Fourteen-year Evaluation of Posterior Zirconia-based Three-unit Fixed Dental Prostheses. A Prospective Clinical Study of All Ceramic Prosthesis
Conditions: Prosthesis Survival
Sponsor: Federico II University

4/4/2020 
 
 
 
Fourteen-year evaluation of posterior zirconia-based three-unit fixed dental prostheses. 
A prospective clinical study of all ceramic prosthesis. 
 
In the present prospective clinical study, 37 patients (16 males, 21 females) needing at least 1 posterior FDP in the maxillary and/or 
the mandibular arches were recruited. The mean age of patients was 45.3 ± 11.6 years, with a minimum age of 21 and a maximum of 
68 years. All patients were recruited at the Department of Prosthodontics of the University “Federico II” of Naples (Italy) from 
November 2004 to April 2005 (baseline) and provided a written informed consent. The study fulfilled the requirements of the 
Helsinki declaration and was approved by the ethical committee of the same University. All the included patients met the following 
inclusion criteria: 
- good general health; 
- ASA I or ASA II according to the American Society of Anesthesiologists; 
- good periodontal health; 
- Angle class I occlusal relationship; 
- minimum of 20 teeth; 
- good oral hygiene; 
- no evident signs of occlusal parafunctions and/or temporo-mandibular disorders. 
Moreover, the abutment teeth had to fulfill the following inclusion criteria: 
- periodontal health (absence of tooth mobility, absence of furcation involvement); 
- proper positioning in the dental arch (tooth axes adequate for a FDP); 
-!sufficient occlusal-cervical height of the clinical crown (≥4 mm) for the retention of a FDP; 
- vital or endodontically treated to a clinically sound state; 
- opposing natural teeth or fixed prostheses. 
The following conditions caused the patients exclusion from the study: 
- subjects preferring implant-supported prostheses; 
-!high caries activity (according to the American Dental Association Caries Risk Assessment Form for age>6); 
- occlusal-gingival height of the abutment teeth <4 mm;
- reduced interocclusal distance or supraerupted opposing teeth; 
- unfavorable crown-to-root ratio (up to 1:1 as minimum ratio); 
- severe wear facets, clenching, bruxism (identifiable during clinical examinations or reported during anamnestic interviews); 
-!presence of removable partial dentures; 
-!pregnancy or lactation. 

