materials
Article Water-Airborne-Particle Abrasion as a Pre-Treatment to Improve Bioadhesion and Bond Strength of Glass–Ceramic Restorations: From In Vitro Study to 15-Year Survival Rate
Luan Mavriqi 1, Francesco Valente 2,3 , Bruna Sinjari 2,3 , Oriana Trubiani 2 , Sergio Caputi 2,3 and Tonino Traini 2,3,*
1 Department of Dentistry, Albanian University, 1001 Tirana, Albania; [email protected] 2 Department of Innovative Technologies in Medicine & Dentistry, University “G. d’Annunzio” of Chieti-Pescara, 66100 Chieti, Italy; [email protected] (F.V.); [email protected] (B.S.); [email protected] (O.T.); [email protected] (S.C.) 3 Electron Microscopy Laboratory, University “G. d’Annunzio” of Chieti-Pescara, 66100 Chieti, Italy * Correspondence: [email protected]; Tel.: +39-08713554143
Abstract: The purposes of this study were to evaluate the efficacy of water–airborne-particle abrasion (WAPA) as pre-etching procedure for tooth surfaces to increase bond strength, and to compare the survival rate of WAPA vs. non-WAPA glass–ceramic restorations with a 15-year follow-up. The occlusal surfaces of 20 human molars were sectioned and flattened. The prepared surfaces areas were subdivided into two parts: one received WAPA treatment (prophy jet handpiece with 50 µm Citation: Mavriqi, L.; Valente, F.; aluminium oxide particles) followed by acid etching (37% phosphoric acid for 20 s/3-step etch-and- Sinjari, B.; Trubiani, O.; Caputi, S.; rinse); the other one was only acid-etched. In total, 108 specimens were obtained from the teeth, of Traini, T. Water-Airborne-Particle which 80 were used to measure the micro-tensile bond strength (µTBS) in the WAPA (n = 40) and Abrasion as a Pre-Treatment to control (n = 40) groups, while the remaining specimens (n = 28) were investigated via SEM to evaluate Improve Bioadhesion and Bond the micromorphology and roughness (Ra) before and after the different treatment steps. The survival Strength of Glass–Ceramic rate (SR) was performed on 465 glass–ceramic restorations (131 patients) comparing WAPA treatment Restorations: From In Vitro Study to (n = 183) versus non-WAPA treatment (n = 282). The bond strength was 63.9 ± 7.7 MPa for the WAPA 15-Year Survival Rate. Materials 2021, group and 51.7 ± 10.8 MPa for the control group (p < 0.001). The Ra was 98 ± 24 µm for the enamel 14, 4966. https://doi.org/10.3390/ control group, 150 ± 35 µm for the enamel WAPA group, 102 ± 27 µm for the dentin control group ma14174966 and 160 ± 25 µm for the dentin WAPA group. The Ra increase from the WAPA procedure for enamel
Academic Editor: Roman Perez and dentin was statistically significant (p < 0.05). Under SEM, resin tags were present in both groups Antoñanzas although in the WAPA they appeared to be extended in a 3D arrangement. The SR of the WAPA group (11.4 years) was 94%, while the SR of the non-WAPA group (12.3 years) was 87.6% (p < 0.05). Received: 4 August 2021 The WAPA treatment using aluminium oxide particles followed by a 3-step etch-and-rinse adhesive Accepted: 27 August 2021 system significantly improved bioadhesion with an increased bond strength of 23.6% and provided Published: 31 August 2021 superior long-term clinical performance of glass–ceramic restorations.
Publisher’s Note: MDPI stays neutral Keywords: dental bonding; acid etching; air abrasion; microtensile bond strength; glass–ceramic with regard to jurisdictional claims in restorations published maps and institutional affil- iations.
1. Introduction Dental adhesive technology has had a great impact in direct and indirect restorative Copyright: © 2021 by the authors. procedures, opening the way to metal-free adhesion and minimally invasive dentistry [1]. Licensee MDPI, Basel, Switzerland. For direct restorations, all procedures are usually performed during the same appoint- This article is an open access article ment, whereas for indirect restorations a provisional phase is necessary. Delayed dentin distributed under the terms and sealing is traditionally performed for indirect restorations, so the dentin is sealed after conditions of the Creative Commons the provisional phase during the cementation appointment. Unfortunately, this technique Attribution (CC BY) license (https:// cannot provide optimal conditions for bonding procedures [2,3] due to tooth surface con- creativecommons.org/licenses/by/ tamination by provisional cement, bacteria and even impression material [4]. To overcome 4.0/).
Materials 2021, 14, 4966. https://doi.org/10.3390/ma14174966 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 16
Materials 2021, 14, 4966 2 of 15 nique cannot provide optimal conditions for bonding procedures [2,3] due to tooth surface contamination by provisional cement, bacteria and even impression material [4]. To over- thesecome inconveniencesthese inconveniences and to and improve to improve the bonding the bonding performance, performance, immediate immediate dentin sealing dentin (IDS)sealing was (IDS) introduced was introduced for adhesive for adhesive restorations restorations [4–10]. [4 The–10] IDS,. The consists IDS, consists of anadhesive of an ad- procedurehesive procedure performed performed immediately immediately after tooth after preparation. tooth preparation. This step This has step demonstrated has demon- improvedstrated improve adhesiond adhesion [7]. [7]. DespiteDespite thethe goodgood levellevel ofof adhesionadhesion achievedachieved forfor directdirect restorationsrestorations andand indirectindirect onesones withwith IDS,IDS, furtherfurther improvementsimprovements toto dentindentin bondingbonding areare desirabledesirable becausebecause thethe finalfinal strengthstrength ofof thethe tooth-restorationtooth-restoration complexcomplex isis highlyhighly dependentdependent onon adhesiveadhesive proceduresprocedures [ 6[6]].. ForFor aa physicalphysical phenomenon,phenomenon, aa rougherrougher dentaldental surfacesurface maymay increaseincrease thethe adhesionadhesion ofof aa restorationrestoration becausebecause itit createscreates aa moremore extendedextended tooth–adhesivetooth–adhesive interfaceinterface [[11]11].. AnAn intuitiveintuitive methodmethod to achieve this could could be be tooth tooth surface surface sandblasting sandblasting [12,13] [12,13. ].Intraoral Intraoral sandblasting sandblast- ingwith with alumina alumina particles particles (Al2 (AlO3)2 Owas3) wasfirst firstdescribed described in 1945 in 1945 by Black by Black [14]. [ 14Initially,]. Initially, it was it wasreported reported that thatthe bond the bond strength strength to the to tooth the toothsurface surface improved, improved, also confirmed also confirmed by recent by recentinvestigations investigations,, and some and authors some authors adopted adopted its use in its clinical use in clinicalprocedures procedures even after even prepar- after preparinging the cavity the cavitywith rotating with rotating instruments instruments [13,15– [18]13,15. Tooth–18]. Toothsandblasting sandblasting was therefore was there- in- foretroduced introduced in restorative in restorative dentistry dentistry as a method as a method of cavity of cavity preparation preparation and called and called “air abra- “air abrasion”sion” [19]. [ 19]. DespiteDespite these these observations, observations, the the application application of of air air abrasion abrasion in in aesthetic aesthetic restorativerestorative dentistrydentistry isis stillstill limited, limited probably, probably related related to to the the discolouring discolouring effects effects on theon dentin.the dentin. Unpub- Un- lishedpublished observations observations showed showed that discoloration that discoloration disappears disappears if the tooth if the surface tooth surfaceis treated is withtreated airborne-particle with airborne abrasion-particle underabrasion a water under jet a (water–airborne-particle water jet (water–airborne abrasion:-particle WAPA) abra- (Figuresion: WAPA)1). (Figure 1).
