This randomized clinical trial investigated the clinical performance of the modified, experimental cements (NHA-GIC and NHA-RMGIC) and compared the performance to their respective control cements (CGIC and RMGIC) in class V cavities at baseline (immediately) and after 3, 6 and 9 months using modified USPHS criteria (retention, color match, surface texture, surface staining, marginal adaptation, marginal discoloration, recurrent caries and post-operative sensitivity).
At the 9 month recall, all restorations, irrespective of material, showed excellent retention [93.3% (Alpha) rating], which confirms the results of previous studies (Loguercio et al. 2003; Franco et al. 2006; Jyothi et al. 2011; Fagundes et al. 2014). The high retention rate reported for both Fuji GC Gold Label 2 (CGIC) and Fuji II LC (RMGIC) might be related to their relatively good mechanical properties and better adhesion mechanism to dental tissue. The auto-adhesive capacity of GICs through the formation of ionic bonds between the carboxyl groups of polyalkenoic acid and hydroxyapatite in the tooth structure might have been responsible for higher retention rate of these materials (Shikumar et al. 2016). Moreover, the micro-mechanical interlocking of the polymer in RMGIC added another adhesion mechanism, which might participate in improving its retention rate (Jyothi et al. 2011). Another factor associated with the favorable retention performance of CGIC and RMGIC, was the fact that these materials have elastic modules close to that of tooth structure (Shikumar et al. 2016). This increases the strain capacity of the restorations and prevents their deformation under occlusal load, thus preserving the adhesion of the materials at the margins of the cavities (Abdalla et al. 1997). The fact that there was no significant difference between the experimental groups (NHA-GIC and NHA-RMGIC) and the control groups (CGIC and RMGIC) regarding the retention rates might be due to the similarity in general mechanical and physical properties between the control and the experimental cements.
The results of clinical performance regarding the color matching showed that there was no statistically significant difference between the four tested restorative materials during the study period. Nevertheless, there were statistically significant changes in the color matching criterion through the 9 months study period in the NHA-GIC and RMGIC groups. There was an increase in prevalence of Bravo scores and a decrease in prevalence of Alpha scores from 3 to 6 in NHA-GIC and RMGIC groups, as well as from 6 to 9 months in NHA-GIC. These results were in accordance with several previous clinical trials (Folwaczny et al. 2001; Sidhu 2010; Lee et al. 2010; Priyadarshini et al. 2017). The poor color match for both CGIC and RMGIC, control as well as experimental groups, could be related to the "chalking phenomenon" of glass ionomers which damages its appearance under dry conditions making the surface appear weak and opaque (Perdigão et al. 2012). Moreover, the presence of porosity and microcracks within the glass ionomer microstructure, might be another factor that affects the color stability of the tested cements. This allowed for the accumulation of oral fluid and stain adsorption and discoloration of the restoration (Priyadarshini et al. 2017). In addition, lack of color stability in GICs could be attributed to the polyacid content in the material, which could be explained by degradation of the metal polyacrylate salts. When GICs were exposed to acid attack (depending upon pH of the patient oral environment), H+ ions diffuse into the glass ionomer component and replace the metal cations in the matrix. These cations later diffuse outwards and get released to the cement surface. Thus, the material might become rough with voids, and undissolved glass particles result in greater water and food colorant absorption (Cardoso et al. 2010). Moreover, RMGIC "Fuji II LC, improved" undergoes color change during polymerization of the resin components as the acid–base reaction is retarded. This delayed acid–base reaction in addition to water sorption by the resin components within RMGIC might result in post-polymerization color change (Adusumilli et al. 2016). Furthermore, the hydrophilic monomers, incomplete polymerization, residual HEMA molecules after light-curing, physical adsorption and desiccation might also affect the color stability of RMGIC and increase its potential body discoloration and surface staining (Cardoso et al. 2010; Hussainy et al. 2018).
In contrast to these results Hussainy et al. (2018) observed close color matching and high color stability of RMGIC "Fuji II LC, improved" to the tooth structure with Alpha scores of 100% and 95%) after 6 months and one year, respectively. The differences in the results might be explained by the carious cervical lesions which were selected for this study rather than the non-carious cervical lesions which were selected for the other studies (Priyadarshini et al. 2017; Hussainy et al. 2018). Therefore, the cavities might be deeper in the present study with subsequent increase in the volume of the restorations. This might result in higher rate of water sorption and more pronounced color change (de Oliveira et al. 2012). Moreover, it was suggested that the color stability of RMGIC was shade dependent (Yap et al. 2006).
