LABORATORY STUDY. Larkin Clark, DDS*1 Martha H. Wells, DMD, MS2 Edward F. Harris, BA, MA, PhD3 Jennifer Lou, DDS4

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1 P E D IA TR IC D E N TIS T R Y V 3 8 NO 1 JAN / FEB 16 LABORATORY STUDY C o m p a ris o n o f A m o u n t o f P rim a ry T o o th R e d u c tio n R e q u ire d fo r A n te rio r and P o s te rio r Z irc o n ia a n d S tainless S teel C ro w n s Larkin Clark, DDS*1 Martha H. Wells, DMD, MS2 Edward F. Harris, BA, MA, PhD3 Jennifer Lou, DDS4 Abstract: Purpose: To d e te rm in e i f aggressiveness o f p rim a ry to o th p re p a ra tio n va ried a m o n g d iffe re n t b ra n d s o f zirco n ia a n d stainless steel (SSC) crow ns. M eth ods: O ne h u n d re d p rim a ry ty p o d o n t te e th w ere d iv id e d in to five g ro u p s (10 p o s te rio r a n d 10 a n te rio r) a n d assigned to: C heng C row ns (CC); EZ Pedo (EZP); K inder K row ns (KKZ); N us m ile (NSZ); a n d SSC. Teeth were p re p a re d, a n d assigned crow ns w ere fitte d. Teeth w ere w e ig h e d p rio r to a n d a fte r p re p a ra tio n. W e ig h t changes se rve d as a su rro g a te m easu re o f to o th re d u ctio n. Results: A nalysis o f variance show ed a significant difference in to o th reduction am ong brand/type fo r b o th the anterior and posterior. Tukey s honest significant d iffe re n ce te s t (HSD), w hen a p p lie d to a n te rio r data, re ve a le d th a t SSCs re g u ire d sig n ific a n tly less to o th re m o v a l co m p a re d to th e com p o site o f the fo u r zirconia brands, which showed no significant difference am ong them. Tukey s HSD test, applied to p o sterio r data, revealed th a t CC re q u ire d sig n ific a n tly g re a te r re m o v a l o f cro w n stru ctu re, w hile EZP, KKZ, a n d N SZ w ere sta tis tic a lly e quivalent, a n d SSCs re q u ire d sig n ificantly less rem oval. Conclusions: Z irco n ia crow ns re q u ire d m o re to o th re d u c tio n th a n stainless ste e l crow ns fo r p rim a ry a n te rio r a n d p o s te rio r teeth. Tooth re d u c tio n fo r a n te rio r zirco n ia crow ns was e q u iv a le n t a m o n g brands. F or p o s te rio r teeth, re d u c tio n f o r th re e b ra n d s (EZ Pedo, K inder Krowns, N us m ile ) d id n o t differ, w hile C heng C row ns re q u ire d m o re re d u ctio n. (P e d ia tr D e n t Z 016;3 8 (l):4 2-6 ) Received J u ly 14, 2015 I Last Revision S eptem ber 26,2015 I Accepted O ctober 3,2015 K E Y W O R D S : Z IR C O N IU M, P E D IA T R IC D E N T IS T R Y, C R O W N S The Rocky Mountain Company introduced stainless steel crowns (SSCs) to pediatric dentistry in For decades, they have outperformed other materials such as amalgam and composite to become known as the least likely restoration to require retreatment.1,2 While durability is a significant advantage of this restoration, the esthetic outcome is not. In 2005, Woo et al. conducted a survey of dental professionals and parents and found that individuals within all groups perceived anterior SSCs as esthetically unacceptable.3 Clinicians are encountering increased parental demands for esthetic restorations in the pediatric dental office.4 As parental involvement in the clinical decision-making process continues to increase, so does the necessity for providers to stay abreast of emerging esthetic techniques and treatment options.3,5,6 For over 30 years, the dental material sciences have spearheaded research into esthetically pleasing restorative materials that would meet or surpass the clinical properties of materials currently in use.7 Today, the dental practitioner has various options for the esthetic restoration of primary teeth, with each technique providing its own sets of advantages and disadvantages based on technical, functional, and esthetic limitations.5,6,8 Open-faced SSCs provide an esthetic option but are limited by chair time, use of multiple materials, isolation requirements, and lack of published clinical data supporting their retention or durability.6,9 In 2010, a poll of pediatric dentists showed that only four percent of those surveyed consider