Jum'ah AA, Creanor S, Wilson NHF, Burke FJT, Brunton PA. Dental practice in the UK in 2015/2016: Part 3. Aspects of indirect restorations and fixed prosthodontics. Br Dent J. 2019; 226:192-196
Randall RC, Wilson NHF. Glass ionomer restoratives: a systematic review of a secondary caries treatment effect. J Dent Res. 1999; 78:628-637
Papagiannoulis L, Kakaboura A, Eliades G. In vivo vs in vitro anticariogenic behaviour of glass-ionomer and resin composite restorative materials. Dent Mater. 2002; 18:561-569
Cury JA, de Oliveira BH, dos Santos APP, Tenuta LMA. Are fluoride releasing dental materials clinically effective on caries control?. Dent Mater. 2016; 32:323-333
Pameijer CH, Jefferies SR. Retentive properties and film thickness of 18 luting agents and systems. Gen Dent. 1996; 44:524-530
Zidan O, Ferguson GC. The retention of complete crowns prepared with three different tapers and luted with four different cements. J Prosthet Dent. 2003; 89:565-571
Heintze SD. Crown pull off test (crown retention test) to evaluate the bonding effectiveness of luting agents. Dent Mater. 2010; 26:193-206
Burke FJT, Qualtrough AJE, Hale RW. Dentin-bonded all-ceramic crowns: current status. J Am Dent Assoc. 1998; 129:455-460
Burke FJT. Maximising the fracture resistance of dentine-bonded all-ceramic crowns. J Dent. 1999; 27:169-173
Burke FJT. Four year performance of dentine-bonded all-ceramic crowns. Br Dent J. 2007; 202:269-274
Hill EE, Lott J. A clinically focused discussion of luting materials. Aust Dent J. 2011; 56:67-76
Burke FJT, Fleming GJP, Nathanson D, Marquis PM. Are adhesive technologies needed to support ceramics? An assessment of the current evidence. J Adhes Dent. 2002; 4:7-22
De la Macorra JC, Pradies G. Conventional and adhesive luting cements. Clin Oral Investig. 2002; 6:198-204
Attar N, Tam LE, McComb D. Mechanical and physical properties of contemporary dental luting agents. J Prosthet Dent. 2003; 89:127-134
Han L, Okamoto A, Fukushima M, Okiji T. Evaluation of physical properties and surface degradation of self-adhesive resin cements. Dent Mater J. 2007; 26:906-914
Burke FJT, Crisp RJ, Richter B. A practice-based evaluation of the handling of a new self-adhesive universal resin luting material. Int Dent J. 2006; 56:142-146
Christensen GJ. Clinical Research Associates. CRA Newsletter. 2003a; 27
Christensen GJ. Clinical Research Associates. CRA Newsletter. 2003b; 27:1-2
Burke FJT, Crisp RJ, Cowan AJ, Lamb J, Thompson O, Tulloch N. Five-year clinical evaluation of zirconia-based bridges in patients in UK general dental practices. J Dent. 2013; 41:992-999
Thompson O, Tulloch N, Crisp RJ, Burke FJT. A case series of zirconia-based bridges luted with a self-adhesive resin luting material at 12 years, in patients in UK general dental practices. Eur J Prosthodont Rest Dent. 2018; (Spec Issue)17-20
The Dental Advisor. 3M™ RelyX™ Unicem Self-Adhesive Resin Cement. 15-year clinical performance. Dental Consultants Inc. 2016;
Burke FJT, Fleming GJP, Abbas G, Richter B. Effectiveness of a self-adhesive resin luting system on fracture resistance of teeth restored with dentine-bonded crowns. Eur J Prosthodont Rest Dent. 2006; 14:185-188
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Luting materials are a central component of indirect dentistry, with the most recently introduced types being resin-based materials. These may be classified into conventional resin luting materials, self-adhesive resin luting materials and ‘smart’ resin luting materials. Their physical properties have been found to be good, with no risk of dissolution in the dilute organic acids found in plaque, such as occurs with phosphate and glass ionomer cements. Uses of resin luting materials include the luting of any indirect restoration, but these materials have facilitated the development of the resin-retained bridge and aesthetic ceramic or composite restorations, notwithstanding the fact that they may provide, for the first time, truly adhesive luting of crowns and inlays.