Regarding the surface texture of the restorative materials, the results showed that there was no statistically significant difference among the four tested restorative materials during the 9 months study period. However, there was an increase in the prevalence of Bravo scores and a decrease in prevalence of Alpha scores from; baseline to 3 in NHA-GIC group and from 3 to 6 as well as 6–9 months for all tested restorative materials. These results were in consistence with those of other studies (Folwaczny et al. 2001; Jyothi et al. 2011; Sooraparaju et al. 2014; Hussainy et al. 2018).
The decrease in the quality of the surface texture of CGIC and RMGIC, control or experimental, might be related to the influence of microstructure and mean filler particles size of the restorative materials on its physical properties and abrasion resistance. In the current study, the rough surface texture for the tested RMGIC (Fuji II LC, improved) might be contributed to the larger and irregular filler particles size which were approximately about (4–5 µm) (Abdalla et al. 1997; Loguercio et al. 2003; Sidhu 2010; Jyothi et al. 2011; Fagundes et al. 2014; Shikumar et al. 2016; Priyadarshini et al. 2017).
Moreover, this value was much higher than the wave-length of the light (350–750 nm), so these particles scatter light and produce opaque materials (Jyothi et al. 2011; Konde et al. 2012). In addition, conventional GIC (Fuji II Gold Label 2) was found to present larger mean particle sizes, more sensitivity to water sorption and longer setting time than RMGIC, which might be responsible for the high surface roughness obtained by GIC (Pacifici et al. 2013).
Another factor might explain the pitted or rough surface of glass ionomers restorations; in the present study glass ionomer cements were used in "powder and liquid" form rather than "encapsulated" form, which was mandatory to allow for incorporation of NHA in a correct way. Manual mixing might incorporate air bubbles and decrease polymer conversion due to inhibition of the setting reaction by oxygen in the bubbles, which introduces voids and porosity leaving behind a rough, plaque retentive surface and staining (Priyadarshini et al. 2017).
In addition, surface roughness of glass ionomers restorative materials may be related to the final surface resin coating (varnish). This resin coat may represent an area of weakness at the resin coat-GIC interface, especially under-functioning force. This might result in bond failure between the GIC and the overlying coating, which creates a high roughness area and a gap on the restoration surface (Yap et al. 2006).
According to the present study, there was a statistically significant, positive relation between surface texture and color match. This observation is in accordance with another previous study (Nassar et al. 2014). This could be related to the rough surfaces of restorations which positively favor the accumulation of dental plaque and staining from dietary habits, resulting in a change in surface properties of the restorations, such as surface gloss and color stability. Moreover, it has been found that an increase in surface roughness results in an increase in the degree of random reflection of light with subsequent decrease in color properties (Pacifici et al. 2013; Nassar et al. 2014). Regarding the surface staining criterion, results showed that there was no statistically significant change through the study period for any of the restorative materials. Moreover, there was no significant difference between the four materials during the 9 months study period. This observation was similar to that of Jyothi et al. (2011), which might be related to the relatively short follow-up duration as surface staining is usually recorded after long period.
The results showed that there were statistically significant changes in marginal integrity criterion through the study period in the CGIC and NHA-GIC groups. Thus, there was an increase in the prevalence of the Bravo score and a decrease in the prevalence of the Alpha score from 3 to 6 as well as from 6 to 9 months for these groups. At the same time, there was no significant change in marginal integrity through the study period in the RMGIC and NHA-RMGIC groups. This result was in agreement with other previous studies (Franco et al. 2006; Fagundes et al. 2014; Adusumilli et al. 2016; Shikumar et al. 2016; Hussainy et al. 2018).
The differences in the marginal adaptation of CGIC (Fuji GC gold label) in comparison to RMGIC (Fuji II LC, improved), could be explained by the sensitivity of CGIC to humidity in the early period, which increases the solubility of the cement. On the other hand, RMGIC sets by instant resin polymerization, which makes RMGICs more resistant to moisture contamination and hence results in better marginal adaptation (Bapna et al. 2002).
Another reason for the inadequate marginal adaptation in GIC groups could be related to the immediate finishing/polishing procedure of the "one-visit treatment." This immediate finishing/polishing could compromise the marginal seal of GIC to the tooth, as the material is still soft during its initial setting (Fagundes et al. 2014).
