Luting Agents Selection of Luting Agents, Part 1

Sajid A. Jivraj, DDS, MSEd; Tae Hyung Kim, DDS; and Terry E. Donovan, DDS

Abstract

The clinical success of indirect restorations is dependent on multiple factors that include he practicing clinician has many choices with regard preparation design, mechanical forces, restorative material selection, oral hygiene, and to luting agents. No cur- rently available luting agent selection of a proper luting agent. The selection of the luting agent is dependent on the spe- is ideal for all situations and cific clinical situation, the type of restoration utilized and the physical, biologic, and handling a careful choice needs to be Tmade based on scientific rationale. properties of the luting agent. Ideal attributes of a luting agent are noted in (Table 1). These will be Although it is important to choose the best luting agent for each clinical situation, far greater discussed in reference to currently avail- able luting agents. variations in physical properties result from improper manipulation of a given luting agent than Adhesion to Tooth Structure 1 exist between different types of cements. One study listed loss of retention as the third-leading The primary function of a dental cement is to seal the restoration to the cause of prosthetic replacement, with failure occurring after only 5.8 years in service.2 tooth. This would occur if the cement The primary purpose of the luting procedure is to achieve a durable bond and to have good would biomechanically or biochemi- cally adhere to the prepared tooth. Zinc marginal adaptation of the luting material to the restoration and tooth. Conventional cements phosphate, which has been the most popular luting agent for the past 100 have always relied upon retention and resistance forms in tooth preparations; Adhesive-type years, does not chemically bond either luting agents offer the clinician an added advantage by bonding to the tooth structure.2 Guest editors / Sajid A. Jivraj, Three main types of conventional “cements” are commonly used, zinc phosphate and the DDS, MSEd, is chairman of the Section of Fixed polyelectrolyte cements polycarboxylate, and glass ionomer cements. Because of its long his- Prosthodontics and Operative tory of successful clinical use, zinc phosphate is considered the gold standard against which D e n t i s t r y , University of Southern California School of Dentistry. He also all other luting agents are compared because of its long clinical history of successful use. maintains a private practice limited to prosthodon- tics in Sherman Oaks and Torrance, Calif. Terry E. Donovan, DDS, is professor and division chair, Currently, two additional types of luting agents have gained considerable popularity. These Primary Oral Health care, University of Southern California School of Dentistry. 1 Author / Tae Hyung Kim, DDS, is chairman of the include the resin-modified glass ionomer cements and resin cements. The resin cement cat- Section of Removable Prosthodontics, University of Southern California School of Dentistry. egory includes light-cured, dual-cured and chemically cured agents. Acknowledgments / The patient treatment (Figures 6-10) was completed by student Ilya Mironov; The purpose of this article is to discuss the ideal attributes of a luting agent and make clinical Sajid A. Jivraj, DDS, MSEd, faculty supervisor. The patient treatment (Figures 13-18) was completed by student Arpita Sharma; Sajid A. Jivraj, DDS, MSEd, recommendations for their use. faculty supervisor.

