Retrospective Analysis of Full-Arch Zirconia Rehabilitations on Dental Implants: Clinical Outcomes and Patient Satisfaction

Abstract

The use of zirconia for full-arch Implant-Supported Fixed Complete Dental Prostheses (ISFCDPs) is common and reliable, with different techniques available for their design and fabrication. This retrospective study investigated prosthetic and implant survival/success rates and patients’ satisfaction for ISFCDPs produced with three different techniques: zirconia-on-titanium (milled zirconia arch glued to a titanium bar), progressive monolithic zirconia (entirely made of zirconia and directly screwed to the implants) and zirconia-on-zirconia (milled esthetic zirconia crowns glued to a milled high resistance zirconia frame). Fourteen patients (five males, nine females) aged 52–80 and treated with 14 ISFCDPs (86 implants) were included in this analysis. The mean follow-up at the time of recall was 36 months. Prosthetic and implant-related success rates were, respectively, 92.86% and 95.35%. No failures have been reported. One case of prosthetic chipping was observed; however, it was successfully repaired intraorally. Patients’ satisfaction was high: 78.57% were completely satisfied, 14.53% very satisfied and 7.14% satisfied. In conclusion, all the patients recommend treatment with ISFCDPs as full-arch prosthetic rehabilitation. The present study demonstrates positive clinical outcomes and high patients’ satisfaction. Further long-term, prospective studies with a larger cohort of patients are needed to confirm the advantages of the different prosthetic designs.

1. Introduction

Zirconia is a polycrystalline ceramic derived from zirconium, first produced in 1914, and implemented for medical use in 1969 for hip head replacement.

Zirconia can be obtained from zirconium through calcination, producing zirconium dioxide (ZrO₂). Zirconium dioxide is then stabilized in a crystalline form through yttrium oxide (Y₂O₃), thus obtaining yttria stabilized tetragonal zirconia polycrystal (Y-TZP) [1].

This material possesses excellent mechanical properties, and has a structure made of molecules organized in three different phases: monoclinic, tetragonal and cubic.

Because of the large number of techniques and materials used to produce monolithic zirconia, the strength of monolithic zirconia is heavily dependent on its brand: the overall strength and the fracture resistance, however, exceed those of zirconia cores layered with conventional ceramic, and are reported as two thirds greater than those of lithium disilicate [2].

Higher yttria molarity enhances the translucency and esthetics of zirconia, but also lowers its strength; thus, high-yttria-molarity zirconia should not be used for more than 3 units in posterior restorations or in particularly stressful occlusal configurations [2]. For this reason, zirconia with a different yttria molarity has a number placed before the “Y” corresponding to “Yttria”; for example, 3Y-ZTP identifies a low-yttria-molarity zirconia, which is tougher but more opaque in color; 5Y-ZTP identifies a high-yttria-molarity zirconia, which is less resistant but has an higher translucency [3].

One of the main uses of zirconia in dentistry is the production of Implant-Supported Fixed Complete Dental Prostheses (ISFCDPs), which are a good therapeutic option to treat completely edentulous patients [4] as a valid alternative to removable dentures [5,6].

Although the prevalence of edentulism is decreasing, [7] its impact on patients’ quality of life remains detrimental, leading to reduced social interactions [8], difficulties in nutrition and reduced self-esteem [9].

Clinical results of ISFCDPs can be influenced by multiple factors, some related to the prosthesis design and others related to the materials used.

The literature provides few clinical trials comparing the clinical performances of ISFCDPs with different materials; however, the available evidence reports good clinical results of ISFDPs made of zirconia, with high survival rates, ranging from 88% to 100% [4,6,10,11].

Zirconia has evolved over the years and, at the present time, both high-translucency/lower-strength zirconia and low-translucency/high-strength zirconia are available on the market, allowing the development of different techniques to produce ISFCDPs.

