Zirconia-based versus metal-based single crowns veneered with overpressing ceramic for restoration of posterior endodontically treated teeth: 5-year results of a randomized controlled clinical study

Abstract

Objectives

The aim of this 5-year randomized controlled trial was to compare the longevity and clinical behavior of single posterior crowns made with pressable ceramic on zirconia and on metal frameworks, and if failures occur, to delineate the contributing factors.

Methods

72 patients, who needed the covering of at least a molar and/or premolar, were included in the study. All teeth were endodontically treated, with absence of periapical lesion or active periodontitis. Ninety single crowns were made with zirconia or metal framework and covered with pressable veneering ceramics. Two independent examiners assessed the survival of restorations at 6 months, 1–4 and 5 years after restoration placement including periapical radiographs, intraoral photographs, and USPHS modified criteria. The statistical analyses were performed with the Kaplan-Meier method.

Results

One core fracture occurred in Zircad/Zirpress crowns and one metal ceramic crown was lost for root fracture. Chipping fracture of the veneering ceramic was detected in 2 metal-ceramic crowns and in 3 zirconia-based crowns. The Estimate Cumulative Survival (ECS) and the Estimate Cumulative Success (ECSs) with standard deviation (SE) were respectively 97,73 ± 2,19 and 92,64 ± 4,14 for zirconia-based crowns whereas 97,44 ± 2,39 and 91,11 ± 4,27 for porcelain fused to metal crowns.

Conclusions

The present randomized controlled trial shows that the survival of zirconia-based and metal-based single crowns is similar over a follow-up period of 5 years. No significant differences in esthetic, functional and biological outcomes were demonstrated between the two groups. The main failure mode was the chipping fracture of the veneering ceramic in both materials. Study number on ClinicalTrial.gov NCT02758457.

Clinical significance

According to the results of this clinical study, zirconia-based rehabilitations with overpressing veneering technique represent a valid alternative to metal-based for posterior single crown restorations.

Introduction

Conventional metal ceramic restorations are recognized as a predictable solution and are still commonly used in prosthetic dentistry with good long-term clinical success .

However, the presence of gold or other metal alloys adversely affects the optical properties of the restoration, tends to cause a graying of the surrounding tissue, and may give rise to allergic or toxic reactions . Moreover, realization of porcelain-fused-to-metal restoration needs a very careful approach, involves many steps, and is consequently more expensive. Moreover, the increased cost of the gold-based alloy has shifted the focus on the metal-free materials and on the CAD/CAM systems.

All-ceramic materials, such as densely sintered aluminum oxide ceramic (Procera AllCeram; NobelBiocare, USA) or lithium disilicate material (e-max CAD; Ivoclar Vivadent, Liechtenstein), have permitted an improved aesthetic appearance but have inadequate strength for especially for posterior application . However, zirconia-based ceramic seems to satisfy both aesthetic and mechanical needs .

In vitro studies of Yttria-Tetragonal Zirconia Polycristals (Y-TZP) samples have shown values of 900–1200 MPa for flexural strength and values of 9–10 MPa m ½ for fracture toughness , also the stiffness of the zirconia had a better effect on the marginal adaptation better than on the fiber-reinforced composite restorations . However, due to the metastability of tetragonal zirconia, stress-generating surface treatments such as grinding or sandblasting are liable to trigger the tetragonal → monoclinic transformation with the associated volume increase, leading to the formation of surface compressive stresses . This transformation toughness leads to an increase in flexural strength but can also alter the phase integrity of the material, increasing the susceptibility to aging through water absorption .

The strength of an all-ceramic restoration depends on the characteristics of the material used, thickness of the crown, design of the restoration, core-veneer bond strength, and cementation . Several short-to-mid-term in vivo studies were conducted with zirconia restorations, proving great clinical performance. In most cases, the studies investigated zirconia fixed partial dentures (FPDs) in the posterior region , while single crowns have a small representation in published clinical trials .

