Review Article

Custom-made root-analogue zirconia implants: A scoping review onmechanical and biological benefits

Miguel Pessanha-Andrade,1 Mariane B. Sordi,2 Bruno Henriques,3 Filipe S. Silva,3 Wim Teughels,4

J�ulio C. M. Souza 3

1Division of Oral Implantology, School of Dentistry, Universidade Fernando Pessoa (UFP), Porto, Portugal2Post-graduate Program in Dentistry (PPGO), Universidade Federal de Santa Catarina (UFSC), Florian�opolis, Brazil3Center for Microelectromechanical Systems (CMEMS-UMinho), University of Minho, Campus Azur�em, Guimar~aes, Portugal4Department of Oral Health Sciences, University Hospitals Leuven, Katholieke Universiteit Leuven, Leuven, Belgium

Received 15 January 2018; revised 18 March 2018; accepted 9 April 2018

Published online 00 Month 2018 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.34147

Abstract: The aim of this study was to conduct a literature

review on the potential benefits of custom-made root-ana-

logue zirconia implants. A PubMed and ScienceDirect biblio-

graphical search was carried out from 1969 to 2017. The

increased interest in zirconia-based dental structures linked

to aesthetic and biological outcomes have been reported in

literature. Recent technological advances have focused on

novel strategies for modification of zirconia-based surfaces to

accelerate osseointegration. However, only a few studies

revealed mechanical and biological benefits of custom-made

root-analogue zirconia implants and therefore further studies

should investigate the influence of different design and sur-

face modification on the performance of such implants.

Custom-made root-analogue zirconia implants have become

a viable alternative to overcome limitations concerning stress

distribution, aesthetics, and peri-implantitis induced by bio-

films. However, further in vitro and in vivo studies on sur-

face–bone interactions and mechanical behavior of zirconia

should be evaluated to reduce clinical issues regarding

mechanical failures and late peri-implant bone loss. VC 2018

Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 00B:

000–000, 2018.

Key Words: dental implant, zirconia, custom-made implant,

root-analogue

How to cite this article: Pessanha-Andrade M, Sordi MB, Henriques B, Silva FS, Teughels W, Souza JCM. 2018. Custom-maderoot-analogue zirconia implants: A scoping review on mechanical and biological benefits. J Biomed Mater Res Part B2018:00B:000–000.

INTRODUCTION

Dental implants are currently used to replace missingteeth.1–4 However, growing concerns regarding aesthetics,titanium hypersensitivity, and corrosion by gradual materialdegradation encourage further research into highly estheticand biocompatible alternatives.5–8 Nowadays, patients lookfor esthetic ceramic restorations. In response to such increas-ing demand, technical developments have led to the introduc-tion of improved ceramic-based structures for implant-supported prostheses.9 The demand for ceramic materialswith tooth-like colors10 and high biocompatibility9 is increas-ing at about 12% per year.11 Zirconia-based materials, one ofthe most commonly used ceramic materials, are increasinglyconsidered by the dental practitioner community due to its

optical and mechanical properties. Therefore, zirconia-basedimplants might have the potential to replace titaniumimplants in several clinical situations.12 Zirconia is a signifi-cantly proper material to peri-implant soft tissue appearanceand morphology than titanium. However, it is well known inthe literature that buccal soft tissue thickness is a confound-ing factor in the appearance.13–16

The main benefits of zirconia compared to titanium implantsinclude no metal aura in cases of deficiency of the buccal bonewall and/or thin mucosal biotype, corrosion resistance, andhypoallergenicity.17,18 The color of zirconia can vary dependingon the oxide content to mimic the color of natural teeth, whichis a significant advantage to overcome aesthetic issues oftitanium-based implant systems.19 Additionally, zirconia is a

Correspondence to: J. C. M. Souza; e-mail: jsouza@dem.uminho.ptContract grant sponsor: Conselho Nacional de Desenvolvimento Cient�ıfico e Tecnol�ogico (CNPq); contract grant number: PVE/CAPES/CNPq/

400254/2014–0

Contract grant sponsor: Fundac~ao para Ciencia e Tecnologia (FCT); contract grant number: NORTE-01–0145-FEDER-000018

