Essay: using RBCs to adhesively cement all-ceramic restorations

According to multiple clinical studies, using RBCs to adhesively cement all-ceramic restorations to surrounding tooth structures improves the brittleness and fracture resistance of the ceramic restorative material. The RBC is thought to interpenetrate any defects or irregularities present on the ceramic surface thereby making the ceramic less susceptible to crack propagation. This results in enhanced strength and clinical performance of the prosthesis (31, 32).
Mechanical properties and optimal functioning of the RBCs depends on the light curing protocol, especially when using light-cured resin cements for luting ceramic restorations, as was done in this study, because these particular cements can only undergo polymerisation using the light that is transmitted through the ceramic restorative material (31). To initiate the curing reaction of light-cured cements, light of an appropriate intensity and wavelength (470-480nm) must activate and excite a sufficient number of photoinitiators so that an adequate quantity of free radicals are generated (32). The generated free radicals can then interact with the C=C bonds within the resin monomer and initiate the polymerisation reaction which results in the formation of a polymer with C-C bonds (32). The conversion from monomer to polymer is known as DC. Adequate DC would consecutively increase mechanical properties of the cement alongside the strength and clinical performance of the restoration (32).
4.1-Analysis of LT data
The presence of a ceramic restorative material above the resin cement would decrease the amount of light that is transmitted to the resin cement for curing. When light encounters all-ceramic restorative materials, some of the light is transmitted whereas the rest of the light is either reflected at the surface or scattered within the material (29). Ceramics can absorb between 40-50% of the transmitted light (29). When the opacity or thickness of the ceramic restorative material is increased, more incident light is absorbed and scattered internally, and subsequently, less light is transmitted through the ceramic material to the resin cement (33).
The first hypothesis of this study, which stated that LT would be decreased through thicker and opaque ceramic disc combinations, was accepted. Ceramic discs of increasing thickness and opacity were observed to have lower LT in comparison to the thin and translucent ceramic discs. The darkest and thickest ceramic specimens tested, (A3.5 at 1.40mm) had reduced LT whereas the lighter and thinner ceramic specimen (A1 at 0.60mm) achieved the greatest LT. The LT values for A3.5 at 1.40mm and A1 at 0.60mm were 12.63% and 37.01% respectively. These findings partly correlated with the results achieved in a study by Filho et al, who studied the effect of ceramic thickness on LT and stated that during light activation, the light that passes through a ceramic is both absorbed and scattered. The study revealed that the degree of light attenuation is decreased with increasing ceramic thickness (24). Furthermore, a study by Meng et al, identified that light intensity of 800mW/cm2 decreased significantly to 160mW/cm2 when LT was measured through a ceramic specimen of thickness 2mm (24).
The opacity of the discs as well as thickness is imperative for achieving adequate LT. The translucency of a ceramic depends on its crystalline structure, light refractive indexes and the thickness of the ceramic (31). An opaque material may be ideal for aesthetically demanding clinical situations but an opaque ceramic contains a higher percentage of chroma and this chroma pigment has the ability to absorb light, therefore less light is transmitted through the ceramic restorative material, or in this study the ceramic disc, to the cement (29).
LT was found to exponentially decrease when the distancing from the curing tip was increased. The amount of light reaching the lower layers of the ceramic restorative material and the resin cement can be greatly decreased when the distance is increased (33).
4.2-Analysis of DC data
The properties of the resin cements are directly related to the degree of polymerisation so it is pivotal that an adequate DC is achieved to obtain a successful clinical performance of the restoration (25).
From this study, it was evident that LT was negatively influenced by increasing the thickness or opacity of the ceramic disc as well as by increasing the distance from the light curing tip. The second hypothesis of this study (that the DC would be lowest in the resin cement which had the thickest, darkest ceramic disc combination above the cement) was accepted. The presence of a dark and thick ceramic disc would influence light attenuation to the resin cement and thus decrease the number of photoinitiators that are activated in the resin cement. The consequence is a lower DC (24). From the results, the DC obtained with a ceramic disc of shade A1 at 0.60mm and A3.5 at 1.40mm was 64.56% and 59.45% respectively.
The DC of the resin cement was found to be affected by factors related to the resin cement and not just the ceramic. The LT to the resin cement is also affected by the opacity and thickness of the resin cement layer. The optical properties of the resin cements are also vital in order to allow maximum LT through to all areas of the cement and to increase the depth of cure (34). The optical properties are influenced by the thickness and opacity of the RBC. In this study, a shade A3 opaque veneer cement was used which contains a resin system consisting of a TEGDMA/BisGMA blend filled up to 47% by weight with a zirconia/silica filler (34). This cement is recommended for use with thin and fairly translucent ceramic restorations to enhance the DC of the cement because this ceramic combination would allow more light to pass through the ceramic and reach the cement. The greater the filler content, the more light scattering within the resin cement (34). As the resin cement used was an opaque shade, it is assumed that these darker shades of resin cements would contain darker pigments which also absorb some of the light (34). The increasing opacity of the resin cement would consequently decrease LT. This was identified in the study as at times, it was difficult to obtain a good signal during FTIR testing due to the opacity of the cement, especially when using thicker resin cement layers. Consequently, thicknesses above 1mm could not be tested. More translucent resin cement shade would increase the LT to the deeper areas of the resin cement and consequently, the DC because more free radicals would be generated via an increased excitation of photoinitiators. However, the increased opacity of the resin cement can be counteracted by decreasing the cement thickness (34).
