This article traces the evolution of the development of adhesive dentistry, focusing on the contributions of various key clinicians and researchers over the past 70 years. The introduction of acid etching, waterproof bonding, and the various generations of adhesive resin are discussed.
Adhesion of restorative materials to replace defective or missing human tooth structure has been and still remains the ultimate goal in dentistry. The development of adhesive dentistry has a very interesting history. It is remarkable to trace the steps in the evolution of this discipline and how each clinician and researcher contributed a part to the overall science.
One of the physical property requirements of a dental restoration is to retain its size and form after it has been placed in a tooth. In the post-World War II era, the choices of dental materials that did just that were amalgam, gold foil, cast gold, fused porcelain and, in certain cases, silicate cement. In 1942, Eugene Skinner1 wrote in reference to silicate cement restorations, "the tooth structure can be imitated with complete satisfaction for the first few months after the restoration is placed, but almost invariably the material discolors and gradually disintegrates in the mouth." Imagine that the acceptability, or standard of care, of a silicate restoration was just a few months. That may have been acceptable to some but not to others. For its esthetic qualities and insolubility properties, acrylic resin was developed to replace silicate cement restoratives.2
The resin filling materials used in the 1940s and 1950s were methyl methacrylates, which had a high shrinkage factor during polymerization. The term "percolation" described the expansion and contraction of the filling material due to thermal changes that resulted in the formation of a space between the filling and the tooth. In Skinner and Phillips' "The Science of Dental Materials,"3 the issue of percolation, which they described as the alternative imbibing and extruding of liquids, was addressed. They stated that there was a difference of opinion as to the significance of the percolation, which actually was marginal leakage. Some investigators regarded the marginal leakage no worse that other restorative materials, while others felt that the effect lead to secondary decay.
In 1952, Nelson et al4 described the volumetric changes of acrylic filling materials that were subject to thermal changes in the oral cavity. Their study consisted of embedding a 36-gauge copper-constantan thermocouple at the pulpal floor of a Class III preparation and then inserting a self-curing resin restoration on a human tooth. The test subject then consumed extremely hot coffee (measured at 60°C) and then cold soft drinks (measured at 4°C), creating a thermal cycling. After accounting for polymerization shrinkage (10%) and the volumetric changes from thermal cycling (90%) they concluded that the space that developed between the restoration and the tooth surface allowed for a constant fluid exchange, which explained the cause of secondary decay around the margins of existing restorations. They called for the need for further investigation into dealing with the percolation problem.
At the 1954 Annual Meeting of the Dental Materials Group of the International Association for Dental Research, Dr. Ernest Rose spoke about the clinical difficulties with resin filling materials; a possible partial solution was to develop a plastic that would adhere to human tooth structure. If the problem with percolation of resin filling materials could be prevented then bonding of the filling material to the tooth structure could be attained.5 Their study consisted of more than 5,500 tests of various materials and found that no materials maintained adhesion to human tooth structure after prolonged water immersion. Their conclusion was that another avenue of finding a way to bond restorative material to tooth structure would need to be explored.
The introduction of adhesive dentistry has always been synonymous with Dr. Michael Buonocore for his work with etching of the enamel with an acid. There is little mention of the pioneers that came before him who laid the groundwork and were the inspiration for Buonocore's research. In 1949, Oskar Hagger, a Swiss chemist who was working for the Amalgamated Dental Company in London, in cooperation with their Swiss subsidiary, DeTrey in Zurich, developed an adhesive system for bonding acrylic resin to tooth structure called Sevriton Cavity Seal®.6 They received a Swiss patent in November 1951. This material used glycerophosphoric acid dimethacrylate and sulphinic acid as the catalyst and was capable of bonding acrylic resin to a tooth cavity. In 1952, with collaboration from Hagger, it was determined by Kramer and McLean7 through the use of staining techniques that the actual bonding mechanism of the Sevriton Cavity Seal to tooth structure was from the penetration of the glycerophosphoric acid dimethacrylate into the dentin, forming an intermediate layer that is now referred to as a hybrid layer. Also in 1952, the actual clinical technique for using the Sevriton system was described by McLean and Kramer,8 who illustrated for the first time the bonding of a self-curing acrylic to dentin using an etchant (glycerophosphoric acid). The understanding has always been that enamel bonding came first and dentin bonding was introduced later, but, actually, dentin bonding was the precursor to enamel bonding and Hagger truly originated acid-etched restorations.
