INTRODUCTION

Development of recurrent caries under different types of restorations is considered a major cause of restorative failure and replacement. It is important to diagnose early lesions in order to prevent severe destruction of hard tissue and to enhance the prognosis for a successful treatment outcome.^1–3 Radiographic detection is the most useful method to diagnose recurrent caries adjacent to restorations in conjunction with clinical examination. Intraoral film and digital radiography are commonly available methods in routine clinical dentistry. Conventional intraoral film consists of silver halide crystals in order to produce analog images. On the other hand, digital intraoral systems include a solid state silicon chip or a photostimulable phosphor plate (PSP). Solid state detectors use a scintillator layer to convert x-rays to light and include a charge-coupled device or a complementary metal oxide semiconductor.^4–7 PSPs absorb and store energy from x-rays. This energy is then released as phosphorescent when stimulated by another light of an appropriate wavelength.⁸

Regardless of the intraoral system used, the two-dimensional (2D) nature of the images limits the information that can be obtained, and their diagnostic value is dependent upon beam angulation, superimposition of anatomical structures, and patient-related factors.⁹ Due to their 2D nature, intraoral techniques may fail to provide enough information in certain cases.^4–9 For example, buccal recurrent caries lesions under restorations are difficult to detect in radiographic examination. Radiopacity, which is greater in amalgam restorations than in enamel, can interfere with the detection of lesions in the lingual and buccal areas.¹⁰

Introduction of cone beam computed tomography (CBCT) enabled dentists to visualize teeth in axial, coronal, and sagittal views with a reduced radiation dose compared with medical CT. CBCT uses a cone-shaped x-ray beam centered on a 2D sensor to scan a 180°-360° rotation around the patient's head to acquire a full three-dimensional (3D) volume of data.¹¹ CBCT systems offer different sensor types, fields of view (FOV), and exposure settings. However, beam hardening and metal artifacts that occur in CBCT images are thought to be a limiting factor in detection of recurrent caries under restorations.^12,13 Although CBCT eliminates many disadvantages of intraoral radiography, it must be taken into consideration that patients receive higher radiation doses compared with intraoral and panoramic radiography.^14,15 Therefore, available CBCT images obtained for different purposes should be used only if conventional methods are not useful for diagnostic accuracy.

Considering the difficulties in detection of buccal recurrent caries under restorations due to the compression of structures in intraoral radiography and occurrence of metal artifacts in CBCT, the goal of this ex vivo study was to assess the performance of intraoral film and digital radiography and two different CBCT systems for the visibility of artificial buccal recurrent caries lesions under various restorative materials. Our null hypothesis was that there was no difference between CBCT and intraoral radiography systems for detection of artificial buccal recurrent caries lesions under various restorative materials.

MATERIALS AND METHODS

A total of 120 caries-free teeth (mandibular premolars and molars) extracted for periodontal and orthodontic reasons were used. Teeth of people who gave informed consent to donate their teeth for research and teaching were obtained from our hospital collection.

Image Acquisition

All teeth were randomly placed in the alveolar sockets of a dry human mandible in groups of eight (two premolars and two molars on left and right hemimandibles), and a 2-cm-thick plastic glove filled with distilled water was placed around the dry mandible in order to simulate soft tissue. Thereafter, teeth were imaged with intraoral conventional radiography, intraoral digital image receptors and two different CBCT units. Intraoral conventional radiographies and intraoral digital images were exposed ortho-radially with a Trophy Trex x-ray unit (Croissy, Beaubourg, France) operated at 65 kVp and 8 mA with a standardized paralleling technique and a focus-receptor distance of 20 cm. Repeated exposures after individual adjustment of the jaw/beam for each tooth were performed under reproducible conditions. Intraoral conventional radiographs were taken with Kodak Insight Film (size 2, E/F sensitivity, Eastman Kodak Co, Rochester, NY, USA) and an exposure time of 0.40 seconds. Films were automatically processed on the same day with fresh chemicals (Hacettepe, Ankara, Turkey) using an Extra-x Velopex (Medivance Instruments Ltd, London, England) in accordance with the manufacturer's instructions. Digital images were recorded using a Digora Optime (Soredex, Tuusula, Finland) PSP digital intraoral system, which includes a feature that automatically erases residual image signals. Image recording was set at a 40-μm pixel size, 14-bit grayscale, 12.5 line pairs per millimeter (lp/mm) spatial resolution and an image-exposure time of 0.20 second. A size 2 imaging plate was used, and the exposed phosphor plates were scanned immediately after exposure. Images of the teeth were obtained from two different CBCT units: 1) Veraviewepocs 3D model X550 (J Morita Mfg Corp, Kyoto, Japan) with a flat-panel detector offering digital 3D, panoramic, and cephalometric imaging options. With the Veraviewepocs 3D system, images were obtained at 60–90 kVp, 3 mA, and an exposure time of 9.4 seconds with a 40 × 40-mm FOV (0.125-mm³ voxel size). 2) With the Kodak 9000 Extra-oral imaging system (Eastman Kodak), images were obtained at 60 kVp, 3 mA, and an exposure time of 13.2 seconds with an 50 × 37-mm FOV (76×76×76-μm voxel size isotropic voxel). Axial scans and multi-planar reconstructions were obtained, and volumetric data were reconstructed using the systems' software programs to provide serial cross-sectional views. A total of four image sets were obtained: 1) Veraviewepocs 3D (Morita), 40 × 40 mm FOV (0.125-mm³ voxel size); 2) Kodak 9000, 50 × 37 mm FOV (76×76×76-μm voxel size); 3) intraoral digital images (Digora Optime PSP, Soredex); and 4) intraoral conventional film images (Kodak Insight Film). Figures 1 and 2 show examples of cross-sectional images obtained by Veraviewepocs 3D and Kodak 9000, respectively. Buccal recurrent caries–like lesions shown by arrows can be detected under each restoration. Figure 3 shows examples of intraoral images obtained by Digora Optime PSP (Soredex). Buccal recurrent caries–like lesions shown by arrows can barely be detected under each restoration. For all four methods, the exposure parameters used for image acquisition were based on pilot studies to ensure optimal image quality with good visibility of the pulpal root canal, enamel, and dentin.