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to the tooth structure or the restor- Table 1 ative material. However, when freshly mixed, zinc phosphate has a very low Desirable properties of a luting agent pH. This acidity allows for excellent 1. Adhesion to restorative material wetting of the tooth and for microme- 2. Adequate strength to resist functional forces chanical attachment to the prepared 3. Lack of solubility in oral fluids dentine. Retention depends on careful 4. Ability to achieve a low-film thickness under cementation conditions design of the tooth preparation and 5. Biocompatibility with oral tissues the quality of fit of the restoration. Several microleakage studies have dem- 6. Possession of anticariogenic properties onstrated significant linear penetration 7. Radio-opaque of silver nitrate from the external mar- 8. Relative ease of manipulation gin along the restoration/tooth inter- 9. Esthestic/color stability face after crown cementation with zinc phosphate cement. The significance of this microleakage will be discussed later in the article (Figure 1).4,5 cement. There is sufficient data to war- 2-5 micron zone of demineralization The polyacrylic-based cements bond rant their use as an alternative to zinc has been described with a 15-second to both enamel and dentine and are also phosphate in luting full-coverage res- application for conditioner. Prolonged claimed to have some affinity for metal torations.8,9 Their adhesion to enamel application results in a deeper deminer- and ceramic surfaces. This category of and dentin is similar to glass ionomer alization zone which resists resin infil- tooth adhesive cements includes poly- cement. An added advantage is that tration. If complete infiltration of the carboxylate and the glass ionomer, and these cements are able to bond to com- collagen by the primer does not occur, resin-modified glass ionomer cements. posite resin. the collagen within the deeper demin- Although they have some ability to With the advent of predictable den- eralized layer will be left unprotected bond to metals, they do not provide tin bonding, the resin cements can bond and subject to future proteolysis and adequate bond strengths to metal or to both tooth structure and restorative breakdown. After demineralization, a ceramic in some of the more demand- material. Adhesion to enamel occurs wetting agent, such as HEMA (hydroxy- ing situations encountered.6,7 through micromechanical interlocking ethyl methacrylate), is applied. HEMA Polycarboxylate cements exhibit of the resin to the hydroxyapatite crys- is bifunctional and hydrophilic, which chemical adhesion to the tooth through tals and the rods of etched enamel. allows it to bond to the dentine, and it interaction of free carboxylic acid Resin to dentin adhesion is obtained is also hydrophobic which allows it to groups with calcium. It is reasonable to by infiltration of resin into etched den- bond to the adhesive.10,11 assume that because of this adhesion, tin producing a micromechanical inter- It is reasonable to assume that lut- polycarboxylate cements would exhibit lock with partially demineralized den- ing agents that present stronger bonds less microleakage. However, microleak- tine, which underlies the hybrid layer. to tooth structure will also demonstrate age studies demonstrate they leak just Adhesion to dentine requires multiple less microleakage. This has been veri- as much as zinc phosphate. The glass steps beginning with the application fied by both in vitro and in vivo stud- ionomer cements form an ionic bond of an acid conditioner to remove the ies.4,5 Restorations cemented with resin to the tooth as a result of chelation of smear layer, open and widen tubules and resin-modified GIC exhibit reduced the carboxyl groups in the acid with and demineralize the top 2-5 microns of microleakage when compared to zinc the calcium and phosphate ions in the dentine. The acid dissolves and extracts phosphate cement. Conventional glass apatite of dentine and enamel.7 the apatite material and opens chan- ionomers also demonstrate significantly The resin-modified glass ionomer nels around the collagen fibers. These less microleakage than zinc phosphate cements also form much stronger bonds channels provide an opportunity for cements. What is the clinical significance to dentine than does zinc phosphate micromechanical retention. An optimal of reduced microleakage? A recent in vivo

150 CDA.JOURNAL.VOL.34.NO.2.FEBRUARY.2006 one should achieve a stronger bond to bond strength, and it was demonstrated the restorative substrate by silanating that delaying the time between silaniza- the ceramic or not. tion and bonding resulted in increased The research on silanization reports bond strength.15 From a practical per- higher bond strengths to the ceramic.13 spective, delaying the bonding time is It reports chemical bonding between the not feasible. ceramic and the resin composite. Use of Other studies have also investigated silane improves the wettability and con- the effect of the post-silanization drying tributes to the covalent bond formation time with a stream of warm air to deter- between porcelain and resin composite. mine if this could increase the tensile It also theoretically supports reinforce- bond strength of composite to ceramic ment of the ceramic through chemical over that produced by room tempera- bonding, theoretically decreasing the ture drying.16 Higher bond strengths Figure 1. Leakage pathway of cast crown. likelihood of fracture. were achieved with warm air and the study evaluated microleakage with cast Those against silanization argue failure mode was cohesive within the gold crowns cemented with zinc phos- that a greater bond strength to the composite. The results of the study phate. The study evaluated eight restora- ceramic is not required, the microme- concluded that use of a miniature blow tions that had provided a mean service chanical bond to the etched ceramic is dryer is effective in enhancing bond of 22 years. The teeth required extraction adequate. Increasing the bond to the strength of ceramic to composite than for periodontal reasons. When conven- restorative substrate results in uniaxial drying at room temperature. tional tests were done on these teeth, the shrinkage of the cement toward the Silane must be used appropriately. typical microleakage associated with zinc restoration and significant contrac- Imperative procedures include properly phosphate was demonstrated. However, tion gaps develop at the tooth cement sealed containers to prevent evapora- there was no evidence of recurrent car- interface. These gaps are thought to tion of solvent, heat drying following ies, sensitivity, or pulpal degeneration. result in microleakage and continue to applications, and a delay in bonding This data calls into question the clinical be a source of sensitivity.14 time. Those who do not use silane significance of microleakage studies.12 If the practitioner decides to silanate, must exercise proper care in etching There is no evidence that improved a number of variables need to be consid- the ceramic with hydrofluoric acid after adhesion to tooth structure improves the ered. When using silane, one mix and clinical try-in and remove ceramic pre- clinical performance of dental cements two mix silanes are available. The silane cipitates that form on the internal sur- for cast restorations. However, one must coupling agents contain a high volume face of the restoration.17 These residues be careful not to extrapolate this to the of various solvents. Improperly sealed have a potential to reduce the bond bonded restoration where adequate seal or open containers will allow evapora- strength of the ceramic to the compos- may play a major role in the survival of tion of the solvents and increase the ite (Tables 2, 3).18 the restoration. concentration of the coupling agent. Indirect composites were introduced If this occurs, the silane may act as a as an inlay/onlay material in an attempt Adhesion to Restorative Material separating medium reducing the bond to improve the mechanical proper- It is also thought that a strong bond strength between the ceramic and the ties over direct restorative materials. to the restoration is desirable. With composite. However, there is no clinical evidence conventional cements this would not Various authors have also evaluated to show that an improvement in physi- be an advantage because when the the effect of silanization of porcelain cal properties translates to an improve- crown loses retention, it is normally on the bond strength to composite. ment in long-term clinical success. A seen that the cement is retained with- The general trend observed was that number of studies have evaluated adhe- in the crown. With regard to ceram- application of a silane coupling agent sion between the resin cement and ic restorations luted with resin-based resulted in improved bond strength. the inlay/onlay material. Anecdotally, cements, there is controversy whether The heat treatment showed increased many clinicians have observed debond-