VC 2018 WILEY PERIODICALS, INC. 1

biologically inert material possessing high biocompatibility thatcan provide osseointegration.2–4 Zirconia-based materials can beconverted into the shape of the tooth root and placed in thesocket immediately after extraction. Thus, strength, biocompati-bility, and tooth-like color of zirconia are desirable features toreplace titanium-based implants.20,21 Nevertheless, zirconia hassome limitations linked to its high elastic modulus (at about240–260 GPa), hardness (at about 1.2 GPa), and ultralow chemi-cal reactivity. The elastic modulus of zirconia is significantlyhigher than that recorded on cortical bone (10–20 GPa) that canresult in stress shielding and peri-implant bone loss. The highhardness and chemical resistance is a limitation for surfacemodification procedure involving conventional grit-blasting andetching procedures. Furthermore, surface modifications on zir-conia by grinding and/or gritblasting must be avoided as theprocess severely affects the fracture strength of thematerial.10,11,14–18

Reports on the concept of replacing teeth by using custom-made root-analogue implants started in 1969. Nonetheless, theself-curing and heat processed polymethacrylate used to fabri-cate the tooth-analogue was encapsulated by soft tissue. Lundg-ren reintroduced the idea of root-analogue implants in 1992.23

Root-analogue zirconia implants have been developed to rees-tablish physiological peri-implant conditions, stress distributionfrom occlusal loading, and aesthetics. The design of the custom-made implant can maintain the stress distribution pattern inthe surrounding bone, due to the design mimicking the alveolarregion.12 However, the stress concentration at the implant-boneregion is still higher than that recorded on teeth-bone due tothe lack of periodontal ligament. The benefits of the root-analogue implants include uncomplicated immediate implantplacement, decreased number of surgeries, and minimal inva-sive approach.2,3,21,23–25 The absence of a microgap betweenimplant and abutment seem to be of benefit since there is non-retentive region for bacterial adherence.8 From a biologicalpoint of view, zirconia has demonstrated a low affinity to bio-film accumulation, small amounts of inflammatory infiltrateand good soft tissue integration, that might reduce the risk forperi-implant diseases.8,26 Nevertheless, root-analogue implantshave some limitations, being restricted to cases of extraction ofperiodontally healthy tooth with appropriate deep socket,atraumatic extraction, sufficient bone support, and absence ofperiapical pathologies.21 Since they are manufactured as one-piece or single-component implants, the surgical placementmay not always meet the prosthodontic requirements to correctdefects on positioning. Additionally, single-piece implants areimmediately exposed to masticatory forces and tonguepressure.8,21,27

In fact, several aspects must be elucidated to better under-stand the clinical use of this new and challenging rehabilita-tion therapy. Therefore, the aim of this study was to conduct ascoping review on the potential benefits of the use of custom-made root-analogue zirconia implants.

MATERIALS AND METHODS

A PubMed and ScienceDirect bibliographical search was car-ried out from 1969 to 2017. The following search items were

explored: “zirconia” and “custom-made” and “dental implants,”zirconia and “root-analogue” and “dental implants”, “zirconia”and “anatomical” and “dental implants”, “zirconia” and “finiteelement” and “dental implants”, “zirconia” and “customized”and “dental implants”, “zirconia” and “mechanical properties”and “dental implants”, “zirconia” and “biomechanical” and“dental implants”. The eligibility inclusion criteria used for arti-cle search were: Meta-analysis; randomized controlled trials;prospective cohort studies; and retrospective cohort studies.

The literature selection accepted the following tests:Microbiological assays; physicochemical characterization;biomechanics by analytical finite elements tests; topographyby scanning electron microscopy (SEM) and atomic forcemicroscopy; stereoscopic pictures analysis; surface chemis-try characterization by X-ray techniques; histomorphometricanalysis; push-in tests; removal torque testing; light micros-copy computer-assisted analysis; ultrasonic wave characteri-zation; spectroscopy analysis; transmission electronmicroscopy analysis; resonance frequency analysis; electro-chemical and wear tests; and in vivo studies performed inanimals or humans under radiographic evaluation.

The title and abstract of the identified articles weregiven a preliminary evaluation to establish whether theymet the inclusion criteria. This evaluation was carried outby two researchers independently, following which a jointdiscussion was scheduled to select the relevant articles.Then, selected articles were entirely read and analyzed con-sidering the purpose of the study.

