Dental Adhesives: a 10 years story

An article by dr. Salvatore Sauro

Overview and basic commentary 

Before any discussion about adhesion to crown and root dentine, it is imperative to state that there is no substantial scientific research evidence-base that can support a specific bonding approach rather than another.

Said that, there are several in vitro studies showing that multi-step bonding systems bonded to dentine can have a longer lasting durability compared to simplified one-bottle systems, in particular with those applied through an etch-and-rinse approach.  Nowadays, only two main categories of bonding systems are available for clinical applications; etch-and-rinse adhesives (ERAs) and self-etching adhesives (SEAs). The latest adhesives are known as  “universal” adhesives that combine etchant, primer, and adhesive in a single solution which can be used both with phosphoric acid etching pre-treatment or as a self-etching adhesive.

However, despite the progress in the formulation of dental adhesive systems over the last ten years, shortcomings such as post-operative sensitivity, premature reductions in bond strength, interface and marginal degradation, and biocompatibility are still considered important issues with such materials. Enzymatic degradation of collagen fibrils within the hybrid layer and hydrolysis of polymers are the major factors thought to destabilise the resin-dentine interface. It is also important to consider that glass ionomer materials can be considered as self-adhesive, which can be used in alternative to conventional adhesive-composite restoration or as dentine substitute materials; these materials are less affected by conventional degradation events commonly occurring at the resin-dentine interface (i.e. hybrid layer) created using adhesive systems. Nevertheless, the use of glass-ionomer-cement as dentine substitute material in resin composite restoration does not have any effect on the survival of resin-composite restorations; after 18-years there was no clinical difference in the survival rate between resin-composite  restoration performed with or without an intermediate layer of glass ionomer cement. However, it would be interesting to see the results of future clinical studies if the use of air-abrasion performed with bioactive glass before bonding procedures may reduce the influence of secondary caries and improve the clinical longevity of composite restoration performed with or without an intermediate layer of glass ionomer cement 

Pathways for the degradation of the bonding interfaces 

The longevity of dentine adhesive restorations still remains a challenge as bonding failures lead to recurrent caries (e.g. secondary caries), and represent the main reason for replacement of direct restorations [1,2]. Indeed, more than 50% of restorations being replaced have been reported to be caused by formation of secondary caries [1,3,4]. The longevity of amalgam is over 20 years, while adhesive/composite restorations are estimated to endure in the patients’ mouth less than 7 years; the failure rate of posterior resin composite restorations after 7 years can be 50% greater than that of high-copper content amalgams [5, 6]. Regarding the restoration of endodontically-treated teeth, It is challenging to evaluate the survival rate since it is influenced by a huge number of variables such as residual volume of structure in prepared teeth, the presence of proximal contact-points, location of the teeth, cuspal coverage restoration in molar teeth and the employment of a post. However, it has been reported that teeth restored with fibre posts have comparable survival rates to those restored with direct or indirect metal posts [7]. Moreover, direct composite restorations in root-filled premolars with Class II cavities performed as well as teeth restored with single unit crowns [8]

The issue about the longevity of the adhesive interface of composite restorations is related to two main mechanisms which have been identified to contribute to resin-dentine hybrid layer degradation: i) Intrinsic or proteolytic degradation of the organic matrix; ii) Extrinsic or hydrolytic degradation of the resin matrix. Both mechanisms are interlinked and occur simultaneously, decreasing the durability of resin-dentine bonds. [9-11]. However, it is important to state that the choice of the material to being used for bonding and restoration represent only one of several factors (patient compliance, complexity of the clinical case, medical history of the patients and so on…) that may influence the longevity and survival of the treatment; the most important factor remains the ability and the skills of the clinician [2].