CPD/Clinical Relevance: Resin luting materials are appropriate for placement of all indirect restorations: the self-adhesive variants have simplified the use of such materials.
Article
Luting materials are a central component of indirect dentistry. These, essentially, fill the void at the interface between the restoration and tooth (or implant) and therefore must fulfil basic mechanical, biological and handling requirements.1
Must not harm the tooth or periodontal tissues that it contacts;
Should adhere to tooth substance and restoration, either chemically, mechanically, micromechanically, or all three;
Must allow a sufficient working time to place the restoration;
Must have sufficiently low film thickness and fluidity to allow complete seating of the restoration;
Must quickly form a hard mass which is strong enough to resist functional forces transmitted through the restoration: this should include high tensile and compressive strengths, high fracture toughness and a modulus of elasticity which is appropriate for stress absorption;
Must not dissolve or wash out, ie must be insoluble in the dilute organic acids found in plaque, given that plaque may collect in patients with suboptimal oral hygiene and/or poorly fitting or poorly contoured restorations;
Have radio-opacity similar to or greater than dentine;
Must maintain a sealed, intact restoration, ie resist water sorption;
In addition, the ideal luting material should be cariostatic, technique insensitive, capable of easy and accurate proportioning if presented in powder/liquid form and, if being used to lute all-ceramic and/or restorations in the aesthetic zone, be available in a range of shades.
Early luting materials
Zinc phosphate cement was introduced circa 125 years ago. Despite its shortcomings, which include solubility in the dilute organic acids found in plaque, and poor tensile strength, it remains in use by some practitioners. In this regard, results of a recent survey of 500 UK-based general dental practitioners (GDPs) indicated that 14.6% of respondents continued to use phosphate cement for cementation of metal-ceramic single unit crowns.3 It could be considered to be a ‘passive’ luting material, insofar that it simply fills the space between the restoration and the tooth, without adhering to either.
The introduction of glass ionomer (GI) cement facilitated the development of a luting material derived from the same technology in the late 1970s. These cements contain an ion-leachable fluoro-alumino-silicate (FAS) glass which reacts with an aqueous poly(alkenoic) acid, with the outer layers of the FAS glass being dissolved, leading to the release of Ca2+, Al3+ and F- ions. While fluoride is released, this is not considered to have any cariostatic effect in vivo.4,5,6 These luting materials proved popular, but again had suboptimal physical properties and, like phosphate cement, were soluble in dilute organic acids.
The resin modified glass ionomer (RMGI) materials, while containing the ingredients of the traditional GI materials, incorporated a monomer into their formulation, such as 2-hydroxyethyl-methacyrylate (HEMA) or bisphenol A-glycidyl methacrylate (Bis GMA): their associated luting materials overcame many of the shortcomings of GI materials, demonstrating improved physical properties and minimizing dissolution in oral fluids and were stated as being used by 28.2% of the respondents in the recent UK survey.3 These materials go through a ‘rubbery’ phase during setting, in a manner similar to phosphate cements: this facilitates clean-up following the placement of the restoration. A summary of the properties and potential uses of currently-available luting materials is presented in Table 1.7 From this, it is apparent that, while any luting material may be used for the placement of any indirect restoration, the number of advantages of the materials in the table increases as one travels towards the bottom of the table. If lack of solubility and good physical properties are considered important, then the RMGI and resin luting materials may be considered optimal, but if potential for adhesion to tooth and restoration are added to these criteria, then only the resin luting materials may be considered optimal. Such adhesion to tooth and restoration may facilitate restoration retention should the preparation geometry and resistance form be suboptimal (vide infra) (as may sometimes be the case if and when a tooth requires, for example, a crown).