A recent retrospective cohort study evaluated 1132 zirconia-based single crowns made with different finishing line preparations over a time period of up to 5 years. The cumulative survival rate of all restorations was 98.1%, while the cumulative success rate was 94.2%. Functional criteria showed the most failures, with only 1 fractured zirconia core, 13 delaminations, and 46 instances of chippings of the ceramic veneering. An association between parafunction and mechanical failure was found in patients with severe parafunction . An interesting systematic review by Sailer et al. compared the survival of all-ceramic and metal-ceramic single crowns after a follow-up period of five years, describing the incidence of biological, technical, and esthetic complications. The survival rates of most types of all-ceramic single crowns were similar to those reported for metal-ceramic single crowns in both the anterior and posterior regions; however, the authors concluded that weaker feldspathic/silica-based ceramics should be limited to applications in the anterior region and the zirconia-based single crowns should not be considered as the primary option because of their high incidence of technical problems .

Moreover, no in vivo study has compared the clinical behavior of pressable veneering ceramics when used on metal and zirconia frameworks. Few in vitro studies showed that among different zirconia veneering porcelains, the highest microtensile strength was obtained with pressable veneering ceramics .

The objective of this study was to compare the 5-year survival and clinical behavior of single posterior ceramic crowns made with pressable ceramic on zirconia or on a metal framework. If failures occurred, the further aim of the study was to delineate the factors contributing to the failure. The null hypothesis stated that the survival of zirconia-based restorations would be no worse that those made with metal-based material.

Materials and methods

Patient selection and inclusion/exclusion criteria

A randomised controlled clinical trial was conducted using a parallel group design. The study protocol was submitted and approved by the University of Bologna internal review board; 12/12/2007 and registered on ClinicalTrial.gov (study number NCT02758457). The study was conducted according to the Declaration of Helsinki and was conformed to good clinical practice (GCP) guidelines. Each patient was provided with written information about the proposed treatment involved, principles of treatment, potential discomforts, risks of the procedures. Each patient signed a written informed consent form prior to clinical examination.

Recruitment

All patients needing covering of at least one molar and/or premolar with a fixed prosthesis single crown were recruited from the Division of Prosthodontics and Maxillo-Facial Rehabilitation of the Department of Biomedical and Neuromotor Sciences of the University of Bologna.

Clinical examination

Before entering the study each patient received an intra-oral examination carried out by two calibrated clinicians with an inter-examiner agreement that was previously determined to be >80% (kappa score 0.87). All patients who met the inclusion criteria were invited to participate in the study and each patient received an identification number.

Inclusion and exclusion criteria

The inclusion criteria for entering in the study were: age raging from 18 to 70 years; minimum of 20 teeth; moderate to good oral hygiene with the ability to perform mechanical oral hygiene techniques including tooth brushing; low to moderate caries risk, and no active periodontal disease. Caries risk assessment was defined taking in consideration clinical indicators such as recently placed restorations, heavy dental plaque and evident tooth decay or white spots. A maximum of four single crowns were placed for each patient. Teeth had to fulfill the following requirements to be included in this study: endodontically treated teeth; absence of active periodontitis; absence of periapical lesions; occluso-gingival dimension of at least 3.0 mm from the interdental papilla to the marginal ridge of the abutment teeth; and ferrule effect of at least 1 mm. The exclusion criteria were: unacceptable oral hygiene practices; allergic reaction or hypersensitivity to ingredients of the adhesive or restorative material; strong parafunction ( e.g. , bruxism with marked wear facets) with or pronounced malocclusion ( e.g. , cross bite); inability to follow preparation guidelines for single crowns. The choice to include only endodontically treated teeth was made for two reasons. First, most of vital teeth don’t need a complete coverage crown and the treatment of choice is an adhesive partial crown restoration. Second, the structural difference between vital and endodontically treated teeth may introduce bias regarding the functional and biological behavior of the tooth/restoration complex.

Randomization

The patients were randomly allocated using a computer generated random assignment (SPSS software version 21; IBM). Patients randomization was done by a researcher and the allocation was hidden from the clinical operators using sealed and sequentially numbered white envelopes.

Data collection

Each patient received a medical record with general information about age, gender, general health, medical history, dental history and oral hygiene practices. During the clinical examination all details on carious lesions, restorations and teeth present were recorded. Plaque index and papilla bleeding scores was were recorded with a graduated periodontal probe at six sites of all posterior teeth .