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Table 2 ing and execution of this restorative service.1 It has been shown that with Reasons in favor of using silane proper resistance and retention form Advantages that long-term clinical service of resin- 1. Higher bond strengths to ceramic bonded fixed partial dentures is at least equal to conventional cemented pros- 2. Provides chemical bonding theses.22 3. Reinforcing the ceramic decreasing propensity of fracture Various methods to develop adhe- sion between a prosthesis and a tooth have been developed. Initially the Table 3 approach was macroretentive but grad- Reasons against using silane ually adhesive procedures involving Disadvantages micromechanical and chemical bond- ing became available.23,24 1. Don’t need a higher bond strength to ceramic Another approach is the use of adhe- 2. Potential for postoperative sensitivity sion promoters such as silica coating, 3. Silane improperly used can act as a separating medium. tin plating, tribochemical coatings, and metal primers, which have been devel- oped to improve the bond between ing between the luting resin and com- resin composites prior to luting (micro- metal and the more conventional Bis posite inlay. filled and hybrids).18 The effect of GMA or urethane dimethacrylate resins. Microsandblasting of the composite hydrofluoric acid, phosphoric acid, mic- An important consideration in adhesion surface is a prerequisite for optimal bond- rosandblasting, and combinations were is whether one is seeking to bond a base ing.19 Indirect composites are also sec- investigated. The results of the stud- metal alloy or a precious metal alloy. ondarily cured and studies have shown ies showed that phosphoric acid and For the resin-bonded fixed partial that this curing causes a significant hydrofluoric acid alone did not produce dentures, the metal is etched, which reduction in availability of the bonds sufficient roughness to create mechani- removes one of the phases and provide and a consequent reduction in bond cal retention. In fact, the use of the micromechanical retention. This pro- strength to composite.20 One author hydrofluoric acid degraded the surface vides a surface onto which composite evaluated the adhesive bond strength of of the composite. With microfilled com- resin can adhere. The composite lut- resin cements to resin composites with posites, the glass particles were embed- ing resins are very similar to compos- and without secondary curing and with ded in the resin and were unavailable ite resin restorative materials in that and without microsandblasting.18 The for etching. Hybrids had higher bond they consist of Bis GMA, or urethane results of the study showed that second- strengths with etching than did micro- dimethacrylate resins, and a glass filler. ary curing only without sandblasting fills. The highest bond strengths were Where these resins are different is that resulted in a decreased bond strength achieved when microsandblasting fol- they are a two-paste system, which are to the resin cement. Sandblasting lowed by etching with phosphoric acid. either chemically or dual-cured. The improved the bond strength. The great- The microsandblasting roughens the filler particle size is less and the filler est bond strengths were achieved with- surface and the phosphoric acid cleans loading tends to be slightly less in order out secondary cure and with sand- any debris (Table 4, Figures 2-6). to ensure a lower film thickness. blasting.21 When using these types of Resin-bonded fixed partial dentures One of the drawbacks of the tech- restorations, microsandblasting of the have an undeserved poor reputation in nique is the reluctance of clinicians fit surface should always be performed the minds of many practitioners who and laboratory technicians to use to prior to bonding. believe that such prostheses have a beryllium containing Ni-Cr alloys. The authors have also evaluated relatively short life span. This reputa- Nonberyllium containing Ni-Cr alloys various surface treatments of indirect tion has resulted from improper teach- do not etch as well.

