The Impact of Repeated Steam Sterilization Cycles on the Efficacy of Chairside Adjustment Kits for Polishing Monolithic Multi-Layered Zirconia Dental Restoration Material

Introduction

Since the introduction of milled blocks in the dental market (year 1980) [1], with subsequent production of computer-aided design computer-aided machining (CADCAM) esthetic restorations, zirconia-based ceramics have overcome the major limitations associated with the use of dental alloys in fabrication of dental restorations [2]. Polymorphism of zirconia allows it to exist in multiple crystalline forms [monoclinic, tetragonal, cubic] that are primarily temperature-dependent (room temperature to 2370°C and above) [3]. Stabilization of zirconia crystals at desired temperatures is achieved by addition of yttrium oxide (Y2O3) or magnesium/aluminum oxide (Mg2O3, Al2O3), which causes non-existing tetragonal and/or cubic crystal phases to exist after sintering at room temperatures [4]. To date, 4 different generations from yttria – stabilized tetragonal zirconia oxides (Y-TZP) have found clinical applications [1st (3Y-TZP) (3% yttrium, 0.25% Al2O3) (partially stabilized zirconia) (PSZ); 2nd (3Y-TZP) (3% yttrium, 0.5% Al2O3) (PSZ) [5]; 3rd (5Y-TZP) (5% yttrium, 0.5% Al2O3) (fully stabilized zirconia)(FSZ) [6]; and 4th generation (4Y-TZP) (4% yttrium, 0.5% Al2O3) [7]. PSZs features a characteristic phase transformation toughening in response to a stimulus that includes chairside adjustment (grinding/finishing/polishing) through inhibiting crack propagation while FSZ has isotropic and large volume cubic crystals than tetragonal grains that enhances its translucency [3,5,6]. The main advantage of first 2 generations was their high flexural strength, while aesthetic effect was accomplished through layering of feldspathic ceramic which produced clinical failures (chipping) [7]. Monolithic zirconia (MLZ) restorations manufactured using single CADCAM block overcame the problems associated with layering of ceramics but lacked an important aesthetic parameter of translucency. Third generation zirconia with its 50% cubic phase (early 98% tetragonal) not only allowed light transmission but also reduced refraction effect of opaque zirconia, thereby improving translucency, with the chief drawback being lower fracture resistance [8], which was overcome by the introduction of 4th generation [6]. Introduction of multi-layered (polychromatic/preshaded) transparent MLZ comprising of 2–4 variable translucent layers within the same restoration was achieved by blending 2 zirconia materials (3Y-TZP and 5Y-TZP), while improved strength was accomplished by reducing cubic crystal phase [9,10]. Multilayer MLZ presents high strength (flexural) 3Y-TZP at restorations cervical region (dentin/body) and high strength translucent 5Y-TZP (high or ultra-high translucent) in the incisal region [11]. This combination overcomes the low flexural strength of 5Y-TZP. MLZ restorations fabricated with preshaded, multilayer, polychromatic, and translucent zirconia thus provides clinicians with advantages of conservative tooth preparation [12], improved flexural strength for fabrication of short-span fixed partial dentures [13], minimal antagonistic tooth wear, translucency-backed aesthetics [14], less fabrication time, and no complication of ceramic chipping [13,14]. For a dental patient, the technology offers substantially lesser clinical time and fewer sessions, with no fear of complications.

