## 4 Discussion
[34], the relative risk of identifying a radiolucent zone under restoration in sound and demineralized dentin were 2 and 1.85 times higher when two adhesive layers were applied. Very experienced dentists may consider the morphology of the radiolucency, trying to distinguish between secondary caries and the adhesive layers, but it remains an educated speculation. To obtain assurance that the radiolucencies are not recurrent caries underneath the composite, many dentists prefer to replace the restoration entirely, which may be completely unnecessary. Radiopacity has thus become a critical property for dental restorations. For this reason, radiopaque glass particles--such as barium, strontium, and zirconia atoms, or microfillers containing yttrium and ytterbium are incorporated into adhesive formulations. The radiopacity of adhesive depends on their filler content and type of filler. Optibond Solo, use in the present study, contains approximately 15% of glass filler. Here, suggestive radiolucencies indicative of secondary caries were observed in 12.6% of the bitewings. A study evaluating radiographs had consistent findings of 15% (119/770) [27]. In another study, suggestive radiolucencies indicative of secondary caries were diagnosed radiographically in 14% of restorations [35]. Examination of bitewings for radiolucent secondary caries is a diagnostic challenge. Because the image only mirrors the extent of demineralization, radiographs alone cannot diagnose secondary caries. However, the radiographic appearance of the demineralization (radiolucency) can assist the dentist in clinically diagnosing secondary caries. Many factors can influence the ability to detect these alterations accurately, such as exposure parameters, type of image receptor, image processing, display system, and viewing conditions. Furthermore, the illusory optical effect of each band appears when there is a vast difference between the radiopacity of the tooth structure and restorative material [36]. Therefore, optical illusions such as the Mach-band effect and phenomena such as the cervical radiolucency (burn-out effect) may be falsely interpreted as caries lesions [34; 37; 38]. In the present study, we have managed to cover the restoration (less radiopacity) with a black square filter during radiographic evaluation to inhibit the false effect on optical receptors. We acknowledge several limitations of our study. First, it is a database review and was limited by the number of patients with adequate radiographs. The radiographs that were excluded were not indicative of clinically unacceptable radiographs. The BW radiographs exclusion was based on parameters that would affect the accurate detection and calculation of the prevalence of atypical radiographic abnormalities. The 83.5% prevalence of atypical findings is an aggressive estimate of the restorative practice because this percentage was based on patients who received restorative treatment from dental students. Dental students are providers under training, and their performance may differ from general dentists with years of experience. Published results in the previous study showed a difference in the resulting restorations between operators [20]. Finally, many studies discussed the presence of atypical radiographic findings in bitewings. However, these variables were discussed separately, and there was limited research on the prevalence of radiographic interlayer gaps [20; 23; 27; 36]. Second is the subject-defined parameters for assessing the adhesive interface. We have not measured the thickness of adhesive space and did not include a quantitative reference for it. Thus, we may have underestimated the number of atypical adhesive interfaces. Likewise, not much information was provided in the records to know if any liners were applied underneath the restoration. Liners would contribute to the radiolucency.