| Dental gypsum casts are the common mold casts poured to produce the prosthesis in dental health-care settings. The dental gypsum casts are possibly contaminated with microorganisms from oral cavity of patients during the cast-pouring procedures, and the untreated dental gypsum casts will probably be the carriers of nosocomial infection. The disinfection methods used to dental gypsum casts must satisfy the efficacy of disinfection and keep their physical properties.Clinical samples were collected to determine the contamination level of dental gypsum casts and the distribution of pathogens. A vitro carrier quantitative germicidal test was conducted to compare disinfection efficacy of UV, ozone and microwave. Dimensional accuracy and compressible strength were tested to evaluate physical change after disinfection. The purpose of this study was to find out the suitable disinfection procedures of gypsum casts in dental health-care settings.This study included three parts. Part Ⅰ the contamination level of dental gypsum casts:each 30 poured gypsum samples were collected from orthodontics and prosthetics patients. The whole samples were transferred into PBS and counted colony-forming units (cfu). The identifications of microbial were conducted by automatic microbial identification system. Part Ⅱ comparison of the disinfection efficacy among three disinfection methods:Three disinfection methods (UV, ozone and Microwave) and 12 procedures were compared. Three common type of gypsum were used to fabricate the lcm3 test carriers and inoculated the carriers with 106 cfu of standard Escherichia coli (8099) and Bacillus subtilis var.niger (ATCC 9372). Artificial saliva was used as organic interferent. The efficacy of 12 disinfection procedures were evaluated by determining the number of colony forming units recovered after disinfection of gypsum casts and calculating the KL value. Part Ⅲ The effect of disinfection procedures on physical properties:APMMA model imitating the rigid fixed bridge was utilized as the mould to make silicone rubber impression.65 experimental casts were poured using three type of gypsum. Dimensional accuracy was determined from the mean percentage deviation of four measurements taken from casts compared with corresponding measurements from the controls. Compressible strength was determined using standard cylinder model according to YY 0462-2003 and compared with controls.The mean contamination level (log cfu/sample) of dental gypsum casts was 4.50±1.27, and the acceptability was only 5.0%. The contamination level of the samples collected from orthodontics and prosthetics patients were not significantly different (P=0.429). According to detective rate, the top three bacteria were Streptococcus mutans (65.0%), Propionibacterium propionicum (43.3%) and Candida albicans (23.3%).The disinfection efficacies of three methods were not the same. The mean KL of Bacillus subtilis var.niger (ATCC 9372) and Escherichia coli (8099) were 0.63±0.12 and 1.59±0.28 exposed to UV radiation for 30min. The difference of gypsum type did not influence the KL value.The log value of killing Escherichia coli (8099) differed from the exposed time to ozone (P<0.001). Exposed to ozone for 120min, Escherichia coli were totally killed. The different point in ozone disinfector test carriers were put did not affect the KL value. However there were negative correlations between gypsum rigidity of the carriers and KL of Bacillus subtilis var.niger (ATCC 9372), Escherichia coli (8099)(Pearson r=-0.805、-0.595) using the120min procedure.The killing Bacillus subtilis var.niger (ATCC 9372) and Escherichia coli (8099) efficacies were acceptable under 3 microwave disinfection procedure (carriers contained 50% water exposed for 15min and contained 100% water exposed for 7 min and 15min. Gypsum rigidity of test carriers did not affect germicidal efficacy (P>0.05). Nevertheless, there were positive correlations between water capacities of carriers, exposed time and KL of Bacillus subtilis var.niger (ATCC 9372), Escherichia coli (8099) (Pearson r=0.805,0.618,0.545,0.762).Compared among 12 disinfection procedure, the procedure which exposed to microwave for 15min with 100% water content was best one at germicidal efficacy, while 30min radiation of UV was the worst.The effects of physical properties after using different disinfection procedures were not the same.4 dimensional accuracy markers were significantly affected by four microwave disinfection procedures (exposure for 3min with 50% water and exposure for 3min,7min and 15min with 100% water); meanwhile,4 disinfection procedures were significantly affected:30min radiation of UV,46min exposure to ozone and 3min,5min exposure to microwave.Water capacities of carriers were positive correlative to the degree of deformation of four indicators after microwave exposed:MD, BL, OG, and IP (Pearson r=0.795,0.816,0.811, and 0.824).No significant decreases in compressible strength were observed after 30min UV radiation and 46min ozone exposure, nevertheless, after other disinfection procedures compressible strengths still met the needs of standard. There were negative correlations between water capacities of carriers and compressible strengths of three gypsum type using microwave procedures (Pearson r=-0.966,-0.958 and-0.955).According to the results of the study, some conclusions were made.1. Dental gypsum casts was contaminated heavily, and the main contaminated bacteria were opportunistic pathogens.2.30min UV radiation did not affect the physical properties of dental gypsum casts, but failed in disinfection efficacy using Bacillus subtilis var.niger (ATCC 9372) and Escherichia coli (8099) as indictors.3. Exposure to ozone for 120min was proved to be the effective disinfection procedure, moreover the effect of physical properties were acceptable to clinical application.4. The disinfection efficacy was negative influenced by the type of gypsum used.5. The gypsum castes pretreated with 50% water content and exposed to microwave for 15min was proved to be the effective procedure satisfied both disinfection efficacy and physical properties.6. The efficacy of microwave disinfection was influenced by the water content of gypsum casts and the exposure time. With increasing in water content and exposure time, the microwave disinfection efficacy was increased.7. The effect of physical properties after using different disinfection procedures was affected by the water content of gypsum casts. The higher water proportion the greater change of dimensional accuracy and more decrease of compressible strength were observed. |
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