Multi-layered MLZ restorations require sintering like any other ceramic, which produces linear shrinkage resulting in 15–30% volumetric changes of the restorations [3,15]. This shrinkage is overcome subjectively by increasing the pre-sintered size of the restoration, which is later compensated for after post-sintering shrinkage. These non-controllable changes in restoration are overcome by clinical adjustment of the restoration on either contacts (proximal/occlusal) and/or contours (emergence profile, margins) [16]. Adjustment is mostly preferred on pre-glazed sintered restoration, but is associated with additional and/or multiple clinical visits by the patients [17]. MLZs also undergo phase transformation and low-temperature degradation upon repeated firings, resulting in weak ceramic surfaces [3,6]. The conventional glazing process after clinical adjustment also has been found to increase wear of antagonist teeth [18,19]. Studies evaluating the comparative differences of surface treatments (glazing and polishing) after surface grinding (clinical adjustment), on wear behavior [18], translucency/color [3,20], surface roughness (smoothness), [19] reported polishing of MLZ produces to have parameters equivalent or better than glazing. The superiority of chairside polishing over glazing in terms of light transmission [21], surface roughness [18,19,22], and gloss [20] reported in studies eventually found alternatives to overcome problems related to repeated glazing in MLZ restorations. Polishing of MLZ has also been reported to have no effect on phase transformation [23]. Polishing is based on tribology, the science of interacting moving surfaces that involves application of various principles (friction, wear, and lubrication). MLZ polishing can be performed either using a 2-bodied abrasion (without lubricant/polishing paste) or 3-bodied abrasion method (with lubrication) [24]. Zirconia, when compared to other glass ceramics, displays higher polishability, due to its homogenous crystal lattice [25]. Earlier polishing of highly translucent monolithic zirconia (HTMLZ), which was performed by conventional finishing/polishing systems, was found to produce greater surface roughness as compared to glazed MLZ [26], and also had shorter survival [27]. Research on polishing of MLZ, including studies on multi-layered preshaded, have established several key factors that influence their final surface roughness and polishing end result. These include polishing system selection and manipulation, as well as general surface roughness determinant factors (abrasive characteristic, handpiece speed, polishing load, application time, and motion type) [28–30]. The outcome of these studies has resulted in various MLZ manufacturers providing individual customized finishing and polishing kits and their respective technique (1-, 2-, or 3-step, with/without polishing paste) for both pre- and post-sintered MLZ [3,31]. Use of polishing paste (both pre-sintered and post-sintered) was also reported to influence the translucency negatively and significantly in HTMLZ, mainly due to abrasives like silicium carbide, zirconia [32], and individual binders [33]. To reduce or eliminate the influence of polishing paste on translucency of HTMLZ, polishing kits provide abrasive-impregnated polishing tips in the form of wheels, points, and cylinders. Depending upon the material to be polished, the tips contain abrasives (diamonds, zirconium oxide, silicon dioxide) with particle size that ranges from 6 mm for diamond to 40 mm for silicon dioxide [34]. Chairside adjustment kits (CAK) are specifically manufactured for individual specific products that in most of the cases cannot be interchanged. The manufacturer of the CAK at times provides detailed instructions on how to use the product tools and under what conditions (speed, motor type). Abrasive particles are bound to the binder through sintering (diamond burrs are strongest due to fusion), vitreous bonding (fused with ceramic), resinoid bonding (cold/heat pressed to resin, used in composite resin finishing kits), and rubber bonding (elastomeric matrix for ceramic polishing kits) [35,36]. While the abrasives themselves (diamond, zirconium oxide) are highly efficient because of their relative hardness, which permits them to resist wear during polishing, the binder (natural/synthetic rubber, nitrile rubber, silicone elastomer) used in polishing tips tends to wear rapidly, limiting their life and efficacy [25]. Surface modifications/treatments on MLZ has been recently reported to produce differences in crystallography, phase composition, and mechanical properties in response to mechanical/thermal treatments like grinding, finishing, polishing, or glazing [37].

