Study On The Wear Of Natural Enamel By Highly-Translucent Monolithic Y-TZP Ceramics Crowns

All-ceramic crowns have become popular in restorative dentistry due to their strength and aesthetics.In traditional all-ceramic crowns,Y-TZP ceramic or alumina ceramic is used as the substructure and a layer of veneering porcelain is fired on the surface to modify its opacity and present proper esthetics.However,the veneering porcelain is readily crushed and chipped during mastication.To resolve this problem,full-contour zirconia crowns without veneering porcelain were developed using a monolithic Y-TZP blank and CAD/CAM technique.The high strength,high hardness,and high fracture toughness of the monolithic zirconia crown give rise to concerns about the wear of the zirconia crown against the opposing tooth.In recent years,a highly-translucent Y-TZP ceramic has been used to produce monolithic ceramic crowns.However,reports about the clinical evaluation of the ceramic,especially about its wear against antagonist tooth,are less.Among these,the quantitative measurement of nature tooth wear was even less and their results were quite different.On the other hand,most laboratory wear studies used the cusps of extracted teeth as a grinding head to wear the plane of the Y-TZP ceramic block.This contact form of teeth on ceramic did not adequately reproduced the crown-tooth contact form in mouth.Therefore,a systematic study on the wear of natural teeth by the monolithic zirconia crown was needed.In this dissertation,combining the clinical evaluation and laboratory testing,the wear of all-ceramic crowns made of highly-translucent monolithic Y-TZP ceramic on the tooth enamel was studied quantitatively and qualitatively.Meanwhile,the predilection wear sites of the antagonist tooth and the possible wear mechanism were analyzed,and the polishing efficacy of commonly used zirconia polishing systems was evaluated.In addition,the clinical influence of monolithic zirconia crowns on the gingival tissue of restored teeth was observed.Methods:1.Clinical evaluation of highly-translucent monolithic Y-TZP ceramic crowns.The purpose of this study was to evaluate the clinical efficacy of posterior monolithic zirconia crown restoration.A total of 49 teeth of 46 patients requiring posterior crown restorations were treated with monolithic zirconia crowns.The crown restorations and its antagonist teeth,corresponding contralateral teeth and its antagonist teeth were evaluated in terms of plaque index,gingival index,exploration depth and the efficacy of the monolithic zirconia crown according to the modified California Dental Association criteria and Smith and Knight tooth wear index immediately after restoration and 2 weeks,6 months,1 year,and 2 years after restoration.The resulting data were statistically processed.2.In vivo quantitative study on the wear of the natural enamel by highly-translucent monolithic Y-TZP ceramic crown and its possible wear mechanism.The Smith and Knight tooth wear index used in first study was unable to evaluate the wear of antagonistic tooth precisely.Therefore,the purpose of this study was to quantitatively evaluate the wear of natural teeth by antagonistic monolithic zirconia crowns and analyze the wear mechanism.Forty-three Chinese patients with 43 posterior crown restorations were screened and enrolled in this study according to an inclusion criterion to evaluate the wear of the monolithic zirconia crown on the antagonistic natural teeth by a combination of quantitative and qualitative methods.All restorations were examined immediately after restoration and 6 months and 1 year after restoration.First,fine impressions of the patients’monolithic zirconia crowns and their antagonist teeth,and the corresponding contralateral teeth and their antagonist teeth,were made.Then stone casts and epoxy resin casts were made respectively,which were then scanned using a three-dimensional（3D）model scanner and digital models were reconstructed and then imported into a 3D analysis software Geomagic Control.The 3D digital morphology of crowns after 6 months and 1 year were overlapped with those immediately after restoration,and then the average wear depth was calculated using 3D deviation analysis.The digital model was further imported into the software of Materialise Magics to determine volume loss.Moreover,the surface of epoxy resin casts was observed under a stereomicroscope to determine wear sites and their morphological features.Finally,micromorphological features were observed under a scanning electron microscope（SEM）to determine the wear mechanism.3.In vitro study on the wear of a highly-translucent Y-TZP ceramic against human enamel.In this part,the wear of the Y-TZP ceramic grinding head against the human enamel block plane was performed using a novel chewing simulator with 49N grinding pressure and 10,000 cycles.The grinding head tested included polished and rough round heads and round heads with a flat tip,which represented two typical contact forms of upper and lower teeth:point-facet contact and facet-facet contact.The average depth loss and the volume