Abstract
Objectives
To test the hypothesis that there is no difference in the in vivo maximum wear of enamel opposing monolithic zirconia crowns, enamel opposing porcelain fused to metal crowns and enamel opposing enamel.
Methods
Thirty patients needing single crowns were randomized to receive either a monolithic zirconia or metal-ceramic crown. Two non-restored opposing teeth in the same quadrants were identified to serve as enamel controls. After cementation, quadrants were scanned for baseline data. Polyvinylsiloxane impressions were obtained and poured in white stone. Patients were recalled at six-months and one-year for re-impression. Stone models were scanned using a tabletop laserscanner to determine maximum wear. Statistical analysis was performed using Mann-Whitney U to determine any significant differences between the wear of enamel against zirconia and metal-ceramic crowns.
Results
Sixteen zirconia and 14 metal-ceramic crowns were delivered. There were no statistical differences in mean wear of crown types (p = 0.165); enamel antagonists (p = 0.235) and enamel controls (p = 0.843) after one year.
Conclusion
Monolithic zirconia exhibited comparable wear of enamel compared with metal-ceramic crowns and control enamel after one year.
Significance
This study is clinically significant because the use of polished monolithic zirconia demonstrated comparable wear of opposing enamel to metal-ceramic and enamel antagonists.
1
Introduction
Zirconia became popular in dentistry because of this material’s excellent mechanical properties , which include high strength, fracture toughness , and biocompatibility . Zirconia was mainly used as a substructure for ceramic-ceramic restorations and required veneering ceramics to obtain proper esthetics because of their high opacity. In general, these ceramic-ceramic restorations exhibited superior esthetic properties compared with their metal-ceramic counterparts .
Despite the excellent physical properties of zirconia, veneer chipping has been identified as a major cause of failure. A systematic analysis of zirconia-based FDPs shows a survival rate of 94.3% . However, when technical complications such as chipping of the veneer ceramic are included, their survival decreases to 76.4% . Heintze performed a systematic review to analyze the survival of zirconia (90%) and metal (97%) supported FDPs after three years. He concluded that veneer chipping was a major cause of failure. The mean long-term survival rate of zirconia frameworks at 10 years is 91.5% with failures attributed to marginal deficiencies and veneer chipping.
To overcome veneer chipping, dental manufacturers developed monolithic zirconia prostheses, which rely on the toughness and strength of the material to eliminate the need for the fracture-susceptible veneering ceramic. Veneer fractures in ceramic-ceramic restorations are believed to be the result of differences in the thermal expansion coefficients of the core ceramic and veneer ceramic and non-uniformity of condensation during ceramic build-up .
One major concern with the use of monolithic zirconia as a restorative material is the abrasive nature against opposing enamel because of this material’s hardness and surface roughness . Several in vitro and in vivo studies were conducted to determine the wear of enamel against zirconia. Numerous in vitro studies showing wear of zirconia against different antagonists, including enamel, have shown zirconia to be comparable to other restorative materials in terms of wear of opposing enamel . However, in vitro studies are hard to compare with each other because of differences in surface finish of material, type of material, method of wear and type of wear analysis used. The limited clinical studies which have been published describe how monolithic zirconia is a viable restorative material in that the wear of antagonist enamel is within the range of acceptable limits . However, since there is a limited number of clinical studies available, there is need for more clinical analyses to further validate the wear compatibility of zirconia with enamel.
The purpose of this study was to test the hypothesis that there is no difference in the in vivo maximum wear of enamel opposing monolithic zirconia crowns, enamel opposing porcelain fused to metal crowns and enamel opposing enamel.
2
Materials and methods
2.1
Study design
A randomized, controlled, clinical trial was designed to analyze the wear of enamel by opposing polished monolithic zirconia crowns and by the polished veneer surfaces of metal-ceramic crowns. This single-blind pilot study involved a total of 30 teeth that required full coverage crowns that opposed natural antagonist teeth.