 
Forty-eight 3-unit posterior zirconia FDPs were fabricated; 11 patients received 2 FDPs each. The pontic element replaced either a first or 
a second premolar or a first molar. Twenty-four FDPs were located in the maxilla replacing 12 premolars and 12 molars; the other 24 
FDPs were placed in the mandible substituting 9 premolars and 15 molars. A first premolar mesial cantilever was designed for a 
maxillary prosthesis. 
Prosthodontic procedures 
Four experienced and calibrated prosthodontists performed all the clinical procedures. The patients were prepared by means of 
professional oral hygiene and core build-ups, endodontic therapies and post-and-core placement if necessary were performed before the 
prosthodontic procedures. Alginate impressions were made in order to obtain study gypsum casts and fabricate diagnostic wax-ups, 
self-polymerizing resin customized impression trays and acrylic temporary restorations. Silicon indexes obtained from the diagnostic 
wax-up were used to check proper tooth preparation and achieve abutments fulfilling the requirements of the CAD-CAM workflow for 
framework production: 
-!margin design: 1 mm circumferential rounded chamfer and rounded cavo-surface angles to prevent stress concentrations; 
- axial reduction: 1.5 mm; 
- occlusal reduction: 1.5-2 mm; 
- total occlusal convergence angle: 10°-14°. 
The slightly subgingival margins of the preparations respected the biologic width. A self-polymerizing resin was used to reline intraorally the 
acrylic resin temporary restorations that were then cemented with a eugenol-free luting agent. Occlusal adjustments of the provisional restorations 
were performed when necessary. After tooth preparation, 10 to 14 days were waited in order to consent the recover of soft tissues from possible 
preparation injuries before making the final impressions. Two non-impregnated retraction cords were positioned around abutment teeth before the 
full-arch impression procedure. Customized autopolymerizing acrylic impression trays and polyether materials were used. A self-polymerizing A-
silicone was used to register the interocclusal relationships. Then, the provisional restorations were cemented. Master casts of super hard gypsum 
were mounted in a semi-adjustable articulator with a die spacer (<30-µm thick) applied at the occlusal and axial surfaces of the abutments, 
starting 1 mm above the preparation margins. The CAD-CAM, was used to digitize the master casts. The sintering shrinkage was compensated by 
enlarging the scanned data by 20-25%. The milling center milled the 1st generation presintered partially stabilized tetragonal zirconia 3Y-TZP 
frameworks from presintered zirconia blanks, then sintered to full density. An ovate pontic was used to replace missing premolars, while 
a modified ridge-lap pontic was designed for missing molars. The minimum retainer thickness was 0.6 mm and the minimum connector 
surface area was 9 mm2. A digital caliper with an accuracy of 0.01 mm was used to measure the framework thickness. Then, the accuracy 
of fit of zirconia frameworks was evaluated intraorally using a silicon disclosing agent; if necessary, geometry adjustments were made on 
the abutments transferring any pressure spot to teeth surfaces. The same experienced dental technician veneered all the frameworks; a 
feldspathic ceramic specifically dedicated to zirconia and a conventional powder build-up veneering technique were used and the 
adequacy of the coefficient of thermal expansion (CTE) of the veneering ceramics was carefully checked. Then, the FDPs were glazed 
and polished. The thickness of veneering ceramics was measured with a digital caliper with an accuracy of 0.01 mm and ranged between 

0.55 and 0.98 mm on the retaining abutments and between at level of the connectors. The final zirconia FDPs were tried-in intraorally to 
evaluate internal and marginal adaption using a silicon disclosing agent. Furthermore, proximal and occlusal contacts were checked with 
articulating ribbon; occlusal adjustments were performed when necessary. Abutments were degreased with 80% ethanol. In order to ease 
the removal of cement remnants, the external surfaces of the FDPs were isolated using liquid paraffin before cementation. The intaglio 
surfaces of zirconia FDPs were conditioned by means of mild sandblasting with 110 µm alumina particles at 0.2 MPa . A resin luting 
agent was used to cement the FDPs and the cement excesses were removed by means of a plastic scaler. If necessary, fine-grit diamond 
burs were used to make further occlusal adjustments and the modified surfaces were meticulously polished with a ceramic polishing 
system. 
Baseline evaluation 
Two external, calibrated and experienced clinicians blind to the prosthodontics procedures performed the baseline evaluation, recorded 7 
days after the cementation of FDPs. As regards periodontal evaluation, tooth mobility, plaque control record, probing pocket depth, 
probing attachment level, bleeding on probing (BOP) at the abutment sites (test) and at the contralateral, not restored teeth (control) were 
assessed. Cold carbon dioxide was used to evaluate pulp vitality of test and control teeth. After making alginate impressions for study 
casts, the clinical evaluators recorded the occlusal relationships between the FDPs and the opposing arches. Clinical photographs of the 
FDPs and periapical X-rays of the abutment teeth were taken. Furthermore, the static and dynamic and the static occlusal contacts were 
checked and recorded photographically. All the patients rated the overall functional and esthetic outcomes of the restorations by means of 
Visual Analog Scales (VASs) ranging from 0 to 10. 
Follow up-examinations 
After the baseline evaluation, all the patients were recalled after 6 months and then annually, over a whole observational period of 14 
years. The same evaluations assessed at the baseline were repeated and the relative data were recorded. Any proximal recurrent decays 
and/or periapical pathologies were checked by means of X-rays. The United States Public Health Service (USPHS) criteria were used to 
report technical and esthetic complications; the FDPs were examined entirely and the worst record was used for rating. 
Statistical analysis 
A dedicated software (SPSS 17, SPSS Inc., Chicago, IL, USA) was used to make descriptive statistics. The 14-year cumulative survival 
rate of the zirconia FDPs was calculated by means of Kaplan-Meier analysis. Two independent curves for patients wearing 1 or 2 FDPs 
were analyzed separately. In order to compare these curves, a log-rank test was performed. The patient receiving the only cantilevered 
restorations was excluded from the statistical analysis, in order not to introduce a pure confounder. The periodontal parameters of control 