Guide through bonding systems currently used in dentistry – Pros & Contras

Adhesives are generally categorised into "generations" in order to outline the different components (e.g. etchant, primer, and adhesive) of all classes of products. Nevertheless, the use of such a classification cannot be universally accepted. Indeed, a great number of adhesive have been lunched on the market so this type of classification has become rather inept and confusing [9]. Contemporary adhesives are classified based on their mode of interaction with dental hard tissues as either etch-and-rinse adhesives (ERAs) or self-etch adhesives (SEAs) [10, 11]. ERAs require the use of a 32-40% phosphoric acid etching to remove the smear layer and demineralised dentine and enamel to create micro-retentions. This is then rinsed, followed by the application of a resin primer, and adhesive (three-step systems) or by a single-bottle self-priming agent (two-step systems). On the other hand, self-etching do not require and pre-etching with phosphoric acid in dentine, but the acidic components (e.g. 10-methacryloxydecyl-dihydrogen-phosphate (10-MDP)) of the adhesive can partially remove the smear layer and expose a very thin layer of demineralised collagen. [9, 11]. Subsequently the primed dentine does not need to be not rinsed and the solvents in the primered-dentine is evaporated with air for approx. 5s, and it is then covered with an adhesive and light-cured (two-step systems). The one-bottle or all-in-one systems are single bottle adhesives that contains functional acidic-methacrylates (e.g. MDP) for etching purpose and cross-linking dimethacrylates for polymerisation. In this latter category we can include the “universal” adhesives that combine etchant, primer, and adhesive in a single solution which can be used both with phosphoric acid etching pre-treatment or as a self-etching adhesive [12].

The bonding performance at short-time period (24hr) of most adhesive currently in use is outstanding, but in vitro studies have shown that the main drawback of “simplified” systems (including modern universal adhesives), is their poor durability over a period superior of 3 months of storage in water, especially for those applied in ER mode [13]. Indeed, an proper penetration of adhesive monomers into the etched dentine is a key requirement to accomplish the formation of a “high quality” and long lasting  hybrid layer [14,15]. Unfortunately, simplified adhesive, including the modern universal adhesives, when applied in ER mode cannot penetrate into acid-etched dentine due to the excess of water within the demineralised collagen fibrils [16-18] (Figure 1).

Figure 1: Scotchbond universal applied in ER mode. It is possible to see a mixed failure (A) and when observing at higher magnifications (B, C, D) on the zone with adhesive failure, it is possible to see zones characterised by several exposed collagen fibrils due to a great lack of resin infiltration .

Such residual water may culminate in nanoleakage within the hybrid layer and nano-phase separation of the adhesive monomers between the polymerised resin; this increases uptake water and jeopardise the mechanical properties of the resin-dentine interface [19-21]. Moreover, It has been demonstrated that the presence of active metalloproteinases such as MMP-2 and MMP-9  and cathepsins within poorly infiltrated hybrid layers plays a crucial play a role in the destruction of the dentine collagen so contributing significantly to the reduction of the durability of the bonding interface [22,23]. Conversely, although self-etching adhesives can also activate such proteolytic enzymes [24], but there is a general consensus that collagen degradation occurs more in ERAs; phosphoric acid etchants demineralise dentine more deeply and more completely, leaving collagen fibrils exposed, and making them more susceptible to proteolytic degradation by the endogenous dentinal enzymes [15, 25]. Indeed when employing milder self-etching adhesives (SEAs) or glass ionomer cements (GICs), the degradation processes at the bonding interface are extremely less compared to etch-and-rinse adhesive. This is mainly due to the fact that milder acidic SEAs and GICs do not totally expose the collagen fibrils and leaves small smear plugs within the dentinal tubules [26-28]. Moreover, it is also noteworthy that the dentine demineralisation when using SEAs or GICs is very superficial (less than 1-2 microns) [9] and therefore the resin-infiltration of SEAs and GICs can occur quite easily, so creating an hybrid layer less porous compared to that created with ERAs [28]. 

As earlier said, contemporary bonding systems can be categorised as: “etch-and-rinse adhesives” (ERAs), “self-etch adhesives” (SEAs) or “glass ionomer-based materials” (GICs). This latter class of materials include classic GICs and RMGICs that can be also used for bonding purpose [11, 29, 30]. The main difference between these systems is their degree of “acidic-invasiveness”. Commonly, ERAs systems cause substantial alteration of the dentine due to the use of phosphoric acid, which exceeds that of SEAs and conventional GIC or resin-modified glass ionomer cements (RMGIC); these latter interact with the dental hard tissues modifying the smear layer and only partially exposing collagen fibrils [9,23]. 