FigureFigure 1.1. ImageImage ofof aa toothtooth surfacesurface treatedtreated withwith airair abrasionabrasion ((AAAA)) andand water–air-particle water–air-particle abrasionabrasion ((WAPAWAPA).). The The red red line line delineates delineates the the boundary boundary between between the the two two treatments. treatments. AA AA shows shows the the effects effects of of black pigmentation of the dentin, while WAPA does not. black pigmentation of the dentin, while WAPA does not.
WAPAWAPA is is a clinical a clinical procedure procedure carried carried out by out means by ofmeans a prophy of a jet prophy handpiece jet handpiece(mounted on(mounted dental chair)on dental applying chair) waterapplying and water powder and directlypowder ontodirectly the on toothto the surface. tooth surface. The tooth The structuretooth structure is conditioned is condit usingioned a using stream a stream of Al2O of3 particles Al2O3 particles generated generated from compressed from com- airpressed and withair and aerosolized with aerosolized water. Thewater. prophy The prophy jet handpiece jet handpiece separates separates the air the and air water and channelswater channels allowing allowing the highly the highly precise precise regulation regulat ofion water of water and and powder powder flow. flow By. By doing doing so,so, it it producesproduces effectiveeffective kinetic kinetic energy energy for for predictable predictable treatment treatment outcomes: outcomes: waterwater and and powderpowder meetmeet uponupon impactimpact withwith thethe toothtooth givinggiving maximummaximum efficiencyefficiency andand minimalminimal aerosolaerosol dispersion.dispersion. The abrasive particles strike the tooth tooth with with high high velocity velocity removing removing smallsmall amountsamounts of to toothoth structure. structure. The The efficiency efficiency of of removal removal is isrelated related to totissue tissue hardness hardness and and the the operating parameters of the device. Like air abrasion, several parameters such as air pressure (fixed on the chair standard at a value of 0.25 MPa), particle size (fixed at 50 µm),
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operating parameters of the device. Like air abrasion, several parameters such as air pres- sure (fixed on the chair standard at a value of 0.25 MPa), particle size (fixed at 50 µ m), quantityquantity of particles passing passing through through the the spout, spout, handpiece handpiece spout spout diameter diameter and angle and angle,, dis- distancetance from from the the tooth tooth (1.5 (1.5–2–2 cm), cm), and and time time of of exposure exposure (10 (10–30–30 s) s) vary vary the the quantity quantity of of tooth tooth removalremoval and and depth depth of of penetration penetration [ 19[19]].. Figure Figure2 2shows shows the the structure structure and and the the functioning functioning of theof the prophy prophy jet handjet hand piece piece used used to carryto carry out out the the WAPA WAPA procedure. procedure.
FigureFigure 2.2. ImagesImages showingshowing the main features features of of the the prophy prophy jet jet handpiece handpiece for for the the WAPA WAPA procedure: procedure: (a) external view of the prophy jet handpiece; (b) Al2O3 particle chamber opened; (c) functioning of (a) external view of the prophy jet handpiece; (b) Al2O3 particle chamber opened; (c) functioning the Al2O3 particle chamber. The black arrows show the flux of the incoming air; white arrows indi- of the Al2O3 particle chamber. The black arrows show the flux of the incoming air; white arrows cate the outgoing air enriched with Al2O3 particles. Water is carried with a tube (**) directly to the indicatetip of the the spout; outgoing (d) prophy air enriched jet handpiece with Al in2O operation3 particles.. From Water the is spout carried of the with handpiece a tube (**) an directly air-jet toenriched the tip ofwith the Al spout;2O3 (aAl (d2)O prophy3) reaching jet handpieceapproximately in operation. 400 km/h is From wrapped the spout in a stream of the of handpiece water (W). an air-jet enriched with Al2O3 (aAl2O3) reaching approximately 400 km/h is wrapped in a stream of waterBased (W). on the above, this study was conceived to evaluate if WAPA pre-etching can provide superior bond strength and extend the clinical service of bonded ceramic restora- tion,Based without on tooth the above, blackening, this study with wasthe purpose conceived of toassociat evaluateing it if with WAPA the pre-etching operative IDS can provideprotocol superior for biomimetic bond strength prosthetic and restorations. extend the clinical service of bonded ceramic restora- tion, without tooth blackening, with the purpose of associating it with the operative IDS protocol for biomimetic prosthetic restorations. Therefore, this study compared the bond strength of an adhesive resin with WAPA followed by the 3-step etch-and-rinse procedure on the tooth surfaces (test) versus the
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Materials 2021, 14, 4966 4 of 15 Therefore, this study compared the bond strength of an adhesive resin with WAPA followed by the 3-step etch-and-rinse procedure on the tooth surfaces (test) versus the conventional 3-step etch-and-rinse technique without WAPA (control). A null hypothesis conventional 3-step etch-and-rinse technique without WAPA (control). A null hypoth- (H0) of any difference in bond strength between the test and control groups was consid- ered.esis (H Moreover,0) of any a differenceretrospective in bondclinical strength long-term between evaluation the test was and performed control on groups 465 glass was– ceramicconsidered. restorations Moreover,: 183 a were retrospective placed using clinical the long-termimproved evaluationIDS protocol was (WAPA) performed and 282 on 465 glass–ceramic restorations: 183 were placed using the improved IDS protocol (WAPA) using conventional IDS (non-WAPA), to verify if establishing WAPA procedure in clinical and 282 using conventional IDS (non-WAPA), to verify if establishing WAPA procedure in protocols leads to the better long-term success of bonded restorations. clinical protocols leads to the better long-term success of bonded restorations.