RESULTS AND DISCUSSION

A total of 611 papers were retrieved by the search on theelectronic database. After reading the title and abstracts, 81were selected for full-test reading and then 59 were evalu-ated as relevant to the purpose of the present study. Theselection of studies is illustrated in Figure 1. The presentstudy assessed the relevant articles published in literatureon the biomechanical and biological benefits of custom-made zirconia implants. The results found in the selectedarticles showed significant enhancement in the stress distri-bution through custom-made root-analogue implants associ-ated to desirable aesthetic outcomes and low risks of peri-implantitis induced by bacterial accumulation. Basically, agood fit between an implant and the bone provided by aroot-analogue implant will raise implant dentistry to a newlevel of truly anatomic implants. Within the selected articles,significant factors highlighted on custom-made root-ana-logue implants are shown in Tables I and II. Key factorslinked to the placement, manufacturing, mechanical behav-ior, and surface conditions are discussed as follow.

Immediate custom-made root-analogue implantsAfter tooth removal, there is an alveolar bone resorptionthat results in loss of bone (buccal plate) and consequentlyloss of the soft tissue contours. This often leads to two dis-tinct complications: first, the use of prostheses supported ornot by implants results in an aesthetic issue due to unde-sired changes in soft tissue morphology; second, the bone

2 PESSANHA-ANDRADE ET AL. ROOT-ANALOGUE ZIRCONIA IMPLANTS

volume can decrease as such that the possibility to place anendosseous implant becomes compromised. Thus, it is cru-cial to preserve the alveolar process dimensions in extrac-tion sites.28 The major advantages of immediate implantplacement are the decrease in treatment time with fewersurgical interventions, overall cost reduction, alveolar boneresorption, and soft-tissue regression due to early functionalload (Table I).29,30 Nevertheless, immediate implant place-ment does not preclude physiological bone remodelingactivity after tooth extraction.31

The tooth extraction site can be slightly larger than thecustom-made implant diameter, resulting in gap between boneand implant in certain regions. The misfit in implant-bone con-tact can be related to crestal bone fracture during the toothextraction as well as on the custom-made surface design toachieve primary stability. This misfit with the extraction socket

requires the use of biomaterials for bone augmentation to pre-vent down-growth of connective tissue or epithelium betweenthe implant and socket.32 Botticelli et al.33 inspected the tissuehealing occurred adjacent to implants placed in bone siteswith a wide marginal defect. In these cases, the new bone for-mation in the test sites resulted not only in the elimination ofthe gap but also in the establishment of a high degree ofbone-to-implant contact (BIC) or osseointegration. Forinstance, the amount of mineralized bone found in the testsites (70.3–75.6%) was similar to that found in the controlsites (74.1%), although the quality of the bone filling the gapwas distinctly different. In the test site within the gap, mostthe bone grown in the test site was immature. Accordingly, agap of 0.5 mm between the bone and the implant candecrease the BIC success rate. Thus, root-shaped implantdesigns are proposed to reduce the gap between implant

FIGURE 1. Search strategy used in this study.

REVIEW ARTICLE

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH B: APPLIED BIOMATERIALS | MONTH 2018 VOL 00B, ISSUE 00 3

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4 PESSANHA-ANDRADE ET AL. ROOT-ANALOGUE ZIRCONIA IMPLANTS

surface and socket walls leading to a prevention of bone loss.Nevertheless, custom-made root formed implants occupyingmost of the extraction socket promoted more pronouncedalveolar bone resorption than standard dental implants in apreclinical study in dogs.34

Kohal et al.35 showed the increase in coronal region vol-ume of the implant neck compensates the loss of peri-implant bone. Additionally, a root-analogue implant canovercome the misfit between implant diameter and alveolarextraction site once the surface design could be improved toachieve the primary stability. Lundgren et al.23 carried out astudy to characterize and evaluate the osseointegration ofroot-analogue implants. In that case, 88% root-analogueimplants were healed-in by contact between bone andimplant with a high degree of predictability. The studyreported that the misfit (gaps) between the bony walls ofthe tooth socket and the root-analogue implants should beavoided. Therefore, a good curettage of the remaining peri-odontal ligament should be done, in order to secureosseointegration of the analogue implant. Gaps located inthe marginal area, lead to the possibility of down-growth ofsupracrestal connective tissue.