Restorative procedures

At the end of the recruitment 72 patients (39 women and 33 men), with an age ranging from 18 to 70 years, were selected for the study. Patients were treated according to their allocated treatment group by five clinicians experienced in fixed prosthodontics. Sample size of 45 crowns for each group was calculated for 80% power, α = 0.05 and anticipated effect size = 0,60 using sample size software (G*Power version 3.00.10, Germany). Ninety posterior teeth were restored with 45 single crowns consisting of a metal framework (IPS d.SIGN 91; Ivoclar Vivadent) supplemented by a pressable veneering ceramic for noble alloy (PoM; Ivoclar Vivadent) and 45 single crowns with zirconia framework (ZirCad; Ivoclar Vivadent) and pressable veneering ceramic (ZirPress; Ivoclar Vivadent).

Before the abutment preparation, the teeth without previous fiber post reconstruction were restored with fiber posts (FRC Postec, Ivoclar Vivadent), luted using a dual curing composite resin cement (Multilink Automix, Ivoclar Vivadent). Abutment teeth were prepared with a chamfer finishing line preparation with air-driven, water-cooled high-speed handpieces. Tooth preparation and finishing were performed using diamond burs (881, 8881, 881EF, 8379; Komet, USA), Dura White stones (CN1 4° convergence), and silicon points (Shofu, Japan) under profuse water-spray cooling. Margins on the posterior teeth were placed either equi- or supragingival when possible, or minimally subgingival to establish a margin on sound tooth structure. The following guidelines were followed for all crowns: axial tooth reduction: maximum 1.5 mm, deep chamfer of 1.0 mm at margin, minimum 1.5 mm occlusal reduction, and 2.0 mm under the cusps. The gingival displacement procedure was performed, if necessary, by placement of a retraction cord (Ultrapak; Ultradent, UT, USA). The impression was taken with polyether impression material (Permadyne Heavy and Light Body; 3 M ESPE, Seefeld, Germany).

After setting the impression tray was removed and the necessary opposing impression and interocclusal records were made. A provisional restoration was custom fabricated in PMMA resin (Splintline; Lang Dental, IL, USA) and cemented with a non-eugenol containing temporary cement (Life; Kerr, CA, USA).

Laboratory techniques

Zirconia-based restorations were fabricated using CAD/CAM procedures from pre-sintered zirconia blanks (ZirCad; Ivoclar Vivadent) and pressable veneering ceramic (ZirPress; Ivoclar Vivadent) according to the manufacturer’s instructions, while metal-based crowns were made using the lost wax technique. Frameworks were cast from a high noble alloy (IPS d.SIGN 91; Ivoclar Vivadent) and were fully veneered with a pressable veneering ceramic (PoM; Ivoclar Vivadent). Zirconia and metal frameworks had a minimum thickness of 0.5 mm and a slow cooling protocol was used to reduce the residual stresses within the porcelain. All crowns were made by the same dental technician.

Luting procedures

After 2 weeks, the patient returned for removal of the provisional restoration and adjustment of the single crown. Proximal contacts were checked with a paper articulation, internal fit of the prostheses was evaluated with Fit-checker (GC Corporation, Tokyo, Japan) the occlusion was evaluated with Accufilm 8-μm articulating paper (Parkell, New York, USA), and any necessary adjustments was made with a diamond bur.

All crowns were luted with Multilink Automix (Ivoclar Vivadent; Schaan) following the manufacturer’s instructions for use. The teeth were cleaned of debris. The surface of the zirconia and metal single crown was coated with 30 μm-aluminum oxide particles modified by silica (CoJet System), abraded, and cleaned (max 1.5 bar).

Evaluation procedures

Two calibrated examiners who were not involved in restoration preparation and placement with an inter- and intra-examiner agreement >80% checked the restorations during the follow up period.

Single crowns were checked according to the modified United States Public Health Service (USPHS) criteria at baseline and after 6, 12, 24, 36 and 60 months post-insertion .

The restorations were visually inspected with a dental mirror and probe. Clinical examination included periapical radiographs of the involved restored teeth, intraoral photographs and measurement of USPHS modified criteria.

At every recall, the two examiners independently filled out a form describing all USPHS modified parameters; if the difference between their evaluations was >20%, the two examiners discussed the case to obtain agreement.

Materials and methods

Patient selection and inclusion/exclusion criteria

A randomised controlled clinical trial was conducted using a parallel group design. The study protocol was submitted and approved by the University of Bologna internal review board; 12/12/2007 and registered on ClinicalTrial.gov (study number NCT02758457). The study was conducted according to the Declaration of Helsinki and was conformed to good clinical practice (GCP) guidelines. Each patient was provided with written information about the proposed treatment involved, principles of treatment, potential discomforts, risks of the procedures. Each patient signed a written informed consent form prior to clinical examination.