2. Materials Materials and Methods EEthicalthical approval waswas obtainedobtained from from the the Ethics Ethics Committee Committee of of the the Albanian Albanian University, Univer- sity,Albania Albania (protocol (protocol code code 278/3). 278/3). Twenty Twenty unerupted unerupted human human mandibular mandibular wisdomwisdom teeth (extracted for orthodontic reasons) were prepared following the technique developed by
Shono et et al. al.as as shown shown in in Figure Figure 3 3(Details (Details can can be be found found in in[20] [ 20). The]). The teeth teeth were were stored stored at 4 °Cat 4in◦ aC 0.15 in aM 0.15 NaCl M solution NaCl solution saturated saturated with thymol with and thymol used andwithin used 1 month within of 1 extraction. month of Briefly,extraction. the Briefly,occlusal the surface occlusal of each surface tooth of eachwas cut tooth and was flattened cut and to flattened expose both to expose the denti bothn andthe dentin enamel, and by enamel, means by of means a semi of-automatic a semi-automatic diamond diamond saw cooled saw cooledwith running with running water (TMA2,water (TMA2, Grottammare, Grottammare, Italy). Italy). The prepared The prepared teeth teeth were were cleansed cleansed in an in ultrasonic an ultrasonic bath bath of physiologicof physiologic solution solution for for10 min 10 min at 40 at °C. 40 ◦OneC. One-half-half of each of each flattened flattened tooth tooth surface surface was pro- was tectedprotected in a inrandomly a randomly chosen chosen direction direction with witha strip a stripduring during the WAPA the WAPA procedure. procedure. The teeth The surfacesteeth surfaces were werethen thendivided divided into two into twosectors, sectors, each each one onereceiving receiving a different a different treatment treatment so thatso that every every tooth tooth was was used used as as its its own own control control to to overcome overcome any any statistical statistical p problemroblem with power and number of specimens. From each tooth sample, eight specimens were obtained but only the best four were selected (control group n = 2 2;; WAPA group n == 2)2) for the microtensile bond strength test.test. A Att the same time, three specimens for each tooth resulting from the cuttings were used for the SEM analysis (Figure3 3B(d–f)).B(d–f)).
Figure 3. Specimen preparation and schematic drawing of the procedures. In ( A) the flattened flattened occlusal area of a specimen shows thethe distinctiondistinction betweenbetween the the treatments. treatments In. In (B (),B), schematic schematic drawings drawing ofs of each each step step a–f. a– Fromf. From each each tooth tooth sample, sample, only only the bestthe best four four specimens specimen outs ofout eight of eight were were selected selected (control (control group group n = 2; WAPAn = 2; WAPA group ngroup = 2) for n the= 2)microtensile for the microtensile bond strength bond strength test and three specimens for each tooth resulting from the cuttings (in black on d and f), were used for the SEM test and three specimens for each tooth resulting from the cuttings (in black on d and f), were used for the SEM analysis. analysis. The WAPA procedure was carried out chairside by means of a prophy unit tooth polisherThe handpieceWAPA procedure (Air Prophy was Unit,carried Renfert out chairside GmbH, Hilzingen,by means of Germany) a prophy with unit 50 toothµm polisher handpiece (Air Prophy Unit, Renfert GmbH, Hilzingen, Germany) with 50 μm aluminium oxide (Al2O3) particles at 0.25 MPa of air–water pressure, perpendicular to aluminiumthe surface oxide (Figure (Al₂O₃)4). A working particles distanceat 0.25 MPa of 1.5 of cmair– andwater time pressure, of action perpendicular of 10 s were to both the surfacestandardized (Figure and 4). controlled.A working distance of 1.5 cm and time of action of 10 s were both stand- ardizedAfter and treatment, controlled. the specimens were carefully cleansed and prepared for total etching and bonding. The 3-step etch-and-rinse adhesive system (OptiBond FL; Kerr, Orange, CA, USA) was used as follows: 20 s of etching with 37% phosphoric acid, abundant rinsing with water spray for 20 s, air drying for 5 s, application of primer (bottle 1) by means of microbrush (Micro Tip Applicator, GC Corp., Tokyo, Japan) with a light brushing motion for 10 s, air drying for 3 s, application of adhesive resin (bottle 2) with a light brushing motion for 10 s and air thinning for 3 s. The bonding resin was light cured with a multi-wave-length light-emitting diode curing lamp (Valo, Ultradent Products, South Jordan, UT, USA) for 40 s at 800 mW/cm2 and with a wavelength of 395–480 nm at 3 mm
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Materials 2021, 14, x FOR PEER REVIEW 5 of 16 tip-to-specimen distance. Finally, the crowns of the teeth were restored using a composite material (Z100; 3M ESPE, Seefeld, Germany) and then cut as previously described.
FigureFigure 4.4. SEMSEM imageimage ofof AlAl₂O₃2O3 particles of 50 µµmm thatthat appearedappeared inin meanmean andand withwith veryvery irregularirregular shapes.shapes. Mag.Mag. 1000×. .
2.1. MicrotensileAfter treatment, Bond Strengththe specimens (µTBS) were carefully cleansed and prepared for total etching and bonding.A total of The 80 3 bar-shaped-step etch-and specimens-rinse adhesive with a bondingsystem (OptiBond area of about FL; Kerr, 1 mm Orange,2 were CA, ob- tainedUSA) was (40 testused and as follows: 40 for control) 20 s of etching and used with to test37% microtensilephosphoric acid, bond abundant strength (rinsingµTBS) (Figurewith water3B(a–f)). spray for 20 s, air drying for 5 s, application of primer (bottle 1) by means of microbrushBefore testing, (Micro eachTip Applicator, specimen was GC carefullyCorp., Tokyo, measured Japan) with with a digital a light calliper brushing (Shimana motion SHAYDC082/83/84,for 10 s, air drying for Toronto, 3 s, appli ON,cation Canada) of adhesive to the nearest resin (bottle 0.01 mm 2) forwith the a cross-sectionallight brushing areamotion to calculatefor 10 s and the air results thinning in MPa.for 3 s The. The load bonding at failure resin was light recorded cured and with the a multiµTBS- waswave measured-length light through-emitting the testingdiode curing machine lamp (MTS810, (Valo, MTSUltradent Co., EdenProducts, Prairie, South MN, Jordan, USA) atUT, 0.5 USA) mm/min for 40 crossheads at 800 mW/cm² speed. and Specimens with a wavelength were fixed of with 395 adhesive–480 nm at cyanoacrylate 3 mm tip-to- (Superspecimen Attack; distance. HENKEL Finally, Ag&Co the crowns KGaA, of Düsseldorf, the teeth were Germany) restored to using the grips a composite of the micro- ma- tensileterial (Z100; device. 3M ESPE, Seefeld, Germany) and then cut as previously described.
