Polyetheretherketone in Implant Prosthodontics: a Scoping Review

REVIEWS

Polyetheretherketone in Implant Prosthodontics: A Scoping Review

Andrea Paratelli, DDS Giammarco Perrone, DDS Department of Prosthodontics and Restorative Dentistry, School of Dentistry, Complutense University of Madrid, Madrid, Spain. Rocío Ortega, DDS, PhD Department of Prosthetic Dentistry, School of Dentistry, European University of Madrid, Madrid, Spain. Miguel Gómez-Polo, DDS, PhD Department of Prosthodontics and Restorative Dentistry, School of Dentistry, Complutense University of Madrid, Madrid, Spain.

Purpose: To undertake a scoping review of the available research on the application of polyetheretherketone (PEEK) in implant prosthodontics, map the available literature in order to highlight possible gaps in knowledge and, if possible, extract clinical guidelines. Materials and Methods: The literature on PEEK in implant prosthodontics published through August 2018 was identified with an online search of MEDLINE (via PubMed), Science Direct, Embase (via Ovid), and Google Scholar databases. Qualitative and quantitative syntheses were carried out for original research studies. Results: The amount of published original research studies was found to be limited. PEEK was found to be applied as a material in the fabrication of implant- supported fixed dental prosthesis (IFDP) frameworks (43%), prosthetic implant abutments (35%), implant abutment screws (15%), and retention clips on implant bars (7%). Only 38% of the studies were clinical studies, while 15% were observational and 47% were in vitro. The studies identified did not permit the estimation of long-term survival nor success rates for any of the prosthetic components. The results only allowed a preliminary short-term assessment of PEEK IFDP frameworks, which presented satisfactory survival but alarming success rates over the first year of service. Conclusion: In light of the paucity of evidence on the viability of PEEK as an implant-prosthodontic material, its use cannot yet be endorsed. Clinicians should heed the suggested protocols to improve mechanical performance and lower the incidence of prosthetic complications. Further high-quality research is needed for an enhanced understanding of the material’s viability. Int J Prosthodont 2020;33:671–679. doi: 10.11607/ijp.6649

lastic materials have a broad range of applications today, and there are many different categories of polymers available. High-performance polymers are plastics Pthat meet stringent requirements for temperature stability, chemical resistance, and mechanical properties. Since their formulations can be altered as required, these materials can be designed favoring any one of these characteristics over the others to meet specific objectives. High-performance polymers find many applications in many different fields, such as engineering, industry, and medicine (ie, medical devices).1,2 Polyetheretherketone (PEEK) is a polycyclic aromatic semi-crystalline thermoplastic polymer with mechanical properties that are favorable for biomedical applications. By incorporating hydroxyapatite particulates into a PEEK matrix, some authors have obtained a composite polymer that showed a tensile strength ranging from 49.0 to 83.3 3 Correspondence to: MPa, which matches the lower limits of cortical bone (50 to 150 MPa). Thanks to its Dr Andrea Paratelli elastic behavior, which is similar to human bone, PEEK is widely used in spinal surgery, Complutense University of Madrid orthopedic surgery, and maxillofacial surgery as a bone and cartilage substitute.4,5 Pza. Ramón y Cajal s/n, 28040 Madrid, Spain PEEK was first introduced in three forms of unfilled versions: PEEK-LT1, PEEK-LT2, Email: [email protected] and PEEK-LT3. Subsequently, fillers such as ceramics, hydroxyapatite, beta-calcium phosphate, and carbon fibers were added to the PEEK formula to increase its bio- Submitted July 26, 2019; accepted June 9, 2020. activity and improve its mechanical properties, leading to materials such as BioHPP ©2020 by Quintessence (Bredent).6,7 Many filled PEEK versions are now being used to manufacture biomedical Publishing Co Inc.