Type of Luting Material
Advantages
Disadvantages
Zinc phosphate
Ease of useEase of clean-upAdequate compressive strengthAdjustable working time by varying mixing protocols
Highly soluble in oral fluidsLow pH, so potential for pulpal irritationNo adhesion to tooth or restoration(s)Technique sensitive mixing technique
Zinc polycaroxylate
Low potential for pulpal irritationSome adhesion to tooth
Highly soluble in oral fluidsLow compressive strengthHigh potential for leakageDifficult clean-up (due to stickiness of material)Adhesion to restorations not possible
Glass ionomer
Adhesion to toothAdequate compressive and tensile strengthsEasy to mix
Moisture intolerance during settingHistory of post-up sensitivityDifficult clean-upLow fracture toughnessAdhesion to restorations not possible
RMGI
Good biocompatibilityFluoride releaseImproved adhesion to tooth (compared to GI)Improved physical properties, particularly tensile strengthLow solubility in oral fluidsEasy clean-up
Expansion of some materials under moist conditions caused fracture of all-ceramic restorations: this problem appears to have been overcomeNo adhesion to restorations
Resin luting materials
Excellent physical propertiesLow solubility in oral fluidsPotential for adhesion to tooth via a dentine-bonding agentTooth-colouredCapable of being bonded to restorationsEnhanced marginal seal
May be technique sensitive if separate dentine-bonding procedure requiredClean-up time is criticalMoisture control is criticalNeed to finish restoration margins
Resin luting materials
The introduction of resin luting materials heralded a new concept in luting because, for the first time, truly adhesive luting of crowns and inlays was possible. Why was this a clinical benefit? Because no clinician can achieve a retentive preparation and the ideal preparation geometry all of the time, especially when treating teeth with reduced coronal height because of wear. Three papers testify to the benefits of adhesion when resin luting materials are being utilized:
Pameijer and Jefferies8 tested 18 luting materials, using extracted premolars with standardized cone-shaped preparations with 33° taper, then constructing gold copings which were cemented with a tensile force being applied after 24 hours. The results indicated that polycarboxylate cement produced the lowest value for retention, that Ketac™ Cem (ESPE), a GI-based luting material, produced retention that was twice that of phosphate cement: also, that dentine bonding and resin cement produced highest values for retention.
Zidan and Ferguson9 made complete crowns prepared with three different tapers and luted with four different cements, in their laboratory. Results indicated that retention of the crowns luted with the adhesive resins investigated were 20% higher at 24-degree taper than the retentive values of conventional cements at 6-degree taper, with the authors concluding ‘as the resin luting materials provided retention that was double the values of zinc phosphate or conventional cements, these results provide an overwhelming indication for the use of adhesive luting’.
Heintze,10 in a systematic review, has provided strong evidence with regard to resin/adhesive luting. He included 18 studies, finding that the most important factors for crown dislodgment were axial height, convergence angle and luting agent and that the frequency of debonding was higher for restorations luted with zinc phosphate than all other types. He advised that, ‘In clinical situations with low mechanical retention, or situations with low preparation height or high convergence angle, the adhesive properties of the luting agent are crucial for the prevention of debonding’.
The dentine-bonded all-ceramic crown was another example of the use of adhesive luting. In this concept,11 the ceramic fit surface was etched with hydrofluoric acid (HF) in order to provide a micromechanically retentive surface, the dentine preparation was treated with a dentine-bonding agent and a resin luting material employed in conjunction with a silane applied to the HF-etched crown (Figure 1). Results of a laboratory study showed that this adhesive concept maximized the fracture resistance of the crown12 and clinical studies indicated good performance.13,14 However, the clinical aspect of these restorations was technique sensitive (Table 2), as it was necessary to use them in conjunction with a dentine-bonding agent, with particular anxieties concerning the possible pooling of the dentine adhesive which had to be cured prior to crown cementation. If that occurred, it would not be possible to seat the crown fully.
Figure 1. The dentine-bonded crown concept, where P = porcelain and D = dentine, with the smaller illustration demonstrating the layers of the interface between the HF-etched ceramic (hatched) and the dentine-bonding agent (black) on the dentine surface and the yellow layer being a silane coupling agent.