2.2
Study intervention
2.2.1
Participant recruitment
Participants that needed full coverage crowns were randomly assigned to receive either a polished (nonglazed) zirconia crown (Lava™ Plus, 3M ESPE, PZ), or a polished (nonglazed) veneer of metal-ceramic crown (GC Initial™, GC America; Argedent 62, Argen, USA, PV). All participants were over 21 years old with no contraindications to dental treatment. These participants were screened for low caries risk, the absence of periodontal disease and no temporomandibular disorders. Each participant needed a crown on either a first or second premolar or first or second molar in any arch. Inclusion criteria for abutment teeth included: restorability with a crown:root ratio of at least 1:1; presence of an opposing natural tooth which was non-restored or minimally restored; the presence of two non-restored or minimally restored teeth opposing each other on the same quadrants as the crowned tooth and the opposing to serve as enamel controls. Minimally restored was defined as teeth which have no restoration greater than a Class II amalgam restoration. The opposing arch did not have a full coverage restoration or a partial denture. Two crowns were the maximum number of crowns for each participant. A random number table was formulated by the statistician to facilitate assignment of teeth to either material group. The clinical coordinator assigned to the study enrolled the participants and assigned them to the material groups. Patients were treated at the University of Florida College of Dentistry Dental Clinical Research Unit. Institutional Review Board approval for treating human subjects using the research protocol was obtained. All participants were required to sign an informed consent form prior to initiating the study.
2.2.2
Crown fabrication
One investigator prepared all the teeth to receive crowns based on design criteria for crown preparation. Provisional material (Protemp™ Plus, 3 M ESPE) was used to fabricate provisionals. Prepared teeth were scanned using a chairside oral scanner (3M True Definition™ Chairside Oral Scanner Digital Impression System, 3M, ESPE). Scans were sent to one laboratory for crown fabrication.
Crowns were received from the laboratory with a polished surface. All crowns were polished using porcelain polishers impregnated with diamond abrasives (Shofu Dura Polish Dia, Shofu Dental Corporation). Try-in and adjustment, if necessary, of each crown were made with a fine diamond bur (8369DF.31.025 FG Fine Football Dialite Diamond, Brassler, USA) on a high-speed handpiece. Crowns were polished with diamond impregnated porcelain polishers in the order of coarse, medium, and fine points (Dialite, Brassler, USA) until a fine lustre was achieved. All crowns were cemented with a resin cement (Rely X™ Unicem Self-Adhesive Resin Cement, 3M ESPE). Participants were not made aware of the type of crown they received.
A baseline examination was performed one week after cementation to ensure that the patient was comfortable with the crown and no further adjustments were needed. When no further adjustments were necessary, teeth were cleaned to remove plaque and saliva. A vinylpolysiloxane impression (Imprint 3, 3M ESPE) was made of the maxillary and mandibular quadrants, where the crown and the opposing tooth are located, to record the occlusal surfaces of each cemented crown and its antagonist tooth. These are the same quadrants where the enamel controls are also located. Photographs of the quadrants were made with occlusal contacts marked by articulating paper (Accufilm ® II double sided articulating film, Parkell Prod Inc). The red paper was used to indicate maximum intercuspation while the black paper was used to indicate contacts in excursive movements. The post-cementation casts were poured with a white gypsum material (GC Fujirock, GC America) to enable optimal scanned image contrast. The participants were asked to return at 6 months and one year. Quadrants were re-impressed during both time periods.
2.2.3
Wear quantification
The maximum wear was quantified as the maximum loss in height. A 3D Laserscanner (CS2, Straumann, Germany) was used to scan the casts at baseline, six months and one year along the x, y and z axes of the casts. These period scans were superimposed against one another using tripodization by identifying three points on the occlusal anatomy which are expected to remain stable ( i.e. marginal ridges). The matching of the two scans was conducted by the software to achieve a match with a standard deviation (SD) less than 25 μm. The scanning accuracy for this type of scanner is reported to be 20 μm . The matching process was repeated until an acceptable SD was achieved. After proper matching was achieved between the period scans, the maximum wear of the crowns and teeth at these time periods were compared and recorded. The wear areas were compared with the clinical photographs to confirm intra-oral contact areas.