152 CDA.JOURNAL.VOL.34.NO.2.FEBRUARY.2006 Table 4 Pretreatment of indirect composite prior to bonding ■ Secondary curing causes reduction in availability of bonds. ■ Microsandblasting improved the bond strengths. ■ Highest bond strengths are achieved with microsandblasting followed by cleaning with phosphoric acid. Figure 2. Recurrent caries beneath amalgam Figure 3. Indirect composite restoration. restoration patient requested esthetic alternative.

Figure 4. Fitting surface of indirect compos- Figure 5. Thirty-seven percent phosphoric Figure 6. Indirect composite on No. 2. ite microsandblasted to improve bond strength. acid used to clean debris on fit surface.

Precious metal alloys also cannot metal surface, these luting agents are Adequate Strength to Resist be etched since they have a relatively referred to as chemically adhesive lut- Functional Forces homogenous microstructure, hence ing resins to differentiate them from Many clinicians believe that it is not possible to use the etch- the Bis GMA resins C&B superbond increased strength of the luting agent ing technique for resin bonding with is one example (based on carboxylic will increase the retention of the cast- these alloys. monomer 4 META). ings on the teeth. Scientific evidence for Because of the trend to move away Another luting agent which has been this belief is lacking and it is becoming from beryllium-containing alloys, modified to contain a phosphate mono- increasingly clear that crown retention laboratories had to find some other mer is Panavia 21 from Kuraray (MDP is a function of resistance and reten- means of bonding to the alloy. The methacryloxyethylphenyl phosphate). tion form coupled with accuracy of fit problem is Bis GMA and UDME res- Resin bonding is facilitated by the high of the casting. Clinical experience with ins do not bond well to etched metal affinity of the carboxylic acid or phos- provisional luting agents and resin- surfaces and rely primarily on micro- phoric acid derivative containing resins bonded fixed partial dentures support mechanical and physical adhesion. In for the metal oxide on the base metal this belief.1 order to improve the adhesive bond alloy, they can provide a durable bond There are substantial differences in to metal, a variety of composite resins to nickel chromium alloys. They have strength between the different groups have been developed in which the low affinity for precious metal alloys, of luting agents. The question always resin component has been modified such as gold and palladium, due to lack arises, if it is stronger, does it mean it to be able to bond chemically to the of surface oxide coating. is better?

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with some dissolving much cement and others dissolve none. The timing of the test is also critical. Glass ionomer cement is quite soluble within the first 24 hours and perform poorly in a 24- hour test. However after the initial 24 hours, glass ionomers are quite resistant to dissolution and hence perform very well in a long-term clinical test. The lat- 1 Figure 7. Short clinical crowns, which ter is more clinically significant. would lack resistance and retention form if pre- The issue should not be the solubil- pared for full-coverage restorations. Figure 8. Preparations must provide ity of the cement but rather the fit of adequate resistance and retention form. the restoration. With excellent fitting restorations solubility is secondary. Cement strengths are generally Many clinicians have experienced compared using the parameters of delivering a definitive restoration with a Ability to Achieve a Low–Film compressive strength and diametral provisional cement only to find that the Thickness Under Cementation tensile strength, with the latter being restoration is very difficult to remove. Conditions considered more important to clini- These provisional cements have much Film thickness is influenced by a cal performance. Compressive strength inferior physical properties than the number of factors including particle tests are done with cylindrical sam- permanent cements, yet still retain the size of the powder, cementation force ples and diametral tensile tests are casting in place over the long term. and technique, viscosity and the use of done using disc-like samples. Neither This calls into question the value of specific techniques such as diespacing, of these tests evaluates the cement in increased strength. venting, or placement of escape chan- the mode, in which it is used, which nels. is a thin film of 25 microns. Testing Lack of Solubility in Oral Fluids ADA stipulations state that luting will reveal that zinc phosphate has the Significant differences in solubil- agents must achieve a film thickness lowest compressive and diametral ten- ity exist between the different luting of no more than 25 microns under sile strength while resin cements have agents. The literature on solubility dem- the conditions of the specification test. values which are much higher. The onstrates the necessity of relying too With this test, a mix of cement is com- clinical significance of these values can heavily on in vitro data to predict clini- pressed between two glass slabs with a be questioned. If the clinician is con- cal performance. Tests are done in which specified amount of force, and then the fronted by preparations with a short the cement sample is immersed in a increase in thickness of the two slabs is wall height, can the strength of resin solute for 24 hours and the weight loss measured. This increase in thickness is cements be used to provide long-term of the sample is recorded or an increase designated film thickness. Most luting retention for restorations? of the cement component in the solute agents can achieve the required film Increased strength of cements is measured. Under these conditions, thickness under the specifications of the will not increase retention of castings zinc phosphate appears to be the least test but the same luting agent may pro- cemented on prepared teeth (Figures soluble and glass ionomer the most sol- duce excessive casting elevations when 7, 8). It may provide retention for the uble. However clinical studies show the the restorations are luted in place.27,28 prostheses in the short term but even- opposite to be true.24-26 The difference What the clinician must understand is tually the cement will undergo fatigue between in vitro and vivo studies can that values reported in trade journals failure and the prosthesis will dece- be explained by patient variability and may not be representative and film ment. Increasing the strength of the timing of the test. Tremendous variabil- thickness is more than just a material cement will not compensate for lack of ity can exist between patients in terms property. Ultimately, it’s how the prac- retention and resistance form.1 of their potential to dissolve cement titioner manipulates the luting agent