In this study, the effect of different RBC thicknesses on DC was measured in which the results obtained showed that increasing the RBC thickness had a negative influence on DC, i.e. DC decreased when the resin cement thickness was increased. This matched the third hypothesis. This is suggested to be because thicker RBC specimens would result in the top few millimetres of the cement being adequately polymerised with the remaining material being poorly polymerised due to decreased LT to the deeper areas of the samples (34). The values ranged from 69.85% for 0.10mm to 63.64% for 1.00mm RBC thickness (10).
After a certain period of time, the DC graph appears to straighten off with no increase. This limited conversion is due to limited mobility of the free radicals and this limits the conversion of the monomer to a polymer (25). With light activation, there is a production of free radicals via the excitation of photoinitiators which enables the initiation step to occur instantaneously (24, 25). During the process of polymerisation, multiple growth centres are produced and the matrix transforms from a liquid to a viscous phase via the production of a polymer network from a monomer (25). The polymer network is highly cross-linked as most of the monomer is converted to polymer (24). The radicals then have limited mobility and this makes it increasingly difficult for monomers to diffuse to the polymeric growth centres. Therefore, fewer polymer growth centres would be formed as the reaction proceeds. As a result, less monomer is converted to polymer (11-13, 35). This is also the reason why the polymerisation reaction never reaches completion (34).
It is clear from the results in this study that increasing the thickness or opacity of the ceramic disc decreases LT through the disc and resultantly, negatively influencing the polymerisation of the RBC (29, 36). LT through the ceramic and to the RBC is vital because if the ceramic material prevents light from reaching the resin cement layer, this leads to inadequate polymerisation of the cement layer (14). The lower LT influences polymer development by primarily decreasing the C=C bond conversion since the polymerisation process is reliant on the light exposure to the resin cement (14). Furthermore, as the level of light irradiance reaching the resin cement is reduced, the development of the polymer network would be affected along with the DC (11-13, 14). A greater degree of light irradiance, as achieved with a thin resin cement layer (0.20mm) or with a thin and less opaque disc (A1 at 0.60mm), would help in the production of more densely cross-linked polymer networks due to the formation of multiple polymer growth centres (14, 28, 37). This would also enable the resin cement to achieve optimal mechanical and chemical properties. Oppositely, a lower degree of light irradiance will result in the formation of loosely cross-linked polymers alongside decreased DC (28). The lower the degree of light reaching the luting agent, the lower the DC and consequently this affects the strength of the restoration and the risk of debonding would be greater in these poorly polymerised RBCs (28, 37).
4.3-Resin-to-ceramic bonding efficiency
The success or failure of the restoration is dependent on the durability of the adhesive bond between the ceramic, the resin cement and the surrounding tooth structure. The adhesive bond eliminates any surface defects by replacing the surface with an interface thus reducing the likelihood of fracture (21, 22). The strength of the bond and the properties of the resin cement are dependent on sufficient resin cement polymerisation (38). The greater the DC of the RBC obtained, it would be expected that the strength of the all-ceramic restoration would increase and thereby improve clinical performance. Alternatively, the presence of an increased amount of monomers decreases both DC and is expected to result in inferior mechanical properties and decreased strength of the restoration (17). Insufficient polymerisation commonly causes early failure of the cemented all-ceramic restoration in the form of debonding or fractures within the ceramic material (32, 36). Hence it was vital to test the BFS obtained with three different ceramic combinations that had different conversion rates- one with the highest, one with the lowest and one with a mid-range DC with the aim of identifying if a greater DC or the application of RBC to the ceramic specimens resulted in a greater BFS.
4.4-Analysis of BFS data
The discs were left for 24 hours prior to BFS testing because RBC polymerisation continues for 24 hours after curing and if BFS was tested straight after curing, then the cement would not have reached the maximum polymerisation possible and as a result the risk of debonding is greater and the optimal values for flexural strength would not have been obtained (38).