The GIC/RMGIC systems are classified as proper self-adhesive materials; that can bond to dentine micromechanically, through infiltration of the collagen network previously exposed by conditioning the dentine using 10% polyacrylic acid (PAA), in combination with chemical bonding obtained by ionic interaction of carboxyl groups from the acid with calcium ions of remaining crystals of hydroxyapatite (HAP) [11, 29]. Furthermore,  glass ionomers are biocompatible materials that release specific ions such as fluoride that can have antibacterial [32, 33] and remineralising properties at the bonding interface [9, 11, 34]. They also have thermal expansion coefficients that match that of dentine and clinically, RMGICs result much more resistant rather than GIC to the tough conditions in the oral cavity such as low pH and thermal stress [35]. Indeed, it was shown that such materials may create durable bond strength after several years of water storage without any drastic loss of micro-mechanical and chemical bonding at the dentine [36, 37]. Moreover, it has been also demonstrated the bond stability after 1 year aging of conventional glass ionomer cements applied to dentine, with or without dentine pre-treatment using a polyalkenoic acid [38].

To conclude, it is important to highlight the possibility to keep etching the enamel (selective etching) even if a SEA or an universal adhesive is employed during the restorative procedures. However, Sato et al. [39] stated that although selective phosphoric acid etching could create a stable adhesive-enamel interface, the bond strength remains dependent on the type of material utilised.

Thus, to the question “what bonding strategy should I use when I restore a cavity after root-canal treatment?” it is possible to affirm that in view of all the studies considered so far, the use of multi-step SEA or ERA system seem to remain the best option to achieve a long-lasting bonding stability. However, in case of employing an universal bonding system, it would be recommendable to use it in self-etching mode associated with a selective-etching approach in enamel. However, it is important to bear in mind  that one of the most “robust” (in terms of bonding longevity) approach is represented by the use of materials based on glass ionomer cements, therefore it should not be excluded the use of such dentine replacement materials without any previous application of adhesive system in dentine.  

What restorative material should  be used for direct restoration of endodontically treated teeth? 

As previously stated at the beginning of this study, there is no substantial scientific research evidence-base that can support a specific bonding restorative approach rather than another in terms of survival. Moreover, the restoration of endodontically-treated teeth is influenced by a huge number of variables so it is quite challenging to evaluate the survival rate of different restorative approach to state which one is the most appropriate; a standardisation in this situation does not exist, especially in the case of direct restorations. Moreover, it is important to recall that teeth restored with fibre posts have comparable survival rates to those restored with direct or indirect metal posts [7] and that direct composite restorations in root-filled premolars with Class II cavities performed as well as teeth restored with single unit crowns [8]. It has been also demonstrated that the use of a glass-ionomer-cement as base prior composite restoration has no negative affect on their survival rate. Indeed, acceptable annual failure rates could be achieved after 18 years when placing an intermediate layer of GIC with no significant difference compared to conventional composite restorations; thus no improvement in survival should be expected based on such a clinical restorative approach [40].

Said that, direct restorations are frequently undertaken using conventional resin composites due to their excellent mechanical and aesthetic properties [41, 42]. Nevertheless, light -curing resin-based materials remain characterised by important drawbacks associated to their polymerisation shrinkage, which can  provoke stress at resin–dentine interfaces and jeopardize the overall longevity of the restoration [43–45]. Indeed, the volumetric contraction of conventional resin composites can cause debonding of some adhesive systems [43, 46, 47]. Accordingly, the sealing between composite and dentine can be seriously compromised with the formation of gaps and marginal leakage; pathways for microleakage of oral fluids, bacteria, and enzymes penetration [43, 48–50]. Such a scenario results in important clinical issues such as marginal discoloration, secondary caries, pulp and root-canal retreatment in endodontically-treated teeth [51, 52]. 

However, there are several clinical strategies that may reduce the stress concentration at the resin–dentine interface during polymerization and possibly increase the longevity of the resin-dentine interface [53]. For instance, flowable composites or glass-ionomer cements (GIC/RMGIC) used as base/liner or dentine-substitute materials may represent a proper method to provide a sort of “stress-absorption” [54,55]; they may prevent stress development at the dentine-bonded interface and reduce the risk for gap formation, microleakage, so promoting secondary caries formation [53,56,57]. Moreover, It is also important to consider that occlusal stress during mastication, swallowing, as well as in cases of parafunctional habits, can exuberate such issue since they can jeopardise the integrity of the bonding interface, making such a structure more susceptible to degradation over time [58, 59]. 