this technique as their first choice.9 While celluloid strip crowns provide a highly esthetic 2D r. Clark is in private practice. West Memphis, Ark., USA. 2Dr. Wells is an associate professor and director. Graduate Pediatric Dentistry Program, and 4D r. Lou is an assistant professor, both in the Department of Pediatric Dentistry and Community Health; and 1Dr. Harris is a professor. Department o f Bioscience Research, all in the College o f Dentistry. University of Tennessee Health Science Center, Memphis, Tenn., USA. Correspond with Dr. Wells at mwells@uthsc.edu option, resulting in the establishment of more natural tooth contours, they are considered extremely technique sensitive by professionals due to the need for adequate isolation and high documented failure rates.5,10 The esthetic facings and decreased technique sensitivity of preveenered SSCs have allowed this restoration to gain favor over the last 25 years.9 Because these crowns are preveenered with esthetic facings, chair time and materials are reduced compared to open-faced SSCs, thus providing a convenient esthetic option for many practitioners.6,9 Today, these restorations are used commonly among pediatric dentists, with parents reporting a high level of satisfaction; however, loss of facial resin can occur.9,11' 14 Prefabricated zirconia crowns are a comparatively newer treatment alternative that allows practitioners to provide patients with superior, highly polished, esthetic results.5,6 Pediatric zirconia crowns were first manufactured by EZ-Pedo Inc., and became commercially available in Currently, there are at least four commercial brands available, each with unique biomechanical properties (Table 1) While long-term clinical research of primary zirconia full-coverage restorations is lacking, it is known that these restorations have mechanical properties similar to metal, cannot be manipulated with crimping or contouring, require a passive fit and, thus, extensive tooth structure reduction, and have documented short-term retention rates as high as 100 percent.5,8,20 According to Oueis et al., the greatest concerns for dentists providing full-coverage esthetic restorative options are crown durability and the need for extensive reduction of tooth structure.9 Individual manufacturers of esthetic zirconia primary crowns readily state that their products require more tooth reduction when compared to alternative non-esthetic SSCs The purpose of this study was to determine if one or more brands of zirconia crowns required more aggressive preparation than other brands or a stainless steel crown. 4 2 Z IR C O N IA PREP

2 P E D IA T R IC D E N T IS T R Y V 3 8 / N O 1 JA N I FEB 16 M e th o d s One hundred primary typodont teeth (Kilgore International, Inc., Coldwater, Mich., USA) were obtained (50 primary maxillary right central incisors and 50 primary mandibular right first molars) and divided into five groups: (1) Zirconia Cheng Crowns (CC; Peter Cheng Orthodontic Laboratories, Inc., Exton, Pa., USA); (2) EZ Pedo Anterior and Posterior V2 (EZP; EZ-Pedo, Inc., El Dorado Hills, Calif., USA); (3) Zirconia Kinder Krowns (KKZ; Mayclin Dental Studios, Minneapolis, Minn., USA); (4) NuSmile ZR (NSZ; Orthodontic Technologies, Houston, Texas, USA); and (5) 3M ESPESSCs (3M ESPE GA, St. Paul, Minn., USA). Each group contained 10 anterior and 10 posterior samples. Each was weighed three consecutive times to the ten thousandths of a gram using a calibrated Mettier Toledo New Classic MF ML 204 digital balance (Mettler-Toledo, LLC, Columbus, Ohio, USA). All measured weights were recorded using an Excel spreadsheet (Microsoft, Inc., Redmond, Wash., USA), and the arithmetic mean was calculated for each sample tooth. The size of the zirconia and SSC crowns used for each group was determined by measuring the mesio-distal width of the to be prepared to receive the restoration (Table 2). Manufacturer s recommendations for both tooth preparation and bur use were reviewed for each brand of restoration. EZ Pedo recommended the use of specific burs, EZ-prep burs (EZ Pedo, Inc., El Dorado Hills, Calif, USA), while