Discussion (3000-4000 words)

Resin-based composite cements are widely and commonly utilised for the cementation of all-ceramic restorations to the prepared tooth structure because clinical studies have identified that using resin cements for this purpose enables the all-ceramic restorations to have an increased fracture resistance in service and thereby improved clinical performance (Kilinc and others). However, this purpose is only fulfilled optimally if the light-cured resin cement is adequately cured to achieve a sufficient degree of polymerisation. In order to initiate the curing reaction, light energy of an appropriate intensity and wavelength must activate and excite a sufficient number of photoinitiators in order to generate an adequate quantity of free radicals. 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 (Dias et al, 2008). An adequate cure would consequently determine the strength of the reinforcement that is achieved via the application of a resin cement to the ceramic specimens.

Light transmission

Talk about LT at different distances. Increasing the distance decreases both LT and DC. Lack of light can be overcome by increasing light intensity or increasing exposure time

In this study, light-cured resin cement was utilised and adequate LT of restorations is even more pivotal for the for the polymerisation of these light cured resin cements because all these cements can utilise is the light that is transmitted through the restoration, or in the case of this study, the ceramic disc (Kilinc and others). The presence of a ceramic disc also restricts the transmission of light to the resin cement as according to Strang et al, a ceramic is capable of absorbing between 40-50% of the curing light (Passos et al, 2013).

In this study, light cured resin cement

The light transmission in this study was proved to be affected by the thickness and opacity of the ceramic disc as well as the distance from the curing tip. The translucency of a ceramic depends on its crystalline structure, light refractive indexes and the thickness of the ceramic (Kilinc and others). An opaque material may be ideal for aesthetically demanding clinical situations but opacity has been shown to affect light transmission as well as the polymerisation of the resin cement. It is clear from the results in this study that increasing the thickness or opacity of the ceramic disc decreases light penetration through the disc (Duran et al, 2012). A ceramic disc of increasing thickness or opacity would allow lower light energy to be transmitted through it and resultantly, lower LT.

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 and consequently, negatively influencing the polymerisation of the resin cement (Passos et al, 2013).

The results also highlighted the fact that the polymerisation was adversely affected with increasing thickness or opacity of ceramic specimens. The presence of a ceramic disc would influence light attenuation and therefore decrease the number of photoinitiators that are activated in the resin cement. The consequence is a lower DC (Filho et al, 2010).

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. The ISO recommend a curing light intensity of 300mW/cm² and the depth of polymerisation should be no greater than 1.5mm. In this study, the curing time used was 30 seconds at an intensity of 800mW/cm² (Akgungor et al, 2005). In the darker shades and thicker ceramic specimens, (give combos) had reduced light transmission as low as values… therefore, the resin cement cured through these groups of ceramic discs were far from possibly achieving adequate polymerisation (Kilinc et al, 2011).

Another option is to use dual-cured resin cements and dentists and clinicians tend to prefer to use dual-cured resin cement systems when cementing all-ceramic restorations because of the important of achieving adequate light transmission through the indirect restoration, which would consequently lead to an optimal degree of conversion 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 consequently improve clinical performance. The chemical polymerisation of dual cured resin cements is expected to guarantee the cure of the material even in the deeper regions where limited light reaches (Indian journal of dental research).

The lower light transmission influences polymer development by primarily decreasing the C=C bond conversion since the polymerisation process is reliant on on the light exposure to both the ceramic but most importantly, the resin cement (Ilday et al, 2012). The lower the degree of light reaching the luting agent, the lower the degree of polymerisation and consequently, the lower the success and longevity of the resin-strengthening mechanism (Pazin et al, 2008).

Light transmission through the ceramic and to the resin cement is vital because if the ceramic restoration/disc prevents polymerisation light from reaching the resin cement layer, causing inadequate polymerisation of the cement layer. This consequently affects the strength of the restoration and the risk of debonding is higher in poorly polymerised cement (Akgungor et al, 2005).