Further efforts to diminish the effect of the shrinkage stress of composites have been proposed, such as the incremental filling techniques.[60, 61] indeed, several studies have demonstrated that such procedural approaches may  reduce cuspal deflection, incidence of enamel cracks or fractures, premature gap and formation at the resin–dentin interface. [60, 62, 63] The rationale of the incremental technique is to avoid the application of composite that can join the opposite walls of the cavity with a large volume of composite. Conversely, shrinkage stress is minimised when there are fewer bonded prepared walls involved during the polymerisation, thus reducing the C-factor. In addition, by incrementally curing either 2-mm thick increments, a higher degree of conversion is expected due to lower light attenuation. [62, 64]

However, there is a growing interest in bulk-fill composites , which have been designed to replace the need for incremental layering, providing simple and fast clinical procedures. This new category of composites is intended to be applied as a single incremental application of 4 to 6 mm thick layer. This simplified strategy is said to be attributed to increased composite translucency, allowing greater light transmission with depth, and to the addition of more reactive photo-initiators. [65, 66] In addition, these materials are claimed to have low shrinkage stress due to inclusion of proprietary stress reliever molecules and polymerisation modulators [65, 67], but unfortunately there are yet too little scientific support for these latter observation, so it is suggested to use an incremental layer technique (up to 2 mm thickness) even when using a bulk-fill resin-composite system (figure 2).

Figure 2: Confocal image of a resin-dentine interface created with the application of a bulk-fill composite. It is possible to see the presence of important gaps probably due to shrinkage of the material during light-curing procedure

In conclusion, to the question “What restorative material should  be used for direct restoration of endodontically treated teeth?” it is possible to answer that in view of all the studies considered so far we can only suggest to use materials with lower modulus of elasticity such as flow composite applied as liner/base at the access of the canals to reduce the stress of the polymerisation of the conventional composite that will be subsequently used to restore the cavity. In case of using bulk-fill sculptable or flow composites, our suggestion is to apply it layer by layer not thicker than 2 mm and light-cure each one of it separately for 20-30 sec. Moreover, the use of GIC-based materials as base or dentine-replacement  material, followed by adhesive/composite restoration would be a further option, especially in those cases where the volume of restoration with one or more walls lost during caries excavation and cavity preparation.

Is there any potential benefit in using air-abrasion before adhesive restoration of endodontically-treated teeth?

Originally, air-abrasion (AB) was proposed as an alternative method to handpiece for cavity preparation, but subsequently it was also advocated for minimally invasive cavity preparation and/or caries removal or for decontamination of the dental substrates before adhesive restorations [68,69]. AB can be considered a kinetic method, which uses a stream of abrasive particles to gently remove dentine and/or enamel via an end-cutting process [70-72]. It has been reportedd to generate a smooth cavity with blurry walls and margins along the cavity [68]. Moreover, it generates no vibrational stress on tooths tructures, so resulting in low disconfort for patients durign air-abrasion procedures [73, 74]. The most common abrasive powder in AB is Alumina (aluminium oxide) [69,75] and recently there has been the introducion of bioactive glass powders, as an alternative abrasive/polishing powder [72, 76].

A recent study has shown that dentine prepared with alumina or other bioactive glass powders was characterised by the presence of smoother dentine walls and indistinct margins compared to the those prepared with burs [76]. It was also shown [68] that the most relevant morphological feature of a cavity prepared through AD is a contour with rounded shape and indistinct walls and margins [77]. Cavities with rounded internal line angles may have less C-factor with consequent a lesser amount of stress concentration along the bonding interface [68, 78]. Conversely, an evident surface roughness characterise the cavities prepared with conventional burs. In this latter situation, when such cavities are restored using conventional resin composites, fatigue failure may happen subsequent to masticatory stress, particularly if the degree of the stress at the interface is sufficient to initiatiate crack propagation [79, 80].