Recruitment

All patients needing covering of at least one molar and/or premolar with a fixed prosthesis single crown were recruited from the Division of Prosthodontics and Maxillo-Facial Rehabilitation of the Department of Biomedical and Neuromotor Sciences of the University of Bologna.

Clinical examination

Before entering the study each patient received an intra-oral examination carried out by two calibrated clinicians with an inter-examiner agreement that was previously determined to be >80% (kappa score 0.87). All patients who met the inclusion criteria were invited to participate in the study and each patient received an identification number.

Inclusion and exclusion criteria

The inclusion criteria for entering in the study were: age raging from 18 to 70 years; minimum of 20 teeth; moderate to good oral hygiene with the ability to perform mechanical oral hygiene techniques including tooth brushing; low to moderate caries risk, and no active periodontal disease. Caries risk assessment was defined taking in consideration clinical indicators such as recently placed restorations, heavy dental plaque and evident tooth decay or white spots. A maximum of four single crowns were placed for each patient. Teeth had to fulfill the following requirements to be included in this study: endodontically treated teeth; absence of active periodontitis; absence of periapical lesions; occluso-gingival dimension of at least 3.0 mm from the interdental papilla to the marginal ridge of the abutment teeth; and ferrule effect of at least 1 mm. The exclusion criteria were: unacceptable oral hygiene practices; allergic reaction or hypersensitivity to ingredients of the adhesive or restorative material; strong parafunction ( e.g. , bruxism with marked wear facets) with or pronounced malocclusion ( e.g. , cross bite); inability to follow preparation guidelines for single crowns. The choice to include only endodontically treated teeth was made for two reasons. First, most of vital teeth don’t need a complete coverage crown and the treatment of choice is an adhesive partial crown restoration. Second, the structural difference between vital and endodontically treated teeth may introduce bias regarding the functional and biological behavior of the tooth/restoration complex.

Randomization

The patients were randomly allocated using a computer generated random assignment (SPSS software version 21; IBM). Patients randomization was done by a researcher and the allocation was hidden from the clinical operators using sealed and sequentially numbered white envelopes.

Data collection

Each patient received a medical record with general information about age, gender, general health, medical history, dental history and oral hygiene practices. During the clinical examination all details on carious lesions, restorations and teeth present were recorded. Plaque index and papilla bleeding scores was were recorded with a graduated periodontal probe at six sites of all posterior teeth .

Restorative procedures

At the end of the recruitment 72 patients (39 women and 33 men), with an age ranging from 18 to 70 years, were selected for the study. Patients were treated according to their allocated treatment group by five clinicians experienced in fixed prosthodontics. Sample size of 45 crowns for each group was calculated for 80% power, α = 0.05 and anticipated effect size = 0,60 using sample size software (G*Power version 3.00.10, Germany). Ninety posterior teeth were restored with 45 single crowns consisting of a metal framework (IPS d.SIGN 91; Ivoclar Vivadent) supplemented by a pressable veneering ceramic for noble alloy (PoM; Ivoclar Vivadent) and 45 single crowns with zirconia framework (ZirCad; Ivoclar Vivadent) and pressable veneering ceramic (ZirPress; Ivoclar Vivadent).

Before the abutment preparation, the teeth without previous fiber post reconstruction were restored with fiber posts (FRC Postec, Ivoclar Vivadent), luted using a dual curing composite resin cement (Multilink Automix, Ivoclar Vivadent). Abutment teeth were prepared with a chamfer finishing line preparation with air-driven, water-cooled high-speed handpieces. Tooth preparation and finishing were performed using diamond burs (881, 8881, 881EF, 8379; Komet, USA), Dura White stones (CN1 4° convergence), and silicon points (Shofu, Japan) under profuse water-spray cooling. Margins on the posterior teeth were placed either equi- or supragingival when possible, or minimally subgingival to establish a margin on sound tooth structure. The following guidelines were followed for all crowns: axial tooth reduction: maximum 1.5 mm, deep chamfer of 1.0 mm at margin, minimum 1.5 mm occlusal reduction, and 2.0 mm under the cusps. The gingival displacement procedure was performed, if necessary, by placement of a retraction cord (Ultrapak; Ultradent, UT, USA). The impression was taken with polyether impression material (Permadyne Heavy and Light Body; 3 M ESPE, Seefeld, Germany).