During the process of light activation, the light passing through the ceramic is absorbed and scattered. Thereby, the light irradiance produced by the light source is attenuated and its effectiveness is reduced as the ceramic thickness increases (Filho et al, 2010). This was proven in a study by Meng et al, in which it was shown that a light intensity of 800mW/cm² decreased significantly to 160mW/cm² after light transmission through a 2mm thick ceramic specimen (Filho et al, 2010).

The greater the LT through a ceramic material, the greater the irradiance reaching the resin cement surfaces and accordingly the greater the DC and mechanical properties (Pick et al, 2010).

The light transmission through a ceramic restoration may affect the degree of polymerization of resin cements, because the amount of light that reaches the cement layer is decreased.10In general, the factors affecting light transmission through a ceramic material include the thickness and shade of the ceramic material, its microstructure, and the presence of defects and porosities. However, the thickness of indirect materials interposed during photo-activation is a key factor for light attenuation, and polymerization generally decreases as thickness increases.12 The physical and mechanical properties of resin cements may be affected by the thickness and microstructure of ceramic restorations. It is important to emphasize that light-cured resin cements should receive an adequate energy density to reach good polymerization and mechanical properties. Furthermore, only effective photo-activation may ensure clinically acceptable hardness levels of dual-cured resin cements, mainly in critical areas. Considering these issues, further studies are needed to find a way to compensate light attenuation aggravated by the interposition of indirect restorative materials.

http://www.duhs.edu.pk/download/jduhs-vol.7-issue-2/9.pdf

DC

Increasing the distance, thickness or opacity of ceramic/resin cement layer would mean that the top few millimetres of the resin cement would adequately polymerise and the remaining material would poorly polymerise due to lack of light (Silva et al, 2009).

Insufficient curing also enhances the solubility of the cement which is the common cause of debonding of the restoration (Dias et al, 2008). As the properties of the resin cements are directly related to the degree of polymerisation, the resin cement used for cementation was measured using FTIR.

Favourable resin cement polymerisation is vital in order to obtain optimal properties of the cement in order to prolong the longevity and clinical performance of the overlying all-ceramic restoration (Ilday et al, 2012). Various studies have stated the importance of a high monomer-to-polymer conversion with a highly cross-linked polymeric network in order to obtain a clinically successful resin-based restoration. The presence of an increased amount of monomers decreases both DC and mechanical properties and the fracture resistance of the restoration (Francescantonio et al, 2013).

The DC% of the resin cement was decreased under the darkest, thickest ceramic combination than under the thinnest, lightest ceramic.

The clinical performance of both the resin cement and the ceramic restoration depends on many important factors, with one of these being the degree of polymerisation (journal of the Serbian chemical society). One way of assessing the DC of the resin cements is through the use of Fourier transform infrared spectroscopy (FTIR) which detects the C=C stretching vibrations directly before and after curing of the resin cement material (Obradovic et al, 2011).

Several factors have been shown to influence the polymerisation of resin-based cements such as the thickness of the resin cement layer, the intensity of the light source as well as the type of light source used, the distance from the curing tip and the duration of curing. The composition of the resin-cement can also affect the rate of polymerisation via factors like the polymer matrix, the filler particles and the coupling between matrix and filler (Obradovic et al, 2011).

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 radical chain ends and this limits the conversion of the pendant methacrylate groups from monomer to polymer (Obradovic et al, 2011). During the process of light activation, 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, i.e. the process of gelation. However, subsequently, the polymer network is highly cross-linked as most of the monomer is converted to polymer which results in the link between the monomers and the oligomers with the network being restricted due to limited diffusion (Filho et al, 2010). With light activation, there is a production of free radicals via the excitation of photoinitiators which enables the initiation step to occur instantaneously. As the propagation phase proceeds, the resin cement changes from a liquid to a viscous gel state, thereby making it increasingly difficult for monomers to diffuse to the polymeric growth centres. As a result, as the polymerisation reaction proceeds, less monomer is converted to polymer (Mendes et al, 2010). The rate of polymerisation reduces as the reaction proceeds due to the formation of fewer polymer growth centres. This promotes the formation of loosely cross-linked polymers and poorer mechanical properties (Silva et al, 2009).