loss of the enamel were measured using noncontact 3D profilometry.The enamel wear surface and its longitudinal-section microstructure were observed under an SEM to determine the possible wear mechanisms.4.Comparison of the polishing efficiency of four polishing systems for Y-TZP ceramic crowns.This study evaluated the polishing efficiency of four commercially popular zirconia polishing systems（KOMET,EVE,TOB,SHOFU）on four brands highly-translucent zirconia ceramic（Weiland,Upcera,Cercon,Audental）,aiming to provide an experimental-based recommendation for clinical selection of zirconia polishing systems.First,the surface of the zirconia ceramic block specimens after milling and sintering process was ground with a fine abrasive diamond bur to simulate the clinical occlusal adjustment,followed by polishing with one of the four polishing systems respectively.Then the surface roughness（Ra）of each specimen was determined using non-contact laser scanning profilometer.Finally,the surface microstructure was observed under a scanning electron microscope and the crystal phase transformations in the surface were assessed using XRD.Results:1.All 46 patients completed the follow-up evaluation and entered the result analysis.According to the modified CDA evaluation criteria,the excellent rate in all restorations was 93.9%～100%in terms of marginal fitness,anatomical morphology,restoration integrity,marginal coloration,and postoperative sensitivity.During each observation period,the plaque index,gingival index,and periodontal exploration depth rating of the restored tooth were not significantly different from those of the antagonist teeth,corresponding contralateral teeth and its antagonist teeth（P>0.05）.Several crown antagonist teeth showed obvious wear in the occlusal surfaces,but they were classified as no change according to Smith and Knight tooth wear index.2.After 6 months,the average wear depth of the highly-translucent Y-TZP ceramic crowns and their antagonist teeth,corresponding contralateral teeth and their antagonist teeth was 17.3±5.23μm,50.03±17.02μm,21.55±7.12μm,and 20.13±6.91μm,respectively.The average wear depth of the highly-translucent monolithic zirconia crowns was significantly smaller than others（P<0.01）.The average wear depth of crown’s antagonist teeth was significantly greater than that of control natural teeth（P<0.01）.The average volume loss of the monolithic zirconia crowns and their antagonist teeth,corresponding contralateral teeth and their antagonist teeth was 0.23±0.05 mm³,0.49±0.12 mm³,0.38±0.11 mm³,and 0.37±0.13 mm³,respectively.The average volume loss of the monolithic zirconia crown was less than others（P<0.05）.The volume loss of crown antagonist teeth was significantly larger than that of control natural teeth（P<0.05）.No significant difference was found in the wear depth or volume among control natural teeth（P>0.05）.After 1 year,the average wear depth of the monolithic zirconia crowns and their antagonist teeth,corresponding contralateral teeth and their antagonist teeth was 27.2±7.63μm,81.57±25.49μm,36.13±11.23μm,and 33.69±10.13μm,respectively.The average volume loss of the monolithic zirconia crowns and their antagonist teeth,corresponding contralateral teeth and their antagonist teeth was 0.31±0.05 mm³,0.61±0.16 mm³,0.47±0.14 mm³,and 0.45±0.15 mm³,respectively.The average wear depth and volume of the monolithic zirconia crown were significantly smaller than others（P<0.01）.Contrastively,the average depth and volume loss of crown antagonist teeth were significantly larger than control natural teeth（P<0.05）.No significant difference was observed in the wear depth or volume among control natural teeth（P>0.05）.All crowns and teeth presented significantly higher loss in depth and volume in one year than in six months（P<0.05）.The observation on epoxy resin cast using a microscope revealed that most wear of antagonist teeth was caused by the occlusal cusps and ridges of monolithic zirconia crowns.The SEM observation of tooth wear traces showed that the wear mechanism of the monolithic zirconia crown on natural teeth was mainly abrasive wear and fatigue wear.3.The average wear depth of the enamel plane surface by polished round head,rough round head,polished round head with a flat tip and rough round head with a flat tip after 10,000 cycles was 31.14±5.19μm,46.75±12.42μm,7.39±2.73μm,and 14.06±3.78μm,respectively.Their average volume loss was（10.49±3.77）×10^-2mm³,（18.10±4.53）×10^-2mm³,（1.19±0.56）×10^-2mm³ and（4.55±1.77）×10^-2mm³,respectively.Both showed significant differences between groups（P<0.01）.The rough round head caused the maximum wear of the enamel and the polished counterpart caused the least wear of the enamel,both demonstrating a significant difference（P<0.01）.Also,significant differences were found between different abrasion time（P<0.05）.The wear of the rough zirconia head on the enamel was significantly greater than that of the polished counterparts（P<0.01）.After 10,000 cycles of wear,SEM showed that the wear pits by polished and rough round heads were more obvious than those by polished and rough heads with flat