The most common ISFCDPs described in the literature are the following [10,12]:

• Zirconia–titanium hybrid ISFCDPs: a monolithic zirconia prosthesis (Katana YML 1050 MPa, Kuraray Noritake, Tokyo, Japan) is glued on a titanium framework and acts as the esthetic and functional portion of the prosthesis, while a titanium framework provides elasticity and force dissipation to the system. The prosthesis is usually screwed directly on the implants or on Multi-Unit Abutments (MUAs), depending on the implant orientation.
• Progressive monolithic zirconia ISFCDPs: a monolithic zirconia prosthesis (Katana HTML 1200 MPa, Kuraray Noritake) is glued on dedicated components and screwed directly on the implants, without the use of a titanium framework. The progressive nature of zirconia is provided by layers with a different yttria molarity, with the more esthetic zirconia on the external section of the prosthesis and stronger zirconia at the core.
• Zirconia-on-zirconia (Zirc-on-Zirc) ISFCDPs are produced with a protocol in which a high-strength and opaque zirconia framework (Katana HTML 1200 Mpa, Kuraray Noritake) is glued on dedicated components to screw it directly on the implants, while highly esthetic zirconia crowns (Katana STML 750 MPa, Kuraray Noritake) are cemented on the zirconia framework using a fluid ceramic fused at high temperatures.

Despite the growing number of patients rehabilitated with zirconia ISFCDPs, there is still a lack of knowledge in the literature regarding the clinical and patient-related outcomes over time.

The primary objective of the present retrospective study was to evaluate the success and survival over time of zirconia ISFCDPs through the evaluation of technical complication rates (i.e., chipping, framework fracture) and biological complication rates (peri-implant tissue health)

The secondary objective was to evaluate different designs of zirconia ISFCDPs and patients’ satisfaction through both an esthetic and functional point of view.

2. Materials and Methods

This study was designed as a retrospective cohort study. All patients previously treated with one zirconia ISFCDP in a private setting, designed with either the zirconia-on-titanium technique, the progressive monolithic zirconia technique or the zirconia-on-zirconia technique, were considered potentially eligible for the study.

Patients’ medical records were examined by two separate investigators (VM and CC), and all the eligible patients were recalled for a follow-up visit in November 2023. The present study was approved by the local ethical committee of the greater north-west area of Tuscany (approval number: 57489/2019) and followed the declaration of Helsinki.

To be included, patients had to fulfill the following eligibility criteria:

• Patients treated with a zirconia ISFCDP.
• Patients following a periodontal maintenance program with at least one recall visit per year.
• Minimum follow-up of 1 year after prosthetic delivery.

Exclusion criteria were the following:

• Patient treated with implant-supported fixed partial prosthesis.
• Patients with uncontrolled systemic diseases, were under treatment with medication that could interfere with bone metabolism, or who received radiation treatment in neck or head area.
• Patients not following a periodontal maintenance program.
• Patients affected by untreated periodontal disease.
• Heavy smokers (>10 cigarettes per day).

Light smoker patients (<10 cigarettes per day) were included in the retrospective study, and all smoker patients were motivated to stop smoking. All patients fulfilling the eligibility criteria were recalled for a follow-up visit. Patients received detailed information regarding the nature of the study and were asked to sign a written consent form.

Each patient was subjected to a clinical examination, and frontal, occlusal and lateral photographs were taken to double-check the clinical examination.

During the clinical examination, the following parameters were evaluated in a dichotomous manner (presence/absence) for the evaluation of the technical complications:

• Buccal soft tissue dehiscence: presence of implant threads clinically visible.
• Implant threads exposure: exposure of implant threads, both in presence of a buccal soft tissue dehiscence or covered by soft tissue (presence of a dark shade under the soft tissue in a position corresponding to an implant).
• Chipping of the esthetic part of the ISCFPDs: presence and position of one or more partial fractures of the pink ceramic.
• Past or present fracture of the framework or functional structure: full framework fracture, cracks or fractures related to the functional part of the ISFCDPs, requiring in-lab repairment.
• Past or present unscrewing of the ISCFPD: requirement of screw tightening during one of the follow-up visits.

Information on the peri-implant health status was evaluated with the following parameters:

• Pocket probing depth (PPD) at implant level: probing was performed using a 15 mm UNC-15 probe (Hu-Friedy) on six sites around each implant (two vestibular, two lingual/palatal and two interproximal, one mesial and one distal). A probing depth of more than 6 mm was considered a sign of peri-implantitis.
• Bleeding on probing at implant level: present in case of bleeding after gentle probing in a nearby site. The presence of bleeding on probing was considered a sign of peri-implantitis.
• Purulent discharge: present in case of purulent discharge appearing after probing, which is considered a sign of peri-implantitis.
• Local signs of inflammation: present in case of clear signs of swelling, redness, pain on probing or spontaneous pain corresponding to an implant; all considered signs of peri-implantitis.

