**2. Materials and Methods**

We reported this bibliometric study in compliance with the Standards for Reporting Qualitative Research (SRQR) [43] and the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [44].

## *2.1. Search Strategy*

An online literature search was conducted in Elsevier's Scopus and Clarivate Analytics' Web of Science (WoS) until 5 October 2020. We used the medical terms (MeSH) feature in the Cochrane Library to obtain the available synonyms for our search terms to create a detailed search strategy (Table 1).


**Table 1.** Search strategy used for each database.


## *2.2. Data Extraction and Bibliometric Parameters*

We used a specially built Excel file (Microsoft, Redmond, WA, USA) to collect the findings of the literature search. The file contained the following information: abstracts, year of publication, indexed keywords, journal name, citations as well as all co-author bibliometric data (H-index, number of papers related to zirconia implant, the total number of papers, citation of paper regarding zirconia implant, and citation of paper regarding zirconia implant). Authors with the highest quantity of clinical studies regarding zirconia dental implants were evaluated and measured the average, the standard deviation, minimum and maximum of topic paper, total papers, topic citations, overall citations, and H-index. Moreover, we evaluated the scientific trend of the included study according to the year of publication and journal details (full title, the impact factor (IF), and rank) based on the Clarivate Analytics report for 2019 with selected categories: "Dentistry, Oral Surgery & Medicine", study design, number of citations received, marginal bone loss, survival rate, failure, and study follow-up.

#### *2.3. Study Selection*

We screened the literature search results in two steps, where the first phase was the screening of the title and abstract by paired reviewers separately. Then, the second phase was a full-text assessment by two expert reviewers (L.F and A.S). The reference list of the studies included in the full-text screening was hand-screened for potential additional studies. In this bibliometric study, inclusion criteria were only clinical studies (either prospective or retrospective) without time restrictions. Exclusion criteria were animal studies, in vitro studies, literature reviews, systematic reviews, short communications, personal opinion, letters, book chapters, and non-English studies.

#### *2.4. Data Analysis*

We used VOSviewer software (version 1.6.8; Leiden University, Leiden, The Netherlands) to visualize a term map analyzing keywords from the data obtained. "Create Map" function was used to analyze the data by using the "Citation" type and setting the unit of analysis as a "number of citations." In the keyword map, the node's size reflects the number of received citations, as the larger size indicates the author with the highest citations. Furthermore, keywords that often appeared together were classified as the same color in network visualization mode [45,46].

#### **3. Results**

#### *3.1. Study Selection*

A total of 1159 references were collected from electronic databases in which (n = 185) were omitted due to duplication. By title and abstract, 968 articles were screened and 841 excluded as irrelevant topics. By the full-text screening of 127 papers, 29 studies were included in this bibliometric study [47–75] excluding the remaining 98 articles because they did not meet our inclusion criteria (Figure 1).

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**Figure 1.** Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flow chart demonstrates the process of literature search and study selection.

#### **Figure 1.** Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flow chart demonstrates the process of literature search and study selection. *3.2. Study Characteristics*

*3.2. Study Characteristics*  The included studies showed wide variability in the study design, presence/absence of a control group, experimental site, type of prosthetic rehabilitation, prosthetic connection (one-piece or twopiece), follow-up period, and different methods for evaluating the effectiveness of research. Although The included studies showed wide variability in the study design, presence/absence of a control group, experimental site, type of prosthetic rehabilitation, prosthetic connection (one-piece or two-piece), follow-up period, and different methods for evaluating the effectiveness of research. Although these differences exist, most studies reported favorable outcomes for the use of zirconia implants in oral rehabilitation. The main characteristics of the included studies are summarized in Table 2.