2.2.2.1. ScanningMicrotensile Electron Bond MicroscopyStrength (µTBS) (SEM) AfterA total cleansing of 80 bar in-shaped an ultrasonic specimens bath with of distilled a bonding water, area the of specimensabout 1 mm² underwent were ob- criticaltained (40 point test drying and 40 in for Emitech control) K and 850 used (Emitech to test Ltd., microtensile Ashford, bond Kent, strength UK) and (µTBS) were then(Fig- mountedure 3B(a–f) onto). aluminium stubs, sputter gold coated in Emitech K 550 (Emitech Ltd., Ashford,Before Kent, test UK)ing, and each analysed specimen via a was scanning carefully electron measured microscope with (SEM) a digital (Zeiss calliper EVO 50(Shimana XVP, Carl SHAYDC082/83/84, Zeiss SMY Ltd., Cambridge, Toronto, ON UK), Canada) equipped to the with nearest a LaB 60.01electron mm for gun the and cross an- Everhart–Thornleysectional area to calculate tetra solid-state the results detector in MPa. (4Q-BSD). The load SEM at operatingfailure was conditions recorded included and the 5.0 kV accelerating voltage, 8.5 mm working distance and a 100 pA probe current for μTBS was measured through the testing machine (MTS810, MTS Co., Eden Prairie, MN, observations under variable pressure (0.75 torr). The images were captured with a line USA) at 0.5 mm/min crosshead speed. Specimens were fixed with adhesive cyanoacrylate average technique using 20 scans. (Super Attack; HENKEL Ag&Co KGaA, Düsseldorf, Germany) to the grips of the micro- SEMtensile Analysis device. and Surface Roughness (Ra) The microstructure morphology of the tooth surface at each step in both types of 2.2. Scanning Electron Microscopy (SEM) treatment was evaluated using additional specimens (n = 4). To evaluate the interface betweenAfter bonding cleansing agent in an and ultrasonic hard tooth bath tissues of distilled qualitatively, water, the specimens specimens (n underwent = 24) from crit- the cuttingsical point were drying used in (Figure Emitech3B(f)). K 850 SEM (Emitech images of Ltd WAPA-treated., Ashford, Kent, vs. non-WAPA-treated UK) and were then enamelmounted and onto dentin aluminium before andstubs, after sputter etching gold as coated well as in the Emitech micromorphology K 550 (Emitech organization Ltd., Ash- ford, Kent, UK) and analysed via a scanning electron microscope (SEM) (Zeiss EVO 50 XVP, Carl Zeiss SMY Ltd., Cambridge, UK) equipped with a LaB₆ electron gun and an Everhart–Thornley tetra solid-state detector (4Q-BSD). SEM operating conditions in- cluded 5.0 kV accelerating voltage, 8.5 mm working distance and a 100 pA probe current for observations under variable pressure (0.75 torr). The images were captured with a line average technique using 20 scans.
SEM Analysis and Surface Roughness (Ra)
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of the tooth–resin interface were obtained. Surface roughness (Ra) was measured on SEM images using Equation (1) for two-dimensional computation,
1 n Ra(z) = ∑(zi − z) (1) n i=1
as the roughness of the mean distance between the roughness profile and its mean line was reported in µm. SEM stereo-imaging was used to reconstruct the surface topography. To obtain accu- rate results it was assured that brightness constancy and alignment were set in such a way that both images had approximately the same brightness and contrast. In brief, stereo pair images were acquired with symmetrical tilt angles of −5◦ and +5◦, and the elevation (relative to the centre of inclination on the specimen) was calculated as a function of the disparity. A horizontal disparity map was finally converted into heights according to the acquisition parameters—-tilt angle, magnification, and pixel size—-with simple trigonometric equations. Disparity d and Height h are related by Equation (2).
θ d = 2 h sin (2) 2
Therefore, the height h (in µm) of a point with a disparity d (in pixels) is related by Equation (3), dp h = (3) θ 2 sin 2 where θ is the total tilt angle, and p is the pixel size or the scale provided by the SEM system in µm.
2.3. Long-Term Survival Rate The survival rate (SR) was performed on 465 glass–ceramic restorations on 131 patients (72 female, 59 male) between 2003 and 2018. The retrospective evaluation considered up to 15 years of follow-up. Causes of failure involved fracture and debonding or secondary caries, while abrasions were not considered among the complications. There were no inclusion/exclusion criteria based on conditions related to the pa- tients. Restorations were not classified for site of placement (anterior or posterior) material (feldspathic ceramic, lithium disilicate, zirconia-reinforced lithium silicate), restoration type (tooth crowns, veneers, inlays, onlays, overlays), or finishing line (e.g., rounded shoulder, chamfer). They were grouped only for different procedures: WAPA (n = 183) versus non-WAPA (n = 282). All the clinical and laboratory steps were performed by the same experienced operator (T.T.). The cementation protocol was performed with the materials and methods described in the previous in vitro part of this study, and under local anaesthesia and rubber dam isolation when necessary. After the teeth were prepared, in the WAPA group, IDS was performed with WAPA, acid etching and the 3-step etch-and-rinse adhesive procedure; in the control group, only acid etching and the 3-step etch-and-rinse adhesive procedure were performed. The oxygen inhibition layer (OIL) was removed with a mounted brush and prophy paste (Detrartrine, Septodont, Mataro, Spain) at low speed. The temporaries (crowns and veneers) were made of acrylic resin (ColdPac, Yates Motloid, Elmhurst, IL, USA) cemented with Temp Bond/Temp Bond Clear (Kerr, Orange, CA, USA) or made of temporary material (Fermit or Telio, Ivoclar Vivadent, Schaan, Liechtenstein) (inlays, onlays, overlays). The workflow was analogical for all the restorations. Prior to cementation, the restorations were treated as follows: - feldspathic ceramic: the bonding surface was etched with hydrofluoridic acid gel 9% (Porcelain Etch, Ultradent, South Jordan, UT, USA) for 1.30 min; Materials 2021, 14, 4966 7 of 15
- lithium disilicate and zirconia-reinforced lithium silicate: the bonding surface was etched with hydrofluoric acid gel 4.5% (IPS Ceramic Etching Gel, Ivoclar Vivadent, Schaan, Liechtenstein) for 20 s. The restorations were washed with distilled water, then the smear resulting from the acid etching was removed with an ultrasonic bath (Puresonic, Kiaccessori, Nola, Italy) in ethyl acetate for 5–10 min and the restoration was stored in ethyl alcohol 96–100% until silane application. Then a silane (Monobond S and Monobond Plus, Ivoclar Vivadent, Schaan, Liechtenstein) was applied by means of a microbrush for 60 s of action on the bonding surface of the restoration and let dry at room temperature. Right before cementa- tion, the temporaries were removed, and the residual eliminated with a mounted brush and prophy paste at low speed. While the acid etching and adhesive procedures were re-performed on the tooth, the only bonding was applied with a microbrush on the bonding area of the restoration. The bonding on the restoration and tooth were not light cured: a thin layer of composite resin and a drop of flowable resin (to facilitate the flow) was applied to the restoration, and then the restoration was placed. To make the composite resin more fluid for proper cementation, the operator waited 1 min after placement to allow the body temperature to heat the composite to ca. 37 ◦C. Excess resin was continuously removed with a probe after the complete placement of the restoration. Then the composite was light cured from 3 different sides (palatal/lingual, buccal, occlusal), for 2 min per side. In the case of veneers, the first light-cured side was palatal/lingual; in the other cases, occlusal. Then the excess polymerized composite resin was gently removed with a curved lancet.
