Year : 2008 | Volume
: 26 | Issue : 4 | Page : 162--167
Evaluation of different sterilization and disinfection methods on commercially made preformed crowns
Y Yilmaz, C Guler
Department of Pedodontics, Ataturk University, Faculty of Dentistry, Erzurum, Turkey
Ataturk Universitesi, Dis Hekimligi Fakultesi, Pedodonti Ana Bilim Dali, Erzurum
The aim of this study was to evaluate the changes caused by different sterilization or disinfection methods on the vestibular surface of four commercially made preformed crowns using stereomicroscopy and scanning electron microscopy (SEM). Preformed crowns (NuSmile Primary Anterior Crown (NSC), Kinder Krowns (KK), Pedo Pearls (PP) and polycarbonate crowns (PC)) were sterilized and/or disinfected by one of the following techniques: no sterilization or disinfection (G1 control group); steam autoclaving at 134 °C (30 psi) for 4 min (G2); steam autoclaving at 134 °C (30 psi) for 12 min (G3); steam autoclaving at 121 °C (15 psi) for 30 min (G4); and ultrasonication in a bath containing 4% Lysetol® AF for 5 min at room temperature (chemical disinfection) (G5). Scanning electron micrographs of the crowns were taken before and after their sterilization or disinfection. The changes on the vestibular surface were then scored for the presence or absence of crazing, contour alteration, fracturing, and vestibular surface changes. The data were analyzed statistically using the chi-square test. No changes were observed before and after sterilization or disinfection in the stereomicroscopic evaluation of the vestibular surface of the crowns. However, all methods in which steam autoclaving was used to sterilize the crowns caused significant (P < 0.05) crazing and contour alterations of the vestibular surface of the crowns when they were examined by SEM. Chemical disinfection using an aldehyde-free disinfectant is the preferred method of disinfection for crowns that have been used previously in other dental patients.
|How to cite this article:|
Yilmaz Y, Guler C. Evaluation of different sterilization and disinfection methods on commercially made preformed crowns.J Indian Soc Pedod Prev Dent 2008;26:162-167
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Yilmaz Y, Guler C. Evaluation of different sterilization and disinfection methods on commercially made preformed crowns. J Indian Soc Pedod Prev Dent [serial online] 2008 [cited 2021 May 9 ];26:162-167
Available from: https://www.jisppd.com/text.asp?2008/26/4/162/44032
Preformed crowns are still commonly used for complete coronal coverage in the treatment of severely carious, malformed or fractured primary anterior teeth. Before fitting the crown, the tooth is prepared by reducing its occlusal, aproximal and peripheral surfaces. During the fitting of these preformed crowns to the reduced tooth, the selected preformed crown might not always fit the prepared tooth properly, and the dentist will need to try differently sized crowns in order to gain the desired fit. During test fitting, unwanted crowns become contaminated with blood and saliva and also may carry many microorganisms such as human immunodeficiency virus or hepatitis B virus, which can cause infection if the crown is used in another patient.  Therefore, the direct and indirect contact surfaces of such crowns need to be sterilized and/or disinfected before their re-use in another patient.  Although there is much information about the sterilization and/or disinfected of dental prostheses, ,,,,,, there are only a few studies on the effect of the sterilization and/or disinfection on preformed crowns. Wickersham et al ,  subjected commercially available preveneered stainless steel crowns (SSCs) to various sterilization and disinfection methods, such as steam autoclaving at various high temperatures and pressures and chemical sterilization (chemiclave sterilization) using formaldehyde vapor and a 2% gluteraldehyde solution. They reported that there were no changes on the vestibular surfaces of the crowns following these sterilization and disinfection methods.
To date, there is no study which has reported changes in the surface texture of the veneer of the preformed crowns after sterilization or application of a disinfectant.
Therefore, the objective of the present study was to evaluate the changes caused by sterilization or disinfection methods on the vestibular surface of four commercially-made preformed crowns using stereomicroscopy and scanning electron microscopy (SEM).
Materials and Methods
For this study, 80 crowns were obtained from four different manufacturers [Table 1]. The study included one control group (G1) and the four experimental groups (G2 to G5) [Table 2]. The control group consisted of five crowns randomly chosen from each crown type. First, the selected crowns were examined under a stereomicroscope at x60 magnification. Second, the selected crowns were then sputter-coated with carbon using an ion coater (SEM Coating Unit E 500, POLARON Equipment Limited, Barcelona, Spain) before undergoing SEM. The vestibular surface of each crown was then examined and photographed from the incisal edge to the gingival edge (2 mm in width) using a scanning electron microscope (JSM-6400, JEOL, Tokyo, Japan). These micrographs were used as the reference data to disclose the presence of changes on the vestibular surface after sterilization or disinfection. The crowns were cleaned by removing the carbon coating using a rotating brush without prophylaxis paste. Each crown in the control group was then returned to its original group of crown type. The four experimental groups consisted of five crowns randomly chosen from the four crown types.