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Articles identified through online search (n = 1,910)

Articles identified from reference lists and systematic reviews (n = 25)

Abstracts screened after excluding duplicates (n = 1,522)

Records excluded after title and abstract screening (n = 1,376)

Articles selected for full-text analysis (n = 146)

Records excluded after full-text analysis (n = 133)

Articles included in qualitative and quantitative syntheses (n = 13)

Fig 1 Flowchart of literature search protocol.

implants.7 Due to its favorable mechanical properties, application of this polymer in implant prosthodontics PEEK has also reached the field of dentistry, where it would be indicated. Therefore, the present scoping re- has been implemented in several areas. Combining its view was conducted in order to systematically map the properties with the ongoing diffusion of CAD/CAM research done in this area, as well as to identify any exist- technology, PEEK components can now be digitally de- ing gaps in knowledge regarding PEEK material behavior signed and then 3D printed, achieving high precision to when applied as an implant-prosthodontic component. meet clinical requirements. The following research question was formulated: What A variety of reviews have discussed the applications is known from the literature about the application of of PEEK in oral implantology and tooth-supported PEEK in implant prosthodontics? prosthodontics, which include components for tooth- supported fixed dental prostheses (FDPs), tooth-sup- MATERIALS AND METHODS ported removable dental prostheses (RDPs), bioactive nanocomposites, and dental implants.8–15 A search of the literature was carried out in the follow- PEEK was initially introduced to the implant prosth- ing databases: MEDLINE via PubMed, Science Direct, odontics field as a material for the fabrication of interim Embase via Ovid, and Google Scholar. The following restoration components such as implant abutments, keyword search strategy was applied: PEEK OR poly- thanks to its fabrication feasibility with CAD/CAM, its etheretherketone OR poly-ether-ether-ketone OR high- affordable price, and its better mechanical performance performance polymer* OR high performance polymer* compared to common resin materials.16 However, no OR BioHPP AND (dentistry OR implant-prosthodontics). review articles focusing exclusively on PEEK’s role in im- As a search filter, only articles published through August plant prosthodontics have been identified. It has been 31, 2018, were considered. Additionally, reference lists reported by several authors that the evidence regarding and previous review articles were also screened for rel- PEEK in dentistry is still limited in quantity2,13; thus, a evant articles. Regarding the Google Scholar database, scoping review seems to be the most appropriate type of only the first 300 results were considered, as suggested review article to conduct on this topic in order to assess by Haddaway et al.17 No review protocol has been reg- whether conducting a further systematic review on the istered for this work.

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Table 1 Studies Included in Qualitative and Quantitative Syntheses

Study, y Subjects Component type/material Agustín-Panadero et al,26 2015 In vitro Resin prosthetic implant abutment

Al-Rabab’ah et al,21 2017 Humans IFCDP framework/titanium-reinforced resin prosthetic implant abutment

Bayer et al,30 2012 Humans Retention clips on implant bars

Bechir et al,22 2016 Humans IFCDP framework

Malo et al,23 2018 Humans IFCDP framework

Maté-Sanchez de Val et al,29 2016 Animals Titanium-reinforced resin prosthetic implant abutment

Nazari et al,24 2016 In vitro Three-unit IFDP framework

Neumann et al,6 2014 In vitro Implant-abutment screws

Ostman et al,28 2008 Humans Titanium-reinforced resin prosthetic implant abutment

Preis et al,25 2017 In vitro Single-unit IFDP framework

Santing et al,27 2012 In vitro Titanium-reinforced resin prosthetic implant abutment

