###### Abstract  
Bitewing radiographic examination of the Class II composite restorations is commonly performed for diagnosis and preoperative planning of posterior teeth. The purpose of this study was to describe the prevalence; location; and characteristics of radiolucency findings associated with proximal class II composite restorations. Bitewing radiographs of proximal composite restorations of healthy adult patients (18 to 88 years old) who underwent restorative care at predoctoral clinics at the University of Maryland School of Dentistry from August 2016 to July 2019 were identified. Atypical radiographic features were categorized by location and associated material (adhesive or composite). In addition, demographic and clinical details (tooth position; restored surface) were recorded. Out of the 669 radiographically examined bietwings of restorations; 16.5% of radiographs showed no atypical findings; and 83.5% had unusual radiographic signs. The atypical radiographic findings were distributed as 16.5% internal voids; 3% overhang; 7.8% interlayer lines; 12.6% secondary caries; 20.7% interfacial gaps; and 23.1% multiple atypical findings. Class II composite restorations have a high prevalence of atypical radiolucency; particularly in the body of the composite; premolars; and disto-oclusal restorations. The information reported here emphasizes the clinical challenge of ascertaining an appropriate diagnosis of the radiolucencies underneath composite restorations. radiography; bitewing; dental composite; dental caries; bonding +
Footnote †: journal: Medical Imaging  
cost-intensive for patients and the health system [7; 8]. In addition to the financial burden imposed on the patient, there are other reasons to avoid replacement, such as the risk of pulp exposure and pulp irritation [9]. In daily practice, the complexity of the diagnosis of radiolucencies is even more challenging. For example, radiolucencies observed at the interface restoration\cavity walls can be predictive of a lack of the adequate seal, which can directly contribute to secondary caries formation [10; 11]. Moreover, an increased adhesive thickness layer or low adhesive opacity may impose some difficulties in diagnosis [2; 11]. Another factor affecting radiographic evaluation is the concentration of radiopacifiers in the composite. Low radiopacity in the resin-based material makes it hard to discriminate between composite and tooth structures [12]. The radiographic limitation of bitewings should also be considered. In a 2D view of a 3D object, a radiolucent lesion could be superimposed on the restoration and be missed. Therefore, the potential influence of radiolucency findings during recall bitewing of class II composites on replacement outcomes remains controversial. From a functional perspective, voids within restorations may negatively affect the material's mechanical properties over time [13]. Reduced resistance to fatigue and wear, due to their contribution to the initiation and propagation of cracks, may lead to premature failure of the restoration when subjected to mechanical and external loads [14; 15]. The presence of voids in the body of composite restorations has also been reported to be a detrimental factor for the clinical longevity of posterior composites [16]. Secondary caries is reported to be the most common reason to replace or repair a defective dental restoration in general practice [6]. If gaps or voids are at the margin, there is a risk of leakage and secondary caries. Detecting carious lesions, or gaps at margins caught by explorer associated or not with reported pain and sensitivity symptoms often assist in precise diagnosis and decision to replace the restoration [17]. However, secondary caries diagnosis without clinical findings can pose a challenge [18; 19], leading to uncertainty regarding the need to replace or repair existing restorations.