After setting the impression tray was removed and the necessary opposing impression and interocclusal records were made. A provisional restoration was custom fabricated in PMMA resin (Splintline; Lang Dental, IL, USA) and cemented with a non-eugenol containing temporary cement (Life; Kerr, CA, USA).

Laboratory techniques

Zirconia-based restorations were fabricated using CAD/CAM procedures from pre-sintered zirconia blanks (ZirCad; Ivoclar Vivadent) and pressable veneering ceramic (ZirPress; Ivoclar Vivadent) according to the manufacturer’s instructions, while metal-based crowns were made using the lost wax technique. Frameworks were cast from a high noble alloy (IPS d.SIGN 91; Ivoclar Vivadent) and were fully veneered with a pressable veneering ceramic (PoM; Ivoclar Vivadent). Zirconia and metal frameworks had a minimum thickness of 0.5 mm and a slow cooling protocol was used to reduce the residual stresses within the porcelain. All crowns were made by the same dental technician.

Luting procedures

After 2 weeks, the patient returned for removal of the provisional restoration and adjustment of the single crown. Proximal contacts were checked with a paper articulation, internal fit of the prostheses was evaluated with Fit-checker (GC Corporation, Tokyo, Japan) the occlusion was evaluated with Accufilm 8-μm articulating paper (Parkell, New York, USA), and any necessary adjustments was made with a diamond bur.

All crowns were luted with Multilink Automix (Ivoclar Vivadent; Schaan) following the manufacturer’s instructions for use. The teeth were cleaned of debris. The surface of the zirconia and metal single crown was coated with 30 μm-aluminum oxide particles modified by silica (CoJet System), abraded, and cleaned (max 1.5 bar).

Evaluation procedures

Two calibrated examiners who were not involved in restoration preparation and placement with an inter- and intra-examiner agreement >80% checked the restorations during the follow up period.

Single crowns were checked according to the modified United States Public Health Service (USPHS) criteria at baseline and after 6, 12, 24, 36 and 60 months post-insertion .

The restorations were visually inspected with a dental mirror and probe. Clinical examination included periapical radiographs of the involved restored teeth, intraoral photographs and measurement of USPHS modified criteria.

At every recall, the two examiners independently filled out a form describing all USPHS modified parameters; if the difference between their evaluations was >20%, the two examiners discussed the case to obtain agreement.

Statistical evaluation

Tables of percentages were prepared to summarize the demographic characteristics of the patients, the distribution of crowns ( Table 1 ), and the baseline data and to illustrate the recall findings. For statistical evaluation, the Kaplan-Meier survival rate was calculated. Aesthetic, functional, and biological USPHS parameters modified by the FDI World Dental Federation study design were collected, and each parameter was ranked in five subgroups.

Table 1
Distribution of the single crown restorations.
Maxillae Mandible Total crowns
PoM ZirCad PoM ZirCad PoM ZirCad
Premolar 12 12 7 13 19 25
Molar 14 10 12 10 26 20
Total crowns 26 22 19 23 45 45

Estimated cumulative survival (ECS) was defined as the single crown remaining in situ with scores of 1–3 for esthetic, functional and biological parameters. This included hairline cracks of the veneering ceramic, marginal gap <150 μm or moderate surface staining which revealed no evidence of retreatment other than maintenance procedures. Failures for ECS analysis were considered when the restorations reached a score of 4–5, requiring the replacement of the restoration.

Estimated cumulative success (ECSs) was defined as the single crown remaining in situ with scores of 1–2 for esthetic, functional and biological parameters. A fracture of ceramic veneering, marginal discrepancy, postoperative sensitivity, crown decementation, and secondary caries with a score of 3, 4 or 5 were considered terminal events for ECSs analysis.

Life table analysis was generated using statistical software (SPSS Statistics 21; IBM).

Results

The study flow diagram is showed in Fig. 1 . Seventy-two patients (39 women and 33 men), with an age ranging from 18 to 70 years, were selected for the study. During the 5 years of clinical service, three patients with five PFM crowns were lost to follow-up because of relocation. The subject recall rate was 94.5%. The average time of follow-up was 65.7 months.

Fig. 1
Study flow diagram.