Kinder Krowns and NuSmile ZR suggested various burs such as tapered diamonds or diamond footballs but did not require specific burs. Cheng did not provide manufacturer recommendations on preferred burs.15'19 The EZ-Prep bur system fulfilled the recommendations of all manufacturers in this study and was used in the preparation of all samples. Each group was assigned two EZ-Prep bur systems: (1) one system for the preparation of the 10 anterior typodont teeth in the group; and (2) one for the preparation of the 10 posterior typodont teeth. Teeth were prepared by a single operator to properly receive the restoration (Figures 1 and 2). The operator and a Table 1. MANUFACTURER-SPECIFIC ZIRCONIA CROWN PRODUCT FEATURES AS OUTLINED ON MANUFACTURERS WEBSITES IN 2015 Brand Sizes C olor/contour Manufacturer recommended bur use Manufacturer reduction requirements Manufacturer suggested cement Key features Cheng Crown 1-6 Single color Left / right contoured No information provided Not specifically outlined/ videos available No information provided Smart polish technology Crimp-lock retentive margins Light and extra light shades EZ Pedo 1-6 Prime SL (space loss) for cuspids and molars Color enhancers are applied by hand to gingival one third of the margin Left/right contoured upper anteriors Universal options for lower anteriors only Promotes use of EZ prep burs Incisal edge/occlusal: mm Circumferential axial reduction: mm ( mm for posterior) Lingual: mm Circumferential subgingival reduction 2 mm below gumline Final seat: passive Pure glass ionomer cements Zir-lock ultra-precision milled retention grooves extending all the way to crown margins Zir plus mirror-polished surface and anti-reflective coating EZ seat contours EZ prep bur system Kinder Krowns 1-6 Mid-sized options for cuspids and molars Short and regular lengths for anterior crowns Single color Universal and left/right contoured available for upper anteriors LP (less prep) option for anterior and posterior crowns Recommendations are provided Kinder Krowns prep kit is available Incisal edge/occlusal: 1.0 mm Interproximal: 1.0 mm Facial/buccal: 1.0 mm Lingual: 1.0 mm Subgingival: feather margin circumferentially Final seat: passive and subgingivally 1-2 mm Resin modified glass ionomer cement Pure glass ionomer cement Wear-kind polish-glaze system Internal retention bands provide increased surface area High gingival acceptance finely feathered margins Fit check crowns available: avoids contamination of the crown actually cemented Prep kit and adjustment kit available NuSmile 0-6 Posterior primary first molars are available in regular or narrow mesiodistal widths Light and extra light color options Left/right contoured anteriors Universal options for lower anteriors only Recommendations are provided Incisal edge: mm occlusal: mm Circumferential axial reduction: mm (or 20-30%) SubgingivaL: feather margin circumferentially 1-2 mm Round all line and point angles Final seat: passive NuSmile biocem Resin cement Resin modified glass ionomer NuSmile try in crowns: avoids contamination of the crown actually cemented NuSmile ZR adjustment burs available Light and extra light shades Z IR C O N IA P R E P 4 3

3 PEDIATRIC DENTISTRY V 38 / NO 1 JAN / FEB 16 Figure 1. Anterior crowns and preparation. L to R: Stainless Steel crown, NuSmileZR, Cheng Crown, EZ Pedo, Kinder Krown. Figure 2. Posterior crowns and typodonttooth preparation. L to R: Kinder Krown, EZ Pedo, Cheng, NuSmileZR, and Stainless Steel crown. Anterior Posterior Figure 3. Average weights of typodonttooth structure removed (+SE) when preparing teeth for crowns, by brand. veteran faculty member calibrated themselves with three trial preparations of each tooth type (anterior and posterior) in which they mutually agreed upon the amount of reduction necessary to achieve a passive fit. These teeth were not included in the study. During the course of the study, the faculty member randomly selected three prepared teeth from each group to verify the proper fit. If the operator, based on faculty evaluation, did not achieve a proper fit, the sample