2.4. Statistical Analysis Descriptive statistics were obtained for µTBS test data and for surface roughness data. The results were statistically inferred using the unpaired Student’s t-test for µTBS test data and a one-way ANOVA and Holm–Sidak method for multiple comparisons for surface roughness data. The Kaplan–Meier survival analysis was used to compare the mean survival rates between the two groups to determine whether the experimental treatment (WAPA) was an improvement over the traditional one (non-WAPA). Statistical significance was assayed using a Log Rank test after means calculation. The level of statistical significance was set at p < 0.05. The statistical analyses were performed using IBM SPSS Statistics v. 3.5 (IBM Corp, Armonk, NY, USA).
3. Results Results of the microtensile bond strength (µTBS) test and roughness (Ra) analysis are collected in Table1 and described in the following sections.
Table 1. Descriptive statistics of microtensile bond strength (µTBS) and roughness (Ra) between WAPA group (water–airborne-particle abrasion and acid-etching) and control group (only acid-etching).
µTBS a Roughness b WAPA Control WAPA Control Dentin Enamel Dentin Enamel Mean 63.9 51.7 160 150 102 98 SD 7.8 10.8 25 35 27 24 SD: standard deviation; a MPa; b Ra (µm).
3.1. Microtensile Bond Strenght (µTBS) The µTBS was 63.9 ± 7.8 MPa for the WAPA group and 51.7 ± 10.8 MPa for the control group. The difference was highly statistically significant (p < 0.001; power of the performed test = 0.999 with α = 0.05). Results are shown in Figure5. Materials 2021, 14, x FOR PEER REVIEW 8 of 16
3.1. Microtensile Bond Strenght (µTBS) The μTBS was 63.9 ± 7.8 MPa for the WAPA group and 51.7 ± 10.8 MPa for the control Materials 2021, 14, 4966 group. The difference was highly statistically significant (p < 0.001; power of the 8 per- of 15 formed test = 0.999 with α = 0.05). Results are shown in Figure 5.
Figure 5. Results of the µTBS of the two different treatments (WAPA and control groups). WAPA: FigureWater–airborne-particle 5. Results of the μTBS abrasion. of the Statistically two different significant treatments difference (WAPA ( pand< 0.001) control with groups). unpaired WAPA:t-test Water(power–airborne = 0.999- withparticleα = abrasion. 0.05). Statistically significant difference (p < 0.001) with unpaired t-test (power = 0.999 with α = 0.05). 3.2. SEM Observations and Roughness Analysis (Ra) 3.2. SEMThe Observations effects of the and WAPA Roughness procedure Analysis on (Ra) tooth structures (enamel and dentin) before anyThe adhesive effects procedure of the WAPA are shown procedure in Figures on tooth6 and structures7. The surfaces (enamel treated and dentin) with WAPAbefore anyfollowed adhesive by acidprocedure etching are appeared shown to in be Figure roughers 6 andand more7. The complex surfaces (Figure treated6b,d) with compared WAPA followedto the surfaces by acid that etching were onlyappear acided etched to be rougher (Figures 6anda–c andmore7c). complex Enamel (Figure without 6b treatment,d) com- paredshowed to the an Rasurfacesof 98 that± 24 wereµm, only but afteracid theetched WAPA (Figure treatments 6a–c and the Ra7c).was Enamel 150 ±without35 µm treatment(Figure7f,g). showed Dentin an Ra without of 98 ± treatment 24 µm, but showed after the a meanWAPARa treatmentof 102 ± the27 Raµm, was while 150 ± after 35 µmWAPA (Figure treatment 7f,g). Dentin it was without 160 ± 25treatmentµm (Figure showed7h,i). a mean The increase Ra of 102 in ± surface27 µm, while roughness after WAPAdue to treatment WAPA on it both was enamel160 ± 25 and µm dentin(Figure was 7h,i). statistically The increase significant in surface (p
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Figure 6. FigureComparison 6. Comparison of enamel of enamel and dentin and dentin treated treated with with acid acid-etching-etching and and WAPA WAPA and and acid-etching acid-etching procedures: procedures: (a) enamel (a) enamel acid-etchedacid-etched for 20 s for, (* 20) rods s, (*) rodsof enamel of enamel prisms prisms appear appear on on the the same same level. level. Mag. Mag. 2000× ;(2000b) enamel×; (b) afterenamel WAPA after and WAPA acid-etch and acid- etch treatmenttreatment for for 20 20 s, s,(* (*)) rods rods ofof enamel enamel prisms prisms appear appear on different on different levels with levels a three-dimensional with a three-dimensional appearance. Mag. appearance. 2000×; Mag. 2000×; (c)( cdentin) dentin after after 2020 s s of of etching, etching, white white arrows arrows indicate indicate open tubules open which tubules are located which on are the located same level. on Mag. the 2000same×;( level.d) dentin Mag. 2000×; (d) dentintreated treated with with WAPA WAPA followed followed by 20 s of by etching, 20 s of white etching, arrows white indicate arrows open tubules indicate which open are located tubules on differentwhich are levels located with on dif- ferent levelsa three-dimensional with a three-dimensional appearance. Mag. appearance. 2000×. After Mag. comparing 2000×. theAfter SEM comparing images (a) vs. the (b SEM) and (imagesc) vs. (d) ( ita) was vs. possible (b) and to (c) vs. (d) it was possiblenote that to the note main that difference the main between difference specimens between that were specimens acid etched that and those were that acid underwent etched acidand etching those andthat WAPA underwent was acid related to the 3D architecture, which was much more present in the WAPA specimens, both for enamel and dentin. etching and WAPA was related to the 3D architecture, which was much more present in the WAPA specimens, both for enamel and dentin. Analysing the resin tag disposition at the interface between the restoration material and dentin in both the WAPA and control group, it is possible to note that in the control group they were shorter and more irregularly arranged than in the WAPA group, where they protruded to greater extent into the dentinal tubules (Figure 10 and Figure S2).