ISFCDPs were defined as “successful” in the case of the absence of prosthetic or implant-related complications, whilst in the case of manageable complications which did not require the full replacement of the prostheses, they were defined as “survivant” [10,13].

Finally, patients’ satisfaction was assessed by Patient-Related Outcomes Measures (PROMs) [14] with a series of questions related to the level of satisfaction with the ISFCDP rehabilitation; the patient could state whether they were completely satisfied, very satisfied, satisfied or not satisfied, and whether they would recommend treatment with ISFCDPs.

• Statistical methods and sample size

Data are expressed as means, standard deviations and percentages. The sample size was calculated on a study focusing on prosthetic technical complications of zirconia ISFCDPs at a 3-year follow-up [15]. On this basis, considering a percentage of complications of 34%, and expecting a lower percentage in our sample, 14 was the minimum number of patients to include in the study.

However, the retrospective nature of the study and the small population are also attributable to its exploratory nature, which is essential to understand if there are differences in performance between the different techniques. The results of the present study may serve as basis for larger prospective studies.

3. Results

Fourteen ASA I-II patients previously rehabilitated with a zirconia ISFCDP were included in the study and recalled for a follow-up visit.

The 14 patients, 5 males and 9 females, were between 52 and 80 years of age (average of 64 ± 8.5 years), and were rehabilitated with 14 ISFCDPs (86 implants in total).

Seven out of fourteen patients were light smokers (50%), whilst one patient was affected by diabetes (7.1%), three patients suffered from hypertension (21.4%), and two patients had previous myocardial infarction (14.3%).

Ten out of fourteen patients received a maxillary ISFCDP, while four patients received a mandibular ISFCDP. All ISFCDPs were supported by a minimum of four to a maximum of eight implants. The mean follow-up time at the time of the recall was 36 ± 10 months (

Table 1. Patient and implant characteristics.

Sample Characteristics	Number
Number of Patients	14
Males	5
Females	9
Mean Age	64
Number of ISFCDPs	14
Maxillary ISFCDPs	10
Mandibular ISFCDPs	4
Maxillary Zirc-on-Zirc ISFCDPs	3
Mandibular Zirc-on-Zirc ISFCDPs	2
Maxillary Progressive Monolithic Zirconia ISFCDPs	3
Mandibular Progressive Monolithic Zirconia ISFCDPs	0
Maxillary Zirconia-on-Titanium ISFCDPs	4
Mandibular Zirconia-on-Titanium ISFCDPs	2
Number of Implants	86
Mean Follow-up Time (Months)	36

).

Regarding the technique used to produce the ISFCDP, five were produced using the zirconia-on-titanium technique, four using the progressive monolithic zirconia technique, and five using the zirconia-on-zirconia technique.

The presence of purulent discharge was reported in three implants (3.49%) supporting a mandibular full arch zirc-on-zirc ISFCDP in a smoker patient.

Regarding probing depth, six implants (6.98% of all implants) had 5 mm of probing depth or more in at least one site: four implants were part of the same rehabilitation on a smoker patient, while the remaining two implants were each part of two other ISFCDPs in non-smoker patients.

Local signs of inflammation were observed in two implants (2.33% of the total), both part of the same rehabilitation on a smoker patient.

Bleeding on probing was reported in five patients out of fourteen (35.6%), who had a bleeding site on at least one implant; the worst case was on a smoker patient with BoP on 20 sites of a total of eight implants, with a BoP of 41%. The second patient showed bleeding on two sites of two implants (5.5% BoP); the third patient on two sites of two implants with 4.7% BoP. The remaining two patients both showed bleeding on one site on one implant with 2.7% BoP.

The mean BoP value, including the smoker patient with a high percentage of BoP-positive sites, was 13.48 ± 15,9%.

One prosthetic complication was reported (7%): one zirconia-on-zirconia ISFCDP was affected by one instance of chipping in a site corresponding to pink gingiva and was repaired intraorally using pink composite.

Six out of fourteen patients showed at least one site of implant spire exposure, but all exposed sites were less than 1 mm in length. All the complications at implant and prosthetic level are shown in

Table 2. Complications at implant and prosthetic level.