2.3
Statistical analysis
All analyses were performed using the R statistical software package (V3.2.4, The R Foundation for Statistical Computing, Vienna, Austria). Since the sample size was small (N < 15 per group for all comparisons), the non-parametric Mann-Whitney U test was used to compare wear between the zirconia and metal-ceramic crown types at six months and one year. To compare antagonist wear to control wear between the two groups, the difference between antagonist wear and control wear as the outcome for each patient was calculated. The mean wear of the two control teeth was used as the control wear for that patient.
2
Materials and methods
2.1
Study design
A randomized, controlled, clinical trial was designed to analyze the wear of enamel by opposing polished monolithic zirconia crowns and by the polished veneer surfaces of metal-ceramic crowns. This single-blind pilot study involved a total of 30 teeth that required full coverage crowns that opposed natural antagonist teeth.
2.2
Study intervention
2.2.1
Participant recruitment
Participants that needed full coverage crowns were randomly assigned to receive either a polished (nonglazed) zirconia crown (Lava™ Plus, 3M ESPE, PZ), or a polished (nonglazed) veneer of metal-ceramic crown (GC Initial™, GC America; Argedent 62, Argen, USA, PV). All participants were over 21 years old with no contraindications to dental treatment. These participants were screened for low caries risk, the absence of periodontal disease and no temporomandibular disorders. Each participant needed a crown on either a first or second premolar or first or second molar in any arch. Inclusion criteria for abutment teeth included: restorability with a crown:root ratio of at least 1:1; presence of an opposing natural tooth which was non-restored or minimally restored; the presence of two non-restored or minimally restored teeth opposing each other on the same quadrants as the crowned tooth and the opposing to serve as enamel controls. Minimally restored was defined as teeth which have no restoration greater than a Class II amalgam restoration. The opposing arch did not have a full coverage restoration or a partial denture. Two crowns were the maximum number of crowns for each participant. A random number table was formulated by the statistician to facilitate assignment of teeth to either material group. The clinical coordinator assigned to the study enrolled the participants and assigned them to the material groups. Patients were treated at the University of Florida College of Dentistry Dental Clinical Research Unit. Institutional Review Board approval for treating human subjects using the research protocol was obtained. All participants were required to sign an informed consent form prior to initiating the study.
2.2.2
Crown fabrication
One investigator prepared all the teeth to receive crowns based on design criteria for crown preparation. Provisional material (Protemp™ Plus, 3 M ESPE) was used to fabricate provisionals. Prepared teeth were scanned using a chairside oral scanner (3M True Definition™ Chairside Oral Scanner Digital Impression System, 3M, ESPE). Scans were sent to one laboratory for crown fabrication.
Crowns were received from the laboratory with a polished surface. All crowns were polished using porcelain polishers impregnated with diamond abrasives (Shofu Dura Polish Dia, Shofu Dental Corporation). Try-in and adjustment, if necessary, of each crown were made with a fine diamond bur (8369DF.31.025 FG Fine Football Dialite Diamond, Brassler, USA) on a high-speed handpiece. Crowns were polished with diamond impregnated porcelain polishers in the order of coarse, medium, and fine points (Dialite, Brassler, USA) until a fine lustre was achieved. All crowns were cemented with a resin cement (Rely X™ Unicem Self-Adhesive Resin Cement, 3M ESPE). Participants were not made aware of the type of crown they received.