in question was reduced to a greater extent until both operator and faculty member were in agreement. When this occurred, two additional samples from that group were randomly selected for evaluation for proper fit, resulting in 50 percent of the samples within that group being evaluated by the faculty member. Additional tooth preparation was necessary for one sample in the KKZ posterior group. Post-weights for each tooth were then obtained in triplicate and recorded in the same manner as pre-weights. Each tooth s change in weight was calculated and used in statistical analysis as a surrogate measure of the degree of aggressiveness of tooth preparation required for the proper fit of each brand s crown or the SSC. The data were normally distributed, as tested by Shapiro-Wilk tests, so differences were assessed with one-way analysis of variance using the conventional alpha of 0.05 to identify any statistically significant difference.21 The source of significance was assessed using Tukey s HSD as the post hoc test.22 Pre-treatment weights of the anterior teeth weighed an average of grams (±0.001 standard deviation [SD]), with all 50 specimens ranging between to grams. The posterior (molar) typodont teeth weights averaged grams (±0.002 SD), with a range of to grams. Results Anterior teeth. Using one-way analysis of variance (ANOVA), there was a significant difference among the brands of crowns, zirconia and SSCs, in the weight of material necessarily removed to properly fit the restoration (degrees of freedom (df) equals four, 45; F equals 166). O f the five brands, the SSCs required significantly less removal of tooth substance (/V0.0001), while there was no significant difference between the four zirconia brands, as tested by Tukey s honest significant difference (HSD) test. The mean difference between the most aggressive reduction by brand of anterior zirconia restoration (KKZ) and the least aggressive (EZP) was a mere grams. The tooth substance removed for the 3M SSC was 48 percent of that required for the composite of the four brands of zirconia restorations (Table 3 and Figure 3). Posterior teeth. Using one-way ANOVA, there was a highly significant difference among brands in the weight of material necessarily removed to properly fit the restoration (df equals four, 45; F equals 194; /VO.0001). O f the five brands, the SSCs required significantly less removal of tooth substance (/VO.0001). Statistically, the Tukey-Kramer post hoc test recognized three different groupings. SSCs required the least removal of crown substance. EZP, KKZ, and NSZ involved an intermediate degree of reduction, while CC required the most. Mean weight loss due to preparing the crowns of the posterior teeth to receive an esthetic zirconia restoration ranged from grams (CC being the most aggressive) to grams (EZP being the least aggressive). The SSCs required the least preparatory crown modification, and tooth substance removed for this restoration was 54 percent of that required for the composite of the four brands of zirconia restorations (Table 3 and Figure 3). Table 4 shows the average weight change (a 44 ZIRCONIA PREP

4 P EDIATRIC D E N TIS T R Y V 3 8 I NO 1 JAN I FEB 16 surrogate measure of tooth reduction) that was required for each brand of crown and type of tooth (anterior or posterior). D is c u s s io n Typodont tooth weights prior to preparation had a standard deviation of grams, meaning that the typical typodont tooth differed less than a thousandths of a gram from others in the sample and were considered homogeneous in size and weight, as received from the manufacturer. This uniformity of the typodont teeth helped achieve statistically significant differences among brands. One limit of this study was that homogeneity (small variability) was much smaller than if biologically real teeth had been used. After reducing the anterior teeth to fit the crowns, there was a significant difference among the zirconia crowns compared to the stainless steel crowns, which is clinically relevant. The stainless steel crown required much less tooth reduction, which means