Schwitalla et al,7 2016 In vitro Implant-abutment screws

Zoidis,20 2018 Humans IFCDP framework

The PRISMA Extension for Scoping Reviews (PRISMA- One independent author (A.P.) carried out the data ScR) checklist was followed as a guideline for the correct charting, with the objective of extracting information reporting of the items required for this type of review.18 regarding sample size, follow-up period, survival rate, An initial assessment of article eligibility was performed success rate, fracture strength of the restoration, fracture by screening titles and abstracts, followed by a full-text strength of the prosthetic component, and complica- analysis. The titles and abstracts of the search results were tions. Descriptive statistical analysis was performed for initially screened by two reviewers (A.P. and G.P.) for pos- the data that were deemed suitable. For human and sible inclusion. To avoid excluding potentially relevant ar- animal studies that reported the necessary information, ticles, abstracts providing an unclear result were included weighted arithmetic means for survival and success rates in the full-text analysis. The full texts of all possibly relevant were calculated. Survival was considered as the presence studies were obtained for independent assessment by of the prosthetic component inside the mouth at the the two reviewers against the stated inclusion criteria. time of observation. Success was considered as absence Disagreements were brought to an independent reviewer of complications at the time of observation.19 In case sur- (M.G.) and resolved through discussion. vival and success rates regarding the same application of To be included in the preliminary evaluation, the fol- the material were available from multiple studies, human lowing criteria had to be met by the article: abstract and animal data were pooled separately. Interexaminer present; related to PEEK in its unfilled and filled presen- agreement was calculated using κ statistics. tations; and related to dentistry. To be included in the qualitative and quantitative RESULTS syntheses, the following criteria had to be met: articles discussing PEEK’s applications in implant prosthodontics A total of 1,910 articles were identified from the three that registered variables linked to the relevant prosthetic databases searched (MEDLINE, Science Direct, Embase component. via Ovid, and Google Scholar; Fig 1). After checking No discriminations were made regarding the manufac- for duplicates, the titles and abstracts of 1,522 articles turing process of the material, its composition, its surface were screened. Of these articles, 146 were selected for modification, or its veneering procedures. Only original a full-text assessment; of those, 13 were included in research papers and case reports were considered for the qualitative and quantitative summaries after a full- the qualitative and quantitative analyses. Historical and text review (Table 1).6,7,20 –30 During screening of the systematic reviews, book chapters, and letters to the abstracts, a κ value of 0.91 for interexaminer agreement editor were excluded from the analyses; however, if was calculated, and during full-text analysis, a κ value considered pertinent, they were used for contextualizing of 0.83 was observed. Disagreements were resolved the material’s background. through discussion between the reviewers. Different

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Table 2 PEEK as IFDP Framework Material Mean overall Mean overall Treatment Study Sample size, Follow-up, Fracture Survival survival Success success Failure phase Application Study design n mo strength, N rate, % rate, % rate, % rate, % Complications pattern Veneering material

Al-Rabab’ah et al21 In vivo 2 24 NA 100 98.87 50 85.05 1 discoloration NA Milled composite resin

3 soft tissue lesions; Bechir et al22 In vivo 35 12 NA 100 88.57 NA Composite resin paste IFCDP 2 unpleasant taste framework 1 framework Malo et al23 In vivo 49 12 NA 98 83.67 7 adhesive fractures Milled composite resin Definitive fracture Zoidis20 In vivo 1 24 NA 100 100 0 NA Milled composite resin Three-unit IFDP Nazari et al24 In vitro 10 NA 1,430 N NA NAp NA NA NA Adhesive fracture Composite resin paste framework Single-unit IFDP (1) 921; (1) Composite resin paste; Preis et al25 In vitro 32 NA NA NA NA NA NA Adhesive fracture framework (2) 1,921 (2) Milled composite resin NA = not applicable.

Table 3 PEEK as Prosthetic Implant Abutment Material Treatment Study Sample Follow-up, Fracture Type of phase Type of abutment Implant system Study design size, n mo strength, N Survival rate, % Success rate, % Complications restoration Titanium-reinforced SKY elegance Definitive Al-Rabab’ah et al21 In vivo 1 24 NA 100 100 0 Lithium-disilicate crown resin abutment (Bredent) Resin abutment NR Agustín-Panadero et al26 In vitro 8 NA 329.4 NA NA NA NA QuickBridge 1 restoration fracture; Ostman et al28 In vivo 74a 6 NA 100 NR Chairside resin restoration (Zimmer Biomet) 2 screw loosening Interim Titanium-reinforced Maté-Sanchez de Val resin abutment SKY elegance Experimental 24 2 NA 100 100 0 NA et al29 RN synOcta (Straumann) Santing et al27 In vitro 32 NA > 2,000 NA NA NA NA NA = not applicable; NR = not reported. aMinimum sample size