In 1955, Buonocore published "A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces."9 He described the advantageous effect of a filling material having the capability of forming a bond to tooth structure so strong that it would eliminate the need for retention and resistance form in cavity preparations. He stated that the possibilities of this "bonding" occurring would come from either exploration into resin materials that had adhesive properties, modifying present materials to make them adhesive, using a coating adhesive between the filling material and the tooth, or by chemically altering the tooth surface such that existing materials would adhere to it. It was this latter approach that Dr. Buonocore pursued after learning of how phosphoric acid increased the adhesion of paint and resin coating on metal surfaces. He thought that enamel could be treated with phosphoric acid just like metal and render it more receptive to filling materials. His experiments resulted in using an 85% concentration of phosphoric acid for 30 seconds, which rendered a decalcified enamel surface.
His article concluded with the fact that there was a remarkable increase in adhesion but he only could theorize as to why this occurred. The mechanism by which the increase in adhesion occurred remained unanswered until 1968 when Buonocore, Matsui, and Gwinnett published "Penetration of resin dental materials into enamel surfaces with reference to bonding,"10 in which they described the "prism-like" tags of resin materials that penetrated enamel surfaces that were conditioned with phosphoric acid. Unconditioned enamel did not exhibit these tags. Based on their results, a call was initiated for a dental restorative material to be developed that had good wettability for enamel surfaces, high surface tension to gain access to the enamel spaces, and, when polymerized, would be tough, impermeable, and abrasion- and bacterial-resistant.10 Even though they produced a stronger bond of the restorative material to the enamel, water immersion testing had a deleterious effect on the longevity of the bonding, a problem that suggested further study was necessary and continues to be investigated today.
The concept of acid etching would not take hold in the dental community for many years after Buonocore's first publications. At the time, the idea of placing an acid on living tissue (dentin) was generally thought of as harmful. But concurrently, the use of zinc phosphate cement was widely accepted. The liquid portion consists of phosphoric acid and water (2:1 ratio), and the powder portion is made up of calcined zinc oxide and magnesium oxide.11 Although zinc phosphate cement has a long track record as one of the oldest cementing agents and is widely used, the thought of phosphoric acid (in cement form) in contact with dentin was of little concern. While some postoperative sensitivity was associated with its use, this could be remedied by placing a cavity varnish on the dentin prior to cementation.12 Little was it known that the dentinal tubules were becoming exposed by the dissolution of the smear layer by the phosphoric acid in the cement. In fact, acidic cements were used long before zinc phosphate. In pre-Columbian times, stone inlays for Mayan Indians were cemented in with acidic cements.13,14
In 1965, R.L. Bowen,15 a dentist working at the National Bureau of Standards in Wahington, DC, affirmed that a strong bond between tooth and restoration would only be achieved with a waterproof bonding mechanism. He stated that studies conducted by Schouboe et al16 showed that various bonding materials had good adhesion, but once they were subjected to water or saliva the adhesion was lost. Methyl methacrylate resins that were used as direct fillings had many problems and limitations since their development in Germany during World War II.17 As stated earlier, polymerization shrinkage that led to secondary decay and severe discoloration were some of the problems with this filling material. Bowen looked at epoxy resins. In fact, epoxy resins were developed in 1937 by a Swiss chemist working for the same company—Amalgamated Dental Company, London, and its associate DeTrey Freres, Zurich—that produced the Sevriton Cavity Seal material.6 However, because of the extended period of curing time he abandoned further investigation17 and instead directed his research to a bonding mechanism that would act as a coupling agent between the filling material and tooth structure. A bifunctional molecule (one end of the molecule was able to bond to dentin while the other end could bond to the filling material), NPG-GMA (N-phenylglycine and glycidyl methacrylate), was introduced.18 The bonding agent was not clinically successful as it only had a bond strength of 1 megapascal (MPa) to 3 MPa. Still, this is considered the first generation of bonding adhesives. One example of a first-generation bonding adhesive was S.S. White's Cervident.19
It was not until the late 1970s that the next generation of dentinal adhesives was introduced. The main components of these unfilled resins was a combination of bisphenol A and glycidyl methacrylate, which had the acronym BisGMA, and HEMA, short for hydroxyethyl methacrylate.18 These bonding agents were an improvement from the first generation but disregarded the smear layer. Their bond strengths ranged between 4.5 MPa and 6 MPa.20,21
There were three basic approaches to adhesives during the late 1970s and early 1980s. One consisted of a 25% citric-acid etchant that actually etched the dentinal tubules. It was sold by Den-Mat (www.denmat.com) under the name of Dentin Bonding System. The second approach was to use a phosphate ester bonding agent, a bifunctional molecule that had the capacity to bond with the calcium in the tooth structure on one end of the molecule and the methacrylate present in the filling material on the other end. These systems modified the smear layer with cleansing agents. Examples of second-generation adhesives were Bondite (Sybron/Kerr, www.kerrdental.com), ScotchbondTM (3M ESPE, www.3MESPE.com), and Prisma Universal Bond (DENTSPLY Caulk, www.caulk.com). The third approach in this category was to use a polyurethane polymer that was unaffected in the presence of moisture. Hydrogen peroxide was used as a cleansing agent but left the smear layer intact. An example of this system was manufactured by Ivoclar Vivadent (www.ivoclarvivadent.us) under the name of Dentin-Adhesit.