A baseline examination was performed one week after cementation to ensure that the patient was comfortable with the crown and no further adjustments were needed. When no further adjustments were necessary, teeth were cleaned to remove plaque and saliva. A vinylpolysiloxane impression (Imprint 3, 3M ESPE) was made of the maxillary and mandibular quadrants, where the crown and the opposing tooth are located, to record the occlusal surfaces of each cemented crown and its antagonist tooth. These are the same quadrants where the enamel controls are also located. Photographs of the quadrants were made with occlusal contacts marked by articulating paper (Accufilm ® II double sided articulating film, Parkell Prod Inc). The red paper was used to indicate maximum intercuspation while the black paper was used to indicate contacts in excursive movements. The post-cementation casts were poured with a white gypsum material (GC Fujirock, GC America) to enable optimal scanned image contrast. The participants were asked to return at 6 months and one year. Quadrants were re-impressed during both time periods.
2.2.3
Wear quantification
The maximum wear was quantified as the maximum loss in height. A 3D Laserscanner (CS2, Straumann, Germany) was used to scan the casts at baseline, six months and one year along the x, y and z axes of the casts. These period scans were superimposed against one another using tripodization by identifying three points on the occlusal anatomy which are expected to remain stable ( i.e. marginal ridges). The matching of the two scans was conducted by the software to achieve a match with a standard deviation (SD) less than 25 μm. The scanning accuracy for this type of scanner is reported to be 20 μm . The matching process was repeated until an acceptable SD was achieved. After proper matching was achieved between the period scans, the maximum wear of the crowns and teeth at these time periods were compared and recorded. The wear areas were compared with the clinical photographs to confirm intra-oral contact areas.
2.3
Statistical analysis
All analyses were performed using the R statistical software package (V3.2.4, The R Foundation for Statistical Computing, Vienna, Austria). Since the sample size was small (N < 15 per group for all comparisons), the non-parametric Mann-Whitney U test was used to compare wear between the zirconia and metal-ceramic crown types at six months and one year. To compare antagonist wear to control wear between the two groups, the difference between antagonist wear and control wear as the outcome for each patient was calculated. The mean wear of the two control teeth was used as the control wear for that patient.
3
Results
Thirty teeth in 25 enrolled participants (20 females, 5 males and no more than two crowns per participant), were included in this study and were seen from 2013 to 2017. There were 16 monolithic zirconia crowns (PZ) and 14 metal-ceramic crowns (PV) analyzed. The consort diagram ( Fig. 1 ) shows a more detailed distribution of the participants. Wear between the monolithic zirconia and metal-ceramic crowns were compared at six months and one year ( Fig. 2 ). There were no significant differences observed at any time point (6 months p = 0.958; 1 year p = 0.367). The wear of the enamel opposing both types of crowns was also compared to determine if one material wore the opposing enamel more than the other ( Fig. 3 ). There were no significant differences observed for antagonist enamel wear across all time periods (6 months p = 0.776; 1 year p = 0.534). The opposing enamel wear was then compared to the wear between two opposing enamel surfaces (control wear) to determine if either material caused an increase in opposing enamel wear. This was computed for by the difference between antagonist wear and control wear for each participant. The mean between the two controls were subtracted from the antagonist enamel wear of the crowns. In Fig. 4 , negative numbers indicate more control wear than antagonist wear and positive values indicate more antagonist tooth wear. The p values are 6 months p = 0.864 and 1 year p = 0.093 indicating no significant difference between the control enamel wear and the antagonist enamel wear. Greater opposing enamel wear was observed for the metal ceramic than the zirconia crown for the first six months. This trend changed for the first-year data with an increase in wear for the enamel opposing zirconia crowns. Fig. 5 a and b are representative images retrieved from the laserscanner for wear comparison at one year. Fig. 5 b shows the crowned tooth on the lower left second molar, the left first molar is serving as the enamel control. The left most image is the baseline image while the middle image is the one year image. The right most image is the superimposed image of the two scans and red marks indicate differences between the two images or possibly wear. In this superimposed image, the red marks are located in peripheral areas which are not indicative of wear because the teeth do not contact in those areas. Fig. 5 a shows the antagonist teeth to the crown and the control. The superimposed image shows more distinct red marks which are possible areas of wear (arrows). No wear facets are visible on any of these scans.