mechanical exposure of the pulp is less likely. However, in general, the poor esthetics precludes its use in the T a b le 2. SIZE OF ANTERIOR A N D POSTERIOR STAINLESS STEEL AN D ZIRCONIA CROWNS SELECTED FOR COMPARISON Brand Measured Measured Size width o f width of used crown (mm) typodont tooth (mm) Anterior 3M ESPE stainless steel crowns R2 (SSCs) Kilgore Zirconia Cheng Crowns 6.2 International primary CR2 Zirconia Kinder Krowns 6.2 tooth E E2S EZ Pedo Anterior Zirconia 6.3 E2 NuSmile ZR 6.3 AIR Posterior 3M ESPE SSCs LRD 4 Zirconia Cheng Crowns 8.6 Kilgore International DLR5 Zirconia Kinder Krowns 8.7 primary S5 EZ Pedo Posterior V2 Zirconia 8.6 tooth S S5 anterior dentition. Statistically, there was no significant difference among the brands of anterior esthetic zirconia restorations, as all required the same degree of reduction. After adjusting for standard error, there was only a three percent difference for reduction required among the anterior brands of zirconia restorations, which would be obscured clinically by inter-patient variability. Clinically, the minor differences in the reduction of tooth structure necessary for the proper fit of the zirconia restorations would go unnoticed and be mitigated by variations in morphology, size of the carious lesion, and gender and race differences. Again, statistically, there was a large difference among the five brands of posterior restorations, with the SSCs requiring the least removal of crown substance, as one would expect. Clinically, mechanical exposure of the pulp is less likely to occur with the use of an SSC in the posterior region. Three brands (EZP, KKZ, and NSZ) required more reduction than SSCs but did not differ significantly from one another, while CC required the largest reduction to fit the crown. After adjusting for standard error, there was only a five percent difference between the mean reduction required for the four brands of posterior esthetic crowns. We contend that this difference among posterior zirconia brands is also clinically trivial. Consequently, clinicians should consider other product features and requirements when choosing among the zirconia brands studied. Minimally invasive procedures generally are preferred in dentistry. T a b le 3. M E A N ±(S TAN D AR D D EVIATIO N ) W E IG H T OF TO O TH SUBSTANCE REM O VED (G RAM S) FOR TH E FIVE BRANDS OF ANTERIOR AN D POSTERIOR CROW NS* Brands Anterior Posterior 3M SSC ±0.0059A ±0.0068c Cheng Crowns ±0.0034B ±0.0082D EZ Pedo ±0.0033B ±0.0042E Kinder Krowns ±0.0027b ±0.0064E NuSmile ±0.0036B ±0.0064E * One-way analysis of variance showed significant difference in tooth reduction between the crowns tested (anterior: df=4, 45; PcO.OOOl; posterior: df=4, 45; P<0.0001). Uppercase superscripts denote weights that are not significantly different. T a b le 4. ANTERIOR A N D POSTERIOR T Y P O D O N T TO O TH W EIG H TS BEFORE A N D AFTER TO O TH PREPARATION FOR THE FIVE BRANDS OF CROWNS TESTED* Brand Anterior primary maxillary right central incisor Posterior primary mandibular right first molar Pretreatment Post-treatment % reduction in weight Pretreatment Post-treatment % reduction in weight 3M ESPE stainless steel crown Kinder Krowns EZ Pedo Cheng Crowns NuSmile ' Each value represents the arithmetic mean of the averaged three repeated weights for each of the 10 specimens. Z IR C O N IA PREP 4 5

5 PEDIATRIC DENTISTRY V 38 / NO 1 JAN / FEB 16 Tliis study found that conventional SSCs required significantly less removal of coronal tooth structure to obtain proper fit, as one would expect. When compared to SSCs, the composite of the four anterior zirconia restorations required more than twice as much tooth structure removal (205 percent). While in the posterior, the composite of the four zirconia restorations required slightly less than twice as much tooth structure removal when compared to the SSCs (185 percent). The primary limitation of this study was its in vitro nature. While zirconia crown preparations are signficantly more aggressive than SSC preparations, determining the clinical repercussions of aggressive tooth reduction, such as mechanical exposure of the pulp, is beyond the scope of this study. In today s society, practitioners and parents are often driven by esthetic concerns when choosing among restorative options, especially in the anterior region. Conclusions Based on it this study s findings, the following conclusions could be made: 1. Zirconia crowns required significantly more tooth reduction than stainless steel crowns in the anterior and posterior primary dentition. 