## 2 Methods and Materials
### Categorization of the Radiographic Findings  
Three hundred thirty-four included images were transferred to PowerPoint (Microsoft) on black slides. Slides were labeled with the sample number, tooth number, and restoration surface. PowerPoint Slides and Excel sheets were linked with a numeric coding system for each sample. A classification was designed to codify the radiographic findings and used for screening. The classification consists of a description of each category intended to reduce ambiguity. Figure 2 illustrates the definition used for findings classification in a bitewing radiographic image. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Location** & **Category** & **Description** \\ \hline \multirow{4}{*}{Body of the restoration Mass of composite constituting the restoration} & No atypical radiographic finding & No radiolucent or radiopaque results suggesting atypical radiographic findings in composite restoration \\ \cline{2-3}  & Internal void/porous & Void = pore Circular volumetric (2D) empty radiolucent spaces located at the body \\ \cline{2-3}  & Interlayer line & Lack of continuity between the composite layers characterized by a thin radiolucent line \\ \cline{2-3}  & Overhang & Excess radiopaque composite in the interproximal area \\ \hline \multirow{4}{*}{Bonding interface Surface (Line) between tooth and composite} & Internal gap–not gingival margin & Lack of continuity, radiolucency between the composite and tooth not involving gingival margins \\ \cline{2-3}  & Internal gap–at the gingival margin & Radiolucency, lack of continuity between the composite and tooth involving gingival margins: usually “notch” shape. \\ \hline \multirow{4}{*}{Secondary caries} & Presence of radiolucency in dentine indicating recurrent caries \\ \cline{2-3}  & Other & More than one radiographic finding \\ \cline{1-1} \cline{2-3}  & & indicating multicategory \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Description of the categories for classification of findings. From the pool of 334 included x-rays, 55 bitewings showed no atypical radiographic findings, and 279 showed atypical radiolucency/radiopacity findings. This category of radiographs sets the appearances classifiable as standard, normal appearance. This atypical radiolucency category refers to radiographs where areas of radiolucency are present. Those divisions are defined by the standard radiographs, together with the written description. The presence or absence of a radiolucent halo adjacent to the gingival, mesial, distal, and pulpal walls was considered to assess the bonding interface. The 279 bitewings were categorized according to the type of material (composite or adhesive) and location (internal or external) (Table 1). The information recorded for each patient included age, sex, tooth, and restored surface. Guidelines were placed to avoid misinterpretation of bond interface radiolucency and secondary caries. Radiolucent areas detected at the cavity preparation walls were considered interfacial gaps. Large radiolucent regions not bound to the preparation structure and extending into the dentine were interpreted as secondary caries. The radiographic prevalence of atypical findings was presented as frequencies and percentages, whereas demographic variables were presented as frequen  
Figure 1: Flow chart showing the selection criteria and categorization of the findings used in this study. cies and percentages when appropriate. Figure 3 displays a panel of radiographic images illustrating some of the radiolucences associated with Class II composite restorations. A. Secondary caries, B. Voids in the body of the composite, C. Interlayer lines within the body of composite, D. External gap, E. Internal gap, and F. Combination of void in the composite and internal gap at the bonding interface. Figure 3: A panel of radiographic images illustrating some of the radiolucences found associated with Class II composite restorations (yellow arrows are highlighting the area of interests).