these differences exist, most studies reported favorable outcomes for the use of zirconia implants in oral rehabilitation. The main characteristics of the included studies are summarized in Table 2. A total of 21 studies evaluated monolithic or one-piece zirconia implants [47–49,51–54,56– 58,60,62,64,66,70–74,76], two of which had titanium implants as their control and showed no significant difference in survival rate and marginal bone loss between groups (*p* > 0.05) [64,70]. Two studies evaluated the immediate loading of zirconia implants [53,64]: one study compared it to the non-occlusal loading procedure [64], while the other study compared it with the standard loading protocol [57]. Furthermore, 26 papers assessed the cylindrical microgeometry of zirconia implants [47–65,67,70–75], while three studies evaluated the root-analog zirconia implants obtained by a threedimensional scan [66,68,69]. However, Akça et al. and Pirker et al. reported the lowest marginal bone loss after two years (0.31 ± 0.24 and 0.5 ± 0.7 mm, respectively), in which Akça et al. used specially A total of 21 studies evaluated monolithic or one-piece zirconia implants [47–49,51–54,56–58,60, 62,64,66,70–74,76], two of which had titanium implants as their control and showed no significant difference in survival rate and marginal bone loss between groups (*p* > 0.05) [64,70]. Two studies evaluated the immediate loading of zirconia implants [53,64]: one study compared it to the non-occlusal loading procedure [64], while the other study compared it with the standard loading protocol [57]. Furthermore, 26 papers assessed the cylindrical microgeometry of zirconia implants [47–65,67,70–75], while three studies evaluated the root-analog zirconia implants obtained by a three-dimensional scan [66,68,69]. However, Akça et al. and Pirker et al. reported the lowest marginal bone loss after two years (0.31 ± 0.24 and 0.5 ± 0.7 mm, respectively), in which Akça et al. used specially designed titanium–zirconia alloy implants [47], and Pirker et al. used specially designed root-analog zirconia implants with a micro-retention surface in a fresh extraction socket [69].

designed titanium–zirconia alloy implants [47], and Pirker et al. used specially designed root-analog

zirconia implants with a micro-retention surface in a fresh extraction socket [69].


Becker et al. (2015) [50] Cionca et al. (2015) [55]

Clinical Oral Implants

Clinical Oral Implants

Research 8 3.723 15 Prospective

Research 8 3.723 43 Prospective

Cohort Study 52 (52 Implants) Two-piece zirconia

Case Series 32 (49 Implants) Two-piece zirconia

implants - - 95.80% 2 Implants 2 years

implants - - 87% 6 Implants 1 year

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**Table 2.** *Cont.*

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one-piece zirconia implants




#### *3.3. Growth of Publications 3.3. Growth of Publications*

In total, 29 clinical studies were published between 2008 and 2020, in which 19 papers (65.5%) were published in the last five years and ten papers published before 2015. The highest number of published studies was in 2015 (*n* = 6, 20.6%) followed by 2013 and 2017 (*n* = 4, 13.7% for each) (Figure 2). In total, 29 clinical studies were published between 2008 and 2020, in which 19 papers (65.5%) were published in the last five years and ten papers published before 2015. The highest number of published studies was in 2015 (*n* = 6, 20.6%) followed by 2013 and 2017 (*n* = 4, 13.7% for each) (Figure 2).

**Figure 2.** Publication trend of the clinical studies on the zirconia implants. **Figure 2.** Publication trend of the clinical studies on the zirconia implants.

#### *3.4. Journal of Publication 3.4. Journal of Publication*

articles.

The clinical studies on the use of zirconia dental implants for oral rehabilitation were published across ten peer-reviewed journals. The journal with the largest number of publications was "Clinical Oral Implants Research" (*n* = 12, 41%), followed by "International Journal of Oral and Maxillofacial Surgery" (*n* = 4, 13.7%) (Figure 3). The clinical studies on the use of zirconia dental implants for oral rehabilitation were published across ten peer-reviewed journals. The journal with the largest number of publications was "Clinical Oral Implants Research" (*n* = 12, 41%), followed by "International Journal of Oral and Maxillofacial Surgery" (*n* = 4, 13.7%) (Figure 3).

