## 4 Discussion  
The present study investigated the tensile bond strength of three different widely used and differently composed universal adhesives on irradiated dentin compared to non-irradiated dentin in the self-etch application mode in vitro. All experimental steps, including specimen preparation and the application of all materials, followed by bond strength testing, were performed by one researcher (dentist) to avoid any impact of different persons. This dentist was trained in advance to become familiar with the study protocol. In some previous investigations, it was shown that the new universal bonding agents led to higher bond strength values when they were used after a preceding etching step [41; 42]. Our study set-up was to evaluate solely the self-etch mode rather than the etch-and-rinse mode, and those research works investigated the effect on the enamel surface, whereas in the present study dentin tissue was tested. According to [PERSON] et al. [43], the evaluation of numerous studies on the adhesion values of universal adhesives on dentin did not reveal any significant overall difference between etch-and-rinse and self-etch utilization. This analysis considered study results on numerous different materials, which differed in their composition, including the functional monomers contained therein, and thus in their chemical and mechanical properties. The remarkable one of these functional monomers was 10-methacryloyloxydelydrogen phosphate (10-MDP or MDP), which was able to bond to the hydroxyapatite crystals contained in the dentin [29]. By linking calcium (Ca), which is released from the hydroxyapatite (HAp) by demineralization, MDP-Ca salts were formed, which were self-assembled into the so-called \"nanolayering\" [44; 45]. These primary chemical/ionic interactions of specific functional monomers with hydroxyapatite followed the \"adhesion-decalcification\" concept [45], which states that carboxylic acids either adhere to hydroxyapatite or decalcify it in accordance with the dissolution rate of the particular calcium salt in the acid solution [46]. The three universal adhesives tested contain different functional monomers that are intended to create the bond to the tooth structure. AdheSE(r) Universal and Futurabond(r)U contain MDP [47; 48], which, according to [PERSON] et al. [29], creates a very stable bond to the hydroxyapatite, which was validated by the low dissolution rate of its calcium salt in water. [PERSON] et al. [49] affirmed the beneficial properties of MDP by evaluating a range of studies. Long-term bonding effectiveness has not been investigated in this current study, but the results of [PERSON] et al. [28] indicated that the bond of universal adhesives in general is not capable of resisting ageing, although it was observed that bonds obtained by the etch-and-rinse mode are more resistant to water-aging than those achieved by the self-etch mode. The bases of fractionated irradiation in conventional radiotherapy are the five Rs of radiobiology: repair, reassortment, repopulation, reoxygenation and radiosensitivity. These factors influence the response of tumors and normal tissue to radiation, and thus the therapy efficiency [50; 51]. In this study, we applied in cooperation with one specialized radiation physicist from the Department of Radiotherapy of our University a total dose of 60 Gy, in 30 fractions of 2 Gy per day, treating 5 days per week over a period of 6 weeks, according to conventional radiotherapy [13; 39; 40]. The cooperation with only one radiation physicist might help to avoid the impact of different involved researchers. Today, there are several different radiotherapy schedules, such as hyperfractionation or accelerated fractionation, which are being evaluated for advantages over conventional fractionation radiotherapy, regarding patient outcome [52; 53; 54], which may therefore be common practice. To perform the in vitro test, the dentin specimens were irradiated directly and homogeneously.