The fourth hypothesis (that stated that the ceramic specimens that were adhesively luted with a resin cement would have greater flexural strengths compared to the uncemented specimens) was also accepted. The results from this study identified that the resin-coated ceramic specimens had the ability to endure greater forces in the form of stress before the specimen fractured. These resin-coated specimens exhibited greater BFS than the monolayered specimens (39). The greatest change between the monolayered and the bilayered ceramic specimens was achieved with the A1, 0.60mm samples in which the mean flexural strength increased from 69.01MPa, for the monolayered samples, to 101.40MPa for the bilayered samples thus highlighting that the application of a resin cement to the ceramic specimens does indeed increase the strength of the restoration. This was also supported by the stastical results which stated that resin-coating significantly increased BFS (p<0.001). The proposed hypothesis that the DC of resin cements does influence the strength of all-ceramic restorations was accepted according to results obtained in this study. Clinically, the thickness and opacity of the ceramic restoration can decrease the light reaching the RBCs and subsequently, the DC (40). Therefore, the bond produced between the ceramic restoration, the resin cement and the surrounding tooth structure will be compromised (40). This was highlighted in this study as resin coating significantly increased the mean BFS of the greatest conversion system (A1, 0.60mm) but not as significantly in the other two groups of ceramic specimens tested. It should be noted that the system with the greatest DC had the greatest change in BFS between the bilayered and monolayered specimens which highlighted the importance of DC on the magnitude of resin-reinforcement that is achieved (41). From BFS testing, it was evident that a decrease in BFS was generated when testing the ceramic specimens (A3.5 at 1.40mm) with the lowest DC. With this lowest conversion system, the flexural strength was only increased from 88.28MPa to 92.12MPa for the uncemented and cemented ceramic specimens respectively. 4.5-Resin-strengthening mechanism The resin strengthening mechanism has been previously been proposed to occur through the production of a resin cement-ceramic interface in which the resin cement penetrates the defects present on the surface of the ceramic (40). Pagniano et al identified that the resin-strengthening mechanism is thought to occur via a process known as crack bridging. The process is thought to occur in one of two ways (40). The first proposition involves the use of the silane coupling agent present in the RBC and it is predicted that the silane enters any cracks present on the ceramic surface and prevents the cracks from spreading further through the ceramic (40). The second theory, which is very similar to the proposal given by Nathanson, suggests that the polymerisation of the RBC via curing induces compressive stresses in the ceramic which is resultantly thought to results in crack closure and stabilisation of any ceramic surface defects (11-13, 39, 40). The statistical results most importantly identified that there was a significant interaction between the resin-coating and the type of ceramic on the magnitude of strengthening observed (p=0.012). The magnitude of strengthening is thought to be reliant on the flexural modulus of the RBC (~5.5??0.7 and 5.4??0.7GPa for Rely-X’ Veneer cement) (11-13). In some journals, this can also be known as the modulus of elasticity (11-13, 40-41). The modulus, in other studies, has been identified to be between 7 and 12 GPa. The modulus of the cement is vital because it is related to how effectively stresses can be transmitted between the all-ceramic restoration and the tooth structure (11-13, 41-42). Moreover, it provides an indication as to how well the cement can resist elastic deformation which ultimate would endanger the integrity of the bonded interface between the ceramic and cement (41-42). Ideally, the resin cement should have an elastic modulus that is between that of dentine and the ceramic restorative material (41-42). Despite the resin-reinforcement, it is suggested by Yesil that failure still occurs and the mode of failure is caused by surface flaws or flaws within the ceramic material, the adhesive layer, or the bonded cement and flaws in the interface (43). Furthermore, in a different study carried out by Thompson et al, the results demonstrated that when clinically failed glass-ceramic restorations were analysed, the majority of these restorations failed because of fractures which originated from flaws and stresses in the adhesive resin cement interface and not from the restoration contact surface (6). 4.6-Limitations Most clinicians implement a curing regime of 400mW/cm2 for 40 seconds and this is thought to be generally enough for adequate polymerisation when applied directly on the ceramic restoration. This is also the settings recommended by the manufacturer. In this study, the curing time used was 30 seconds at an intensity of 800mW/cm2 (37). Clinically, the thickness of the resin cement used for cementing all-ceramic restorations is 100-150??m which is below the 0.20mm used in this study (6, 43, 37). As the resin cement thickness used in this study was not what is used in clinical situations, the results of this present study cannot predict or summarise the performance of an all-ceramic restoration in clinical situations (6, 43, 37). Therefore, this could be overcome by performing a study that represents more closely the thickness of the ceramic and cement used clinically as well as the curing protocol used by dentists in order to understand the DC obtained using this protocol and to correlate these results to the strength obtained (6, 43). The strength values may be different due to the absence of polishing in the ceramic specimens that were tested for BFS. The discs used in LT testing were polished whereas the ceramic discs tested for BFS were not polished. Polishing the discs modifies the surface as it result in a more even surface with no porosity (11-13). In BFS testing, the internal fit surfaces of the ceramic specimens were roughened via acid etching to promote adhesion. The strength values obtained may have been affected by the absence of polishing