The majority of publications were published in Q1 journals (*n* = 25, 86%), while the journal with the highest impact factor was "Journal of Clinical Periodontology" (IF = 5.241), which had two The majority of publications were published in Q1 journals (*n* = 25, 86%), while the journal with the highest impact factor was "Journal of Clinical Periodontology" (IF = 5.241), which had two articles.

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**Figure 3.** Contribution journals in clinical research on zirconia implants. **Figure 3.** Contribution journals in clinical research on zirconia implants.

#### *3.5. Study Design and Level of Evidence 3.5. Study Design and Level of Evidence*

All included studies were prospective, while the most common study design of clinical research on zirconia implants was cohort study (*n* = 12, 41.4%), followed by case series (*n* = 9, 31%), and RCT (*n* = 5, 17%). According to the hierarchy of evidence levels (Is) [77,78], the available evidence supporting the use of zirconia implants is 17% level II, 41.4% EL IV, and the remaining EL VI. All included studies were prospective, while the most common study design of clinical research on zirconia implants was cohort study (*n* = 12, 41.4%), followed by case series (*n* = 9, 31%), and RCT (*n* = 5, 17%). According to the hierarchy of evidence levels (Is) [77,78], the available evidence supporting the use of zirconia implants is 17% level II, 41.4% EL IV, and the remaining EL VI.

#### *3.6. Contribution of Countries and Institutions 3.6. Contribution of Countries and Institutions*

The majority of the studies originated from institutions in Germany (*n* = 14, 48.3%), followed by Switzerland, (*n* = 6, 20.7%), and Austria (*n* = 5, 17%), where the most productive institution was the Medical Center of University of Freiburg (*n* = 8, 27.6%), followed by the Center of Dental Medicine, University of Zürich (*n* = 5, 17%). While many of the included studies were funded, the most funding support for included research was provided by VITA Zahnfabrik—H. Rauter GmbH & Co. KG, Bad Säckingen, Germany (*n* = 5, 17%) (Table 3). The majority of the studies originated from institutions in Germany (*n* = 14, 48.3%), followed by Switzerland, (*n* = 6, 20.7%), and Austria (*n* = 5, 17%), where the most productive institution was the Medical Center of University of Freiburg (*n* = 8, 27.6%), followed by the Center of Dental Medicine, University of Zürich (*n* = 5, 17%). While many of the included studies were funded, the most funding support for included research was provided by VITA Zahnfabrik—H. Rauter GmbH & Co. KG, Bad Säckingen, Germany (*n* = 5, 17%) (Table 3).


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implants.

Figure 6).

#### *3.7. Bibliometric Assessment 3.7. Bibliometric Assessment 3.7. Bibliometric Assessment*  A total of 29 articles with total citations[Scopus] ranged from 0 to 176 (mean 57.28 ± 42.18), while

A total of 29 articles with total citations[Scopus] ranged from 0 to 176 (mean 57.28 ± 42.18), while the number of citations[Scopus] received by each paper ranged from 0 to 69 (mean 21.3 ± 20). The top-cited study was the RCT of Cannizzaro et al. (2010) (*n*[Scopus] = 69) [54], followed by the prospective case series of Payer et al. (2012) (*n*[Scopus] = 61) [66], and Pirker et al. (2009) (*n*[Scopus] = 58) [69]. A total of 29 articles with total citations[Scopus] ranged from 0 to 176 (mean 57.28 ± 42.18), while the number of citations[Scopus] received by each paper ranged from 0 to 69 (mean 21.3 ± 20). The topcited study was the RCT of Cannizzaro et al. (2010) (*n*[Scopus] = 69) [54], followed by the prospective case series of Payer et al. (2012) (*n*[Scopus] = 61) [66], and Pirker et al. (2009) (*n*[Scopus] = 58) [69]. the number of citations[Scopus] received by each paper ranged from 0 to 69 (mean 21.3 ± 20). The topcited study was the RCT of Cannizzaro et al. (2010) (*n*[Scopus] = 69) [54], followed by the prospective case series of Payer et al. (2012) (*n*[Scopus] = 61) [66], and Pirker et al. (2009) (*n*[Scopus] = 58) [69]. However, the author with the highest number of clinical research on zirconia implants was