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Figure 7.FigureComparison 7. Comparison between between SEM SEM images images of of enamel enamel and and dentin dentin after WAPA after WAPAand acid and-etching acid-etching:: (a) enamel prisms (a) enamel after prisms WAPA procedure and acid-etching. Mag. 924×; (b) enamel after WAPA procedure before acid-etching. The WAPA pro- after WAPA procedure and acid-etching. Mag. 924×;(b) enamel after WAPA procedure before acid-etching. The WAPA cedure introduce a 3D dimension to the surface. Mag. 2500×; (c) enamel (*) and dentin (**) with WAPA procedure (top procedureside) introduce and without a 3D WAPA dimension procedure to the (bottom surface. side). Mag. Mag. 2500230×;× (d;() dentinc) enamel after (WAPA*) and procedure dentin (** and) with acid- WAPAetching. procedureSev- (top Figure 7. Comparison between SEM images of enamel and dentin after WAPA and acid-etching: (a) enamel prisms after eral dentin tubules appeared open on dentin surface with different directions. Mag 2500×; (e) dentin after WAPA proce- side)WAPA and without procedure WAPA and acid procedure-etching. Mag. (bottom 924×; (b side).) enamel Mag. after 230WAPA×;( procedured) dentin before after acid WAPA-etching. procedure The WAPA and pro- acid-etching. Several dure before acid-etching. Mag. 4000×; (f) 3D reconstruction of enamel roughness after WAPA and acid etching proce- cedure introduce a 3D dimension to the surface. Mag. 2500×; (c) enamel (*) and dentin (**) with WAPA× procedure (top dentin tubulesdures. The appeared Ra was 150 open ± 35 on µm; dentin (g) 3D surfacereconstruction with differentof the surface directions. roughness Mag of enamel 2500 (brighter;(e) dentin orange after side) WAPAand den- procedure side) and without WAPA procedure× (bottom side). Mag. 230×; (d) dentin after WAPA procedure and acid-etching. Sev- beforeeral acid-etching. dentintin (darker tubules orange Mag.appeared side) 4000 open treated;( onf ) dentinwith 3D reconstructionWAPA surface procedure with different of (top enamel directions. side) and roughness Mag without 2500 ×WAPA after; (e) dentin WAPA procedure after and WAPA (bottom acid proce- etching side); enamel procedures. The Ra wasdure 150 treatedbefore± 35 acid withµ-m;etching. only (g) acid 3D Mag. etching reconstruction 4000× ;showed (f) 3D reconstruction a Ra of theof 98 surface ± 24 of µm;enamel roughness (h) roughness3D reconstruction of after enamel WAPA of the (brighter and surface acid etching orangeroughness proce- side) of dent andin dentin (darker treated only with acid-etching. The Ra was of 102 ± 27 µm; (i) 3D reconstruction of the surface roughness of dentin after orangedures. side) The treated Ra was 150 with ± 35 WAPA µm; (g) 3D procedure reconstruction (top of side) the surface and withoutroughness WAPAof enamel procedure (brighter orange (bottom side) and side); den- enamel treated with tin (darkerWAPA orange and acid side) etching treated procedures.with WAPA Theprocedure surface (top Ra side)was ofand 160 without ± 25 µm. WAPA procedure (bottom side); enamel onlytreated acid etching with only showed acid etching a Ra showedof 98 a± Ra24 of µ98m; ± 24 (h µm;) 3D (h) reconstruction 3D reconstruction ofof the surface surface roughness roughness of dent ofin dentin treated only with acid-etching.treated only The withRa acidwas-etching. of 102 The± Ra27 wasµ m;of 102 (i) ± 3D27 µm; reconstruction (i) 3D reconstruction of the of surfacethe surface roughness roughness of of dentin dentin after after WAPA and acid WAPA and acid etching procedures. The surface Ra was of 160 ± 25 µm. etching procedures. The surface Ra was of 160 ± 25 µm.
p α Figure 8. Results of One-way ANOVA ( = 0.004; power = 0.881 with = 0.05) and Holm–Sidak pairwise multiple comparison procedures for Ra among both treatments and tissues. WAPA: water–
airborne-particle abrasion and acid-etching; Control group: acid-etching only. A statistically signifi- cant difference (p < 0.05) was present between WAPA and Control group for both enamel and dentin tissues. No statistically significant difference (p > 0.05) was present between different tissue in the same group of treatment. Materials 2021, 14, x FOR PEER REVIEW 11 of 16
Figure 8. Results of One-way ANOVA (p = 0.004; power = 0.881 with α = 0.05) and Holm–Sidak pairwise multiple comparison procedures for Ra among both treatments and tissues. WAPA: water– airborne-particle abrasion and acid-etching; Control group: acid-etching only. A statistically signif- icant difference (p < 0.05) was present between WAPA and Control group for both enamel and den- tin tissues. No statistically significant difference (p > 0.05) was present between different tissue in the same group of treatment.
After the restorative procedures, the differences between the two groups were mainly visible in the dentin, where resin tags appeared (Figures 9 and 10). The WAPA group interface profile appeared to be much more extended for size, length and a more complex spatial arrangement than the one in the control group (Figure 9b,c). The qualita- tive analysis of the surface profile at the dentin/bonding interface compared to the den- Materials 2021, 14, 4966 tin/enamel junction (DEJ) demonstrated a similarity between the WAPA and DEJ (Figure11 of 15s 9d and S1). However, in both WAPA and the control groups there was intimate contact between the restorative material and the tooth.