Indirect restorations require clinical adjustments, some of which are performed intraorally once the restoration is seated, which makes sterilization of finishing/polishing kits necessary for each patient. Sterilization using antiseptics and disinfectants has been reported to be ineffective in eliminating pathogenic organisms and additionally result in corrosion of the tools [38]. Steam sterilization using autoclaves with its proven efficacy is considered to be the standard sterilization procedure in dentistry, where chances of cross-infection are very high [39]. Two of the most common steam sterilization cycles used are standard steam sterilization (121ºC, 15 min, 1 bar, pre-vacuum, not for immediate use) and immediate (flash) sterilization (134ºC, 3 min, 2 bars, pre-vacuum, for immediate use) cycles [40]. Finishing/polishing tools used for composite resin have been reported to have a reduction in their functional effectiveness with a significant effect on surface gloss and roughness after sterilization [41]. Lacerda et al reported that the influence on the polishing points could be as low as 5 cycles of steam sterilization [42]. Elastomeric matrix used as binder in polishing points has been reported to become softer than at its first application [41], with decreased tear-resistance and tensile strength [43,44], and undergo thermal degradation (oxidation and hydrolysis) with cracking and leaching of components [40]. Eventually, such changes result in loss of abrasive particles from polishing tip surfaces, thereby affecting their performance cycle. Shah et al studied the effect of sterilization on rubber elastomers and found an increase in surface hardness with steam sterilization (autoclave cycle type and number) [45]. Multiple-use finishing polishing tips that contain silicone rubber as binder, irrespective of sterilization, have demonstrated significantly reduced polishing outcome (surface roughness) than new polishing kit [41]. Loss of abrasive with concomitant increased surface hardness of the elastomeric binder matrix after repeated steam sterilization of HTMLZ chairside adjustment kit (CAK) could therefore inadvertently result in surface roughness during polishing of HTMLZ, which is considered worth investigating and forms the basis of this study. Surface roughness values (Ra – average surface roughness) beyond the threshold level (Ra=0.25 μm) [46] on MLZ restorations impacts its translucency [10,11,14,15,20], contrast ratio [3,11], abrasive ability [18,19,22,23], biofilm accumulation [47], and prevention of plaque retention [3,14,46]. Surface roughness can be measured using different devices, such as profilometers and microscopes (atomic force, optical, and scanning electron)] [48]. Average surface roughness (Ra) values over the complete surface are the most widely used parameter for studying surface texture, while root mean square roughness (Rq), which represents standard deviation of surface heights, is more sensitive than Ra (it provides microscopic measurement) and can be used clinically to determine the film thickness that can substantially decrease surface roughness either through glazing or polishing.

This in vitro study therefore was aimed to investigate the influence of 2 common steam sterilization techniques (standard and immediate/flash) and number of sterilization cycles (1 to 20 cycles) on the efficacy of manufacturer-recommended CAK to produce acceptable surface smoothness in HTMLZ. We hypothesized that differences between standard and immediate sterilization techniques would exist due to diverse time-temperature relationships, and as the number of sterilization cycles increases, the CAK would lose its ability to produce smooth surfaces with the acceptable threshold (0.25 μm). Alternately, the null hypothesis for the study was be that there would be no differences between the 2 sterilization techniques of CAK on surface roughness and the number of sterilization cycles on the functional efficacy of CAK.

Material and Methods

ETHICS:

This in vitro experimental study was proposed to the ethics committee of college of dentistry, Jazan University as part of the requirement for research conducted by intern students under the direct/indirect supervision of staff of the department of prosthetic dental sciences during academic year 2022 to 2023. The research was duly approved by the college committee, with registration number: CODJU 22081.

STUDY DESIGN:

This study was an in vitro investigation using a comparative intervention approach between control and experimental groups. The independent variables were CAK-related procedures (grinding, finishing, polishing) and 2 steam sterilization techniques (standard, flash) of CAK [Diacera (EVE)]. Dependent variables were the outcome on Ra and Rq roughness values complemented with scanning electron microscopy (SEM). Figure 1 presents the flowchart of the study, showing hypothesis generated and tested, variables (dependent and independent), intervention and outcome. Both operators who measured the parameters were blinded to the type of specimen, group, and research outcome.