tip.The roughness of the wear pits of two polished zirconia heads were smoother than those of the rough counterparts.The surface of wear scratches showed many typical fatigue wear tiny pits when observed under SEM.At subsurface of the pits,within 20～30μm under the pits,micro oblique cracks were found within enamel,mainly between the enamel rods.4.The polishing efficiency of the four polishing systems on highly-translucent Y-TZP ceramic was significant different.The lowest roughness（0.18±0.06 to 0.32±0.15μm）of the four zirconia ceramic blocks was obtained with the EVE system and the highest roughness was obtained with the TOB system（0.80±0.14 to 1.12±0.13μm）（P<0.05）.The TOB system demonstrated almost no polishing efficacy on zirconia ceramic because no obvious roughness change was observed before and after polishing（P>0.05）.The SEM showed that no polishing system was able to completely eliminate the surface scratches and grooves of zirconia ceramic produced by grinding procedure and EVE polishing system produced a much more flat and smoother surface than other polishing system.The polishing efficiency of polishing system seems to related to their abrasive grits size and size distribution because the abrasive grits size of EVE three-level polishing tools was in a reasonable distribution.The scanned three-dimensional topographic of zirconia blocks was consistent with SEM results.The XRD results showed that surface roughening and the following polishing processes did not lead to obvious phase transformation of surface tetragonal zirconia.Conclusion:1.During the 2-year observation period,the highly-translucent monolithic zirconia crowns maintained excellent anatomical shape and integrity,excellent marginal fitness and marginal color,no postoperative sensitivity and secondary caries,no obvious adverse effect on gingival tissue and high patient satisfaction.Although the highly-translucent monolithic zirconia crown demonstrated an excellent clinical outcome,several zirconia crowns caused visible wear to the antagonist teeth.The Smith and Knight tooth wear index was too rough to evaluate tooth wear precisely and a more accurate measuring method need.2.The highly-translucent monolithic zirconia crown itself presents less wear in the clinic,but it causes more wear against antagonist natural teeth and the wear increases with time.The occlusal cusps and ridges of monolithic zirconia crown cause obvious local wear of the antagonist teeth,therefore removing improper cusps and protrusions of the occlusal surface of zirconia crown by grinding and polishing should be a preventive method to avoid excessive wear of antagonist teeth.The wear mechanism of highly-translucent monolithic zirconia crown on natural teeth is mainly abrasive wear and fatigue wear.3.The wear of the highly-translucent zirconia ceramic on the tooth enamel was closely related to the shape of zirconia restorations.The flat-top head with a large contact area had less wear on the enamel,showing more abrasive wear but less fatigue wear.The round-top head with a small contact area had more wear on the enamel and showed more fatigue wear.The surface roughness of zirconia ceramic heads was closely related to the abrasion of the enamel.The head with higher roughness produced more wear of enamel than that of the head with lower roughness and showed more abrasive wear and fatigue wear.4.The in vitro grinding simulating the clinical grinding process significantly increased the surface roughness of highly-transparent zirconia ceramic.Four commercially available polishing systems had different polishing efficiency on the zirconia ceramic.The EVE polishing system was more effective than the other systems and TOB polishing system presented a worst polishing efficiency.The simulated clinical grinding and polishing process did not lead to phase transformation of the surface of the zirconia.The innovations of this study:1.This study was unique in systematically exploring the wear of highly-translucent monolithic zirconia crowns on natural teeth in vivo by a combination of quantitative and qualitative methods.After one year of application,the zirconia crown caused obvious wear against the antagonist teeth,which primarily occurred at the sites where the occlusion cusps and ridges of zirconia crown were in close contact with antagonist tooth during chewing.The wearing surfaces presented abrasive wear and fatigue wear.2.The wearing of highly-translucent zirconia heads against the natural enamel surface was studied in vitro in a chewing simulator initiatively in this study.The results confirmed the findings of the clinical study,showing that the zirconia ceramic head with small contact area led to noticeable wear of the antagonist natural teeth.The wear extent was closely related to the contact area and the surface roughness of the zirconia ceramic head.Highly polishing surface of the cusp could significantly reduce the wear of the natural enamel.3.The wearing mechanism of zirconia ceramic crowns against the natural teeth,which involved primarily abrasive wear and fatigue wear,was investigated and analyzed.