The ECS ± standard error (SE) from 1 to 5 years was 97.73 ± 2.19 for zirconia-based crowns and 97.44 ± 2.39 for porcelain-fused-to-metal crowns, whereas the ECSs decreased to 91.11 ± 4.27 and 92.64 ± 4.14 respectively. Table 2 shows the results, and Tables 3 and 4 show the life-table analysis according to Layton .

Table 2
Five-year results of the single crown restorations in terms of esthetic, functional, and biological properties following the USPHS parameters.
Properties Parameters PoM ZirCad TOTAL
Aesthetic properties
Surface luster 1. Surface luster comparable to enamel 38 44 82
2. Slightly dull, not noticeable if covered with film of saliva 1 1 2
3. Dull, cannot be masked by saliva film 1 0 1
4. Rough surface, unacceptable plaque retentive surface 0 0 0
5. Quite rough, unacceptable plaque retentive surface 0 0 0
Surface staining 1. No surface staining 39 43 82
2. Minor staining 1 2 3
3. Moderate surface staining, also present in other teeth 0 0 0
4. Surface staining present on the restoration and is unacceptable 0 0 0
5. Severe staining and/or subsurface staining 0 0 0
Color stability and translucency 1. Good color match. No difference in shade and translucency 34 40 74
2. Minor deviations 6 5 11
3. Clear deviation but acceptable. Does not affect aesthetics 0 0 0
4. Clinically unsatisfactory but can be corrected by repair 0 0 0
5. Unacceptable. Replacement necessary 0 0 0
Anatomic form 1. Form is ideal 40 45 85
2. Form is only slightly affected 0 0 0
3. Form differs but is not aesthetically displeasing 0 0 0
4. Form is affected and unacceptable aesthetically. Intervention necessary 0 0 0
5. Form is completely unsatisfactory and/or lost. Replacement needed 0 0 0
Functional properties
Fracture and retention 1. Restoration retained, no fractures/cracks 38 43 81
2. Small hairline crack 0 0 0
3. Two or more or larger hairline crack 0 0 0
4. Chipping fractures which damage marginal quality or proximal contacts 2 3 5
5. Partial or complete loss of restoration 0 1 1
Marginal adaptation 1. Harmonious outline, no gaps, no discoloration 38 45 83
2. Marginal gap (50 μm) 2 0 2
3. Gap <150 μm not removable 0 0 0
4. Gap >250 μm or dentin/base exposed 0 0 0
5. Filling is loose but in situ 0 0 0
Wear 1. Physiological wear equivalent to enamel 35 42 77
2. Normal wear with only slight difference to enamel 4 1 5
3. Differing wear rate to enamel but within the biological variations 0 2 2
4. Wear considerably exceeds normal enamel wear 1 0 1
5. Wear is excessive 0 0 0
Contact point/food impact 1. Normal contact point 38 41 79
2. Slightly too strong but no disadvantage 0 0 0
3. Slightly too weak, no indication of damage to tooth or gingivae 2 4 6
4. Too weak (100 μm) and possible damages. Repair possible 0 0 0
5. Too weak and/or clear damage and/or pain/gingivitis. 0 0 0
Patient’s view 1. Entirely satisfied 37 44 81
2. Satisfied 3 0 3
3. Minor criticism of aesthetics. No adverse effect. 0 1 1
4. Desire for improvement 0 0 0
5. Completely dissatisfied and/or adverse effect include pain 0 0 0
Biological properties
Secondary Caries 1. No primary or secondary caries 38 45 83
2. Very small and localized 1 0 0
3. Large area of: demineralization, erosion or abrasion/abfraction 1 0 0
4. Caries with cavitation, erosion in dentine, abrasion/abfraction 0 0 0
5. Deep secondary caries or exposed dentine not repairable 0 0 0
Periodontal response 1. No plaque, no inflammation, no pockets 27 32 59
2. Little plaque, no inflammation (gingivitis) no pocket development 4 3 7
3. Plaque accumulation at acceptable level. Gingival bleeding acceptable. 7 6 13
4. Plaque accumulation not acceptable. Gingival bleeding not acceptable 2 4 6
5. Severe/acute periodontitis. 0 0 0
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Jun 17, 2018 | Posted by in General Dentistry | Comments Off on Zirconia-based versus metal-based single crowns veneered with overpressing ceramic for restoration of posterior endodontically treated teeth: 5-year results of a randomized controlled clinical study
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