study designs were identified. In vivo clinical studies prostheses (IFCDP); three-unit IFDPs; and single-unit consisted of 38% (5 studies) of the total, but only 2 IFDPs. Single-unit and three-unit IFDPs located posteri- had a controlled design, and only 1 was randomized. In orly accounted for information regarding only fracture vivo observational studies consisted of 15% (2 studies) strength, while no information about fracture strength of the total, while in vitro research consisted of 47% (6 was recorded for elements located anteriorly, nor for studies) of the total. IFCDPs. In partial IFDPs, the failure pattern was found The applications for PEEK as a material for the manu- to be from adhesive fracture between the framework facture of components involved in implant-prosthodontic and veneering materials in all the samples tested for rehabilitation procedures were: IFDP frameworks (43%); fracture resistance. prosthetic implant abutments (35%); implant-abutment From the four in vivo studies reporting information on screws (15%); and retention clips for attaching dental IFCDPs, 87 prostheses were identified.20–23 The weighted prostheses to implant bars (7%). Studies on the applica- arithmetic means showed that, over the first 12 months tion of PEEK as a material for the manufacture of implant following prosthetic rehabilitation, IFCDPs had a mean healing abutments were not included in this paper, since survival rate of 98.87% and a mean success rate of they were considered more closely related to the surgi- 85.05%. An incidence rate of 16% was identified for the cal rather than the prosthetic aspect of the treatment. appearance of complications over the first 12 months. Table 2 lists the studies involving PEEK as a frame- Of those complications, 64.28% were found to be of a work material for IFDPs. No in vivo studies providing mechanical nature, while 35.72% were biologic. Among a control group were found; only two case series.20,21 the mechanical complications, 77.8% involved adhesive Two uncontrolled clinical studies22,23 and two in vitro fracture between the framework and the veneering studies were identified.24,25 The prosthetic design units materials, 11.1% involved fracture of the framework identified were implant-supported fixed complete dental material, and 11.1% were reports of discoloration of the

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Al-Rabab’ah et al21 In vivo 2 24 NA 100 98.87 50 85.05 1 discoloration NA Milled composite resin

prosthesis. Among the biologic complications were the showed 100% success and survival rates over the first appearance of soft tissue lesions and unpleasant taste. 6 months.30 Table 3 lists the studies that discuss PEEK applications as a prosthetic implant abutment material. One case DISCUSSION report was identified for the application of PEEK as a definitive prosthetic implant abutment,21 while two in The limitations of this review include the search strat- vitro studies and two clinical studies were identified for egy selected for the analyzed databases, which was PEEK interim prosthetic implant abutments.26–29 The intentionally designed to be more general, as one of use of PEEK in a definitive titanium-reinforced resin the objectives was to identify all possible applications prosthetic implant abutment had only one case report, of the material in the field of interest. For the same rea- which showed 100% survival and success rates over 24 son, another limitation relating to reproducibility of the months.21 One in vivo controlled animal study, which online search may be the inclusion of Google Scholar included 24 subjects, reported a 100% survival rate over among the analyzed databases. Its search algorithms a 2-month period where PEEK was used in an interim often lead to wide amounts of results; however, this titanium-reinforced resin prosthetic implant abutment.29 made it possible to map the literature not indexed in Studies involving PEEK as a material for the fabrication the other databases searched in this review. of implant-abutment screws and round clips for denture According to the evidence obtained, PEEK applications retention over implant bars are listed in Table 4. Only two in implant prosthodontics may be summarized as follows. in vitro studies concerning PEEK as an implant-abutment screw material were found.6,7 Using PEEK for retention IFDP Material clips to attach the prosthesis to round implant bars was The role of PEEK as an IFDP component was the one re- discussed in only one randomized clinical trial, which ported most often. All of the studies that discussed PEEK

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Table 4 Other PEEK Applications in Implant Prosthodontics Study Sample Follow- Fracture Survival Success Application Study Material design size, n up, mo strength, N rate, % rate, %

(1) Unfilled PEEK; In (1) 10; (1) 149.5; Neumann et al6 NA NA NA (2) 30% CFR-PEEK vitro (2) 10 (2) 145.2 Implant abutment (1) 15% short CFR-PEEK; screw (1) 152.7; (2) 40% short CFR-PEEK; In Schwitalla et al7 NR NA (2) 157.3; NA NA (3) 50% continuous vitro (3) 191.7 CFR-PEEK

Prosthetic In retention Bayer et al30 Unfilled PEEK 30 6 NA 100 100 vivo systems NA = not applicable; NR = not reported; CFR = carbon fiber–reinforced.

