Experimental Study On The Effect Of Properties Of Type IV Gypsum Mixed With Different Disinfectants

ObjectEffective disinfection of dental gypsum model can eliminate the cross-infectionand ensure the stability of model performance. The disinfect method that gypsummixed with disinfectant is province, simple, cheap and efficient. It is expected tobecome the outreach program of the disinfection of oral model. This study researchedon influence of dimensional changes, compressive strength, bending strength, surfacehardness of type IV gypsum model mixed with three disinfectants, to provideexperimental basis for clinical application.Materials and Methods1. Study on dimensional changes1.1Sample preparationA mold with two prepared abutments of three-unit fixed bridge was fabricated(BL, bl--buccolingual distance, OH--occlusogingival distance, OO ’–centerdistance of two abutments). An impression of the mold was taken for making gypsum sample. Samples were made by type IV gypsum mixed with distilled water、2%glutaraldehyde、0.5%sodium hypochlorite and500mg/L Trichloroisocynuric Acid(TCCA) disinfectant respectively,10samples for each group. Samples were removed(45±1) min after mixture and placed in experimental environment for standby.1.2Precision measurement of sample size24hours after mixture of type IV gypsum, to measure OO′、BL、bl、OH of themetal mold and gypsum samples by VHX-100three-dimensional measurementsystem. Each sample was measured three times to get the mean value. The mean valuewas substituted into the related formula to calculate the dimensional changes.2. Compressive strength test2.1Sample preparationA two-separated cylindrical brass mold was made, diameter (20±0.2)mm andhigh (40±0.4)mm. Type IV gypsum samples mixed with distilled water and2%glutaraldehyde were prepared respectively,20samples for each group.10samples for0.5%sodium hypochlorite group and500mg/L TCCA group respectively.(45±1)min after mixture, samples were removed and placed in experimental environment forstandby.2.2Compressive strength test1h and2h compressive strength test of samples were completed by an electronicuniversal testing machine. The loaded force on each sample with a speed of (5±2)kN/min was gradually increased until the specimen compressively fractured. Themaximum force was recorded by computer.3. Bending strength test3.1Sample preparationA metal standard mold in size of50mm×10mm×5mm was prepared. Animpression of the mold was taken for making gypsum sample. Type IV gypsum samples mixed with distilled water and2%glutaraldehyde were prepared respectively,20samples for each group.10samples for0.5%sodium hypochlorite group andTCCA group respectively.(45±1) min after mixture, samples were removed andplace them in experimental environment for standby.3.2Three-point bending testBending strength test was completed with a load speed of0.5mm/min by CMT-6301micro-controlled universal testing machine. The maximum force was recordedby computer.3.3Scanning electron microscopy (SEM) of the fractured surfacecharacteristics of gypsum samplesMorphology of the fractured surfaces of gypsum samples was observed by SEM.4. Surface hardness test4.1Sample preparationThe sample was in the same size of the bending strength test mentioned above,10samples for each group respectively.4.2Knoop hardness test24hours Knoop hardness of samples was test by a microhardness instrument.Hardness was determined by loading each specimen surface3times, for15seconds ofa force of100g. Then the diagonal length of indentation of each sample was measuredthree times to get the mean value under a400-time microscope. The mean value wassubstituted into the related formula to calculate the Knoop hardness.Statistical analysis1. Comparison of dimensional changes and Knoop hardness among four-groupsamples: One-Way ANOVA2. Comparison of compressive strength among four-groups: Kruskal-Wallis H testfor several independent samples, and Comparison of compressive strength betweentwo groups: Wilcoxon test for two independent samples. 3. Comparison of bending strength among four-group samples: One-WayANOVA, and comparison of compressive strength between two groups: LSD-test.Results1. Study on dimensional changesThere were no significant difference in dimensional changes among four groupsof distilled water,2%glutaraldehyde,0.5%sodium hypochlorite and500mg/L TCCA(P>0.05).2. Compressive strength test2.1Results of1h compressive strength test1h compressive strength value of glutaraldehyde group was less than those ofdistilled water,0.5%sodium hypochlorite and500mg/L TCCA groups. There weresignificant difference in1h compressive strength among glutaraldehyde group andother groups (P <0.0083), while the difference of1h compressive strength amongthe other groups were not statistically significant (P>0.0083).2.2Results of1h and2h compressive strength of distilled water group andglutaraldehyde groupThere was no significant difference in compression strength between distilledwater1h group and glutaraldehyde2h group (P>0.0083). There were significantdifference in compressive strength among the other groups (P <0.0083).3. Bending strength test3.1Results of1h bending strength test1h bending strength value of glutaraldehyde group was less than those ofdistilled water,0.5%sodium hypochlorite and500mg/L TCCA groups. There weresignificant difference in1h bending strength among glutaraldehyde group and othergroups (P <0.05), while the difference of1h bending strength among the othergroups were not statistically significant (P>0.05). 3.2Results of1h and2h bending strength of distilled water group andglutaraldehyde groupThere was no significant difference in bending strength between distilled water1h group and glutaraldehyde2h group (P>0.05). There were significant differencein bending strength among the other groups（P <0.05）.3.3Results of SEM of the fractured surface characteristics of gypsum samplesSEM of glutaraldehyde1h group sample showed that the section was neat, thecrystal was obviously short. Crystals of distilled water1h group, sodium hypochlorite1h group, TCCA1h group and glutaraldehyde2h group staggered similar degree ofdisorder. Crystals of distilled water2h group were bigger, that arranged parallel orstaggered.4. Knoop hardness testThere were no significant difference in surface Knoop hardness of samplesamong the four groups (P>0.05).Conclusions1. There are no effects of2%glutaraldehyde,0.5%sodium hypochlorite and500mg/L TCCA on24h dimensional changes and24h surface hardness of typeIV gypsum.2. There are no effects of0.5%sodium hypochlorite and500mg/L TCCA on1hcompressive strength and1h bending strength of type IV gypsum.3.1h compressive strength and1h bending strength of type IV gypsum mixed with2%glutaraldehyde may decrease obviously, but its2h compressive strength and2h bending strength can increase greatly.4.2%glutaraldehyde,0.5%sodium hypochlorite and500mg/L TCCA can be usedas water substitutes for mixing type IV gypsum. It is suggested to removegypsum model from the impression1hour after mixture with0.5%sodiumhypochlorite or500mg/L TCCA,2hours with2%glutaraldehyde.