Try-in paste
Clean fitting surface and silanize
Clean tooth with pumice, isolate
Apply dentine-bonding agent
Apply dual cure luting agent to crown
Place with gentle finger pressure
Remove excess luting material
Light cure and finish margins
Check occlusion and polish
Resin-based luting materials have been available since 1952: however, they have undergone substantial development since that time. It has been considered that they are unique insofar that a polymer matrix forms to fill and seal the tooth-restoration space, whereas other luting materials are true cements derived from mixing a powder and liquid to form a hydrogel matrix.15 They have excellent physical properties and are insoluble in the dilute organic acids found in plaque1 (Figure 2). In addition, a literature review has identified these as the ‘go to’ luting material for all-ceramic restorations16 (Figures 3 and 4). Principal disadvantages are the reported price being 175 times that of phosphate cement17 and clean-up of marginal excess more difficult than phosphate. However, the physical properties of resin luting materials have been demonstrated to be superior to those of phosphate and GI cements,18 with the resin cements displaying a ‘good combination of high flexural strength and high modulus of elasticity which, when coupled with adhesive capabilities, make these materials essential in difficult retention situations and veneers’.18
Figure 2. Dissolution of a conventional luting material at the margin of UL2. The discoloured margin at UL1 could have been prevented by use of an all-ceramic crown.Figure 3. Replacement of crown of poor shade match with a dentine-bonded crown using a resin luting material of the same shade as the crown. (a) Dentine-bonded, all-ceramic crown at UL1: crown at fit appointment. (b) Crown in (a) after 5 years, with satisfactory margins, ie no dissolution of the luting material.Figure 4.
(a) Patient complaint, ‘Metal ceramic crown feels thick to my lip, and I don't like the colour!’. (b) Crown at UR2 removed, replaced by all-ceramic crown luted with resin cement of the same shade. Patient content because crown feels less bulky and shade match improved.
Self-adhesive resin luting materials
The advantages and disadvantages of resin luting materials have been presented in Table 1. In this regard, it is apparent that the advantages of resin luting materials relate mainly to the good physical properties of these materials, while the disadvantages relate to their technique sensitivity in clinical practice. The development of a resin luting material which could overcome these disadvantages would therefore have ‘the best of both worlds’. Such a material was introduced a decade and a half ago, with the introduction of RelyX™ Unicem by 3M ESPE, Seefeld, Germany (hitherto termed Unicem in this paper). In this regard, self-adhesive resin luting materials have recently been reviewed to coincide with the 15th anniversary of the first self-adhesive resin luting material, Unicem (3M Oral Care).19,20 Since this material did not require the use of a separate bonding agent, it substantially simplified adhesive luting with resin luting materials. Today's version of the dentine-bonded crown may therefore be a ceramic such as e-max® (Ivoclar Vivadent, Leichtenstein), whose fit surface is capable of providing micromechanical retention when etched with HF, luted with a self-adhesive resin luting material.
The current self-adhesive resin luting materials are two-part materials: the earliest version of Unicem required capsule mixing and delivery by an auto-mixing dispenser, but newer versions have used a much simpler auto-mixing tip. These self-adhesive resin luting materials have a built-in self-etch primer which facilitates adhesion to dentine. According to Ferracane et al,21 one component of the material is comprised of conventional mono-, di- and/or multi-methacrylate monomers that are used in a variety of resin-based dental materials, such as Bis-GMA, urethane oligomers of BisGMA or UDMA. Again, according to Ferracane et al, the acid-functionalized monomers currently utilized to achieve demineralization and bonding to the tooth surface are still predominantly (meth)acrylate monomers with either carboxylic acid groups or phosphoric acid groups. These monomers facilitate an early change upon polymerization from hydrophilic to hydrophobic. Uniquely among the group of self-adhesive resin luting materials, Unicem has been demonstrated to achieve complete neutralization after 48 hours, which was not the case for the three materials that it was tested against.22 For a complete review of the chemistry of the self-adhesive resin luting materials, readers are directed to the paper by Ferracane et al.21
Given that the development of self-adhesive resin luting materials meant that there was no longer a need to carry out a separate dentine-bonding step, it could be anticipated that clinicians considered the material to be less technique sensitive than conventional resin luting materials. This is confirmed in a couple of studies. In a ‘handling’ evaluation by 13 members of the UK practice-based research group, the PREP Panel,23 144 restorations were placed in the study, and Unicem was rated higher by the evaluators for ease of use than both the pre-trial resin-based cement and ‘conventional’ cements. Ninety-two percent (n = 12) of the evaluators considered that it was ‘very advantageous’ for the material under evaluation not to require etching and bonding prior to cementation. In addition, none of the evaluators reported any incidence of patients with post-operative sensitivity following placement of restorations with the material, and this may be considered to be a clinical advantage. The results of the PREP Panel study are in broad agreement with those reported in the July 2003 CRA Newsletter,24 in which 64% of the 28 evaluators who assessed the handling of Unicem stated that this product would replace products that they had used previously, with 96% of evaluators rating it as excellent or good and worthy of trial by colleagues.24 The CRA also assessed the clinical performance of Unicem when used by 68 evaluators in the placement of 4820 restorations, with follow-up from a few months to one year.25 The incidence of post-operative sensitivity was reported to be ‘very low’ and there was a very low incidence of debonding (0.1%).