as a definitive IFDP material applied the polymer to the on only four studies, it should also be noted that all of manufacture of frameworks, which were subsequently the included studies present short follow-up periods; veneered with resin materials.20–25 however, the results are comparable to those achieved by The resistance to fracture of PEEK IFDPs has been metal-acrylic, metal-porcelain, and zirconia IFCDPs.33–35 described as inferior to nickel-chromium and zirconia, On the other hand, the success rates for PEEK IFCDPs but clinically acceptable. However, it should be men- showed more alarming data, as a 16% incidence rate tioned that the information reported by the in vitro for the appearance of complications was observed at studies included in this review refers to static fatigue 12 months. Resin-veneered polymethyl methacrylate loads, and no information has been provided about (PMMA) and ceramic-veneered zirconia IFCDPs were their performance under cyclic fatigue stress, which is reported as showing annual complication rates of 6.6% a more relevant indicator for the behavior of oral re- with a 90.6% survival rate, free of complications, at 12 habilitations. The fabrication design and type of resin months.37 These figures suggest that PEEK IFCDPs incur veneer also impacted the resistance to fracture, favoring greater risk. The incidence of complications reported cemented restorations over screw-retained ones and here, however, should be interpreted considering certain veneering with milled composite resin over composite limitations—namely, only 87 PEEK IFCDPs were identi- paste.25,31 Therefore, in situations where the choice of fied, with a follow-up at 12 months for 84 prostheses screw-retained or a cemented design is equally indicated, and at 24 months for 3 prostheses. Besides the limited cemented restorations are believed to account for bet- number of cases, the 12-month follow-up period pro- ter mechanical performance, and regarding the choice vides limited clinical significance in the case of definitive of veneering procedure and material, milled composite prostheses. resin should be preferred. Different fabrication proce- Biologic complications, which were unpleasant taste dures have also been shown to influence the mechanical and soft tissue lesions, made up 35.72% of the total properties of the prosthesis. Restorations manufactured cases observed. However, it should be noted that the using CAD/CAM–milled PEEK appear to be more resis- authors of the study reporting taste alterations stated tant to fracture than elements made using other fabrica- that some of the patients were also suffering from sys- tion methods.32 temic conditions at the time of evaluation,22 thus adding IFCDPs have been shown to provide highly successful another level of bias to the incidence rate for complica- treatment for patients with edentulism.33 PEEK IFCDPs tions. PEEK has been previously appointed as a bioinert may offer a reasonable meeting point between the material38; however, Trindade et al recently observed affordability and ease of repair of metal-acrylic IFDPs that this material triggers higher immune activation with and the better mechanical performance of zirconia and respect to titanium during the process of osseointegra- metal-porcelain IFCDPs.34–36 tion.39 The authors suggested that the knowledge of Despite the limited number of cases reported, based the material’s chemical inertia was most likely based on the results discussed in this review, PEEK IFCDPs on in vitro studies, since information regarding the role seem to have satisfactory survival rates (98.87%) at 12 of the immune system in the downregulation of the months. Besides the fact that this statement is based inflammatory process during the PEEK-host interaction

676 The International Journal of Prosthodontics © 2020 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER. Paratelli et al was not available.40 It is not yet clear whether the soft An in vitro study conducted by Santing et al reported tissue lesions observed in this review may be related to that composite resin crowns supported by titanium- PEEK’s interaction with the host tissues or to confound- reinforced PEEK abutments are capable of bearing static ing factors from the designs of the included studies; loads up to 2,000 N.27 however, in light of the results mentioned above, this Among the limitations of this paper was the impos- might be worthy of further investigation. sibility to estimate success and survival rates for PEEK Regarding mechanical failures, 77.8% consisted of prosthetic implant abutments due to the paucity of in adhesive fracture between the framework and veneering vivo information and the heterogeneity of the systems materials. Failure patterns in single- and multi-unit IFDPs identified. However, it can be deduced from the results also mostly consisted of adhesive fractures between the that PEEK titanium-reinforced implant abutments could framework and veneering material,24,25,31 which cor- provide superior results in terms of mechanical resistance roborates the findings observed for IFCDPs. This should when applied as either an interim or a definitive abut- indicate satisfactory resistance for the framework, even ment. Therefore, use of a titanium core is preferred to though, to date, no durable and predictable bonding the resin-only version of the component. The thickness strategy between a PEEK framework and composite and manufacturing process of the interim restoration veneer has been found. Future investigations should could be more significant to the outcome of treatment focus on finding the right balance between supplement- with PEEK abutments than they would be for titanium ing the PEEK formula with fillers and choosing the right abutments. Future research should focus on the me- surface-conditioning protocol and veneering procedure chanical resistance of titanium-reinforced PEEK abut- to obtain optimal mechanical performance. ments, as well as long-term in vivo outcomes.

