoid the errors induced during model scanning and CAD files designing.The results showed: 1.The frameworks not trough heat treatment twisted away from base plate on Zaxis,and shrink within XOY plane.After exposed to heat treatment.The frameworks deform otherwise,they twist closer to base plate on Z-axis,and expand within XOY plane.2.Independent Mann-Whitney U test for deviation of point 4 between tested groups showed that the deviation of frameworks in Group Ct[0.8713(0.6659,1.0885)] as well as Group C4[0.8851(0.8376,1.1536)] compared to Group C0[0.5317(0.3803,0.5494)] was significantly larger,and that the deviation of frameworks in Group T4[0.5146(0.4426,0.662)] was significantly larger than that of T0[0.1797(0.1691,0.2627)].Besides,the deviation in group C0 was also larger than that in group T0.(P<0.05)3.Multivariate linear regression showed that for frameworks made of cobalt chromium alloy,standardized regressive coefficient for whether through heat treatment(0.724)was larger than that for other factors(change of holding temperature: 0.507,build direction: 0.453).For those made of titanium,the coefficient for whether through heat treatment(0.743)is also larger than other factor(change of atmosphere during heat treatment)4.no differences of fit was found after adding stanchions,changing shape of support structure and changing of gaseous environment.[Conclusion] 1.Heat treatment could reduce deformation of implant frameworks,result in better fit.Heat treatment could also change deformation patterns of frameworks.2.In this study,frameworks made of titanium show better fit than those made of cobalt chromium alloy.3.When heat treated for an hour,frameworks held at 1150℃ showed better fit those held at 950℃. |
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