Clinical evaluations which may be considered relevant, and which have examined the clinical performance of self-adhesive resin luting materials, include the following:
The PREP Panel have completed a 5-year clinical evaluation of 42 three-unit zirconia-based bridges which were luted with Unicem.26 Most recently, it was possible to examine the margins of 8 bridges placed in the original PREP Panel study at 12 years, using USPHS criteria, with the results indicating that, of the 16 bridge retainers, no unsatisfactory margins were noted, and concluding that follow-up indicated good performance of the luting material, as assessed by examination of the restoration margins.27
The Dental Advisor28 recently published a 15-year clinical performance review of a total of 2226 inlays, onlays, crowns/bridges and zirconia-based crowns and bridges luted with Unicem or Unicem 2. Good performance was noted for lack of marginal discoloration (96% and 98% no reported discoloration, respectively, for restorations luted with Unicem and Unicem 2) and less than 1% post-operative sensitivity for restorations luted with Unicem and ‘even lower’ for restorations luted with Unicem 2, with such sensitivity typically subsiding after two weeks. These positive findings were summarized as ‘proven to be very reliable’.
Kauling and colleagues have recently carried out a literature review which included nine papers describing the clinical performance of Rely X Unicem (in particular, marginal adaptation), with the results being positive.20
Regarding laboratory studies, Burke and colleagues29 compared the laboratory fracture resistance of dentine-bonded ceramic crowns luted with Unicem with those luted with a conventional resin-based material, with the results indicating no significant difference. Ferracane et al21 have also reviewed the laboratory performance of Unicem, with the findings being positive, concluding that ‘this review of the chemical and physical properties of self-adhesive resin cements suggests that these materials may be expected to show similar clinical performance to other resin-based and non-resin-based dental cements.’
Other manufacturers have developed self-adhesive resin luting materials, with a recent survey of 500 UK dentists showing that 13.1% of respondents are now using a self-adhesive resin luting material.3 This successful adoption to the UK market may not be considered surprising, given the ease of use of the material, with no primer needed, its versatility in terms of indications, including the luting of fibre posts, and clinical results indicating low frequency of post-operative sensitivity and low rates of marginal discoloration.26 Finally, technique tips while using self-adhesive resin luting materials include:
Mechanically clean the tooth (for example, using a pumice and water slurry);
Do not overdry the tooth;
Seat under gentle finger pressure;
Light cure the margins (will improve the physical properties of the material as compared with self curing).
In summary, self-adhesive resin luting materials have become an easy to use and reliable part of the armamentarium for all indirect restorations with a good volume of clinical and laboratory evidence to back up their performance.