Pirker and Kocher2 reported successful clinical use of amodified root-analogue zirconia implant for immediate sin-gle tooth replacement. A right maxillary premolar in 64-year-old patient was removed and a custom-made root-ana-logue roughened zirconia implant with macroretentions wasproduced and placed into the extraction socket 4 days later.The authors concluded that a good fit between implant andhost bed by additional retentions was an important factorto decrease bone resorption. Later, Pirker and Kocher3

reported two novel approaches for dental root replacementin humans and evaluated the use of zirconia root-analogueimplants prospectively in 18 patients. The clinical trial indi-cated that immediate implantation of a root-analogue replicaallows instantaneous support of soft tissue and limited func-tional load, resulting in socket preservation with minimalbone loss. The authors confirmed the need of macroreten-tions and implant diameter reduction next to the corticalbone to provide primary stability and excellent osseointe-gration of immediate root-analogue zirconia implant.

Pirker et al.4 described a successful immediate replace-ment of a two-rooted tooth with an individualized two-rooted zirconia implant. A 50-year-old female with chronicalapical periodontitits of the left mandibulary first molar wasextracted and substituted by a root-analogue implant. Theauthors reported several advantages of this type of implant.The similar topography of the extracted tooth root elimi-nates the need for conventional bone drills and other trau-matic preparatory procedures for implantation. Zirconiashowed high biocompatibility and strength required to be asuitable material for dental implants. The zirconia brittle-ness is not a major problem in dental root-analogueimplants since they are wide based with a diameter wellabove 3 mm. In addition, the design mimicking the previoustooth can be achieved by Computer Aided Designing (CAD)as illustrated in Figure F22.T

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:89

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Bone and soft-tissue trauma are reduced by adaptingthe root to the extraction socket instead of adapting thebone to a preformed standardized implant.3

Manufacturing and placement of root-analogue implantThe manufacturing of custom-made root-analogue implantscan be achieved by Computer Aided Design/Computer AidedManufacturing technique (CAD/CAM)19 as well as on addi-tive manufacturing technology combined with cone beamcomputed tomography focusing on the tooth as illustratedin Figure 2. Currently, the widely applied system for fabrica-tion of three-dimensional (3D) zirconia parts is based oncomputer numerical control milling of an unsintered whitemonoblock and subsequent firing into a sintered high-strength ceramic. Three-dimensional-printing technology isfeasible to fabricate root-analogue zirconia implants leadingto a production free of cracks and pores. Also, that allowscustomization without the use of molds required for ordi-nary milling technique.25,36,37 The manufacturing of custom-made root-analogue implants has been reported before andafter extraction. On both cases, the root-form of the extrac-tion surgical site is preserved by minimizing trauma, leadingto a faster healing of the surrounding bone.32,38

In the cases that the fabrication of the custom-maderoot-analogue implant happens after extraction, the surgical

site is cleaned by curettage followed by sterile physiologicalsaline solution and then an iodonform-soacked cottomgauze is placed in the wound. The root can be laser-scanned after extraction and then, the root-analogue implantis produced by CAD/CAM.2 The root-analogue implant iscleaned in an ultrasonic bath containing 96% ethanol for 10min, packaged and sterilized in a steam sterilizer beforeplacement in the surgical site.2 After 1–8 days of the toothextraction, the iodoform cotton gauze is removed, and thealveolar socket curetted and flushed with sterile physiologicsaline solution.2–4,32,39 The custom-made root-analogueimplant may be placed into the socket by using finger pres-sure, following gentle tapping with a hammer and a mallet.Palpation and percussion is used to check primarystability.2–4,32,39

In the cases of manufacturing of custom-made root-ana-log implant before tooth extraction, a computed tomographyscan is obtained from the patient dentition. Such informa-tion is enough to provide a CAD model of the teeth whichare going to be extracted.12,40,41 In this case, the model isthen used to produce a root-mimicking design along withtwo main design features: functionally graded porosity andadvanced abutment design.12 Finally, the CAD model is usedto produce the implant by CAM or by electron beam meltingor by other additive manufacturing technique. The implant

FIGURE 2. Design and clinical application of a zirconia root-analogue dental implant. Courtesy of Dr. Wolfgang Pirker, Face your face Handels

Ges.m.b.h., www.bioimplant.at (Vienna, Austria).