Adequate polymerisation of the resin cement is crucial for stability, optimal mechanical properties and the clinical performance of the indirect all-ceramic restoration. Furthermore, a greater degree of polymerisation would result in a greater bond between the resin-cement and the ceramic and consequently, maximum bond strength (Ilday et al, 2012).

Along with using thinner or less opaque ceramic and resin cement layers, the cure depth of the resin cement, the degree of conversion and consequently the strength of the restoration can be increased by longer light exposure times or increasing the light intensity (Ilday et al, 2012). The latter two factors were not evaluated in this study. Insufficient polymerisation commonly causes early failure of the cemented all-ceramic restoration (Duran et al, 2012).

BFS

Flexural strength were found to show a relationship with conversion of double bonds with the resin cement (Ozturk et al, 2005).

Ceramic specimens luted with a resin luting agent exhibited greater flexural strengths than the specimens without any form of luting agent (Pagniano et al, 2005). Look at pagniano journal for more info- if space left.

The discs were left for 24 hours prior to BFS testing because the cement undergoes polymerisation 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 therefore the risk of debonding and poor flexural strength is greater (Akgungor et al, 2005).

The magnitude of strengthening is reliant on on the flexural modulus of the resin cement. This can also be known as the modulus of elasticity. (value of resin cement according to dr Addison journal). The modulus, in other studies, has been identified to be between 7 and 12 GPa. The elastic modulus of the cement is vital to study because it is related to how effectively stress can be transmitted between the all-ceramic restoration and the tooth structure. 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. Ideally, the resin cement should have an elastic modulus that is between that of dentine and the ceramic restorative material (Braga et al, 2002).

Flexural strength of brittle materials is likely to be more affected by surface defects or imperfections such as porosity, cracks and other related flaws.

Specimen failure is thought to initiate at the bottom surface of the specimen and all ceramic specimens were placed with the non-irradiated surface facing towards the load application and therefore a lower DC would lead to a reduced flexural strength (Pick et al, 2010).

From BFS testing, it was evident that a decrease in BFS generated at the resin-ceramic interface when testing the ceramic specimen (A3.5 at 1.40mm) with the lowest DC.

Resin coating significantly increased the mean BFS of the greatest conversion system 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 which highlights the importance of DC on the magnitude of resin-reinforcement that is achieved (Fleming et al, 2012).

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 (Yesil, 2009).

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 and most of the fractures initiated from flaws and stresses originating from the adhesive resin cement interface and not from the restoration contact surface (Yenisey et al, 2009).

Clinically, the thickness and opacity of the ceramic restoration acts as a barrier to light penetrating the methacrylate resin-based composite cements. Therefore, the durability of the bond produced between the ceramic restoration and the resin cement as well as the interface between the cement and the surrounding tooth structure will be compromised. For resin-based composites, a maximum value of DC is ideally wanted in which there is complete conversion of the monomer double bonds to network contributory single bonds. However, the conversion is normally between 45-70% because vitrification stops the reactions by inhibiting diffusion (Isgro et al, 2011).

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 in BFS were not polished. Instead, the internal fit surfaces of the dental ceramics were roughened to promote adhesion. The strength values obtained may have been affected by the absence of polishing of dental ceramic specimens prior to mechanical testing. (see Isgro et al, 2011) for more info.

Give values of % increase between cemented and uncemented samples and the mean values.

Look at Pagniano et al, 2005 for information on how the interaction between cement and ceramic affects BFS. Add if word count available.

Future work

Look at effect with different light output- look at Duran et al, 2012 journal.

If space left, look at Pazin et al journal for info on degree of cross-linking.

Look at SEM images

Look at Molin et al, 2006 and Isgro et al, 2011 for info on contraction stresses for BFS

In conclusion, 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. Furthermore, it will compromise the clinical success and longevity of the restoration. Sufficient DC would also 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 (Kim et al, 2009; Yan et al, 2010; Braga et al, 2002).