OPERATIONAL DEFINITION:

Clinical adjustment refers to various steps that include grinding, finishing, and polishing. Grinding is contextually defined to denote the act of correcting disharmonies in various contacts and contours of a MLZ restored by removing the excess on the surface [49]. Finishing refers to intentional, selective and controlled wear of restorative material while polishing is defined as reducing the surface roughness created during finishing, to the extent that the surface feels smooth and looks glossy [49]. Steam sterilization is the physical process of absolute elimination of microbial viability on an inert surface using an autoclave (in this case the finishing and polishing kit) [40]. Ra is the metric mean of the total deviations from a mean plane on the specimen surface that represents the average roughness value evaluated over the entire specimen surface. Rq is the metric mean of the absolute values of 5 highest peaks and valleys from mean plane within the sample specimen surface area.

SAMPLE SIZE: The total study and individual group sample size was statistically calculated using NQuery software (Informer Technologies, USA, V7) (formula N=2 σ2×(Zα+Zβ) 2/@2) [50]. The criterion set in the software was an effect size (D2=.28), study power assumption (80%) and type 1 error rate (a=0.05) based on earlier studies [3,18]. The calculation yielded the total sample size to be 100 specimens with 50 samples in 2 main groups and 10 specimens in each subgroup. To overcome the loss of sample due to manufacturing defect or impaired intervention 2 samples in each group were kept as subsidiary in case of the need of replacement.

: ZGX is a CADCAM milled, preshaded multilayer highly translucent MLZ (type II, class 5 – DIN EN ISO 6872) that is available commercially in 8 A to D vita shades with addition of bleach. The color of the blank is thus integrated into different shades of color and translucency that is distributed from cervical to incisal areas. The chosen shade (A2) in this study represents a combination of vita shade A1 and A2. The material is available as blanks in the shape of a disk with different heights (range 12 to 25 mm). Before processing, the manufacturer recommends calibration (specimen dimensions, sintering temperature) so that appropriate sintering temperature is chosen for specific dimensions. Designing of the specimens was performed by recommended software (Ceramill Mind DRS) followed by transfer for sample positioning of each block in the 5-axis milling machine (vhf Camfacture AG, Ammerbuch, Germany) using other software (Dentalcam, v 8.0). Milling of each specimen was done following recommendations (800 W, 60 000 rpm spindle; dry milling, 3 μm repetition accuracy) using a range of cutting tools (Table 1). ZGX recommends a nesting of the specimen that is based on the desired shade and translucency [51]. For the study, nesting was accomplished by placing the ceramic blank exactly in the vertical center with reference of thickness of the specimen. One hundred disk-shaped specimens with equal dimensions (10 mm diameter, 3 mm thickness) were procured from pre-sintered ZGX blanks. The specimens were milled in auto-dry mode followed by the recommended sintering program [program 7 (P7) zolid gen -X/DRS/bion 1 h]. A standard sintering program that included a heat-up, dwell/holding time, and cooling phase (Table 1) with different temperatures and rates was used.

CHAIRSIDE ADJUSTMENT (GRINDING, FINISHING AND POLISHING): The manufacturer recommends grinding to be performed using diamond grinding points (grain size 40 μm) and finishing/polishing using diamond impregnated polishing bits. Standardization of grinding was achieved by connecting the high-speed handpiece to pressure capsule gauge which controlled pressure on each specimen. Diamond bur (NTI-Kahla GmbH, Ome-City, Tokyo) was applied at 160 000 rotations per minute in a unidirectional movement with handpiece fixed to a surveyor. The specimens were marked from the outside with 2 points between which the operator had to run the specimen horizontally in single direction on an even platform. Finishing/Polishing of the specimens was carried out as per manufacturer’s recommendations of the CAK (Eve Diacera (Ernst Vetter, GmbH, Keltern, Germany), having a set of 8 tools with 2 coarse, 3 medium, and 3 fine. Each point was run at manufacturer-recommended speed (Table 1). Polishing was done for a period of 90 s using the same assembly as that for finishing with the difference being the change of motor driven handpiece. One direction polishing for a period of 60 s was done for each specimen in each subgroup. All specimens after adjustment were rinsed with air water spray (30 s) followed by cleaning in ultrasonic cleaner (TUC-150, Telsonic AG; Bronschhofen; 60 s, triple-distilled water), and drying in room-temperature air.