Experimental Procedures and Assessment
Details of the different sterilization or disinfection methods that were used in this investigation are summarized in [Table 2].
All crowns in the four experimental groups were examined under a stereomicroscope at x60 magnification before and after a particular sterilization or disinfection method. Scanning electron micrographs were taken of all crowns in the four experimental groups before and after a particular sterilization or disinfection method as described for the control group. The vestibular surfaces were then scored for the presence or absence of a change using the criteria of Wickersham et al ,  with modifications. These criteria were crazing (internal cracks of vestibular surface), contour alteration (changes in vestibular surface contour such as melting, flow or flattening), fracturing (chipping) and vestibular surface changes (crazing + contour alterations). The scoring for each crown was performed under the stereomicroscope while a dental explorer was moved gently over the vestibular surface, and from scanning electron micrographs of the vestibular surface of each crown.
To validate examiner reproducibility for assessing each criterion, all micrographs were re-evaluated after one week by the same examiner. Kappa statistics were used to measure the extent of agreement between the repeated measurements performed by the same examiner. The Chi-square test was used in order to determine whether there were any statistically significant differences between the effects of the sterilization or disinfection methods with respect to crazing, contour alterations, fracturing, and vestibular surface changes of the crowns. In addition, this statistical test was used to identify which type(s) of crown and/or treatment(s) was responsible for any observed differences. The critical value for statistical significance was set at P =0.05.
The strength of the agreement between the two assessments for each criterion was almost perfect (Kappa value = 0.9).
No changes were observed after sterilization or disinfection in the stereomicroscopic evaluation of the vestibular surfaces of the crowns.
Micrographs of the vestibular surfaces of the crowns in the control and experimental groups are displayed in [Figure 1],[Figure 2],[Figure 3] and the results of the evaluation of scanning electron micrographs of the vestibular surface changes of the crowns following sterilization or disinfection are summarized in [Table 3]. All the methods in which steam autoclaving was used to sterilize the crowns caused surface changes when the crowns were examined by SEM. Furthermore, combined ultrasonication and chemical disinfection was the method of sterilization and disinfection that caused the smallest number of changes to the vestibular surfaces of preformed crowns.
Fractures were not observed in any of the crowns; therefore, no statistical analysis was performed for this criterion. There were statistically significant differences ( P P a preformed crown for a severely carious, malformed or fractured primary anterior tooth is performed either by measuring the mesiodistal width of the crown of the tooth to be treated or by trial and error. Tried and unfitted crowns in the clinical setting should be sterilized or disinfected to prevent cross-contamination by viruses and bacteria before their insertion into the mouth of another patient. Furthermore, sterilization or disinfection method should be effective and should not damage the material composition of the crown. 
This study investigated the changes to the vestibular surface of esthetic performed crowns that were created by different sterilization or disinfection techniques. In SEM micrographs, according to all evaluation criteria the least surface change was obtained by using combined ultrasonication and chemical disinfection (G5) [Table 3]. Although the manufacturer of PC crowns has not recommended any sterilization or disinfection method for their product, the manufacturers of KK and NSC preveneered SSCs recommend chemical disinfection with gluteraldehyde whereas that of PP preveneered SSCs recommends steam autoclaving for their products. However, Wickersham et al , found that the use of gluteraldehyde results in a color change in the esthetic constituents of NSC and KK crowns and a marked decrease in the resistance to fracture of KK crowns.  They and others have also reported that chemical disinfection of SSCs with gluteraldehyde had other disadvantages: the time needed for effective disinfection is long; , it is expensive; the solution progressively loses its disinfectant activity over a 14 day period; it causes corrosion of some metals; and it is a skin and mucosal irritant. , In this study, we used Lysetol® AF (Schülkeand Mayr GmbH, Norderstedt, Germany) as the chemical disinfectant for several reasons: it is suitable for rapid disinfection; it is aldehyde-free; and it does not cause metal corrosion. We did consider using either a solution of hypochlorite, chlorine or chlorhexidine as the chemical disinfectant in our study because each chemical is widely used to disinfect dentures. However, it has been reported that (a) disinfectants containing sodium hypochlorite are moderately cytotoxic;  (b) chlorine-based disinfectants can cause metal corrosion, although they are not cytotoxic  and (c) chlorhexidine gluconate can change the mechanical properties of restorative materials.  For these reasons, we decided not to use these chemical disinfectants in our study.