154 CDA.JOURNAL.VOL.34.NO.2.FEBRUARY.2006 rather than the physical property of the adhesive and curing, an impression is centrations of fluoride must be released luting agent itself. made. It appears that immediate den- over a period of time, and the material tine bonding has several advantages itself should not suffer from any signifi- Biocompatibility With Oral Tissues by sealing the dentinal tubules prior to cant degradation. Of the conventional When luting agents are used they impression-making: cements, the glass ionomers have been will invariably contact a large area ■ No contamination of the den- reported to have long-term fluoride of dentin, hence the susceptibility to tal tissues by impression material or release and cariostatic activity of these producing postoperative sensitivity or cement, cements has been proposed.33 However, pulpal inflammation is a very impor- ■ Stabilization of the adhesive layer even if fluoride is released, one must tant consideration. prior to subjecting the adhesive inter- question just how much fluoride is An ideal luting agent would not be face to stresses, and released from the margins of a well- harmful to the dental tissues. It was ■ Reduction of postoperative sensi- fitting restoration, and whether this long thought that cements containing tivity. amount of fluoride has any significant phosphoric acid cause pulpal inflam- impact. Scientific studies have been mation as a result of their low pH. For inconclusive in showing that the thin many years clinicians applied copal var- It is likely that all film of cement at the margin of a resto- nish over the prepared tooth to protect ration has any significant clinical thera- the pulp from the acidity of zinc phos- commonly used peutic value as a cariostatic agent. phate cement. Research has challenged dental cements elicit this long-held belief and it is likely no long-term pulpal Radio-opacity that all commonly used dental cements An ideal luting agent should be radio- elicit no long-term pulpal response and response and hence opaque to enable the practitioner to dis- hence meet the criteria for biocompat- meet the criteria for tinguish between the cement, the tooth, ibility.29 biocompatibility. and the restoration. Combinations of Postoperative sensitivity has also composite luting cements/and or glass been rightly or wrongly attributed to ionomers may show gap-like features the luting agent used. Clinical symp- because of difference in radio-opacity. It toms such as sensitivity after crown It appears to be a perfectly rational is important that dental cements have cementation are more likely because way to seal and protect the dentino- greater opacity than dentine. of microleakage than pulpal inflam- pulp complex, prevent sensitivity, and It is impossible radiographically to mation resulting from damage caused bacterial leakage during the provisional detect excess luting agent if the mate- by the luting agent. Well-controlled phase. rial is radiolucent. In practice, luting clinical trials using a strict protocol for agents come in a wide range of radio- cementation have demonstrated that Possession of Anticariogenic opacities. the sensitivity is clearly operator-related Properties One study showed zinc phosphate and can essentially be prevented with Many luting agents have been to be the most radio-opaque. The dual proper technique.30,31 described as having anticariogenic polymerized and conventional glass ion- A concept which has been intro- properties. A number of these have omer showed the same as human enam- duced during the last few years is the been marketed on this premise. Many el. The RMGI are intermediate between idea of “immediate dentin sealing.” manufacturers claim that their specific enamel and dentine. The autopolymeriz- This has been primarily advocated for brand of cement releases fluoride, but ing luting agents had similar radio-opac- adhesive-type restorations. It has been the clinical efficacy of such claims has ity to dentine and were the lowest.34 demonstrated that effective adhesion not been investigated. can be achieved by immediately apply- The fact that a material contains flu- Relative Ease of Manipulation ing the adhesive following tooth prepa- oride does not necessarily endow it with One of the most important attributes ration.32 Following application of the anticariogenic properties. Sufficient con- of any dental material is that it be user