of the specimens prior to mechanical testing (11-13, 40). This could be overcome by either polishing or not polishing all the ceramic discs for all the tests but it would be more preferable to polish one side of the discs as it would more closely resemble clinical situations as the outer surface of the restoration is usually polished to give more aesthetically pleasing results (40). Although the BFS testing was simple to perform, the values obtained do not reflect the actual fracture strength that would be expected to be obtained in clinical situations because of different environmental and loading conditions in the oral cavity (44). During BFS testing, the load applied to the ceramic specimens was applied in roughly the same position and with identical force but in clinical situations, the direction and power of the masticatory forces vary considerably (45). Additionally, the reliability of the results obtained in this study, especially the BFS tests, could be increased by increasing the sample size. Besides, the resin cement-ceramic samples could be subjected to a thermocycling regime that represents the forces witnessed in the oral cavity to estimate how long these samples would last in the oral cavity if they were to be placed (44-46). Another limitation could be the use of light-curing resin cements as these can only be cured using light (21). An improvement could be to use dual-cured resin cement systems, which can be cured by both light and chemical means, when cementing all-ceramic restorations because of the importance of achieving an optimal DC of the cement layer, and the chemical reaction of dual-cured resin cements theoretically guarantees a satisfactory polymerisation of the cement as it could compensate for the lack of DC obtained via photo-activated means
which would improve clinical performance (21-24). This is expected to guarantee the cure of the material even in the deeper regions where there is a lack of light (44, 46). An additional drawback was that this study was performed at room temperature but the rate of polymerisation as well as the DC is dependent on temperature, so the DC could be measured at different temperatures in which the temperatures of the oral cavity should be taken into consideration (23). The different temperatures would influence the speed of activation of the photoinitiators, the mobility of the radicals, the polymerisation rate and finally the DC (23). 4.7-Clinical applications Over the past 30 years, there has been an increasing shift towards metal-free restorations due to the increasing demand from patients for aesthetically pleasing restorations in the posterior region along with the anterior regions (30). Therefore, various all-ceramic systems have been developed with the aim of producing restorations with superior aesthetics, biocompatibility and longevity (30, 31). This study involved the use of glass ceramics and even though in the last decade there has been extensive development into the use of other ceramic materials, resin luted glass ceramic restorations such as crowns, veneers, inlays and onlays are still the most widely used option by clinicians in cosmetic dentistry due to their superior aesthetics (39). The use of adhesive resin cements as luting agents for the cementation of all-ceramic restorations may be an important method for improving the brittleness and fracture resistance of these ceramic systems (39). Adhesively cementing these restorations using resin cements would enable the restoration to withstand greater masticatory forces thereby improving clinical performance. The restorations also have excellent mechanical integrity which would allow them to be suitable for use in both anterior and posterior regions of the mouth (47). The success rate of resin-bonded glass inlays and onlays is expected to be 92% over 8 years (10). According to other studies, the reported survival rates of all-ceramic restorations are between 88-100% after service for 2-5 years, and can be a maximum of 97% after 5-15 years (10). This does not match the longevity that has been achieved with metal restorations which is currently a problem and this problem occurs because of the brittleness and low fracture toughness of the ceramic restorative material. Inadequate polymerisation of the RBC would further reduce the success rate due to the luting cement not having optimal mechanical properties and thereby cannot prevent crack propagation as effectively through the already brittle ceramic material (10). Accordingly, the ceramic material would be more prone to failure by fracture or debonding (10). In this study, it was possible to see the DC obtained with different shades and thicknesses of cement and ceramic but in clinical situations, the clinician cannot see the resin cement once the ceramic restoration is placed above the resin cement and have to rely on curing as close as possible to the restoration using the curing regime provided by the manufacturer which is commonly at an intensity of 400mW/cm2 for 30-40 seconds (10). If the resin cement is not adequately cured beneath the restoration, the resin-strengthening mechanism would be less sufficient and hence the longevity of the restoration will be compromised (10). However, it is difficult to predict the degree of curing of the resin cement in clinical situations and it is difficult to exactly state how long the restoration will last as different patients have different oral conditions. Laboratory tests are vital in order to predict the thicknesses and opacities with which adequate curing and accordingly, optimal strength of the resin-coated restorations can be achieved (10, 47). These experiments can also be utilised to estimate the lifetime and failure of the restoration. As a result, the clinician has to make an informed evidence based decision that is appropriate to the clinical situation specific to the patient to achieve maximum resin cement polymerisation so that the failure is minimised and clinical success is increased (10, 47). 4.8-Clinical consequences Adequate polymerisation is desirable to reduce problems associated with post-operative sensitivity, microleakage, risk of recurrent caries, discolouration, in addition to decreased mechanical, chemical and physical properties of the resin cement (10, 48, 49). 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