Complications	Number (%)
Implants showing purulent discharge	3 (3.49%)
Sites with PPD > 3 mm but <5 mm	15 (2.7%)
Sites with swelling and redness	2 (0.3%)
Sites with bleeding on probing (BoP)	26 (4.7%)
Chipping	1 (pink gingiva) (7.1%)
Patients with at least one implant spire exposure.	6 (42.86%)

.

The overall prosthetic success rate was 92.86%, with a survival rate of 100% and an absence of failures, whilst the implant success was 95.35%, with a survival rate of 100%.

Finally, 78.57% of the patients declared they were completely satisfied with the results obtained, 14.53% declared themselves to be very satisfied and 7.14% declared themselves to be satisfied; in addition, all patients would recommend rehabilitation with ISFCDPs (

Table 3. Complications stratified considering prosthetic design.

Complications	Zirc-on-Zirc ISFCDPs	Progressive Zirconia ISFCDPs	Zirconia-on-Titanium ISFCDPs
Presence of purulent discharge (over total n° of ISFCDPs)	1/5	0/3	0/6
Presence of sites with PPD >3 mm but <5 mm (over total n° of ISFCDPs)	2/5	2/3	0/6
Presence of sites with swelling and redness (over total n° of ISFCDPs)	2/5	2/3	0/6
Sites with bleeding on probing (over total n° of ISFCDPs)	2/5	2/3	0/6
Chipping (over total n° of ISFCDPs)	1/5	0/3	0/3
Presence of at least one implant spire exposure (over total n° of ISFCDPs)	2/5	2/3	2/6

,

Table 4. PROMS results.

Satisfaction	Percentage
Completely satisfied	78.57%
Very satisfied	14.53%
Satisfied	7.14%
Not satisfied	0%
Would recommend treatment with ISFCDPs	100%

).

4. Discussion

The results of the present retrospective study are in line with the current literature, confirming that the use of zirconia for ISFCDPs showed good clinical outcomes, with high survival rates (88–100%). The major prosthetic complication (superficial chipping) was easily and directly restorable by the clinician with no necessity for laboratory repair [10].

Technical complications are more commonly reported in implant-supported prostheses than in those supported by natural teeth. Therefore, the design, material choice and production processes of ISFCDPs represent critical factors influencing the prosthesis’ success over time [16].

The prosthetic design should allow for optimal load distribution and the minimization of implant micromovements; in particular, when immediate loading is considered, a rigid structure provides an optimal distribution of occlusal load, which prevents stress on the peri-implant bone, thus reducing osteointegration impairment [17].

It should be noted that most of the tissue-related complications (high probing depth, BoP, recessions) were reported on a single smoker patient who did not follow through with re-education in order to stop the smoking habit.

Regarding tissue-related complications, a statistical comparison between different prosthetic designs could not be performed due to the low incidence rate: through the simple observation of available data, however, it appears that different prosthetic designs do not influence the incidence of tissue-related complications; smoking, on the other hand, can represent a factor influencing soft-tissue complications.

Regarding prosthetic complications, only one incidence of the chipping of the pink gingiva on a zirc-on-zirc ISFCDP was reported for one patient: this may be due to the fact that pink gingiva is stratified and sintered upon the zirconia framework.

• Zirconia–titanium hybrid ISFCDPs (Figure 1)

It has been observed that zirconia frameworks show higher strain concentration compared to titanium frameworks, which may translate into a higher incidence of framework fracture over time [18]. Additionally, titanium is biocompatible and possesses high resistance to mechanical stresses and corrosion [18]. The use of a titanium framework and a monolithic zirconia superstructure allows for both parts to be CAD-CAM designed and milled, with highly polished interfaces and an anatomically ideal transmucosal part, both in terms of form, low bacterial adhesion and good cell adhesion [19]. The titanium framework is designed to leave at least 2 mm for the zirconia superstructure and obtain the ideal stress distribution [18]. The presence of two different strain patterns in the same prosthesis, however, may expose patients to some long-term risk for both superstructure chipping and the fracture of either the framework of the superstructure [13].

• Progressive (or multi-layer) Zirconia ISFCDPs (Figure 2)

Progressive (or multi-layer) monolithic zirconia ISFCDPs have shown promising clinical outcomes, eliminating the possibility of chipping of layered ceramic and providing a homogenous structure capable of distributing the occlusal load properly [13].