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friendly and relatively easy to manipulate. polymerization shrinkage and their sen- techniques such as slaking, incremental It is important that cements be sitivity to moisture. Using resin cements mixing, use of a chilled slab, and mix- mixed according to manufacturer’s with restorations that have subgingival ing over a wide area to dissipate the recommendations and with meticu- margins is problematic. Removal of the heat of the exothermic reaction. lous attention to detail, far greater excess becomes difficult because of the With resin cements there is a variations occur from improper mixing hardness of the cement and its adhesion choice between dual-cured and light- rather than selection of type of cement. to the tooth. cured resins. Cements may be handmixed or come Resin-modified glass ionomer The light-cured resins have some in pre-dosed capsules or syringes. The cements are less technique-sensitive purported advantages in that work- auto-mix cartridges are an advantage than the resin cements and in auto-mix ing time is increased, the ability to because they allow a consistency of cartridges, can prove to be an extremely remove excess, and reduced finish- mix, convenience, and less clean up is ing time. Dual-cured cements have required. Disadvantages include great- traditionally been used when ceramic er expense and the inability to vary the thickness did not allow light pen- viscosity. The working time etration for maximal conversion of Amongst the conventional luting and setting time are the luting cement. Disadvantages of agents, zinc phosphate appears to be dual-cured cements include porosity the least technique-sensitive. A specific considerations in the from mixing, reduced working time, protocol is required with the use of zinc choice of luting agent, degree of conversion, and color insta- phosphate, and as long as these recom- the longer working bility due to amine degradation. One mendations are followed long-term suc- author investigated both dual- and cess will be achieved.1 time being needed for light-cured cements in regard to con- Polycarboxylate cements are also long-span prostheses version rate under cerec inlays.36 more technique-sensitive. They exhibit vs. single-unit The following parameters were eval- a thixotropic behavior where an appar- uated: the effect of ceramic thickness, ently viscous mix flows readily under restorations. use of a light reflecting wedge, and pressure. However, they exhibit a rapid varied the time of curing. Following increase in film thickness that may curing, Vickers hardness at the pulpo- impede proper seating of a casting. efficient way of delivering cast resto- axial wall was measured. It was con- During setting, polycarboxylate cements rations. One of the disadvantage of cluded that dual-cured cements offered go through a rubbery stage, and at this RMGIC is the hydrophillic nature of the no advantages over the light-cured time should remain undisturbed during polyhema, which results in increased cements, provided an extended cur- setting to prevent it from being pulled water sorption and subsequent plastic- ing mode 120 seconds was used. One away from under the margins. ity and hygroscopic expansion. The question which often arises is curing One disadvantage of polycarbox- continual water resorption does have of the luting agent beneath excessive ylate cements is they exhibit plastic deleterious effects. Potential for sub- thicknesses of ceramic. An alternative deformation and thus the cement is stantial dimensional change contrain- approach to avoid excessive thicknesses not suited for use in areas of high dicates their use with all-ceramic type of ceramic is to build the tooth up in masticatory stresses or in cementa- restorations. composite material. tion of long-span prostheses.35 Their The working time and setting time The Young’s modulus of composite is use is confined to single units in low are considerations in the choice of lut- more like dentin as opposed to ceramic, stress areas. These cements may also ing agent, the longer working time which is a more enamel-like material. be used to secure long-term provi- being needed for long-span prosthe- The core of the tooth may be built up sional restorations. ses vs. single-unit restorations. With to minimize thickness of ceramic in the Resin cements are extremely tech- conventional luting agents, the work- definitive restorations. Traditionally, cli- nique-sensitive because of their inherent ing time can be varied by utilizing nicians removed tooth structure to elim-