STEAM STERILIZATION (AUTOCLAVE): Ten CAKs [Diacera (EVE) Ernst Vetter, GmbH, Keltern, Germany] were used for fulfilling the study protocol. One kit was meant for each subgroup to make sure that each group was finished and polishing equally without having used the polishing tips on other samples. The kits were then divided into 2 main groups (n=5 each) depending upon the steam sterilization technique (standard group (Gp) S, immediate/flash group Gp F). A common autoclave (Melag, Vacuklav 23 B, Keltern, Germany) was used for all kits. Details of the 2 cycles are presented in Figure 1 and Table 1. Each CAK was steam sterilized 20 times, and depending upon the number of autoclave cycles the ZGX samples were further subdivided [autoclave cycle (AC) 1, AC5, AC10, AC15, AC20]. Each kit in Gp S and Gp F were used for grinding, finishing and polishing at these 5 steam sterilization cycles. Between sterilization cycle (2–4, 6–9, 11–14, 16–19), separate set of ZGX specimens were ground, finished and polished to simulate clinical scenario. Based on autoclave cycling of the finishing kits, the ZGX specimens represented each group (Gp S=S1, S5, S10, S15, S20; Gp F=F1, F5, F10, F15, F20) (n=10 each) (Figure 1). For each group, the subgroup after first sterilization cycle acted as a control (Gp S1, Gp F1, n=10 each), while the remaining 4 subgroups were experimental (n=10 each).

SURFACE ROUGHNESS MEASUREMENT (QUANTITATIVE ANALYSIS): Samples of 10 subgroups were evaluated for surface roughness after undergoing a predesignated autoclave cycle of the finishing/polishing kits (cycles 1, 2, 5, 10, 15, 20) in both groups (Gp S, Gp F) using a profilometer (JEOL, Tokyo, Japan) interconnected to a desktop computer with Vision 32 software (Veeco, NY). The specifications of the profilometer and the relative calibrations are presented in Table 1. Profilometer calibrations were performed to make the stylus tip scan an approximate length of 0.75 mm. For each ZGX specimen, the profilometer tracing was recorded at a predesignated center of the specimen. Three measurements for each specimen were taken. The average of these 3 mean surface measurements was then calculated in terms of score of a single specimen. Measurement of surface roughness was performed at 20x (200 μm x 200 μm) using arithmetical means Ra (average surface roughness) and Rq (root mean square roughness). Other arithmetical means (Rp, Rt, Rv) were also generated and were used to verify the results manually using different surface roughness measurement formulas [52]. The operator of the profilometer was blinded during analysis when randomly choosing 3 areas of a particular specimen.

SURFACE TOPOGRAPHY (SEM) (QUALITATIVE ANALYSIS):

A randomly selected specimen from 3 predetermined subgroups (control, 10^th cycle, 20^th cycle) was analyzed for surface topography using scanning electron microscopy (SEM) (JSM, 6360 LV, JEOL Corp., Tokyo, Japan). These groups were selected according to the anticipated changes, which were based mainly on the differences in mean surface roughness values (Ra). For each analysis, specimens were rinsed under distilled water, followed by drying and fixing on a cylinder (13 mm diameter, 10 mm height). Following sputter coating (gold palladium alloy, SPI-Module sputter, USA), micrographs were created at magnification (5000×) in SE mode to illustrate sample roughness.

STATISTICAL ANALYSIS:

Following all analyses, data obtained were entered into Microsoft Excel, where they were corrected, refined, and coded using SPSS version 22.0 software (IBM, Armonk, NY, USA) on a desktop computer (Lenovo, CT55AG7) using Windows 10 Pro. Mean values and standard deviations were obtained for all subgroups after testing the raw data for normality using the Kolmogorov-Smirnov test. To determine the differences in average surface roughness mean values (Ra) and RMS roughness (Rq) between the experimental and control subgroups, one-way analysis of variance (ANOVA) was performed. Tukey’s honest significant difference (HSD) test was used as a follow-up to one-way ANOVA to assess the significance of differences in means between paired groups. All differences were considered to be statistically significant at P≤0.05.