Compared to translucent monolithic zirconia, progressive monolithic zirconia presents different strengths depending on which depth of the prosthesis volume is considered: in the inner part of the prosthesis, or core, a tougher and more opaque layer of zirconia is present; on the outer part of the prosthesis, a less resistant but more translucent layer of zirconia is present [13].

The presence of a stronger inner core and a more esthetic outer layer potentially avoids the main issue of full-arch esthetic monolithic zirconia ISFCDPs, which exhibit a higher incidence of prosthetic fracture compared to high-strength opaque monolithic zirconia [19,20]. The production of a monolithic zirconia ISFCDP requires a digital design, which is then milled: prosthetic components are then glued to the prosthesis, which is screwed directly onto the implant platforms. Prosthetic design must allow for an optimal distribution of forces and adequate gingival adaptation, allowing effective cleaning procedures and avoiding food impaction. However, the literature regarding this technique is scarce: more long-term studies on the performance of progressive (or multi-layer) zirconia ISFCDPs are still needed [13].

• Zirconia-on-zirconia (zirc-on-zirc) ISFCDPs (Figure 3, Figure 4 and Figure 5)

The protocol adopted for the present zirc-on-zirc ISFCDPs was designed by one of the authors (M.S). It consists of a digitally designed and 3D-printed wax up of the ISFCDP, which is then adapted on the master model obtained by impressions of the patient’s arch. A temporary ISFCDP is then produced and used for the provisional phase of the restoration. After adjustments, if any, the temporary restoration is scanned intra-orally and refitted to the master model.

Based on the digital model of the temporary ISFCDP, the framework is milled from a hard-strength white zirconia disk. The framework shows abutments for crown cementation corresponding to the final teeth position, and pink ceramic is stratified and baked on the lowest part of the ISFCDP, which will be in contact with the patient’s soft tissues. The final crowns are milled using high-translucency, multi-layered 750 Mpa, sintered, polished and cemented to the framework through the fusion of a dedicated ceramic. Gingival margins and characterizations of the anatomy of both teeth and artificial pink tissues are created by analogically stratifying dedicated ceramics fused at 940 °C.

The zirc-on-zirc protocol allows for two different types of zirconia to be used, thus obtaining an optimal strain distribution and excellent esthetic results.

However, despite our findings, this study has several drawbacks such as the small sample size, the retrospective nature of the study and heterogeneous follow-up times. The average follow-up period of 36 months may not be sufficient to extensively evaluate the performance of different prosthetic designs, and the sample size is too small to perform meaningful statistical comparisons.

For these reasons, further studies with a larger population and a prospective randomized controlled trial study design are needed to confirm the findings of the present study.

Currently, there is a growing number of studies investigating the use of zirconia for ISFCDPs; the follow-up times, however, are short, and the sample sizes are limited [21].

A recent narrative review observed a great heterogeneity in the definition of prosthetic success and in the prosthetic design of zirconia ISFCDPs, thus making it difficult to effectively compare the success rates obtained in the present study with the current literature [10]. It can be stated, however, that the prosthetic complications reported in the present study are in line with the most common complications reported for ISFCDPs [10].

5. Conclusions

The results of this retrospective study showed that zirconia ISFCDPs, regardless of the design and production, possess high success and survival rates at short-term follow-up. A single prosthetic complication was reported among all cases, and the prosthetic designs allowed for easy cleaning and maintenance by the patients.

Zirconia ISFCDPs were also successful in terms of patients’ satisfaction and had a positive impact on masticatory and phonatory function and overall esthetics.

The zirc-on-zirc protocol also appears to be as successful as more traditional ISFCDP designs: it is important to note, however, that the use of zirconia for ISFCDPs requires a deep technical knowledge of the material, and prosthetic design and production processes should be investigated further in order to understand which technical and design processes allow for the optimal performance over time of zirconia ISFCDPs. Clinicians approaching zirconia ISFCDPs should familiarize themselves with the material’s unique properties and manufacturing processes, and collaborate with experienced dental technicians.

It is important to emphasize that further research is required, especially through long-term prospective studies with a higher number of included patients and a direct comparison with alternative zirconia ISFCDP designs.