6 PESSANHA-ANDRADE ET AL. ROOT-ANALOGUE ZIRCONIA IMPLANTS

undergoes post-manufacturing processing steps beforebeing sent to the dental office.40,41 Thus, on a clinical pointof view, the implantation is accomplished in one dental visit.The implant can be ready upon the initial visit of the patientwhere the dentist can carefully remove the damaged toothand insert the implant with minimal to non-surgical sitepreparation.12

Regarding the additive manufacturing technique, bulkproperties of custom-made root-analogue implants can betailored to improve stress distribution decreasing peri-implant bone loss, and to enhance osseointegration.42–45 Forinstance, an implant structure can be produced within agradual transition in chemical composition from a compactzirconia at the inner regions of the implants toward to abioactive and softer material on the outer region.42–44 Thebioactive materials such as hydroxyapatite enhance thebone healing while adjust the overall stiffness/strength ofthe implant considering the cortical and trabecular peri-implant bone elastic modulus.42–44 In fact, custom-madeimplants produced by Functionally Graded Materials (FGM)approach can avoid the stress shielding phenomenon andstimulate a physiological remodeling activity of trabecularperi-implant bone.42–45 A combination of materials anddesign relies heavily on obtaining a good balance betweenthe implant stiffness/strength and bone remodeling activity.Additive manufacturing technologies can provide a desirablevariation in stiffness from the coronal to the apical implantregion ends for different base materials within differenttopographical aspects.

Given the wide variety of individual oral conditions andclinical situations, custom-made dental implants can over-come the limitations of the available standard designs andthe patient’s oral conditions. Furthermore, rehabilitationtime could be reduced, presenting a promising prospectivefor implant dentistry.37

Mechanical and biomechanical assessment of materialsfor dental implantsA success-rate of a dental implant is determined by severalaspects related to the implant, surgery, prosthetic, andpatient conditions. The type of material used is also a keyfactor to the implant osseointegration and clinical success-rate. Commercially pure titanium (cpTi) is the most com-mon material used in the last 20 years, but zirconia hasgrowingly become a potential material in implant den-tistry.46 Zirconia-based materials have appeared in dentistryfor metal-free structures due to an excellent biocompatibil-ity, improved esthetic results, high flexural strength, fracturetoughness, and high chemical resistance.3,4,47

The finite element method (FEM) has become anincreasingly useful tool for the prediction of the effects ofstress on implants and the surrounding bone.48,49 FEM pro-vides an accurate analytical model of a dental implantessential to produce realistic solutions using appropriateengineering software. FEM can simulate the stress distribu-tion around implants and determine a proper design to dis-sipate the stresses from occlusal forces.50 Bone tissue isknown to remodel its structure in response to applied

stress. Variations in the internal state of stress in bonedetermine whether constructive or destructive bone remod-eling takes place. On the one hand, low stress levels arounda dental implant may result in atrophy like the loss of alveo-lar crest after the removal of natural teeth. In contrast,abnormally high stress concentrations in the supporting tis-sues can result in patient discomfort, pressure necrosis, andthe eventual failure of the implant system.46

Choi et al.46 evaluated the biomechanical behavior ofTi6Al4V and PS-ZrO2 dental implants inserted into thehuman mandible during clenching using a 3D anatomicallyrealistic finite element model. Ti6Al4V and PS-ZrO2 dentalimplants were modeled as cylindrical structure with a diam-eter of 5.26 mm and length of 12 mm, and placed in thefirst molar region on the right hemimandible. On Ti6Al4Vdental implants, the maximum tensile stress, compressivestress and von Mises stress values recorded were at 11.02,12.39, and 11.37 MPa, respectively. On PS-ZrO2, the maxi-mum tensile stress, compressive stress, and von Misesstress values recorded were at 14, 15.3, and 14.2 MPa,respectively. The results revealed an increase of 2–3% inthe tensile and compressive stress mean values while vonMises stress increased in 8% in the bone-to-implant inter-face when PS-ZrO2 dental implant was used instead of Ti-6Al-4V dental implants (Table I).

Fuh et al.51 investigated the effects of different threaddesigns on the bone around Yttria stabilized Tetragonal Zir-conia Polycrystal (YTZP) and cpTi implants. A total of 18finite element models comprising two implant materials(YTZP or titanium), three thread designs, and three interfaceconditions were assessed considering the stress distributionon bone tissue. In the immediately loaded implant, thestress was highly concentrated at one site of the peri-implant bone. Also, zirconia implant can reduce the bonestress in the crestal cortical region (Table I).

In a study by Gungor and Yilmaz,16 the purpose was toevaluate the distribution of stress through YTZP and cpTiimplant-supported-prostheses located in the anterior maxil-lary region. Two different implants composed of YTZP andcpTi were assessed considering the support of a three-crown partial fixed dental prostheses composed of lithiumdissilicate or YTZP. The study concluded that stress distribu-tion was lower around YTZP implants than around cpTiunder horizontal loading although similar stress values forYTZP and cpTi were reported for oblique loading (Table I).