Results

SURFACE ROUGHNESS (AVERAGE AND ROOT MEAN SQUARE):

The quantitative mean surface roughness values of HT ZGX specimens after sterilization of finishing polishing kits after various sterilization cycles are presented in Table 2. The lowest Ra values were observed in subgroup F1 (m=0.2076) (control) in Gp F, while in Gp S the lowest Ra values were observed in subgroup S5 (mean=0.2384). The highest Ra values among all subgroups were seen in subgroup S20 (m=0.6116), suggesting that finishing kits when sterilized with standard cycle (121ºC, 15 min) produced more surface roughness at the end of 20 sterilization cycles. There was less change in Ra values from cycles 1 to 20 when finishing kits were sterilized using the flash cycle (0.2076 to 0.4127) than when sterilized with the standard sterilization cycle (0.2397 to 0.6116). Rq values, which represents the standard deviation of the surface heights, showed less deviation in subgroup F5 (m=1.767) for group F and subgroup S10 (m=1.5009) for group S. The one-way ANOVA comparison of the means of different independent groups showed significant differences (P≤0.05) for both Ra and Rq among the 5 sterilization cycles in both groups. The Tukey HSD post hoc pairwise comparison test results are presented in Table 3, which shows which subgroups differed significantly for Ra and Rq surface roughness parameters within each group. For Ra, the post hoc test showed 9 subgroup comparisons differed from each other, and the distribution of differences between subgroups varied in both groups. Gp S showed differences between subgroups after cycle 15, while Gp F showed differences between subgroups after cycle 10. For Rq, which describes surface roughness more appropriately, the post hoc test showed fewer pairs of subgroups differed significantly in Gp F than Gp S. Figure 2 shows a 2-dimensional surface topographic image using a profilometer for lowest Ra values in each subgroup. The color-coded scale shows the roughness values with lower values represented by lighter shades and higher roughness values depicted by dark shades.

SCANNING ELECTRON MICROSCOPY:

The scanning electron microscopy images of a single random specimen for Gp S and Gp F at control, cycle 10, and cycle 20 are shown in Figure 3. The images at both control period shows smooth surfaces without any residual defects, while the image taken at steam sterilization cycles 10 and 20 of finishing kit showed the spread of surface defects and smooth surfaces. At both cycles, the specimen in Gp F showed fewer surface defects than that of Gp S.

Discussion

MANUFACTURER’S RECOMMENDED POLISHING SYSTEM:

Our study results also support the fact that for HTMLZ one should not use routine adjustment ceramic kits. This is due to the differences in abrasive type used in the adjustment kit for HTMLZ. Routine porcelain polishing systems contain ceramic abrasives, but for HTMLZ the recommended abrasive in the polishing systems is different (diamond, zirconium oxide, or silicon dioxide), which has been reported to have higher hardness than other ceramic abrasives [28,37]. The abrasive used for polishing zirconia contains diamond abrasive, whose higher abrasive efficacy is attributed to its high hardness values (H=10) [53]. High hardness allows it to resist wear, which is directly related to its ability to maintain sharpness in polishing systems. The diamond used in the abrasives is in the form of dust or particles with different sizes (grits) that are either coated within a rigid matrix or impregnated within bonded elastomeric matrix (rubber tips) [35,37,41] or also used as polishing paste/slurry [33]. However, it should also be noted that the surface topography of a diamond abrasive used in polishing systems plays a significant role in maintaining its efficacy [54]. Surface topography of diamond coatings does influence its performance, including the level of friction [55], wear, and biocompatibility [56]. Use of diamond polishing pastes (3-bodied) for polishing of ceramics have been found to round the profile shape, which in turn lowers the height of only maximum rough peaks. Despite reducing the metric mean height of the surface profile, polishing paste use of ceramics influences the overall average surface roughness values, which is why most ceramic polishing systems for ceramics are 2-bodied [57]. Polishing paste has been recently investigated for use with highly translucent zirconia (Prettau Anterior; Zirkonzahn), and was found to have less surface roughness than other groups that did not use polishing paste [37]. However, the authors did not mention whether polishing paste is recommended by manufacturers and at what stage was polishing paste used (pre- or post-sintering). Pffererle et al [33] studied different polishing tools (felt wheel, goat hair brush, polishing paste, finishing kit) for white-stage zirconia (Ceramill Zolid HT) and found the felt wheel produced the highest translucency, while 2-step polishing produced less surface roughness, and polishing paste reduced translucency.