2. EZ Pedo, Kinder Krowns, Cheng Crowns, and NuSmile brands were statistically equivalent esthetic choices in the anterior region, based on aggressiveness of preparation. 3. In the posterior region, Cheng Crowns required the removal of significantly more tooth substance to obtain proper fit when compared to the other brands of zirconia restorations. References 1. Randall RC, Vrijhoef MM, Wilson NH. Efficacy of preformed metal crowns vs. amalgam restorations in primary molars: a systematic review. J Am Dent Assoc 2000; 131: Randall RC. Preformed metal crowns for primary and permanent molar teeth: review of the literature. Pediatr Dent 2002;24: Woo D, Sheller B, Williams B, Mancl L, Grembowski D. Dentists and parents perceptions of health, esthetics, and treatment of maxillary primary incisors. Pediatr Dent 2005;27: Zimmerman JA, Feigal RJ, Till MJ, Hodges JS. Parental attitudes on restorative materials as factors influencing current use in pediatric dentistry. Pediatr Dent 2009;31: Townsend JA, Knoell P, Yu Q, et al. In vitro fracture resistance of three commercially available zirconia crowns for primary molars. Pediatr Dent 2014;36: Ashima G, Sarabjot KB, Gauba K, Mittal HC. Zirconia crowns for rehabilitation of decayed primary incisors: an esthetic alternative. J Clin Pediatr Dent 2014;39: Zarone F, Russo S, Sorrentino R. From porcelain-fusedto-metal to zirconia: clinical and experimental considerations. Dent Mater 2011;27: Walia T, Salami AA, Bashiri R, Hamoodi OM, Rashid F. A randomised controlled trial of three aesthetic fullcoronal restorations in primary maxillary teeth. Eur J Paediatr Dent 2014; 15: Oueis H, Atwan S, Pajtas B, Casamassimo PS. Use of anterior veneered stainless steel crowns by pediatric dentists. Pediatr Dent 2010;32: Tate AR, Ng MW, Needleman HL, Acs G. Failure rates of restorative procedures following dental rehabilitation under general anesthesia. Pediatr Dent 2002;24: Shah PV, Lee JY, Wright JT. Clinical success and parental satisfaction with anterior preveneered primary stainless steel crowns. Pediatr Dent 2004;26: Beattie S, Taskonak B, Jones J, et al. Fracture resistance of 3 types of primary esthetic stainless steel crowns. J Can Dent Assoc 201 l;77:b Fuks AB, Ram D, Eidelman E. Clinical performance of esthetic posterior crowns in primary molars: a pilot study. Pediatr Dent 1999;21: Ram D, Fuks AB, Eidelman E. Long-term clinical performance of esthetic primary molar crowns. Pediatr Dent 2003;25: NuSmile. Clinician s #1 Choice for esthetic crowns. Available at: Accessed June 13, (Archived by WebCite at: http: //www. webcitation.org/6qj7dbriq ) 16. Cheng Crowns. Pediatric crowns. Available at: chengcrowns.com/products. Accessed June 13, 2014 (Archived by WebCite at: 6QJAxeGhW ) 17. Kinder Krowns. Esthetic pediatric crowns. Available at: Accessed June 13, (Archived by WebCite at: http: //www. webcitation.org/6qjbbhe0s ) 18. EZ Pedo. Anterior technique. Available at: ezpedo.com/anterior-technique-2.html. Accessed June 13, (Archived by WebCite at: citation.org/6qjbjn9kl ) 19. EZ Pedo. Posterior technique. Available at: pedo.com/posterior-technique-2.html. Accessed June 13, (Archived by WebCite at: org/6qjbqujtt ) 20. Planells del Pozo P, Fuks AB. Zirconia crowns: an esthetic and resistant restorative alternative for ECC affected primary teeth. J Clin Pediatr Dent 2014;38: Shapiro SS, Wilk MB. An analysis of variance for normality (complete samples). Biometrika 1965;52: Kramer CY. Extension of multiple range tests to group means with unequal numbers of replications. Biometrics 1956;12: ZIRCONIA PREP

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