FigureFigure 9.9. RepresentativeRepresentative SEMSEM imagesimages forfor thethe qualitativequalitative evaluationevaluation ofof lineline profileprofile atat the the interface interface betweenbetween dentindentin andand resin:resin: (a(a)) red-line red-line profile profile of of the the dentin/enamel dentin/enamel junctionjunction (DEJ)(DEJ) ofof aa soundsound tooth;tooth; (E) enamel; (D) dentin. Mag 500×; (b) blue-line profile of the interface between dentin and resin for (E) enamel; (D) dentin. Mag 500×;(b) blue-line profile of the interface between dentin and resin for the WAPA group. (B) resin; (HL) hybrid layer; (D) dentin; black arrows: resin tags inside dentinal the WAPA group. (B) resin; (HL) hybrid layer; (D) dentin; black arrows: resin tags inside dentinal tubules. Mag 500×; (c) green-line profile of the interface between dentin and resin for the control × tubules.group. (B) Mag resin; 500 (HL);(c )hybrid green-line layer; profile (D) dentin. of the Mag interface 500× between; (d) comparison dentin and among resin line for profiles. the control The group. (B) resin; (HL) hybrid layer; (D) dentin. Mag 500×;(d) comparison among line profiles. The Materials 2021, 14, x FOR PEER REVIEWsimilarity between the red and blue profile lines and the substantially different trend for the12 green of 16 similarityline appears between evident. the red and blue profile lines and the substantially different trend for the green line appears evident.
FigureFigure 10.10. Representative SEM observations of the tooth-restorationtooth-restoration interface for both WAPA andand controlcontrol groups:groups: ((aa)) SEMSEM imageimage ofof bothboth typestypes ofof preparationpreparation areare visible:visible: thethe WAPAWAPA is is identified identified by by square squarec ,c while, while control control is is identified identified by by square square (b b).. Mag.Mag. 851851××;;( (bb)) Higher magnification magnification of control groupgroup (3000(3000××).). At At the the interface, between dentin (**) andand thethe restorationrestoration materialmaterial (*)(*) thethe resinresin tagstags appear appear to to be be shorter shorter and and less less developed developed if if compared compared with with those those of of (c );(c ();c) ( Higherc) Higher magnification magnification of theof the WAPA WAPA group group (4000 (4000×).× At). At the the interface, interface, between between dentin dentin (**) (**) and and restoration restoration material material (*), (*) the, the resin resin tags tags appeared appeared to beto be more extended inside the dentin tubules. more extended inside the dentin tubules. Analysing the resin tag disposition at the interface between the restoration material and dentin in both the WAPA and control group, it is possible to note that in the control group they were shorter and more irregularly arranged than in the WAPA group, where they protruded to greater extent into the dentinal tubules (Figures 10 and S2).
3.3. Long-Term Survival Rate The SR for the WAPA group at 14.1 years was 94%, while the SR for the non-WAPA group at 14.4 years was 87.6%. Mean calculation is shown in Table 2. A difference of 6.4% was statistically significant (p < 0.05) (Figure 11).
Table 2. Means calculation for clinical WAPA and non-WAPA groups.
Mean 1 Treatment CI 95% Estimate SE LB UB WAPA 14.12 0.29 13.56 14.68 non-WAPA 14.44 0.09 14.27 14.61 Overall 14.34 0.09 14.17 14.52 1 Estimation is limited to the largest survival time if it is censored. SE: standard error; CI 95%: 95% confidence interval; LB: Lower Bound; UB: Upper Bound. Data are expressed in years.
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3.3. Long-Term Survival Rate The SR for the WAPA group at 14.1 years was 94%, while the SR for the non-WAPA group at 14.4 years was 87.6%. Mean calculation is shown in Table2. A difference of 6.4% was statistically significant (p < 0.05) (Figure 11).
Table 2. Means calculation for clinical WAPA and non-WAPA groups.
Mean 1 Treatment CI 95% Estimate SE LB UB WAPA 14.12 0.29 13.56 14.68 non-WAPA 14.44 0.09 14.27 14.61 Materials 2021, 14, x FOR PEER REVIEW 13 of 16 Overall 14.34 0.09 14.17 14.52 1 Estimation is limited to the largest survival time if it is censored. SE: standard error; CI 95%: 95% confidence interval; LB: Lower Bound; UB: Upper Bound. Data are expressed in years.
(a) (b) Figure 11. KaplanKaplan–Meier–Meier survival survival analysis analysis of of 465 465 glass glass–ceramic–ceramic restorations restorations (131 patients, 59 males, 72 female) for up to 15 years (2003–2018):(2003–2018): (aa)) WAPAWAPA treatment,treatment, n n = = 183;183; ( b(b)) control control (non-WAPA (non-WAPA treatment), treatment) n, =n 282.= 282. For For both both graphs, graphs, the they axisy axis is iscumulative cumulative survival survival (%), (%) and, and the thex axis x axis is follow-up is follow (years).-up (years). The differenceThe difference was statistically was statistically significant significant (Log Rank (Log test, Rankp < test, 0.05). p < 0.05). 4. Discussion 4. Discussion As stated previously, the novelty of the present study was to test and demonstrate afterAs a lengthystated previously, time the clinical the novelty efficacy of ofthe the present WAPA study procedure was to whentest and associated demonstrate with afterthe IDS a lengthy protocol time for the bonded clinical ceramic efficacy restorations. of the WAPA For procedure this reason, when this associated study consisted with the of IDStwo protocol parts: the for first, bonded an in ceramic vitro study, restorations. and the second, For this an reason,in vivo thisevaluation study consisted after a lengthy of two parts:time. Wethe aimedfirst, an to integratein vitro study the laboratory, and the andsecond clinical, an resultsin vivo for evaluation a higher level after of a evidencelengthy time.to overcome We aimed the widespreadto integrate dichotomousthe laboratory behaviour and clinical that results separates for in-vitroa higher evidence level of fromevi- denceclinical to applications. overcome the widespread dichotomous behaviour that separates in-vitro evi- denceThe from results clinical of applications. the in vitro part of the study rejected the hypothesis under test. The WAPAThe procedure results of led the to in an vitro increase part of thethe bondstudy strength rejected betweenthe hypothesis the restoration under test. and The the WAPAtooth of procedure 12.2 MPa led (23.6%). to an Thisincrease improvement of the bond was strength statistically between significant the restoration (p < 0.001) and andthe toothassociated of 12.2 with MPa an (23.6%). increase This in the improvement roughnessat was the statistically tooth tissues/resin significant interface (p < 0.001) of about and associated54%, which with was an also increase statistically in the significant roughness (p at< the 0.05). tooth tissues/resin interface of about 54%, Ourwhich results was also agreed statistically with the significant majority of (p the < 0 studies.05). that tested sandblasting on tooth surfacesOur [results15,17,18 agree,20,21d]. with However, the majority some authorsof the studies reported that no tested difference sandblasting in bond on strength tooth surfacesbetween [15,17,18,20,21] sandblasted and. However, non-sandblasted some authors teeth although reported they no diff usederence self-etching in bond adhesives strength between sandblasted and non-sandblasted teeth although they used self-etching adhe- sives instead of etch-and-rinse