Coating of Sandblasted and Acid-EtchedDental Implants With Tantalum Using

Vacuum Plasma SprayingXian Zhou, MD,* Xiulian Hu, DMD,† and Ye Lin, DMD‡

Although dental implants havebecome common prostheticsfor missing teeth, the need for

improved success rates and less com-plications has received increasingattention. Titanium (Ti) and its alloysare widely used as biomaterials in den-tistry because of their excellent bio-compatibility. However, the inherentsurface bioinertness of Ti and its alloyshinders osseointegration between theimplant and surrounding tissues.1 Var-ious methods including grit blasting,acid etching, coating, and electro-chemical anodization as well as bioac-tive methods have been developed tomodify the surface of Ti dental im-plants.2–6 However, improved osseoin-tegration is difficult to achieve. Evenbioactive surface modifications do notalways result in improved osseointe-gration. In addition, inflammation ofsoft and hard tissues around theimplant caused by oral microflora per-sists. Periimplantitis is considered oneof the main risk factors affecting thelong-term survival rate of implants.Similar to periodontitis, this condition

is commonly caused by the attachmentof bacterial plaque and a hostresponse, resulting in bone resorptionand even implant loss. A recent reviewsummarized strategies for improvingthe antimicrobial functionalization ofdental implants.7 However, most ofthese strategies have only been testedin vitro under static conditions. Theexploration of novel types of dentalimplants with excellent osteogeneticand antibacterial properties is of cur-rent interest.

Tantalum (Ta) is a rare metal thathas attracted increased attention inimplant dentistry because of its highcorrosion resistance and excellent bio-compatibility. A bacterial adhesionassay revealed lower bacterial adhesion

of pure Ta compared with commonlyused biomaterials in orthopedic im-plants.8 The antibacterial activity ofTa has also been reported in other stud-ies.9,10 Moreover, previous studieshave demonstrated the excellent biolog-ical performance of Ta with a porousstructure11–14 and even its predictabledental treatment outcomes.15–18 Fordecades, Ta has been applied in ortho-pedics as a load-bearing metal19–21;however, the difficulty in manipulatingsolid Ta has limited its application. At-tempts have been made to develop anopen-cell porous biomaterial witha structure similar to trabecular bone,known as porous Ta trabecular metal(PTTM).22 This material is fabricatedon a foam-like vitreous carbon scaffold

*Master Degree Candidate, Department of Oral and MaxillofacialSurgery, Peking University School and Hospital of Stomatology,Beijing, China.†Associate Professor, Department of Oral Implantology, PekingUniversity School and Hospital of Stomatology, Beijing, China.‡Professor, Department of Oral Implantology, Peking UniversitySchool and Hospital of Stomatology, Beijing, China.

Reprint requests and correspondence to: Xiulian Hu,DMD, Department of Oral Implantology, PekingUniversity School and Hospital of Stomatology, No. 22Zhongguancun South Avenue, Haidian District, Beijing100081, China, Phone: 86-10- 82195344, Fax: 86-10-62173402, E-mail: Sunhu33@163.com

ISSN 1056-6163/18/02702-202Implant DentistryVolume 27 � Number 2Copyright © 2018 Wolters Kluwer Health, Inc. All rightsreserved.

DOI: 10.1097/ID.0000000000000727

Purpose: The objective was toprepare tantalum (Ta)-coated sand-blasted and acid-etched (SLA) dentalimplants using vacuum plasmaspraying (VPS) and to analyze theirmorphologies.

Materials and Methods: TwelveSLA implants were coated with Tausing VPS. The topographies of thecoatings and Ta/SLA surface interfaceswere examined using scanning electronmicroscopy. The thickness at 4 loca-tions for 6 Ta-coated and 6 uncoatedSLA implants and pore sizes of theneck, central, and root areas of Ta-coated implants were measured. SPSSv20.0 was used for statistical analysis.

Results: The Ta coatings wererough and consisted of pitted struc-

tures with various pore sizes; nocracks were observed. The Ta/SLAsurface interface was tightly bonded.The 95% confidence interval of theTa coating thickness was (114.0759,129.3574). The maximal pore diam-eter was concentrated at 200 to 400nm.