In a study performed by Himmlov�a et al.,52 a mathemati-cal simulation of stress distribution was used to determineinfluence of length and diameter to dissipate stress. A FEMwas carried out to simulate masticatory forces. The resultshighlighted a decrease in stress of 31.5% for implants diam-eter at 4.2 mm. In conclusion, an increased implant diame-ter decreased the maximum von Mises equivalent stressaround implant neck more than for an increase in theimplant length, due to a more favorable distribution of thesimulated masticatory forces.

Mobilio et al.14 compared the stress in bone around zirco-nia and titanium implants on loading. An one-piece YTZPimplant and a replica of the same implant composed of cpTi

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were embedded in two self-curing acrylic resin blocks. Loadsof 50, 100, and 150 N, with orientations of 30, 45, and 60degree to the implant axis were applied. Strain under all load-ing conditions on both samples was measured. 3D virtual rep-licas of both implants were reproduced using the FEM. Bothimplants revealed different biomechanical behavior. Titaniumimplant revealed higher stress values on the cortical bonewhile the YTZP implant showed higher stress values on thetrabecular bone. The stress magnitude were similar in bonefor both implants in all cases even if the stiffness of YTZP wastwice higher than that of titanium. On the mechanical point ofview, YTZP was thus a feasible substitute for dental implants(Table I). Only a few studies reported the stress distributionfor root-analogue zirconia implants.53 Considering the vari-able design of root-analogue implants, there are several limi-tations by using FEM to evaluate the influence of stressdistribution.54 The studies have to be targeted to random cus-tom shapes or limited to a case follow-up.

In a clinical study, Anssari Moin et al.54 selected a rightupper human canine in a 64-years-old individual to create a3D surface model of a root-analogue implant. Based on thestandard triangulation language, five different (targeted)press-fit design root were built: nonmodified standard, tar-get press-fit prism, targeted press-fit fins, targeted press-fitplug and targeted press-fit bulbs. Two different loadingswere applied to stimulate anterior masticatory forces. Thestress mean levels caused by oblique loading were higherwhen compared to vertical loading. The study concludedthat the optimization of standard root design, preferablyfins or bulbs, would have a positive effect on stress distribu-tion and lower stress concentration on peri-implant bone(Table I).

Previous studies have evaluated the effect of cyclic load-ing ranging from 10 to 200 N on one-piece zirconiaimplants in different simulated oral conditions.55–57 Thecompressive strength of one-piece zirconia implants wasassessed after fatigue testing at 50,000 cyclic loading on600 N.55 Failure mode of one-piece zirconia implants wascharacterized by crack initiation mainly at the tensile bend-ing side55 as expected considering the ceramic nature of thespecimens. A previous study has reported an adverse effectof abutment preparation on the strength of one-piece zirco-nia implants after fatigue testing at 1 million cyclic loadingfrom 10 to 200 N.56 Fracture was not detected after fatiguetesting in such conditions. However, the strength of one-piece zirconia implants decreased from about 1164 N downto 953 N after abutment preparation.56 Another studyreported the decrease of strength of one-piece zirconiaimplants after abutment chamfer preparation and increasingloading cycles from 1.2 to 5 million at 98 N.57 The meanstrength of one-piece zirconia implants after 5 million cycleswas lower (at 1364 N) than that (2044 N) recorded after1.2 million cycles. Therefore, the mean strength of one-piecezirconia implants was negatively affected after abutmentpreparation revealing values at 967 N for 1.2 cyclic loadingand 884 N for 5 million cycles.57 In fact, the abutment prep-aration is often required to enhance prosthetic crown adhe-sion although that procedure can negatively affect the

mechanical behavior of zirconia structures. Such findingsalso revealed that the loading magnitude determine thefracture of zirconia implants since the risk of fracture is lowon occlusal loading at anterior teeth regions.