As previously stated, new bioactive powders have been formulated to being used in air-polishing/cutting procedures as an alternative to alumina [72, 76]. It was demostrated that dentine air-abrasion performed using bioactive glasse (BAG) van criate a “bio-reative”smear layer [9], which can remain available at the interface when restoations are perfoemd glass-ionomer cements or SE adhesives; their biactivity can contribute to the reduction of the degradation processes that occur at the bonding interface over time [81,82]. Indeed, a recent study showed that air-abrasion performed with alumina has no negative effect on the immediate bonding performance of SE adhesives, but the bonding longevity may be jepardise due to degradation procecess at the bonding interface over time [21]. Conversely,  the presence of BAG at the interface may protect the hybrid layer thanks to the hydrated silica Si(OH)₄ produced such a biaoctive substance once in gets in contact with water, saliva, blood [83]. Subsquenlty, this latter may serve as template for mineral precipitation (e.g. Ca and PO) which then maturates into apatite [84, 85]. Such a  remineralisation process may reduce the proteolytic degradation caused by MMPs [86-88]. Moreover, BAG have an antibacterial effect that may be attributed to the release of specific ions (e.g., fluoride, zinc calcium and phosphate), that have a toxic effect on the cells and cause neutralization of the local acidic environment [89], due to a local increase in pH that is not well tolerated by many oral bacteria [90]. 

However, the hypotesis that may justify the use of bioglass before retorations of endodontically treated-teeth is based on the posibility of such biactive materials to repair gaps via apatite remineralisation at the interface, so avoiding bacterial leakage and reinfection of the root-canal system, with a consequent need for endodontic retreatment.  Indeed, a recent study showed that a resin-based material doped with BAG, was able to reduce significantly the extent of bacterial biofilm penetration into preexisting marginal gaps. Moreover, the release of BAG ions into the gap can help control the local gap chemistry and create an antimicrobial environment that slows biofilm development and propagation [91].

It is important to bear in mind that the presence of gaps may facilitate accumulation of biofilm within the restoration-tooth margins [92, 93]. Moreover, cyclic loading due to mastication is a known potential cause of margin failure and gap propagation [94, 95]. On the other hand, polymerization shrinkage of resin composites during light-curing procedure may cause stresses on the interface that increase the chance of interfacial failure [93]. In other words, gaps at the restoration-tooth interface seem to be inevitable, and that represent a real and constant risk factor that may jeopardise the longevity of our restorations 

Thus, to the question “Is there any potential benefit in using air-abrasion before adhesive restoration of endodontically-treated teeth?” it is possible to answer that in view of all the studies considered so far, the use of air-abrasion with alumina or BAGs before restoration may create a bonding substrate smooth and a cavity with rounded walls that can help in reducing the effect of polymerisation shrinkage of composite. However, Alumina seems to have no effect on the longevity of the bonding interface, while BAGs, can offer some possible benefits in terms of antibacterial effects and remineralisation/repair of the gaps at the bonding interface. 

Figure 3: Confocal image of a bonding interface created with the application of a resin modified glass ionomer cement (GIC, Riva LC, SDI, Australia) on dentine air-abraded with SELECTA Kinetic, a polycarboxy-lated zinc-doped bioglass created by Velopex International, Harlesden, London, UK. It is possible to see the presence of minerals precipitation at the bonding interface that closed the gaps probably due to shrinkage of the material during light-curing procedure

The use of root canal posts in endodontically-treated teeth

Anterior teeth, premolars and molars are subjected to a different biomechanical stress in terms of load direction. For instance, maxillary anterior teeth are typically exposed to high shear stress, so making  this area potentially at a higher risk of failure compared to posterior teeth [96]. This is the main reason why it is believed that posts should be used in this area; unfortunately, there is not much scientific evidence to support such an assumption [7]. Indeed, adhesive-luted root canal posts seem to offer very little reinforcement to the endodontically treated teeth [97, 98), but they may increase the anchorage of core build-ups and coronal restorations [99, 100]. However, the determination of when to use a post essentially depends on the coronal hard tissue loss, as well as the type of tooth and the restoration that will be performed; these are more normally used for indirect restorations rather than direct restorations [101]. The existing literature does not indicate any exact level of structure loss at which a post contributes to the survival of a root-canal treated teeth [7]. The clinical evidence for a positive effect of using post remains scarce and controversial. For instance, it was demonstrated that only a significant positive effect on survival rate of post used in teeth with no remaining dentinal walls [102]. Moreover, two further studies showed after 17 years no difference in outcome for post-retained and post-free restorations, both in indirect [103] and direct restorations [104]. Mannocci et al. [105] demonstrated that premolars with Class II cavities treated endodontically presented no difference in survival rate when restored using carbon fibre posts and direct composite or an amalgam Nayyar core.