Conclusion: SLA dental im-plants were successfully coatedwith Ta using VPS. The nanopo-rous structure of these implantsmay facilitate osseointegrationcompared with uncoated SLAimplants. (Implant Dent 2018;27:202–208)Key Words: morphology, coatingthickness, bonding strength, scan-ning electron microscopy

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using chemical vapor diffusion withhydrogen and chlorine gases.23

PTTM-enhanced Ti dental implantshave been designed and intro-duced15,16,24 with PTTM material inthe middle part of the implant, such asthe Tapered Screw-Vent Implant pro-duced by Zimmer Dental Inc., Carls-bad, CA. Its trabecular bone structureprovides an enhanced surface for os-seointegration. However, little isknown about its biological reactions inthe complex oral environment, and therelatively high cost and production dif-ficulties limit its wide application.

Vacuum plasma spraying (VPS) isa relatively economical method forfabricating porous coatings in an atmo-sphere with very high melting temper-atures. Studies have demonstrated thatthe preparation of porous metal struc-tures usingVPS is a promising strategyfor bone regeneration. Tang et al25

successfully fabricated porous Tacoatings on Ti substrates using VPS.These porous Ta coatings enhancedthe osteogenic differentiation andbone regeneration of bone marrowstromal cells. A hydroxyapatite–silver coating on Ti implants was alsoprepared using this technique.26 Thesefindings indicate that VPS may beapplicable for the fabrication of porousTa coatings for dental implants.

In this study, we aimed to prepareporous Ta coatings on sandblasted andacid-etched (SLA) dental implantsusing VPS. The initially rough surfaceof the SLA implants was expected toenhance the bonding strength of theinterface. Experiments were performedto investigate the morphological char-acteristics of the Ta-coated SLA surfa-ces compared with uncoated SLAsamples. This part of study is to preparefor subsequently analyzing the effect ofthese coatings on osseointegration andantibacterial activity in the animalexperiment.

MATERIALS AND METHODS

In this study, SLA implants werecoated with Ta using VPS. The initialmicrostructures of the Ta coating sur-faces without histologic staining werecharacterized using scanning electronmicroscopy (SEM; Hitachi S-4800,Tokyo, Japan).

Sample Preparation

Materials. The SLA implants weredesigned and manufactured by WeihaiWEGO Jericom Biomaterials Co., Ltd.as substrates for the preparation of theTa-coated implants. The implants hadslightly tapered bodies with an outerdiameter of 4 mm and a length of 8 mm(Fig. 1).

Ta metal powders (purity 99.9%;Zhuzhou Jiabang RefractoryMetal Co.,Ltd., Zhuzhou, China) with particlesizes ranging from 5 to 15 mm wereused.

Experimental EquipmentA VPS system (Sulzer Metco,

Winterthur, Switzerland) was used forfabrication of the Ta coatings witha spraying distance of 270 mm. Table 1lists the spraying parameters used toprepare the Ta coatings. SEM (HitachiS-4800, Tokyo, Japan) was used toexamine themorphology and determinethe thickness of the Ta coatings.

SEM ExaminationThe initial surfaces of the Ta-SLA

implants were examined using SEM atvarying magnifications. Sites were ran-domly selected from the neck, central,and root areas of the Ta-coated samplesto determine the morphological differ-ences of the different regions.

Twelve samples (6 Ta-coated and 6uncoated SLA implants) were selectedto determine the thickness of the Tacoating layer. Each sample was verti-cally fixed on the workbench as far aspossible with conductive adhesive tape.However, the thickness of Ta coatingscould not be directly determined usingSEM because the interface between theTa coating and SLA surface could notbe clearly distinguished. To overcomethis issue, a reference line was selected,as shown in Figure 2 for both the Ta-coated and uncoated SLA implants.Distance from the surface margin tothe reference line of the cross sectionin the neck area was measured as thethickness. The thickness was measuredat 4 sites for each sample (Fig. 3). These2 sets of data were statistically ana-lyzed, and the difference of these valueswas the thickness of Ta coatings.

Another 6 Ta-coated samples wereselected, and 3 parts were classified foreach sample, including the neck, cen-tral, and root area (Fig. 4, A–C). Aswell, 6 uncoated SLA implants wereserved as control. These samples werehorizontally fixed on the workbenchwith conductive adhesive tape.