Moreover, GIC cervical restorations can be abraded more easily by tooth brushing with dentifrices compared to RMGIC restorations, which exhibit higher resistance to toothbrush wear (Rekha et al. 2012). Also, GICs are more subjected to erosion and microhardness loss over time in comparison to RMGICs. This could be explained by matrix dissolution that occurs in the periphery of the glass particles of CGICs and could result in dissolution of the siliceous hydrogel layer (Rekha et al. 2012; Meral and Baseren 2019). All these factors might contribute to the poor marginal adaptation observed in the GICs groups.
Contradictory results to this study have been reported by Sidhu (2010), who found unsatisfactory marginal integrity results for RMGIC restorations. The author attributed that to the hygroscopic expansion of glass ionomer components, which might result in minor marginal fractures at the interface between the tooth and the restoration. This difference in observations could be explained by multiple variables contributed between different study designs that might affect the outcome, such as cavity design, size of restorations and operator variability. Additionally, other factors, such as the bonding capacity of the restorative material, light-curing device and technique, follow-up period, pH and temperature fluctuation in the patient's mouth could also explain the difference in results between the studies.
There was no significant change in the marginal discoloration criterion through the study period for any of the tested materials. This is in agreement with findings from other studies (Abdalla et al. 1997; Franco et al. 2006; Lee et al. 2010; Sidhu 2010; Perdigão et al. 2012; Fagundes et al. 2014; Adusumilli et al 2016; Shikumar et al. 2016; Priyadarshini et al. 2017; Hussainy et al. 2018). The cavosurface marginal discoloration might be considered a sign of microleakage, which occurs when there are marginal gaps. The absence of marginal discoloration with CGIC and RMGIC, control and experimental groups, was considered as an indication of good bonding of these materials to tooth structure without microleakage (Jyothi et al. 2011).
The results showed absence of failures due to secondary caries with the four tested restorative materials after the 9 months follow-up period. This result is in accordance with other previous studies (Franco et al. 2006; Jyothi et al. 2011; Fagundes et al. 2014). Secondary caries at the margin of the restoration are rarely detected in GIC restorations. This might be related to a demineralization inhibition effect of GICs due to continuous fluoride release over time, which possibly produces a cariostatic action (Dias et al. 2018).
The present clinical trial found no significant difference in post-operative sensitivity among the tested groups during the study period. However, some patients exhibited mild hypersensitivity immediately (at baseline) in the RMGIC, GIC and NHA-RMGIC groups. This finding was in agreement with Gurgan et al. (2015); and Hussainy et al. (2018). Post-operative sensitivity might be attributed to several factors, such as operative trauma, marginal leakage and desiccation. The absence of post-operative sensitivity in the current study may be explained by the absence of marginal leakage together with the clinically acceptable properties of materials that minimize the hydrostatic fluid movement within the dentinal tubules (Hussainy et al. 2018). However, the presence of immediate hypersensitivity (at baseline) may result from mechanical irritation during the restorative procedures or following the finishing and polishing procedure. In this study, the initial hypersensitivity was decreased over time and had disappeared completely at the 3 months follow-up.
In the present study, the cumulative survival rate at the 9 months follow-up for the four tested materials NHA-GIC, NHA-RMGIC, CGIC and RMGIC was 100%. This is in agreement with previous short-term evaluations performed with similar restorative materials (Yap et al. 2006; Nassar et al. 2014) or even with long-term evaluations (Loguercio et al. 2003; Franco et al. 2006; Jyothi et al. 2011; Fagundes et al. 2014). These studies found that glass ionomer cement and resin-modified glass ionomer considered the most retentive materials for the management of cervical lesions.
This insignificance difference between the experimental NHA groups and control groups in the current study could be attributed to two main reasons: First, the tested materials may not really differ, but possess similar mechanical and physical properties, and thus show no clinical difference in survival rates. Secondly, a 9 months follow-up period can be considered a very short period to evaluate the long-term clinical behavior of dental restorative materials (Gӧstemeyer et al. 2016).
Finally, the null hypothesis of the present study was accepted, as the 9 months clinical performance of the modified NHA-GIC and NHA-RMGIC did not differ from that of CGIC and RMGIC. However, further long-term clinical studies are required.