adhesives, as we did, and detected lower μTBS values [22,23]. This methodological difference could be relevant since some authors reported that sandblasting increased the bond strength of a restoration only if it were associated with a total etching procedure [19,21]; however, resin tags without WAPA do not contribute to dentin adhesion in self-etching adhesives [24]. These topics are of extreme importance since the present SEM qualitative analysis showed that the enamel surface of the WAPA group was generally more irregular, while the dentin surface showed more open dentinal tubules (Figure 6). At the interface between the tooth and the restoration, the resin tags were more extended inside the dentin tubules, and this survey could be, in our opinion, the key factor to explaining the major bond strength of the tooth restoration (Figure 10). Thus, since 3-step adhesive systems perform the best and still are the gold standard for cementing indirect restorations [25], even more so when WAPA is performed, it is better to carry out a total etching adhesive procedure rather than a self-etching one to generate more open dentinal tubules and provide longer resin tags that can provide superior mi-
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instead of etch-and-rinse adhesives, as we did, and detected lower µTBS values [22,23]. This methodological difference could be relevant since some authors reported that sandblasting increased the bond strength of a restoration only if it were associated with a total etching procedure [19,21]; however, resin tags without WAPA do not contribute to dentin adhesion in self-etching adhesives [24]. These topics are of extreme importance since the present SEM qualitative analysis showed that the enamel surface of the WAPA group was generally more irregular, while the dentin surface showed more open dentinal tubules (Figure6). At the interface between the tooth and the restoration, the resin tags were more extended inside the dentin tubules, and this survey could be, in our opinion, the key factor to explaining the major bond strength of the tooth restoration (Figure 10). Thus, since 3-step adhesive systems perform the best and still are the gold standard for cementing indirect restorations [25], even more so when WAPA is performed, it is better to carry out a total etching adhesive procedure rather than a self-etching one to generate more open dentinal tubules and provide longer resin tags that can provide superior microtensile bond strength. It would be desirable if future investigations focused on the correlation among resin tag number, length, and disposition in influencing bond strength after WAPA. Our observations on surface roughness are in agreement with Patcas et al., who reported a rougher enamel surface after sandblasting and acid etching compared to etching procedure alone [26]. Nevertheless, our results disagreed with those of Chinelatti et al., who reported no differences in enamel between sandblasting and sandblasting followed by acid etching. Regarding the effects of sandblasting on dentin, the same authors reported, according to our study, that dentinal tubules were more open when the acid etching procedure was carried out after sandblasting [17]. Although not statistically significant, it is interesting to note that, as expected, after WAPA the dentin Ra (160 ± 25 µm) was higher than the enamel Ra (150 ± 35 µm), principally because of its less tough structure. On the other hand, the enamel prisms and mature matrix offered superior wear resistance to high-velocity sandblasting with Al2O3 50 µm particles. The major limitation of the in vitro part of the present study is related to the absence of dentinal fluid pressure. However, this limitation was overcome by the results obtained from the in vivo part since it is a mere simulation of reality. It demonstrated that glass– ceramic restoration cemented with WAPA has a superior SR than restoration without WAPA for up to 15 years of service. The difference of 6.4% was statistically significant (p < 0.05). To the knowledge of the authors, this is the first study to evaluate the clinical performance of a considerable number of glass–ceramic restorations following a WAPA procedure with a long-term follow up; hence, the outcomes are highly valuable. The clinical results confirmed the results of the in vitro part of the study, validating the cementation protocol used in the study and nullifying potentially affecting variables. It must be noted that survival analysis was performed only on groups where WAPA was performed or not. It therefore would be an additional benefit for future evaluations to analyse groups with different selection criteria, such as patient-related conditions (e.g., bruxism), base material of the restoration (feldspathic ceramic, lithium disilicate, zirconia-reinforced lithium silicate) or kind of restoration (tooth crowns, veneers, inlays, onlays, overlays), to learn more about the impact of WAPA in specific clinical situations. From a clinical point of view, it can be stated that the WAPA procedure is a useful tool in minimally invasive dentistry to prevent tooth blackening and improve the adhesion of the bonded restorations to sclerotic dentin, which is usually problematic. However, these speculations need further scientific investigations.
5. Conclusions The results of this study indicated that tooth surfaces treated with 50 µm of aluminium dioxide WAPA and a 3 step etch-and-rinse adhesive procedure had increased surface roughness and an microtensile bond strength of 12.2 MPa (23.6%) without detrimental tooth blackening. These outcomes were positively confirmed in the clinical scenario, where glass–ceramic restorations cemented with an operative IDS protocol with WAPA Materials 2021, 14, 4966 14 of 15
showed superior long-term clinical performance than when WAPA was not performed. Therefore, WAPA can be considered an effective pre-etching procedure associated with the operative IDS protocol for the long-term success of biomimetic prosthetic restorations in clinical dentistry.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/ma14174966/s1, Figure S1: SEM image showing the dentin-enamel junction architecture between dentin (D) and enamel (E) at high magnification. SEM, Mag. 10k×, Figure S2: Additional SEM image of the interface between dentin (D) and resin (B) of the WAPA group: it is possible to note the resin tags departing form the resin and the hybrid layer (HL) and developing into the dentinal tubules (black arrows). Mag. 4400×. Author Contributions: Conceptualization, T.T.; methodology, L.M. and T.T.; validation, L.M. and T.T.; formal analysis, T.T.; investigation, T.T.; resources, S.C. and T.T.; data curation, F.V., B.S. and T.T.; writing—original draft preparation, L.M. and F.V.; writing—review and editing, F.V., O.T., T.T. and S.C.; visualization, F.V. and B.S.; supervision, T.T. and B.S.; project administration, O.T. and S.C.; funding acquisition, T.T. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Prof. Tonino Traini ex 60% University of Chieti funds. Institutional Review Board Statement: The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Ethics Committee of Albanian University, Albania (protocol code 278/3, 7 May 2021). Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: Data are available upon request to the corresponding author. Conflicts of Interest: The authors declare no conflict of interest.
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