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RESULTS

Table 1 shows the intraobserver κ coefficients calculated for each observer by image type. Intraobserver κ coefficients ranged from 0.536-0.609 for the intraoral film (Kodak Insight), from 0.517-0.691 for the digital intraoral (Digora Optime), from 0.613-0.649 for the Veraviewepocs 3D, and from 0.582-0.628 for Kodak 9000 images, suggesting moderate and strong intraobserver agreement for observers 1 and 2 and strong intraobserver agreement for observer 3. Tables 2 and 3 show the interobserver κ coefficients for both the first and second readings by image type, respectively. Higher interobserver agreement was obtained from the CBCT images when compared with the intraoral images. Poor and moderate interobserver agreement was found for the first and second readings for the intraoral Kodak Insight film images (from 0.339-0.470) and for the digital intraoral Digora Optime images (from 0.337-0.522). In general, moderate interobserver agreement was found for the first and second readings for the Veraviewepocs 3D (from 0.465-0.603) and Kodak 9000 (from 0.458-0.617).

Table 1:  Intraobserver Agreement Calculated for Each Observer by Image Type

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Table 2:  Interobserver κ Coefficients Among Observers for the First Readings

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Table 3:  Interobserver κ Coefficients Among Observers for the Second Readings

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The areas under the ROC curves (Az values) for the different observers, readings, and image types were calculated and are given in Table 4. The Az values of both readings of all three observers were highest for the Veraviewepocs 3D followed by the Kodak 9000, except for the second reading of the third observer. Az values of the CBCT images were higher than those of the intraoral images. Se, Sp, PPV, NPV, and FPR for each observer and their two readings are presented in Table 5. Also, higher sensitivity values for CBCT systems compared with intraoral images were obtained.

Table 4:  Az Values, Their Standard Errors (SE), 95% Confidence Intervals (CI), and Significance Levels (p) for Each Observer

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Table 5:  Sensitivity (Se), Specificity (Sp), Positive Predictive Value (PPV), Negative Predictive Value (NPV), and False Positive Ratio (FPR) for Each Observer and Their Two Readings

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Comparisons between modalities are given in Table 6. No differences (p>0.05) were found between the Az values of the Kodak Insight film and those of the digital intraoral Digora Optime images for all observers. Also, there was no statistically significant difference (p>0.05) between the two CBCT systems (Kodak 9000 and Veraviewepocs 3D). Statistically significant differences between Az values for the intraoral Kodak Insight film images and Veraviewepocs 3D images were found for both readings of observer 1 (first reading: p=0.007, second reading: p=0.011) and observer 2 (first reading: p=0.003, second reading: p=0.023). Statistically significant differences were also found between the Az values for the digital intraoral Digora Optime and Veraviewepocs 3D images for both readings of observer 1 (first reading: p=0.001, second reading: p=0.027), observer 2 (first reading: p<0.001, second reading: p=0.001), and observer 3 (first reading: p=0.002). There was only a significant difference for the second reading of observer 2 between Kodak 9000 and Kodak Insight film (p=0.027) and between Kodak 9000 and the digital intraoral Digora Optime system (p<0.001).

Table 6:  Modalities Compared by Using Z-Test, With a Significance Level of α = 0.05 (Statistically Significant p Values Are Written in Bold)

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When visibility of buccal recurrent caries under four different restorative materials for each imaging modality was taken into consideration for all observers, no statistically significant difference (p>0.05) was found among different restorative materials for each imaging modality except for the comparison of composite and amalgam restorations obtained from Veraviewepocs 3D images (observer 1; p=0.011 and observer 2; p=0.003).