For each part, 4 sites were exam-ined using SEM at 310.0k magnifica-tion. Because of the pitted structures onthe surface, some pores buried in theseareas were not completely visible.Therefore, the maximum diameter ofthese pores was difficult to measure.In the same visible field of each site,the sizes of the clearly exposed poreswere measured.

Fig. 1. Photograph of SLA implant before Tacoating. The implants had slightly taperedbodies with an outer diameter of 4 mm anda length of 8 mm. The surface was obtainedafter SLA treatments. They were designedand manufactured as substrates for thepreparation of Ta-coated implants.

Table 1. Vacuum Plasma SprayingParameters for Preparation of TaCoatings

Spray ParametersNumericalValues

Ar gas flow rate (slpm) 40H2 gas flow rate (slpm) 10Spray distance (mm) 270Pressure (Pa) 104

Powder carrier gas (Ar)flow rate (slpm)

2.0

Powder feed rate (g/min) 20Current (A) 650Voltage (V) 60

Ta-coated surfaces were fabricated with a spraying distance of270 mm. Ta powder feed rate was 20 g/min. This sprayingprocess was completed in a vacuum chamber with a low-vacuum pressure of 104 Pa.

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Statistical AnalysisThe data obtained were analyzed

using statistical software (SPSS v20.0;IBM, New York, NY). Paired sample t

tests were performed to compare thedifferent thicknesses of the differentsites. A P value below 0.05 was consid-ered statistically significant. Indepen-dent sample t tests were performed todetermine the 95% confidence interval(CI) of the difference of the populationmean for the thickness of the Ta-coatedand uncoated SLA implants. A box plotwas made to evaluate the distributionrange of the pore size for each part(neck, central, and root).

RESULTS

Ta coating implants were success-fully prepared using VPS (Fig. 5), andtheir microstructure, Ta coating thick-ness, and pore size were evaluated.

Morphology of Ta Coating SurfacesThe SEM images in Figure 6, A–I

show the initial surface morphology ofa porous Ta coating prepared usingVPS with a spraying distance of270 mm. Similar to the surface ofuncoated SLA implants (Fig. 7, A andB), the surface consisted of a rich pitted

structure and pores of different sizes. Novisible surface cracks were observed onthe deposited layer (Fig. 8).Moreover, noobvious cracks were observed for thecross section of the Ta-coated sampleswith mechanical cutting. The interfacebetween the Ta coating and SLA surfacewas tightly bonded.

Ta Coating ThicknessThe thickness at each site was mea-

sured for the Ta-coated and uncoatedSLA samples (Table 2). The statisticalanalysis indicated that these 2 sets of datawere characterized by normal distribu-tions (Table 3). The results of the pairedsample t tests revealed no significant dif-ference between the groups (P . 0.05)(Table 4). These 2 sets of data were alsoanalyzedwith independent sample t tests.The results indicated that the 95% CI ofthedifferenceof thepopulationmeanwas(114.0759, 129.3574), which was re-garded as the 95% CI of the Ta coatingthickness.

Pore SizeThe pore sizes of Ta coatings and

SLA surfaces were determined usingSEM at310.0k magnification (Fig. 9).Themaximum diameter of pores on Ta-coated surface was on the nanoscale.The SEM images revealed that the dis-tribution of the diameter of poresranged from 40 to 857 nm, which wasconcentrated at 200 to 400 nm. In addi-tion, the minimum diameter of smallerpores below 40 nm was not distinctlyvisible for measurement at 310.0k

Fig. 5. Photograph of the Ta-coated SLAimplant. Ta coatings were successfully pre-pared using VPS. The thread-form structureof the implant surface was still clearly visible.

Fig. 4. Three parts (A: neck; B: central; C: root) were classified for measurement of pore sizesof SLA and Ta-coated samples. (A) It is the neck part of the Ta-coated implant. (B) It is thecentral part of the Ta-coated implant. (C) It is the root part of Ta-coated implant. For each part,4 sites were randomly selected and pore sizes of each site were examined using SEM at310.0k magnification.

Fig. 2. Black arrow showed the reference line for thickness measurement. Interface between SLAlayer and pure Ti substrate was selected as the reference line. (A) It showed the reference line foruncoated SLA implants. (B) It showed the reference line for Ta-coated SLA implants. Distance fromthe surface margin to the reference line of the cross section in the neck area was measured as thethickness.