156 CDA.JOURNAL.VOL.34.NO.2.FEBRUARY.2006 inate undercuts so that the preparation because the dentin is not etched, the significant difference to dentin. Acid would allow a single path of insertion. smear layer is not removed, and the etching raised the microtensile bond Today, clinicians are able to block out dentinal tubules remain closed. The strength to that of the control cement undercuts with composite and avoid multistep of etching, priming, and but was detrimental to the dentine the unnecessary destruction of tooth bonding are not required so there is bonding effectiveness. This was due to tissue so that a single path of insertion little risk of overdrying, overly moist inadequate infiltration of the collagen may still be realized. In this manner, the dentin and generation of nanoleakage mesh. thickness of ceramic can be optimal for by inadequate preparation of the primer SEM evaluation showed that Rely use of light-cured resins.37 and bonding system. X cement interacted only superficially Another category of luting agents One group of authors evaluated with enamel and dentine, and applica- that has recently been introduced is the the microtensile tensile bond strength tion, using some pressure, is required auto-adhesive group. It is not the pur- for close adaptation of the cement to pose of this article to describe the dif- the cavity wall. There was negligible ferent mechanisms of adhesion of resin chemical bonding. cements but briefly just to describe the There is no three categories. ideal luting agent Esthetics and Color Stability Etch and rinse, self-etch adhesive Esthetics, although not a major con- along with a low-viscosity resin com- for all procedures; sideration with metal and metal ceram- posite, and the self-etch, which also and choice is ic restorations does become an issue includes the self-adhesive resin. This dependent on with translucent porcelain restorations. third category of resin cement is becom- Light transmission and color stability of ing very popular with practitioners physical properties, the luting agent are important in this because of the reduced chairtime and technique regard. Expanded kits of resin cements a simpler application protocol. It com- sensitivity and with tints, opaques, and multiple shades bines the adhesive and resin in one are tailored to anterior ceramic restora- product eliminating the need for pre- evidence base. tions and supposedly allow subtle shade treatment of both tooth and restorative corrections to be made. Caution should material prior to cementation. be exercised in this approach. In prac- The adhesive properties are based on of RelyX Unicem(3M Espe) to enamel tice, the color of a try-in paste may dif- acidic monomers that demineralize and and dentin.38 The experimental pro- fer significantly from the luting agent. infiltrate the tooth substrate resulting in tocol also evaluated the interaction of Of three shades of three bands tested micromechanical retention. A second- this material with dentin by means of by one author, all but two had easily ary reaction of this cement is to provide high resolution electron microscopy. detectable color differences.39 For ante- chemical adhesion to hydroxyapatite. The authors compared the microtensile rior esthetic restorations, color of the Several other purported advantages bond strength to enamel and dentine restoration should be developed with from manufacturers include: with and without etching after 24- intrinsic characterization techniques ■ It is dual-cured and achieves a hour water storage compared to the and the restorations bonded with a uni- bond to tooth structure similar to that bonding effectiveness of the control versal luting agent. achieved by multistep adhesives, cement Panavia F (Kuraray). The inter- Color stability over time should be ■ Mechanical properties are sup- face between the dentine and the luting considered.40,41 The amine accelerator posedly superior to zinc phosphate and agent was also examined with a scan- necessary for dual cured polymeriza- glass ionomer cements, ning electron microscope. tion can cause the color to change over ■ It has excellent moisture toler- The results showed that microten- time. This in itself may be another ance and manufacturers state that a sile bond strength was significantly reason for choosing light-cured resins rubber dam need not be used, and lower than that of the control cement over dual-cured when bonding esthetic ■ Little risk of postop sensitivity to etched enamel, while there was no restorations.

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Clinical Recommendations delivery systems make dispensing and ally impossible to expose the cement There is no ideal luting agent for all clean up much easier. From a practical to an adequate amount of light to procedures; and choice is dependent perspective, it is easier to mix than zinc enable it to set. The cement should be on physical properties, technique sen- phosphate cement. Long-term clinical opaque to mask the color of the metal sitivity and evidence base. The type of data also warrants its use. that may alter the shade of the abut- restoration being fabricated also has Polycarboxylate cements should ment teeth. an important role in the selection of a only be used for single-unit restoration. luting agent with the requirements of Plastic deformation over time limits Porcelain Veneers bonded restorations being very different their use when luting fixed-partial den- The luting agent of choice here is to that of cast restorations. tures. With polycarboxylate cements, light-cured resin cement. Dual-cured Physical properties of a luting agent, the setting reaction proceeds rapidly cements exhibit color instability over although somewhat important, cannot and the mixing should be completed time and can affect the esthetics of the be used as the sole basis for selecting a within 30 to 40 seconds. restoration in the long term. This lut- cement based on discussions earlier. ing agent should also be radio-opaque Two criteria to look at would be evi- Dowel and Cores (Figures 9-14). dence-base and technique-sensitivity. It It appears from what has been dis- The type of composite to use also is reasonable to make the statement that cussed that luting of cast dowels should requires consideration. unless a specific indication for a given be carried out with zinc phosphate. ■ Heavily filled composites are luting agent exists, the least technique- Glass ionomers should not be used desirable. The viscosity of these can sensitive material should be utilized. because they do not attain adequate be reduced by warming the carpule The following is a brief summary strength in their early setting stage, and of composite contained in a clear, of clinical recommendations. A more frequently such dowels do not fit the plastic, waterproof bag in hot water. detailed analysis of luting of all ceram- canal space with precision. This may The concern with this is that seating ic restorations will follow in Part 2 of result in excessive film thicknesses of may be a little more difficult and the the article. GIC, which may weaken it. risk of fracture with excessive seating There is a tendency to go toward force exists. The advantage of using Gold Castings and Metal Ceramic bonding of cast dowel and cores. a slightly heavier filled composite is Restorations Excellent retention can be obtained that it makes clean up much easier for There are a number of luting agents using the proper technique. This tech- the operator. available when seating a well-fitting nique would involve etching the inter- ■ Composites with low filler content cast restoration. nal of the canal, applying a hema- have worked very well in the past but ■ Zinc phosphate would be consid- based primer and then the activated excess is more difficult to control and ered for its long clinical history of use. monomer.42 The activated monomer increased time is required for clean up. Also, it has a long working time when is applied to the dowel, which is then correctly mixed zinc phosphate materials coated with adhesive resin and then the Ceramic Inlays/Onlays are indicated for multiunit fixed-partial dowel is inserted. However, if retrieval The advantages of light-cured res- dentures, as well as full-arch restorations. of the dowel is required for endodon- ins have been discussed previously. ■ Glass ionomer could be used but tic retreatment, it becomes a difficult There is easier clean up, command set, the variables of the mixing procedure endeavor. Practitioners should exercise and a homogenous mix. With dual- should be controlled. caution in this approach. cured resins, there is limited working ■ Glass ionomer does have a short- time and the possibility of porosity on er setting time in comparison to zinc Resin-Bonded Fixed-Partial mixing. Excessive bulk of ceramic can phosphate. Dentures be avoided by building the tooth up ■ Resin-modified glass ionomer Chemical cured resin cements such with composite prior to preparation so cement is also appealing because of as Panavia should be used. The chemi- that optimal thicknesses of ceramic are its user friendly nature. The auto-mix cal cure is essential because it’s virtu- attainable.