DISCUSSION

To our knowledge, up until now, no previous study has compared CBCT and intraoral radiography in detecting buccal recurrent caries under different types of restorations. In the present study, composite restorations and thermoplastic polymer were used as nonradiopaque restorations, whereas amalgam and full crown were used as radiopaque restorations. Comparison between radiopaque and nonradiopaque restorations was considered useful in terms of assessing beam hardening and metal artifacts. Metal artifacts, which are seen as dark and light streaks on tomographic images, can seriously degrade the visual quality and interpretability of CBCT images. It is accepted that image degradation increases with the number of metal restorations in the jaws, whereas small voxel size, limited beam, and true alignment of x-ray beam decreases image degradation.¹⁶ Although metal artifacts seen in CBCT images are claimed to be limiting factors in the diagnosis of caries under restorations, we found better Az values for CBCT images compared with intraoral images. This can be explained by the fact that with CBCT, it is possible to view teeth and related structures in axial, coronal, and cross-sectional views. Besides, in the present study, CBCT units with limited FOVs and small voxel sizes were chosen that could reduce metal artifacts. With the Veraviewepocs 3D, slightly higher values were found compared with the Kodak 9000. This difference, although without significance, may be due to sensor, software, or hardware specifications. In our notion, digital intraoral and CBCT systems must be evaluated by using their dedicated software as in the present study because software is probably the most important component of the digital systems. Software capability can not be detached from digital radiographic imaging systems. In addition, when the visibility of buccal recurrent caries under four different restorative materials for each imaging modality was taken into consideration for all observers, no statistically significant difference (p>0.05) was found.A study¹⁷ found no difference between intraoral film (Ekstraspeed Plus, Eastman Kodak) and direct digital radiography (Sidexis, Siemens, Bensheim, Germany) in the detection of small artificial lesions induced by a demineralization buffer gel system at the crown margin, similar to our findings. Authors recommended the use of hydroxyethyl cellulose for creating artificial caries lesions. Also, artificial cementum and dentin lesions were slightly easier to diagnose than enamel lesions. Authors concluded that radiography was not considered a reproducible and safe method for characterization of the demineralization process localized at the crown margin—especially in enamel—due to the low interexaminer agreement among three calibrated observers. Therefore, the importance of a thorough visual and tactile examination was emphasized.¹⁷ In the present study, a standard 1-mm-diameter red wax was used under restorations in order to simulate small buccal recurrent caries that are difficult to diagnose in routine clinical and radiographic examination. A similar method in an attempt to create artificial caries lesions was used in a previous study.¹⁸ Standardization of artificial buccal caries lesions was thereby provided to some extent, because our aim was only to compare different radiographic techniques in detecting artificial lesions, instead of histological validation of caries. In the present study, higher Az values for CBCT images compared with intraoral images may be attributed to the sharp round margins of the defects; however, sharp-margined defects were imaged by all systems assessed.

Another study¹⁹ found that a thorough clinical examination was more reliable than intraoral radiography in detecting recurrent interproximal caries at crown margins of full restorations. This may be due to the masking effect of full restorations and limited information gathered from intraoral radiography. On the other hand, for nonrestored teeth, radiographs often rendered evidence of caries lesions that were not diagnosed during the clinical examination.¹⁹ However, findings of the mentioned study are debatable because only one observer clinically and radiographically evaluated teeth. Therefore, it is not possible to assess the reproducibility of the results. The present study only compared different radiographic systems in detecting artificial buccal recurrent caries under different restorations. Visual and clinical examination findings were not in the scope of the present research.

It has been postulated that the marginal gap between the restoration and dentin is the main reason for recurrent caries' development, despite the fact that even with highly sophisticated technology, there is always a marginal gap.¹ However, existence of a clinical marginal defect alone is not a reason to replace a restoration, given that not all defective margins cause recurrent caries under restorations. Of the 822 teeth with defective restorations in the total sample, 86% (709 of 822) were free of radiographic recurrent caries.¹ On the other hand, an increased likelihood of defective over intact restorations to display radiographic recurrent caries was found. Approximately 14% of the defective restorations were associated with radiographic recurrent caries, compared with 5% for the intact restorations.²⁰ Similarly, colors next to restorations are not always predictive of secondary caries. Stained composite margins and ditched amalgam margins are not necessarily signs of decay, although they indicate a greater risk.^21,22 In light of these findings, radiographic diagnosis of caries under restorations is an important aid to clinical examination.

It must be taken into consideration that patients receive higher radiation doses with CBCT compared with intraoral and panoramic radiography. Radiation doses from CBCT scans vary substantially among devices, FOVs, and other technical factors.^14,15 In view of concerns regarding radiation exposure, a smaller FOV results in a less effective dose and should be used for dental images, whereas a larger FOV should be restricted to cases in which a wider view is required.²³ Although radiation exposure was not an issue for this ex vivo research, we used CBCT with a limited FOV and small voxel sizes in order to assess teeth that could possibly increase the observer's ability to detect artificial buccal recurrent caries lesions under restorations. Values produced in the present study may not apply to CBCT images taken for other indications and with different settings.

CONCLUSION

Higher Az and sensitivity values were obtained with Veraviewepocs 3D and Kodak 9000 images compared with both intraoral images, which performed similarly in the diagnosis of artificial buccal recurrent caries under restorations. Available CBCT units and images can be useful in the diagnosis of buccal recurrent caries under restorations.