and test teeth between the baseline and the 14-year follow-up were evaluated using the Wilcoxon test, with a level of significance set 
at p< 0.05. 
Results 
After 14 years of clinical function, no patient was lost at follow-up or censored and consequently all the 48 3-unit zirconia FDPs were 
available for examination. The survival rate was 98% while the success rates were 91% and 99% for patients wearing 1 and 2 FDPs, 
respectively, as reported in the Kaplan-Meier graph considering complications as events. 
There were no statistically significant differences between the survival curves of patients with 1 or 2 FDPs, as shown by the log-rank 
test (p< 0.05). During the entire period of observation, 5 minor cohesive fractures of the veneering ceramics were observed (10.4%): 
at the 1-year recall, the first chipping was detected on the distal connector of a maxillary premolar; after 2 years of clinical function, 
the examiners detected 2 more chippings of the veneering ceramics, 1 on the occlusal surface of a mandibular molar and 1 on the 
distal connector of a maxillary molar in a patient wearing 2 FDPs; at the 9-year recall, the examiners observed 1 mesial-lingual 
chipping on a maxillary first premolar and 1 mesial-lingual chipping on a mandibular first premolar. The patients did not notice such 
cohesive fractures since the chipped areas did not impair function; consequently, the surfaces were carefully rounded and polished 
and the FDPs remained in situ for further observation. 
Moreover, 1 decementation (2.1%) of a mandibular FDP was detected after 10 years of clinical function; after thoroughly cleaning 
and degreasing both the abutment teeth and the restoration, the FDP was cemented using the same resin cement. After 11 years of 
serviceability, 1 catastrophic fracture (2.1%) was detected in the same patient experiencing the previous decementation; the abutment 
teeth were intact and the soft tissues were stable, consequently, the restorations was replaced by a new zirconia FDP. 
At the baseline, 82 abutments (85.5%) were vital and they all remained vital, after 14 years of observation. The follow-up 
examinations showed no significant differences in the average periodontal parameters between test and control teeth. Moreover, 
neither signs or symptoms of proximal decay nor radiographic evidence of periapical pathologies were detected during the entire 
follow-up period. 
The mean values of the outcomes reported by patients were recorded using VAS judgments: the overall functional score was 9.3 (± 
1.4) while the average esthetic value was 9.0 (± 0.9), where 0 meant “not satisfied at all” and 10 meant “fully satisfied”. All the 
patients declared to be pleased with chewing efficiency and esthetics of the restorations, although slight gingival recession were 
observed in some cases; this report was probably due to the absence of grey metal frameworks that allowed the patients not to 
complain about esthetics. Excluding the subject who experienced a catastrophic fracture, all the patients in which chipping occurred 
did not report any significative functional discomfort, apart from minimal surface roughness that was polished and sporadic food 
impaction on the lingual aspect of connectors and in contact areas. As regards mechanical resistance to fracture, all the frameworks 
but the fractured one scored Alpha. In terms of occlusal wear, 6 restorations opposing natural teeth rated Bravo; 2 of them opposed 
previous chipped restorations. The Wilcoxon test performed for the analyzed periodontal parameters at baseline vs 14-year recall on 

test and contralateral control teeth showed no statistically significant differences (p>0.05) for probing pocket depth, probing 
attachment level, plaque control record and bleeding on probing.