It is also important to consider the type and the material that the posts are made of. For several years, metal-based posts such as gold alloy or cobalt-chromium-based cast posts and cores have been employed in clinic to restore endodontically-treated teeth, with a high success rate > 84% at 10 years [106, 107]. However, the main disadvantage of such posts is related to the amount of dental substrate that must be removed for the preparation of an adequate post space, along with the risk of coronal leakage during temporary restoration. Nowadays, clinicians have the possibility to decide between rigid post materials such as titanium, stainless steel, gold alloys, zirconia or elastic ones like carbon, glass or quartz fibres. In particular, posts based on glass and quartz fibre are typically embedded in an epoxy or methacrylate resin to achieve a level of aesthetic superior to that of metal-based or carbon fibre ones. 

It has been advocated that post should form a monoblock with the root-dentine when applied using adhesive luting procedures [108], so that the similar modulus of elasticity of the different compartments (e.g. post, luting cement and dentine) may favour the distribution of stress more uniformly  so reducing the incidence of fractures of roots compared to posts made of more rigid materials [109]. 

Unfortunately, there is scarce clinical evidence to support that the use of fibre posts will result in less incidence of fractures [110, 111]; prospective clinical also demonstrated the both rigid and flexible posts have no benefit on tooth survival and restoration success [107, 112]. Again, the main factors that influence the long-term survival of root-canal treated teeth is the amount of remaining coronal tooth substance; excessive removal of root dentine structure during post space preparations should be avoided [113, 114]. Recently, customized fibre bundles that can be adhesively adapted to irregular root canals even without post space preparation have been developed in order to avoid unnecessary dentine removal, but the evidence on the survival of teeth restored with such technique is scarce and inconsistent [115; 107].

Adhesion and luting of posts to root canal dentine

Some specific bonding issues related to imperfect polymerisation, inadequate adhesive application, along with incomplete and/or shrinkage of resin-based luting agents, are the main reasons why teeth restored with root canal posts commonly fail in clinical [116]. Root canal post can be luted using self-adhesive resin cements (SACs) or through the use of etch-and-rinse and self-etch adhesives. However, adhesive procedures inside the root canal results quite difficult because of restricted access and visibility in root canals [117]. Moreover, issues related to the high C-factor [108, 118] and those related poor light transmission through fibre posts during light-curing procedures [2019] should not be undervalued. Conversely, the use of dual-curing systems seems to guarantee a better level of proper polymerization with lower shrinkage, especially when materials based on glass ionomer cements are employed [212]. These latter cements are well known to undertake a dynamic ionic exchange with the surrounding microenvironment for a relatively long time [34, 120] and to create a stable chemical bonding to dental tissues [11, 9, 121]. Moreover, as previously described, the use of bioglass in air-abrasion system before followed by GIC restoration may protect the bonding interface and repair it in case of  gaps via apatite remineralisation, so avoiding bacterial leakage and reinfection of the root-canal system [57, 91]. A recent randomized controlled clinical trial showed greater debonding after six years in conventional resin cements applied using an etch-and-rinse adhesive compared to SAC cements [122].

It is important to consider that during the preparation of the post space preparation inside the canal a specific smear layer is created and it is usually thicker than the smear layer created during endodontic procedures; it consists of dental debris and residues of gutta-percha as well as sealer particles [123]. This is the reason why it is often recommended the use of phosphoric acid to remove the smear layer; this latter can impair the bonding longevity of adhesive systems in root canal dentine due to enzymatic and hydrolytic degradation [87, 124]. Again, the use of air-abrasion may contribute in the decontamination of such a particular dentine substrate, as well as replacing that thick smear layer with a more “adhesive-friendly” smear layer, or with a bioactive one when bioglasses are used as abrasive powders [9, 76]

Application of adhesives and luting cements into the root canal should be performed carefully as the creation of porosities and voids within the cement layer may also affect the long-term stability of the bonding interface [125]. Moreover, the use of EDTA as dentine pre-treatment before bonding procedures seems to offer some benefits only when using self-adhesive systems [126], while it seems to reduce the bond strength of etch-and-rinse based adhesive systems [127]. Conversely, the use of ethanol and chlorhexidine as dentine pre-treatment before bonding procedures to improve the long-term bonding performance seems to be controversial [128 – 130]. In conclusion, no clinical recommendations can be provided, other than ensuring a clean and adequately dried dentinal surface to have an ideal and standardised adhesive luting procedure [7].