The reason for the low bond strengths was the adhesives were not bonding to the dentin surface but rather to the smear layer that was left behind after cavity preparation.22 The smear layer, composed of debris caused by frictional heat and deformation during the cutting of dental tissues,23 impeded the penetration of the resin adhesive. There was some resistance in the dental community to placing an acid on the dentin because it was believed that pulpal damage could occur.24,25 The success of dentin bonding agents was limited by the presence of the smear layer.26 On the other hand, Fusayama27 advocated that postoperative sensitivity was a result of inadequate dentin bonding, and thorough simultaneous etching of the dentin and enamel with phosphoric acid would improve the sealing of the dentinal tubules. But, still, the standard of care at that time was opposed to this method.23 If acid etching was to be used, Wilson and Kent (who developed glass ionomer) recommended the placement of a glass-ionomer liner over the dentin to protect it from the acid penetration.28
A third generation of resin adhesives was developed in the late 1980s that addressed the smear layer by significantly modifying it, allowing resin penetration into the underlying dentin. Most of these systems consisted of three parts: a conditioner, primer, and adhesive. The conditioner was either a weak organic acid (maleic acid) or a low-concentration inorganic acid (phosphoric or nitric acid). The amount of smear layer dissolution was dependent on the acid, the concentration, and the time of contact. The conditioner demineralized the peritubular and intertubular surface dentin, resulting in exposure of the collagen fibers.18 According to Pashley and Livingston,29 these conditioners increased the permeability of the dentin by 4 to 9 times.
The primer consisted of a bifunctional monomer in a volatile solvent such as acetone or alcohol. The bifunctional monomer was hydrophilic on one end, giving it an affinity towards the hydrophillic dentin. The other end of the monomer was hydrophobic with similar molecular configuration to the adhesive resin. Therefore, the primer, acting as a link, increased the wettability of the conditioned dentin and promoted greater contact between the adhesive resin and the underlying dentin. Examples of these bifunctional primers included HEMA (hydroxyethyl methacrylate), 4-META (4-methacryloxyethyl trimellitate anhydride), NPG (N-phenylglycine) and NSMA (N-methacrylol-5-aminosalicyclic acid).
The adhesive was either unfilled or partially filled resin that combined with the primer and created a hybrid layer. This hybrid layer was 1-µm to 5-µm thick and infiltrated a portion of the dentinal tubules, forming resin tags.30 First described by Nakabayashi31 in 1982, this hybrid layer was neither dentin nor resin but a combination or hybrid bioengineered structure32 and essential for resin bonding. The composite filling material was able to bond to the methacrylate groups in the adhesive.
One example of third-generation adhesive systems was ScotchbondTM Multi-Purpose (3M ESPE). The conditioner was maleic acid but was changed to phosphoric acid in 1994 because of its increased ability to dimineralize enamel. There was a second version of this system, ScotchbondTM Multi-Purpose Plus, that included a dual-curing catalyst and a ceramic primer. Other examples included XR Bonding System (Sybron/Kerr), Gluma® (Heraeus Kulzer), Tenure (Den-Mat), and Syntac Classic (Ivoclar Vivadent).