Fig. 3. Schematic diagram showed theselected 4 sites (A–D) for thickness measure-ment. They were selected equally from thecircular cross section in the neck area of boththe Ta-coated and uncoated SLA samples.Then the data obtained were subtracted todetermine the thickness of Ta coating.

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Fig. 7. Initial morphological characteristics of the uncoated SLA implants. (A) A multilevelmicroporous structure could be observed at 31.00k magnification. The SLA surface was richof various size pores. (B) The pore diameters were examined at 32.00k magnification. Themaximum pore size ranged from 20 to 60 mm and the minimum pore size was from 2 to 6 mm.So the micron-sized Ta particles (5–15 mm) could crystallize between these multiple holes.

Fig. 8. Surface morphology of Ta coatings at3500 magnification. No visible cracks wereobserved on the Ta-coated SLA surface. Thisindicated a compact structure of Ta coatingsurface.

Fig. 6. Initial morphological characteristics of Ta coatings at varying magnifications (without histologic staining) were showed. (A) The SLA surface wasfully covered by Ta coatings and implant threads were clear (3100 magnification). (B) Ta-coated surface was dense and tight (3500k magnification). (C)No visible cracks were observed on the Ta-coated surface at 31.0k magnification. (D) SEM image showed circular pitted structure at 32.0k mag-nifiction. (E) The microstructure of pores was clearly observed (35.0k magnification). Some pores were buried in these pits. (F) Pores on the Ta-coatedsurface were in a circle-like shape. The maximum diameter of the pores was on the nanoscale (310.0k magnification). (G) It showed the porous structureof Ta coatings at 315.0k magnification. Pores of various sizes distributed on the Ta coatings and connected with each other. (H) SEM image showeda pit with a diameter of 50 mm (320.0k magnification). (I) The pit was nearly in a circular shape and surrounded by pores (330.0k magnification).

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magnification. As to the control group,a multilevel microporous structure ofuncoated SLA surface could beobserved. The maximum pore sizeranged from 20 to 60 mm, andthe minimum pore size ranged from 2to 6 mm.

DISCUSSION

Because of their favorablemechan-ical and biological properties, Ti and itsalloys are widely used in implant den-tistry. Factors including osseointegra-tion and the amount of bacterialcolonization around the implants affectthe final outcome of dental treatment.

To achieve an increased survival rate,numerous techniques have been intro-duced to improve the biological prop-erties of Ti implants. However,inflammation-induced implant loss re-mains challenging.27 In the study, Tawas selected for coating Ti implantsusing VPS. Previous studies have dem-onstrated the excellent bone ingrowthof porous Ta in orthopedics. Moreover,porous Ta even performed well fora revision surgery for an infected totalknee arthroplasty14 and promoted os-seointegration under diabetes condi-tions.28 However, little is known aboutits biological and antibacterial activitiesin the complex oral cavity. The chang-ing microbiota and pH values in salivarequire higher stability and antibacterialactivities for the implant surface. ThisTa-coated dental implant is expected tobe a new form of enhancement for boneformation and antibacterial properties.Moreover, compared with the high costof manufacturing PTTM,17 VPS isa much simpler and relatively econom-ical method. The experimental resultsindicate that Ta-coated implants withporous structures were successfullyfabricated using VPS (Fig. 5).

In the experiment, SLA dental im-plants were used as substrates withcellular morphological surfaces(Fig. 7). The micron-sized Ta particles(5–15 mm) crystallized between multi-ple holes (2–6 mm, 20–60 mm) on thesurface of the SLA implants. Reclaruet al29 observed that the porous mor-phology of coatings was affected by thecoating process parameters, resulting indifferent corrosion resistance. By ad-justing the experimental spray parame-ters, rough surfaces rich in pores ofdifferent sizeswere prepared.No crackswere observed on theTa coating surfaceor Ta coating/SLA surface interface.The tightly bonded interface indicatesa positive bonding strength between theTa coating and SLA surface, animportant factor for the long-term sta-bility of implants. To further examinethe porous structured surface, thethicknesses of the Ta coatings weredetermined. Statistical analysis re-vealed no significant differencebetween the groups, suggesting thata uniform coating thickness can be ob-tained using the spraying parameters

Table 3. Normality Test (Shapiro–Wilk) for Thickness Data

Samples N P

TaSLA 24 0.534SLA 24 0.621

N, sample size.Results of Shapiro–Wilk test indicated that these 2 sets of data (thickness of both Ta-coated and uncoated SLA samples)were characterized by normal distributions. So paired sample t test can be performed to compare the difference between these 2groups.