Most ceramic polishing systems are customized to the ceramic they are recommended for, which is based on previous studies that indicated different ceramic materials need different polishing systems to achieve an acceptable finish [58]. The EVE Diacera polishing system contains 6 flexible abrasive tools in varying shapes that are intended exclusively for finishing and polishing. The other 2 (coarse) are rigid that are primarily used to grind excess ceramic. These flexible abrasive tips are manufactured by molding different size distributions within an elastomeric matrix, which can be either a natural, synthetic, or silicone rubber. The decreased efficacy in polishing is more due to the changes in the elastomeric matrix than the abrasive. The elastomeric matrix loses flexibility upon repeated sterilization, especially in the presence of water (steam), and with polishing there are physical changes in the shape of the tips. Friction on these tips makes them to lose abrasive particles, which exposes the mold in which the particle was embedded. As elastomeric matrix becomes hard [49], the edge of the mold becomes the source of creating roughness on a ceramic rather than being the source of polishing the ceramic, except for the binder degradation for implant abutments, which are impregnated with different growth-promoting factors [59].

INFLUENCE OF STERILIZATION:

Regarding the overall influence of sterilization on CAK, we found that as sterilization cycles increased, the Ra and Rq values increased, with more changes being produced after cycle 10, irrespective of the sterilization technique used. Standard sterilization cycle uses lower temperatures and pressures but require more time than flash sterilization. Analyzing the results in Gp S, it is evident that less temperature and pressure during sterilization does not increase the efficacy of CAK in terms of creating smooth surfaces on HTMLZ ceramic. On the contrary, the lower Ra values from cycles 1 to 20 (0.2076 to 0.4127) in Gp F as compared to those in Gp S (S1 – 0.2397 to S20 – 0.6116) show that the duration of sterilization has more influence on the efficacy of polishing systems than temperature and pressure, since flash sterilization has a 3-min duration as compared to 15 min in the standard cycle. Other differences in sterilization cycles like heat-up and cool-down times, which directly affect differential expansion of the product being sterilized, can also influence the efficacy of the polishing system being sterilized [40]. Flash sterilization requires longer heat-up and cool-down times, which in turn reduces the exposure time during the cycle and, more importantly, removes the longer influence of moisture from the sterilizer, which produces less distortion in the products being sterilized [40]. Most polymers undergo chemical reactions in the presence of steam, including oxidative degradation and hydrolysis, which causes most problems. Elastomers are flexible polymers that differ from polymers in terms of elasticity. No single sterilization method exists that is compatible with all healthcare products (devices, materials, drugs, polymers) [40]. Lacerda et al [42] studied the effect of 2 disinfection and sterilization methods on the silicone rubber-based finishing instruments for composite resin, showing that the SEM images (150×) after 5 cycles of polishing and sterilization produced visible changes in the polishing tips compared to the unused ones. They attributed these changes to loss of a superficial polymer-rich layer of elastomeric matrix of finishing tips, which contains less filler particles (abrasives), which further substantiates that changes in elastomeric matrix are associated with polishing tip efficacy. Gupta et al [60] studied the effect of autoclaving on translucency of multilayer composite shade guides and concluded that autoclaving affected both color and translucency parameters of the shade guide upon repeated sterilization.