The fourth generation of dental adhesives was characterized by the complete removal of the smear layer.18 Both the enamel and dentin could be etched with phosphoric acid simultaneously; this method was referred to as the "total-etch" technique. Although Fusayama had published the "total-etch" concept in 1980 (2 years before Nakabayashi), it was controversial in the United States and Europe due to the fear of pulpal irritation.32 It was later determined that the sensitivity produced from this technique was from the bacterial leakage that was a result of inadequate sealing of etched cavities.33
One significant change from the traditional bonding technique was that the dentin surface was left moist to prevent collagen collapse after etching. Prior to this discovery, the technique was to dry the dentin after rinsing the etchant with water and allowing the primer to re-wet the dentin surface. Kanca found that if the dentin was kept moist with water after the acid etching step, postoperative sensitivity decreased and the bond strengths increased.34,35 Similar independent studies by Gwinnett36 and Sugizaki,37 published at the same time in different publications, showed that moist bonding enhanced the bond strength to dentin. The phrase "wet bonding" was coined for this technique. Examples of fourth-generation products included OptiBond (Sybron/Kerr), which later became OptiBond® FL, and All-Bond (Bisco Inc, www.bisco.com), which later became All-Bond 2. These fourth generation systems proved to be successful and helped promote the start of the "cosmetic dentistry" revolution.38
The technique sensitivity of "wet bonding" led to more simplified systems. This gave rise to the fifth generation of resin adhesives, which consisted of the "one-bottle systems" and the self-etching primer bonding systems.18 The "one-bottle" system combined the primer and adhesive, which was placed on the tooth cavity after etching the enamel and dentin with phosphoric acid. Realistically, it was a two-step technique but was marketed as a one-step. The self-etch system eliminated the acid-etching step by lowering the pH of the formulation enough to etch through the smear layer.32 Combining the three steps (etch-primer-adhesive) reduced the working time and decreased the number of steps for the clinician. Although these systems led to a less meticulous dentin bonding procedure, some studies suggested that the fourth-generation systems performed better.14,39-41 One concern was the etching effect of the lower pH formulas on enamel compared to the even lower pH of the phosphoric acid. Studies showed that the one-bottle system that incorporated the separate etching step had lower leakage than the self-etching systems.18,42 Examples of fifth-generation systems are OptiBond® Solo (Sybron/Kerr), Gluma® One Bond (Heraeus Kulzer, www.heraeus-dental-us.com) and Single Bond (3M/ESPE).
The elimination of phosphoric acid etching gave rise to the creation of the sixth generation of resin adhesives in the early 2000s. Referred to as "self-etching" adhesives, an acidic primer was substituted for phosphoric acid to condition the tooth.43 What popularized the self-etching adhesives was the reduction of postoperative sensitivity due to the exclusion of the phosphoric acid etchant.44,45 There were two types. Type I applied the primer and adhesive in separate steps and was light-cured or dual-cured.43 Type II was the first system to be self-etching, self-priming, and self-bonding all in one bottle but was light-cure only.43 The bond strengths of this adhesive generation varied between 18 MPa and 23 MPa.43 However, bond strength and microleakage are two different entities. An adhesive with high bond strength does not necessarily have a low microleakage.46 Waldman et al46 compared fifth- and sixth-generation methods and found the one-step adhesives lacked an efficient tissue-adhesive hybridized layer for adequate sealing between the dentin and the restorative. An increase in microleakage was found with the sixth-generation adhesive.
In 2002-2003, to make the bonding process even more convenient and less technique-sensitive, manufacturers placed all three steps-etching, priming, and bonding-into one bottle. The introduction of the one-bottle resin adhesive systems denotes the seventh generation. Early tests showed the newest generation of adhesives did not perform at the same level as the fourth generation.47 The fourth generation (three-step: etch-prime-adhesive) is considered the gold standard.14,38,48 Improvements have continued with the seventh generation of bonding agents because of their popularity and ease of use by clinicians. The success rates of the one-bottle resin adhesives have significantly improved and are now approaching levels of the fourth generation.49-54
Examples of seventh-generation resin adhesives include OptiBond® All-In-One (Sybron/Kerr), Xeno® IV (DENTSPLY Caulk), ClearfilTM S3Bond (Kuraray) and iBond® (Heraeus Kulzer).