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However, the author with the highest number of clinical research on zirconia implants was Kohal R.J. (*n* = 10), followed by Spies B.C. (*n* = 8) and Vach K. (*n* = 6), while the top-cited author of clinical studies on zirconia implants was Kohal R.J. (*n*[WoS] = 155), followed by Arnetzl G. (*n*[WoS] = 91), Koller M. (*n*[WoS] = 87), Payer M., and Jakse N. (*n*[WoS] = 86 for each) (Figures 4 and 5). However, the author with the highest number of clinical research on zirconia implants was Kohal R.J. (*n* = 10), followed by Spies B.C. (*n* = 8) and Vach K. (*n* = 6), while the top-cited author of clinical studies on zirconia implants was Kohal R.J. (*n*[WoS] = 155), followed by Arnetzl G. (*n*[WoS] = 91), Koller M. (*n*[WoS] = 87), Payer M., and Jakse N. (*n*[WoS] = 86 for each) (Figures 4 and 5). Kohal R.J. (*n* = 10), followed by Spies B.C. (*n* = 8) and Vach K. (*n* = 6), while the top-cited author of clinical studies on zirconia implants was Kohal R.J. (*n*[WoS] = 155), followed by Arnetzl G. (*n*[WoS] = 91), Koller M. (*n*[WoS] = 87), Payer M., and Jakse N. (*n*[WoS] = 86 for each) (Figures 4 and 5).

**Figure 4.** Network analysis of the authors with the largest number of clinical studies on zirconia **Figure 4.** Network analysis of the authors with the largest number of clinical studies on zirconia implants. implants.

**Figure 5.** Bibliometric variables for authors with the highest number of topic papers. **Figure 5.** Bibliometric variables for authors with the highest number of topic papers.

**Figure 5.** Bibliometric variables for authors with the highest number of topic papers. The authors' H-indexScopus ranged from one to 79 (mean 22.67 ± 19.96), and the author with the most bibliometric characteristics was Hämmerle C.H.F., who had 364 publications (two of which were clinical studies on zirconia implants) with 8311 total citations and H-indexScopus = 79 (Table 4 and The authors' H-indexScopus ranged from one to 79 (mean 22.67 ± 19.96), and the author with the most bibliometric characteristics was Hämmerle C.H.F., who had 364 publications (two of which were clinical studies on zirconia implants) with 8311 total citations and H-indexScopus = 79 (Table 4 and Figure 6). The authors' H-indexScopus ranged from one to 79 (mean 22.67 ± 19.96), and the author with the most bibliometric characteristics was Hämmerle C.H.F., who had 364 publications (two of which were clinical studies on zirconia implants) with 8311 total citations and H-indexScopus = 79 (Table 4 and Figure 6).

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**Table 4.** General bibliometric variables for authors with the largest number of topic papers.


160

**Figure 6.** Box plots summarize the bibliometric variables of the authors with the largest number of **Figure 6.** Box plots summarize the bibliometric variables of the authors with the largest number of studies.