Resin luting materials for resin-retained bridges
The resin-retained bridge technique would not be possible without the use of resin luting materials which facilitate adhesion to the sandblasted (metal) framework and also to the etched enamel of the abutment tooth/teeth. Readers are directed towards comprehensive texts on the resin-retained bridge (also called the resin-bonded bridge or Maryland bridge) for example, that by Ibbetson.30 In that regard, the resin-retained bridge may be considered as the ‘ultimate’ adhesive, minimal intervention, indirect restoration with no risk of pulpal irritation, given that the preparation of the abutment tooth/teeth is minimal or nil and success rates may be considered satisfactory.31 The ‘typical’ resin-retained bridge employs metal-ceramic systems (ie a metal framework) which can be bonded to the (phosphoric-acid) etched enamel, although some workers have also reported good success rates using all-ceramic zirconia bridges.32 Therefore, resin luting materials are the materials of choice, given that they will bond to the etched enamel of the abutment tooth/teeth, but it is necessary to employ systems which also demonstrate adhesion to the metal substrate. Initially, this meant the use of chemically-active resins.7 Today, this means the use of resin luting materials that will achieve chemical bonding to the metal substrate: those containing the resin 10-MDP (which has a demonstrable adhesion to metal),33 or which employ systems which include a bonding system which contains 10-MDP. The Panavia® (Kuraray, Osaka, Japan) luting materials would appear appropriate for this task, plus luting agents which are marketed with a 10 MDP-containing bonding agent.
In addition, given the potential problem of dark metal shine-through of metal-ceramic, resin-retained bridges, which may cause darkening of abutment teeth, the resin luting material should be relatively opaque, so that metal (dark) shine through is not a problem.
Finally, readers are reminded that, although our adhesive systems have improved greatly over the years, it is advised that the achievement of resistance and retention form into the tooth preparation will help protect the bond that is achieved. This includes the paralleling of adjacent surfaces of abutment teeth for fixed-fixed bridges, but might include the provision of slots and grooves for single abutments/cantilever preparations, such as have been described by Kern and colleagues in their paper on the successful ten-year outcome of zirconia cantilever resin-retained bridges.34 These workers included a light cervical chamfer, a small proximal box and small pinhole in their preparation in order to aid resistance/retention form (Figure 5) and, for luting, they utilized either Panavia® 21 TC (Kuraray) (n = 94 bridges) or Multilink Automix after application of metal/zirconia primer (Ivoclar Vivadent) (n = 14 bridges). Failure rates were 4.2% and 14.2%, respectively, for these luting materials, and an overall reported success rate (survival with complication) of 92.0% at 10 years. These data may be a pointer towards the future of resin-retained bridges, given that the zirconia ‘wing’ can be made as thin as 0.6 mm and is tooth-coloured, thereby precluding the difficulty of metal shine-through alluded to above.
Figure 5. Retentive features for resin-retained bridge wings (a light cervical chamfer, a small proximal box and small pinhole) utilized by Kern et al.34 (Adapted from Kern et al34).
’Smart’ resin luting materials
Several manufacturers have developed ‘smart’ resin luting materials which contain a component which facilitates the polymerization of an unpolymerized dentine-bonding agent (from the same manufacturer) when this has been applied to a tooth. The advantage being that, when the dentine-bonding agent is not polymerized prior to the seating of an indirect restoration, there is no risk of pooling of the bonding agent at internal line angles, which could prevent full seating of the indirect restoration. One example of this is the resin luting material RelyX™ Ultimate (3M) which, when used with Scotchbond™ Universal (3M) in the placement of an indirect restoration, facilitates the polymerization of (unpolymerized) Scotchbond™ Universal bonding agent on the dentine surface. Other manufacturers may be expected to introduce materials with a similar functionality.
Conclusions
The conclusion, based upon this review, is that resin luting materials are the material of choice for luting all restorations, with the self-adhesive resin luting materials providing low technique sensitivity, although the conventional resin luting materials may be indicated for the luting of aesthetic restorations, given that their increased variety of shades (as compared with the self-adhesive resin luting materials) may be more readily matched to that of the actual restoration. Resin luting materials are contemporary luting materials. Their physical properties are superior to ‘conventional’ luting materials, they are capable of improving the retention of restorations which may be of suboptimal retention and resistance form, and their previous technique sensitivity has been overcome by the introduction of the self-adhesive resin cements. This review has demonstrated that there is clinical and laboratory evidence to back up these statements.