Recently, a new product was introduced that could be considered a next generation of bonding agents simply because it is a combination of a resin adhesive and flowable composite. VertiseTM Flow (Sybron/Kerr) utilizes the adhesive technology of the OptiBond bonding system that creates a self-etching, self-adhesive, flowable composite. This eliminates the two-step process where the adhesive is placed first followed by a composite liner.
Modifications to resin adhesives have been simplified to either a one- or two-bottle system. The only other improvements to the systems at this point are the contents of the bottles. Creation of the hybrid layer, which is the goal of the resin adhesive, involves penetration of primer monomer into the tooth substrate. There are roughly 30 different types of monomers.55 One of the newest improvements to the adhesive arena is OptiBond XTR (Sybron/Kerr), which is actually a modification of sixth-generation resin adhesives. Glycerophosphate acid dimethacrylate is the monomer in OptiBond XTR and has been used in Sybron/Kerr's resin adhesives since the third-generation adhesive XR Bonding System. As previously mentioned, this same type of monomer was also used in the very first resin adhesive, Sevriton Cavity Seal in 1951. The modification is the addition of acetone to the water and alcohol in the solvent, which creates a more acidic primer, providing a more aggressive etching pattern of the enamel. According to the manufacturer, an increased etching of uncut enamel allows for the development of deeper and larger resin tags, resulting in higher bond strength when compared to Clearfil SE Bond (Kuraray) (Figure 1, Figure 2 and Figure 3). In addition to the deep resin tags (up to 70 µm), lateral branches have also been observed in the dentinal tubules (Figure 4). A major advantage for indirect restorations is that the film thickness is approximately 5 µm (Figure 5), compared to Clearfil SE Bond at 35 µm (Figure 6).
One of the major sources of incomplete resin curing for most resin cementing systems that occurs at the composite resin cement/resin adhesive interface is the neutralization of the benzyl peroxide reduction-oxidation (redox) initiator system. Uncured acidic monomers that remain in the oxygen-inhibited layer of the adhesive interfere with the polymerization process. During cementation the acidic groups from the oxygen-inhibited layer (adhesive) compete with the peroxides for the aromatic tertiary amines of the overlying resin (cement).56 This competition prevents the tertiary amines from participating in the redox reaction, which results in incomplete polymerization.57 This same mode of action occurs in light-cure versions as well. According to the manufacturer of OptiBond XTR, the chemistry was changed to prevent incompatibility with adhesive systems by replacing the tertiary amines with a proprietary amine-free initiating system. This bonding agent is universally compatible with both self-cured and dual-cure resin cements and composites.
There is a fundamental difference between fourth- and sixth-generation dentin adhesive hybrid layer formation.60 The obvious difference is the use of phosphoric acid etchant with the fourth generation, which is used to eliminate the smear layer (formed during cavity preparation), whereas the sixth generation incorporates the smear layer into the bonding process. The inclusion of the phosphoric acid etching during the application of the dentin adhesive also creates a level of greater technique sensitivity. Phosphoric acid etching not only removes the smear layer, but it also decalcifies the intertubular and peritubular dentin, thus opening the dentinal tubules and increasing their permeability.61 Several factors determine the depth of dentin decalcification, including the concentration, pH, and viscosity of the phosphoric acid as well as the application time.62 Air-drying dentin that has been acid-etched can collapse and shrink 65% in volume.32 The etched dentin matrix is composed of a collagen fibril network that will collapse when water is removed. The amount of water that remains after air-drying is critical. Insufficient amounts of water will collapse the dentin matrix to a point that results in an impermeable organic film that impedes resin infiltration.63 Some resin infiltration will ensue, forming resin tags, but voids will be created in areas where the resin cannot penetrate. Sano et al64 described the microporous zone that occurs in deeper demineralized areas lacking the infusion of monomer as nanoleakage. These zones can be created by over-etching the dentin where there is insufficient infusion of the monomer. This combination allows for the ingress of water, leading to hydrolytic activity of the bond.65
On the other hand, excess amounts of water can lead to dilution of the monomers, which limits the infiltration and interferes with the polymerization.66 The question of what is "too dry" or "too wet" is difficult to answer because optimal "wetness" fluctuates among different product manufacturers.32 The "wetness" factor is one of the reasons for the sensitivity in the technique when using fourth-generation adhesives.67