Zirconia surfaces for custom-made root-analogue dentalimplantsA main factor that strongly influences wound healing at theimplantation site is the morphology of the implant surface,which subsequently affects osseointegration.58 Several chemi-cal and physical methods have been used to modify implantsurfaces considering increase in roughness, wettability, andbioactivity.19,59 Indeed, studies have shown that implantswith rough surface have a higher resistance to removal torquewhen compared to smooth surface implants.12 Smooth sur-face implants are not generally used since such implantsreveal a lower contact area of interaction with bone tissue.60

Consequently, the clinical use of YTZP dental implants is lim-ited due to the manufacturing process of YTZP structuresinvolving a morphological enhancement of the surface asfound on titanium implants.61

There are two methods that can be used for modifyingsurface texture, which are classified as ablative or additive.Ablative methods consist in removing material from the sur-face while additive methods involve addition of materialonto the surface. Two ordinary methods are currently usedfor ablating titanium-based implants, namely grit blastingand acid etching.62,63 Additive techniques on titaniumimplants involve physical vapor deposition or electrochemi-cal methods to chemically modify the surface in a specificenvironment containing bioactive materials. Hanawa62 hasreported the functionalization of titanium surfaces by usingceramics or polymeric materials.

In a study performed by Langhoff et al., two mainapproaches for surface modifications on titanium and zirco-nia were assessed.64 At first, microroughness was achievedby gritblasting and acid-etching and, then bioactive coatingcomposed of calcium phosphate, bisphonate, and type-1 col-lagen were applied on the rough surface. Six different typesof dental implants within a core based on YTZP or titaniumwere tested for osseointegration. After two weeks, zirconia-based implants showed a higher BIC (77%) compared totitanium-based implants (57–61%) as seen in Table II. Anincrease in BIC was detected on the pharmacologically andchemically modified titanium implants.64

In a previous study, twelve custom-made titanium (con-trol group) and twelve custom-made YTZP implants (testgroup) were placed in the extraction sites in six monkeysafter five months of tooth extraction.65 Titanium implantsurfaces were grit-blasted with Al2O3 and then acid-etchedwith H2O2/HF. YTZP implants were only grit-blasted sinceacid etching cannot modify YTZP. After 5 months of loading,BIC was recorded around 72.9% for titanium and 67.4% forYTZP implants (Table II). In conclusion, the custom-madezirconia implants osseointegrated to the same extent ascustom-made titanium control implants.

A previous study investigated the modification of tita-nium and YTZP surfaces by an ablative method and

8 PESSANHA-ANDRADE ET AL. ROOT-ANALOGUE ZIRCONIA IMPLANTS

TA

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JOURNAL OF BIOMEDICAL MATERIALS RESEARCH B: APPLIED BIOMATERIALS | MONTH 2018 VOL 00B, ISSUE 00 9

TA

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10 PESSANHA-ANDRADE ET AL. ROOT-ANALOGUE ZIRCONIA IMPLANTS

compared the BIC percentage to that on titanium.20 A totalof 24 YTZP and 24 titanium implants were treated by acid-etching procedure and placed into the tibia of 12 G€ottingenmini pigs. BIC was analyzed by SEM after 12 weeks. Onboth groups, a successful osseointegration was found, andsimilar BIC values were noted for YTZP and titaniumimplants (Table II). Moreover, no interposition of an interfa-cial connective tissue layer or foreign body reaction wasdetected in any specimen. Chahine et al. also compared theendosseous healing between YTZP and titanium implantsregarding roughness.12 On histomorphometric evaluation,there was an increase in BIC throughout the assessmentperiod for both YTZP and titanium implants. After 1 weekof healing, BIC percentages were recorded at 35.5610.8%for YTZP and 47.76 9.1% for titanium implants. After 4weeks in situ, BIC percentages for YTZP implants increasedto 45.3615.7% while 58.669.5% were recorded for tita-nium implants. After 12 weeks, BIC percentages increasedfor YTZP (71.467.8%) and titanium surfaces (82.9610.7%),as seen in Table II. Those BIC results are quite similar to theresults recorded in another study in minipigs.66

Han et al. evaluated the biomechanical and histologicalaspects of a ceria-stabilized zirconia-alumina nanocomposite(NanoZr) in comparison with that of YTZP in Sprague-Dawley rats.67 The average BIC percentage within the bonemarrow area for YTZP was at 25.26% while NanoZrOshowed a BIC at 31.51% after 2 weeks. After 4 weeks, BICwas at 38% for YTZP while 46.78% was recorded for Nano-ZrO. Finally, BIC was recorded at 47.88% for YTZP and at56.81% for NanoZrO after 8 weeks. On cortical bone, themean BIC percentage values were 38.86 and 58.42% for 2weeks, 66.82 and 57.74% for 4 weeks, and 79.91 and78.97% for 8 weeks, for YTZP and NanoZrO, respectively.Animal studies have shown that YTZP and pure titaniumhave similar BIC although the morphological aspects of thesurfaces play a significant role on the osseointegration pro-cess (Table II).