The results of the present study demonstrated that all sterilization methods using steam autoclaving caused surface changes when the crowns were examined by SEM. This finding may be explained by the properties of the esthetic constituents used in the manufacturing of dental crowns. PC crowns are made of a polycarbonate resin which is combined with microglass fibers. The heat distortion point of PC crowns is 132.2 o C  and the temperature used for steam autoclave sterilization in the G2 and G3 treatments was 134 o C. In G4 sterilization, the duration of the sterilization period was longer (30 min) than those used in the G2 and G3 sterilizations (4 min and 12 min, respectively), and the crowns were exposed to a lower temperature (121 o C) than the heat distortion point of PC crowns. This longer duration could be the reason for the occurrence of the surface changes after steam autoclaving.
The esthetic features of PP crowns are achieved by coating the outer surface of an aluminum shell with an organic enamel powder that contains polytetrafluoroethylene (PTFE), a synthetic fluoropolymer. The melting point of pure PTFE is 327 o C. The manufacturer of PP crowns claims that expansion or shrinkage of the esthetic materials in the crowns does not occur after autoclaving. However, we found that autoclaving at 121-134 o C caused changes on the surfaces of PP crowns. The exact cause of these is unknown but they may be due to impurities in the component chemicals or the manufacturing process of PP crowns because the esthetic material is a combination of organic enamel powder and PTFE.
The consistency of the resin composites in performed crowns varies considerably. NSC and KK crowns are constructed using a hybrid resin composite after sandblasting the vestibular surface. When we compared the surface changes of NSC and KK crowns, we found crazing and vestibular surface changes in these crowns and the extent of these changes were almost identical. All methods of steam autoclaving can potentially cause changes in the esthetic constituents of the vestibular surface of preveneered crowns. During autoclaving, degradation of resin itself and the hybrid resin composite (the mixture of the resin matrix and filler) can occur and may be the cause of the resultant surface changes. In addition, we suspect that autoclaving may also cause some changes at the resin composite material-sandblasted stainless steel interface. This notion is supported by the results of Wickersham et al ,  who reported that steam autoclaving may adversely affect both the bond interface and the composition of the resin composite.
All crowns were examined under a stereomicroscope at x60 magnification and we did not find any changes in any of the evaluation criteria. This finding is in agreement with those of Wickersham et al.  In contrast, we observed surface changes in the crowns when they were examined by SEM. Accordingly, stereomicroscopy is not a good method to evaluate surface changes of preformed crowns after their sterilization and/or disinfection.
It should be emphasized that all dental restorations should have highly polished surfaces because rough surfaces accumulate and retain more plaque than smooth surfaces.  Using crowns with rough surfaces increases the likelihood that periodontal tissues will become inflamed. Kopel and Batterman reported that use of methylmethacylate causes color changes and crazing in PC crowns and crazing can lead to plaque formation.  Van Dijken et al ,  have also reported that rough surfaces may develop with time due to alterations in the hybrid composite resin and be the cause of the resultant plaque formation. In our study, we found that sterilizing and/or disinfecting NSC and KK crowns, especially sterilization, resulted in rough surfaces of the esthetic constituent. As noted by Van Dijken et al ,  this development may promote the accumulation of dental plaque. Further in vitro studies are required in order to evaluate the extent of dental plaque accumulation on the esthetic constituents of crowns after different methods of sterilization or disinfection.
PTFE has a low surface energy and when used in crowns, the accumulation of dental plaque is reduced when compared to crowns that do not have PTFE as a constituent. Thus far, no study has tracked the accumulation of dental plaque on PP crowns over time. However, some researchers investigated the adhesion of hepatitis C virus and Streptococcus mutans to PTFE-coated materials and the results of these studies demonstrated that PTFE-coated materials reduced the adhesion of the microorganisms. For example, Maetani et al ,  reported that the early adhesion of S. mutans was decreased on a surface coated with PTFE. In our study, we found that steam autoclaving resulted in the formation of rough surfaces on PP crowns. These rough surfaces increase surface energy and we suspect that the increased surface energy of the rough surfaces of PP crowns that develop following steam autoclaving will be one of the reasons for the enhanced accumulation of dental plaque over time.
Sterilization and disinfection results in crazing, contour alterations and vestibular surface changes of preveneered SSCs when examined by SEM. However, no fracturing was noted. Chemical disinfection in an ultrasonic bath is the preferred method of sterilization of preveneered SSCs because it less damaging to their vestibular surfaces.
The authors thank Diane Johnson Krueger and NuSmile Crowns for their financial support. In addition, funds for the purchase of Kinder Krowns and Pedo Pearls Crowns were provided by the Department of Science Research Projects, Atatürk University.
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