An advantage to the sixth-generation dentin adhesives is that the dentin can be thoroughly dried before the application of the self-etch primer. This nullifies the "wetness" factor concerning technique sensitivity.32 Unlike the fourth-generation adhesives where the dentin substrate is first demineralized and then stabilized with subsequent application of the primer, the sixth-generation adhesive simultaneously demineralizes and primes the dentin.60
The hybrid zone of the sixth-generation adhesive is typically thicker due to the incorporation of the smear layer into the hybrid layer, as opposed to the removal of the smear layer through etching and rinsing that occurs with the fourth generation.44,68
Prevention of marginal leakage is one of the primary objectives of any adhesive system. When assessing an adhesive system's ability to prevent microleakage there are several factors to consider. The physical characteristics of the material should be measured as well as the source of light polymerization, the tooth and surface location, adherence to the manufacturer's instructions, and clinical technique of the practitioner.69-71 Owens et al72 evaluated microleakage in vitro of self-etch and total-etch systems. A total of eight different systems were included in the study. The teeth included enamel and dentin margin locations, and the application of each system strictly followed the manufacturer's instructions. They concluded that there were no significant differences in the amount of dye penetration among the group in the area of dentin margins. There was, however, less leakage in the enamel margin locations. Their findings agree with other similar studies.73-75 Further evaluation of the enamel margins found the total-etch systems to have less microleakage than the self-etch systems. This is also in agreement with other similar studies.76,77 The similarity in the microleakage results at the enamel margin location could be attributed to the effect on the enamel by the phosphoric acid etchant versus the less acidic monomers in the self-etching systems.72 Scanning electron microscopy (SEM) shows greater penetration of total-etch primers after the enamel is conditioned with phosphoric etchant.78,79
Self-etching adhesives have simplified the dentin bonding technique. Notwithstanding, the effect of the milder acidic monomers (pH = 2.7) has decreased the effect on the enamel.80 Composite restorations bonded with self-etch adhesives typically suffer from marginal adaptation problems at the enamel surface.81 As previously stated, the milder acidic primers in self-etching systems do not remove the smear layer over the dentin surface but instead incorporate it in producing a hybrid layer. The total-etch system removes the smear layer over the enamel surface and creates microscopic spaces in the enamel that allow penetration of the resin and form resin tags that constitute the retention (mechanical bond) of a resin composite.82 Since the milder acidic self-etch systems do not remove the smear layer from the dentin surface, they do not remove it from the enamel surface either. In turn, the amount of primer penetration into the enamel is compromised.81 Mine et al83 studied the effect of the bur-type used in the cavity preparation and its influence on the debris that comprises the smear layer over the enamel. Their study concluded that a milder acidic primer performed better when an extra-fine diamond bur (15-µm grit-size) was used as opposed to a regular diamond bur (100-µm grit-size). Ermis et al84 found similar results in their study. Hannig et al85 recommended fine grit diamond bur (30 µm) be used for tooth preparation if a self-etching adhesive system was to be used. There are several manufacturers that recommend the use of phosphoric acid etchant on unground enamel (confined to the enamel only) prior to the application of the adhesive. This step will increase the penetration of the resin into the etched enamel surface.86 Cleaning the unground enamel with pumice is an alternate to using the phosphoric acid etch.87 Bond strengths are similar to those found with using the phosphoric acid-etchant step.86
Therefore, to improve the enamel marginal adaptation of a sixth-generation self-etching resin adhesive, consider the use of a finer grit diamond bur, an acidic primer with a low pH, and pumicing the enamel margins prior to application of the adhesive. Further study is necessary.Click here to view a case study.
Over the span of 60 years, the quest for adhesive dentistry has continuously been the search for the ideal product and technique whereby a restoration can easily be placed and last indefinitely. Presently, even though the goal is to minimize the number of bottles and steps, taking a step back and improving on a previous generation can be more significant and effective in the way dentistry is practiced.
The author would like to thank Ruth Egli for her editorial contribution. All ceramic work was completed by the author.
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This CE lesson was supported by an unrestricted educational grant from Kerr Corporation.
Gregg A. Helvey, DDS
Adjunct Associate Professor
Virginia Commonwealth University School of Dentistry
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