Two rough surfaces of titanium and YTZP were alsoassessed after placement in Sprague-Dawley rat femurmodel.68 Four groups of implants were tested: machinedYTZP (m-YTZP), rough YTZP (r-YTZP), machined titanium(m-Ti), and electrochemically roughned titanium (TiUnite)surfaces. For 14 days of bone healing, the BIC percentageswere at 23.2% for m-Ti, 30.9% for m-YTZP, 36.4% for TiUn-ite and 45.3% for r-YTZP. After 28 days, the BIC percentagesincreased for m-Ti at 39.4%, 46.6% on m-YTZP, 55.2% onTiUnite and 59.4% on r-YTZP. No significant differencescould be found within the groups after 28 days of healing(Table II).

Another previous study reported the osseointegration offusio-sputtered YTZP implants in comparison with titaniumimplants in a biomechanical and histomorphometric study.69

After 4 weeks of healing process, fusion-sputtered YTZPimplants demonstrated significantly higher BIC compared tothose of both titanium and control YTZP implants. Themean BIC percentage recorded on fusion-sputtered YTZPwas at 69.66% when compared to 62.83% for titanium and56.94% for YTZP implants. After 8 weeks, the BIC

percentages for fusio-sputtered YTZP implants remainedstatically higher than that for titanium and YTZP implants.After 12 weeks, both fusion-sputtered and titanium implantsdemonstrated comparable BIC percentages (Table II).

A good number of studies confirmed the osseointegra-tion of zirconia to be like or even better than titanium. Theresults show biological benefits in using zirconia implantsfor custom-made root-analogue implants. Such studies alsoshowed that zirconia performance can be highly dependenton surface preparation and every new surface modificationshould be tested regarding aging and fatigue before clinicaluse.47,70,71 Roughening the surface of machined zirconiaimplants enhances bone apposition and enhances the capa-bility to withstand shear stress. On balance, the osseointe-gration of zirconia implants is promising consideringaddictive manufacturing techniques.47 Such developmenthas been introduced in YTZP implants ranging from amicro- to a nanoscale level to enhance osseointegration.

CONCLUSIONS

The relevant articles analyzed in the present literaturereview reported significant findings on custom-made root-analogue zirconia implants. Within the literature review, themain outcomes from the studies on root-analogue zirconiaimplants can be drawn as follow:

� The time of dental implant placement is determinant onthe alveolar bone remodeling. Immediate root-analogueimplant might prevent a loss of alveolar bone volumewith maintenance of peri-implant soft tissues leading toan improved esthetic and functional prosthetic result;

� Zirconia-based materials have shown increased interestto replace titanium-based structures considering opticaland biological properties. The optical properties ofzirconia-based structures can mimic the color and trans-lucence while their chemical composition enhances thebiocompatibility to soft and bone tissues;

� Mechanical properties of zirconia-based materials areevaluated by in vitro tests to predict the long-termstrength of implant-supported prostheses. Regarding dif-ferent clinical conditions, finite element analysis can alsobe used to assess the biomechanics of custom-madeimplants related to several aspects such as materials,design, loading, and maxillofacial positioning of implants.Root-analogue implants could promote a proper distribu-tion of stresses trough the materials towards to the bone,what might decrease the early peri-implant bone loss;

� The design of the custom-made implant can maintain thestress distribution in the surrounding bone, due to thegeometrical mimicking of the alveolar region. Thecustom-made design can decrease the bone resorptiondue to stress shielding associated to peri-implant inflam-matory reactions. However, the stress distribution magni-tude is still high for peri-implant bone due to the lack ofperiodontal ligament;

� zirconia-based surfaces are currently studied to achievemorphological features that enhance the adsorption of

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JOURNAL OF BIOMEDICAL MATERIALS RESEARCH B: APPLIED BIOMATERIALS | MONTH 2018 VOL 00B, ISSUE 00 11

proteins and then the migration of osteogenic cells. Themajor concern is based on the high chemical resistance ofzirconia-based surfaces, which cannot be modified byusual acid etching procedures. It should be highlightedthat the technological development of zirconia-basedmaterials can promote novel ways to modify root-analogue implant surfaces leading to an enhancement ofosseointegration. Further studies should be performed ondifferent modification techniques of root-analogue zirco-nia implants to validate the percentage of osseointegra-tion along the time of bone healing.

CONFLICT OF INTEREST

No conflict of interest.

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