Implant Abutment Material Implant Abutment Screws Several in vitro and in vivo studies reported using pros- The proportional advantages given by PEEK abutment thetic implant abutments composed entirely or partially screws are related to their shock absorption properties, of PEEK.21,27–29,41–48 However, only five studies discussed as well as their easier removal in case of fracture due to abutment-related variables that were of interest for this their lower friction coefficients. When tested for frac- paper.21,26–29 ture strength compared to titanium, PEEK screws were In terms of mechanical performance, PEEK resin im- shown to be 5 times less resistant, and no differences plant abutments have been described as significantly were found between the unfilled and carbon fiber–filled less resistant to fracture than titanium abutments.26 versions of the material. However, no description of the Their higher shock absorption capability does not nega- carbon fiber design was provided by the authors.6 In a tively affect stress distribution in the implant or the pe- more recent study, Schwitalla et al7 reported that PEEK ripheral bone when compared to other restoration and screws filled with short carbon fibers (with a composition abutment materials.44 Nevertheless, these abutments of up to 40%) showed similar mechanical behavior to showed higher stress concentration in the restoration what was reported by Neumann et al6 and also observed crowns, which is especially relevant in the case of central a 20% increase in tensile strength when the PEEK was incisors where the restoration is thinner, thus suggesting filled with a continuous carbon fiber composition of at that PEEK resin implant abutments may require specific least 50%. precautions when it comes to the design of a crown It has been demonstrated that the addition of PMMA restoration.27,4 4 cement improves the mechanical resistance of carbon PEEK titanium-reinforced resin implant abutments fiber–reinforced PEEK screws.49 have been used in cases of immediate loading by sev- The design of the connection between the implant eral authors.21,28 These systems are composed of a ti- and the restoration is another factor that could affect the tanium core and an overlying PEEK sleeve, which may mechanical performance of the abutment screw. Since be removable or not, depending on the design of the PEEK abutment screws have a higher tendency to incur element. More recently, an abutment composed of a plastic deformation than titanium, the relations between titanium core with a ceramic-reinforced PEEK sleeve has different implant connections and screw designs should been introduced to the market.21 Even if supported by also be investigated further. only one animal study, the evidence seems to indicate The upcoming challenge for PEEK abutment screws that this abutment is superior to titanium in terms of will consist of finding the right balance in the filler com- soft tissue adaptation. This abutment is also suitable position in terms of quantity and structure, the abut- for the “one abutment—one time” concept, meaning ment material, and the implant connection design. To that titanium-reinforced PEEK abutments could also be date, scientific evidence lacks the consistency required used as both provisional and definitive abutments.29 to recommend them for clinical use.

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Prosthetic Retention Systems ACKNOWLEDGMENTS Plastic retention clips have been studied in several sce- narios to investigate their retention of tooth-supported The authors did not receive any funding from third-party sources for RDPs. They have been described as less retentive than the realization of this scoping review. Regarding the studies included commonly used materials such as cobalt-chromium, but in this review (n = 13), 3 (23%) were industry sponsored, 1 (7%) was nonsponsored, 7 (54%) did not report funding sources, and 2 (15%) still retentive enough to serve their clinical purpose. PEEK were public-sponsored. The authors report no conflicts of interest. resin clips have been shown to be potentially reliable as a retention clasp for anchoring tooth-supported RDPs with a 10-year service life; however, it seems that it is REFERENCES necessary to increase their diameter to twice that of the metal clasps in order to obtain sufficient long-term . 1 Kurtz SM, Devine JN. 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Literature Abstract

Oral Saliva and COVID-19 Outbreak pneumonia announced in Wuhan, China, in December 2019, had its causative factor classified as a new coronavirus (SARS- CoV-2). Since saliva can host several viruses, including SARS-CoV-2, the transmission chance for viruses through saliva, particularly those causing respiratory infections, is unavoidable. COVID-19 can be detected through salivary diagnostic testing, which has many advantages for medical care professionals and patients. It should be noted that not only does saliva offer an ecologic niche for the colonization and development of oral microorganisms, but it also prevents the overgrowth of particular pathogens, such as viral factors. The aim of this study is to gather all the information about saliva and its association with COVID-19 for health care professionals across the world. Baghizadeh Fini M. Oral Oncol 2020;108:104821. References: 23. Reprints: Maryam Baghizadeh Fini, [email protected] —Steven Sadowsky, USA

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