#### studies. **4. Discussion**

**4. Discussion**  The present study carried out a bibliometric evaluation of clinical research on zirconia implant rehabilitation, highlighting the significant heterogeneity of the included studies, which revealed considerable variations in methodology, technical approaches, follow-up, and control group The present study carried out a bibliometric evaluation of clinical research on zirconia implant rehabilitation, highlighting the significant heterogeneity of the included studies, which revealed considerable variations in methodology, technical approaches, follow-up, and control group involvement. Our findings indicate that there is a trend for zirconia implants in oral rehabilitation as there has been an increase in about 180% of the studies published in the last five years.

involvement. Our findings indicate that there is a trend for zirconia implants in oral rehabilitation as there has been an increase in about 180% of the studies published in the last five years. The included studies reported a survival rate for zirconia implants ranging from 87% to 100% with follow-up periods from one to 7.8 years, while the least survival rate reported in RCT by Siddiqi et al. was 67.6% after one-year follow-up (i.e., 16 zirconia implants failed out of 68) [70]. This RCT aimed to study the effectiveness of zirconia vs. titanium implants restored with one-piece ballabutment in mandibular and maxillary overdentures, while this high decrease in the survival rate was for both groups (i.e., 67.6% for zirconia implants and 66.7% for titanium implants); the outcomes The included studies reported a survival rate for zirconia implants ranging from 87% to 100% with follow-up periods from one to 7.8 years, while the least survival rate reported in RCT by Siddiqi et al. was 67.6% after one-year follow-up (i.e., 16 zirconia implants failed out of 68) [70]. This RCT aimed to study the effectiveness of zirconia vs. titanium implants restored with one-piece ball-abutment in mandibular and maxillary overdentures, while this high decrease in the survival rate was for both groups (i.e., 67.6% for zirconia implants and 66.7% for titanium implants); the outcomes of maxillary rehabilitation were worse than the mandible, while no mechanical fractures of the fixtures were reported [70].

of maxillary rehabilitation were worse than the mandible, while no mechanical fractures of the fixtures were reported [70]. Although one-piece and two-piece zirconia implants have been evaluated, the lower marginal bone loss and higher survival rates were observed in studies of one-piece zirconia implant rehabilitation on a single tooth or three element prosthetic rehabilitation [59,61]. However, the studies did not report any differences in the marginal bone loss and survival rate between the single crown and the fixed multiple zirconia implant recovery, while the prosthetic connection appears to have no apparent effect on these parameters [48]. Additionally, Lorenz et al. showed no significant difference in marginal bone loss with a total of 83 zirconia implants compared to natural teeth after 7.8 years of Although one-piece and two-piece zirconia implants have been evaluated, the lower marginal bone loss and higher survival rates were observed in studies of one-piece zirconia implant rehabilitation on a single tooth or three element prosthetic rehabilitation [59,61]. However, the studies did not report any differences in the marginal bone loss and survival rate between the single crown and the fixed multiple zirconia implant recovery, while the prosthetic connection appears to have no apparent effect on these parameters [48]. Additionally, Lorenz et al. showed no significant difference in marginal bone loss with a total of 83 zirconia implants compared to natural teeth after 7.8 years of function [47], and the marginal bone loss was similar in the other studies, which was less than 1 mm in the first year and stabilized in subsequent functional loading [47–49,52,56–58,64,69,73,74]. Moreover, the prospective study by Kniha et al. contained the largest sample size of the included studies involving 81 patients with 105 implants for fixed rehabilitation, who reported a significant decrease of 0.66 ± 0.30 mm with a survival rate of 100% after three years [58].

However, the most common complication (70%) was the failure of implant osseointegration as 17 studies reported a loss of at least one implant in the first six months [48–51,53–56,59–61,63,64,66,67, 69,70,72–74].

As previously reported for titanium dental implant threads, microgeometry appears to have a significant effect on the osseointegration of zirconia implants [79,80], whereas a more retentive surface resulted in an increased survival rate compared to a sandblasted surface only [68,69].

Although all clinical research included in this analysis was screened and selected from the Scopus and Web of Science databases, which may avoid restriction in each database [39,81], our investigation has further limitations. First, the year of publication, which is a reliable indicator of the number of citations received, as older papers receive more citations than recent publications because there is more time to cite them, regardless of their impact [82,83]. Second, open access policies have a significant influence on the citations received in the evaluated papers [84–86], as a result, we found large heterogeneity in Topic/Total Citations% and co-authors' H-index.