Table 4. Paired Sample t Test Was Used to Analyze the Thickness DifferenceBetween 2 Groups

Samples Pair N P

SLA SLAAdSLAB 5 0.877SLACdSLAD 5 0.325SLAAdSLAC 5 0.078SLABdSLAD 5 0.963

TaSLA TaSLAAdTaSLAB 5 0.282TaSLACdTaSLAD 5 0.143TaSLAAdTaSLAC 5 0.492TaSLABdTaSLAD 5 0.186

N, sample size.The results revealed no statistical significance (P . 0.05), suggesting that a uniform coating thickness can be obtained usingVPS.

Fig. 9. Measurement of the pore sizes of Ta-coated surface at 310.0k magnification.Microstructure of VPS Ta-coated surfacewas examined using SEM. Pore sizes werewith a concentrated distribution at 200 to400 nm on the nanoscale.

Table 2. Surface Thickness of Each Site for Ta-Coated and Uncoated SLA Samples

Samples

SLA (mm) TaSLA (mm)

A B C D A B C D

1 55.6 53.3 54.5 56.5 191 211 198 2062 54.5 51.5 54.8 53.2 194 183 173 1853 50.4 51.6 50.3 49.9 156 158 157 1594 51.3 53.1 50.6 49.8 155 161 144 1485 53.4 55.9 52.2 54.7 176 190 187 1806 49.8 50.5 47.3 52.1 168 169 159 170

The thickness was measured at each site for Ta-coated and uncoated SLA samples using SEM at 3500 magnification. These 2 setsof data were statistically analyzed and the difference of these values was the thickness of Ta coatings.

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applied in thiswork.However, thefixedangle of the samples may result inunavoidable errors. The relationshipbetween the thickness and bondingstrength lacks sufficient evidence.

Unlike the SLA surface containinga distribution of micron-sized pores, theTa coating surface was flatter and con-sisted of dense nanosized pores. Pre-vious studies revealed normalosteoblastic growth on the surface ofnanoporous alumina containing poreswith an average diameter of approxi-mately 160 nm.30 The pores on the Tacoating surface had similar diameterswith a concentrative distribution of200 to 400 nm. Although smaller poreswere present, theywere ignored becauseof the measurement difficulty. Alkali-heat-treating of Ti with a nanotopo-graphic surface has been reported toenhance gingival fibroblastic collagensynthesis and result in periodontal-likeconnective tissue attachment. More-over, thepreparationofananocrystallineTa surface resulted in superior mechan-ical properties and corrosion resistancecompared with coarse-grained Ta.31

Thus, this nanostructured porous Tacoating surface is expected to be bene-ficial for bone formation and is expectedto perform well in the oral cavity.

CONCLUSION

The feasibility of using VPS toprepare Ta coatings on SLA dentalimplants was demonstrated in thiswork. The nanoporous structure ofthese novel implants may be effectivein facilitating osseointegration com-pared with the structure of uncoatedSLA implants. Further studies areneeded to analyze the effect of theseTa coatings on biological reactions andantibacterial activity.

DISCLOSURE

The authors claim to have nofinancial interest, either directly orindirectly, in the products or informa-tion listed in the article.

APPROVAL

This article is Part 1. Animal studywill be reported in Part II afterward. The

experimental animal protocol has beenapproved by the Biomedical EthicsCommittee, Peking University. Theapproval number is LA2016313.

ROLES/CONTRIBUTIONS

BY AUTHORS

X. Zhou: Performed all experi-ments and contributed to manuscriptwriting. X. Hu: Contributed to experi-mental design. Y. Lin: Provided finan-cial support for this study.

ACKNOWLEDGMENTS

The work was supported by theNational Basic Research Programof China (973 Project) (No.2012CB933900). The authors wouldlike to thank Dr. Youtao Xie for hisassistance with VPS technology. Theauthors also thank Dr. Shenghui Cuiand Mr. Huiliang Zhang for theirassistance with SEM observation.The authors express gratitude to Dr.Wenjing Bai for her assistance withthe statistical analysis.

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