EVE DIACERA CAK:

Our baseline Ra values for polishing system EVE Diacera are similar to those obtained by Huh et al [30], who compared the 2 MLZ polishing systems (Meisinger and EVE Diacera) and found lower Ra values with EVE Diacera. On the contrary, Park et al [28] reported higher Ra values with EVE Diacera than other systems, the differences in results being attributed to differences in number of polishing instruments used in other polishing systems and due to sterilization. Jumah et al [61] found that 4-step polishing systems lowered Ra values, resulting in fewer surface flaws as compared to 1-, 2-, and 3-step polishing systems on fully and partially stabilized zirconia. Zucuni et al [62] also investigated 2 different polishing systems (EveDiacera and Viking) on Y-TZP (Zenostar T, Ivoclar-Vivadent) and found higher Ra values (0.326–0.839) in polished specimens than in the control group (0.221), and these values were higher than at baseline. The difference in values can be attributed to the difference in polishing technique. Grinding was performed using the coarse EVE Diacera tool, while we used a diamond point for grinding MLZ. The manufacturer’s instructions for ZGX specify that grinding should be done using a diamond burr, whereas the coarse tool in the kit is recommended for removing excess porcelain [51]. Excess surface roughness beyond clinically acceptable thresholds affects HTMLZ translucency and enhances the other factors that decrease translucency [63] and its color [64].

RQ SIGNIFICANCE:

We also substantiated our findings on Ra by measuring and presenting the values of Rq, which describes surface roughness by depicting the standard deviation of the height distribution on the surface [65]. They represent the distribution (skew, kurtosis) of surface roughness. In brief, Ra represents only an estimate of surface roughness, while it cannot distinguish peaks from valleys. Rq values allow this distinction; therefore, lower values for Ra with higher values of Rq indicate the pattern of maximum heights present on the surface and thus provides a clearer picture of the roughness pattern associated to a particular surface. Rq is a statistical representation of what is observed in SEM images. Figure 3 presents the SEM images of 2 different sterilization methods at different cycles. The images for standard cycle roughness areas are wider and larger than those observed in flash-sterilized specimens. The Rq parameter has wide applications in machine mechanics, where it is used to determine how much lubrication will be required to minimize friction between 2 mechanical parts [66]. When applied to ceramics, Rq values indicate the thickness of the overglaze layer that needs to be applied to cover roughness. Higher Rq values require more glaze thickness than lower Rq value specimens. Clinically, these values recommend that if a clinician is adjusting a HTMLZ with a kit that has been sterilized using standard sterilization technique more than 10 times, glazing may be a better option than polishing the restoration with that particular CAK.

STRENGTHS AND LIMITATIONS OF THE STUDY:

To the best of our knowledge, this study is the first to investigate the influence of steam sterilization on the efficacy of the MLZ polishing kit. The study presents not only the average surface roughness values, but also the root mean square values, which provide standard deviations of surface roughness. Study limitations are that the wear of the polishing tip from first specimen to the tenth specimen might influence the outcome, which should be considered in further studies, but for such studies, using 1 kit for each sample can be expensive. The study is also limited by the device used, which was a profilometer, whose tips are usually large and do not allow measuring defects smaller than the tip size.

Conclusions

Within the scope/limitation of the materials and methods used in this study, one may conclude that the efficacy of CAK exhibits a decline in reproducing smooth surfaces on HTMLZ after 10 sterilization cycles. The findings from this study support that repeated heat sterilization reduces effective polishing when using a chairside adjustment kit on HTMLZ. Therefore, to achieve clinically acceptable average surface roughness (Ra) and root mean square roughness (Rq), no more than 10 flash heat sterilization procedures should be performed. Standard sterilization cycles can be used if the same recommendations are followed.