158 CDA.JOURNAL.VOL.34.NO.2.FEBRUARY.2006 Figure 9. Preoperative composite veneers Figure 10. Teeth Nos. 8 and 9 prepared and Figure 11. Porcelain restorations etched on teeth Nos. 8 and 9. dentin immediately sealed. with 10 percent hydrofluoric acid for 90 seconds.

Figure 12. White chalky color denotes Figure 13. Veneers bonded under rubber Figure 14. Final postoperative situation formation of ceramic residues following etching. dam isolation. with ceramic veneers on Nos. 8 and 9 and com- These residues must be removed to improve bond posite restoration on No. 10. strengths. Precipitates removed by placing veneers in ultrasonic bath for three to four minutes. Internal of veneer should have a clean surface.

References / 1. Donovan TE, Cho G, Contemporary All-Ceramic Crowns cal properties, longevity and suitability evaluation of dental cements. Compend 20(3):197- ■ Luting of all-ceramic crowns is in clinical situations. It appears that a 219, March 1999. 2. Walton JN, Gardner FM, Agar JR, A survey of dependent on the substrate that is being single agent is not suitable for all appli- crown and fixed-partial dentures failures: length of utilized. cations. Physical properties should not service and reasons for replacement. J Prosthet Dent 56:416-21, 1986. ■ Ceramic restorations available be a sole criteria for selection because 3. Brien WJ, Dental materials and their selec- today are either etchable or non-etch- improvement in many of the apparent- tion. Third ed., Chicago. Quintessence 1999. 4. White SN, Sorensen JA, et al, Microleakage able based on the core material. ly important physical properties has not of new crowns and fixed-partial denture luting ■ Etchable are the silica-based automatically resulted in an improve- agents. J Prosthet Dent 67:156-61, 1992. 5. White SN, Yu Z, et al, In vivo microleakage ceramics feldspathic, leucite-reinforced, ment in clinical performance. of luting cements for cast crowns. J Prosthet Dent and glass ceramics. To date, no single luting agent can 71:333-8,1994. 6. Richter WA, Mitchem JC, et al, Predictability ■ Non-etchable are the nonsilica- completely compensate for the short- of retentive values of dental cements. J Prosthet Dent based ceramics such as aluminum oxide, comings of preparation retention 24:298-303. 7. Wilson AD, et al, Mechanism of adhesion zirconium oxide. Part 2 of this article and resistance form or ill-fitting, low- of polyelectrolyte cements to hydroxyapatite J Dent will discuss in detail luting protocols for strength restorations. Practitioners must Res 62:590-2, 1983. 8. Yoneda S, Morigami M, et al, Short-term these types of restorations. be aware of the virtues and shortcom- clinical evaluation of a resin-modified glass iono- Luting agents possess varied, com- ings of each type of luting agent and mer luting cement. Quintessence Int 36(1):49-53, January 2005. plex chemistries that affect their physi- select them appropriately. CDA 9. Van Dijken JW, Resin modified glass iono-

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