## 4 Discussion  
The preclinical assessment of a modified dental material has to be experimentally evaluated to determine its biological behavior prior to its clinical application. The use of in vivo models provides a complex systemic interaction between the biomaterial and the underlying different cell types. Rat molars have been shown to be valid experimental models for evaluating pulpal reactions after capping with different biomaterials [7; 19; 21]. Rats have a similar oral bacterial flora to that of humans [22]. Rat molars are anatomically and histologically similar to human molar teeth [23]. Furthermore, the healing process after direct pulp capping in rats is comparable to that observed in humans [24]. Dentinal bridge formation in rats can be shown histologically within four weeks after direct pulp capping [18; 19], making the rat a suitable model for short-term evaluations. Two different time points (one and four weeks) were selected in the present study to assess the effect of  
Figure 4: Histologic features of maxillary first molars subjected to pulp exposure and capped with (**A**) iloprost, (**B**) MTA, and (**C**) MTA-iloprost at week four. H&E stain \(\times\)200 magnification. The star indicates tertiary dentin. Figure 3: Histologic features of maxillary first molars subjected to pulp exposure and restored with glass ionmer without the use of pulp capping agent at week four. Representative photomicrographs showing (**A**) pulpal degeneration and (**B**) calcified bridge formation. H&E \(\times\)200 magnification. the combined use of MTA and iloprost on the progress of the pulpal healing process, in accordance with previous studies [18; 19]. MTA is a bioactive material that is biocompatible, radiopaque, has low solubility, and induces dentin bridge formation. Like any other material, MTA has a number of drawbacks. It has a long setting time, high cost, and potential of discoloration. The metal oxide content of MTA has been reported as the cause of discoloration [25]. Several methods have been introduced to overcome it, such as sealing dental tubules of the pulp chamber by bonding agent before MTA application [26], placing MTA below the cementoenamel junction, and using MTA when esthetic is not a concern, like in posterior teeth. MTA is considered the gold standard among other calcium silicate-based materials, and that is why it was selected in our study. Moreover, MTA's physical and chemical properties have been shown to be affected by the acidic environment [27; 28], therefore, a composite was avoided and resin-modified glass ionomer was used to cover MTA. In the present study, a nontreated control group was included to differentiate between the self-repair and material-induced repair processes. At week one, all teeth in the control group showed moderate to severe pulpal inflammation, which was not due to cavity preparation per se, whereas the other groups did not show the same degree of inflammation. However, the inflammatory response within the control group may be attributed to the cytotoxicity of the resin- modified glass ionomer [29]. At week four, slight to moderate inflammation was present with signs of pulpal degeneration, which may imply a late stage of chronic inflammation. Some of the control samples showed hard tissue formation. The formation of mineralized tissues in the control group has been previously reported in studies using rat molars [18; 20]. This may be explained by the sealing ability of the resin-modified glass ionomer material [30]. Pulp has the inherent capability of healing and forming dentin bridges in the absence of bacterial microleakage [31]. Initial pulpal inflammation is a prerequisite for healing and regeneration [32]. However, promoting the resolution of the inflammatory response would favor the dental regenerative process [33]. With time, specimens in the experimental groups showed less inflammation and greater formation of continuous mineralized bridges, indicating the biocompatibility of MTA and iloprost.