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). Sufficient DC would improve the biocompatibility of the restoration and most importantly, the resin cement layer by reducing the number of residual monomers that are leached into the oral environment (42, 48, 49).