Potential impact of metal crowns at varying distances from a carious lesion on its detection on cone-beam computed tomography scans with several protocols

Intraoral radiography is an essential tool in the clinical detection of dental caries lesions. While the bitewing and periapical techniques can sometimes yield images with overlapping structures, they are nonetheless effective in detecting caries lesions and assessing their extent.1 In contrast, cone-beam computed tomography (CBCT) offers 3-dimensional views of dental structures, yet its usefulness in identifying caries lesions is subject to debate.2, 3, 4, 5 Additionally, CBCT exposes patients to comparatively high doses of radiation, which limits its indication for this diagnostic task. Although CBCT should not be prescribed solely for the evaluation of dental caries, any caries detected incidentally during a CBCT scan performed for other reasons should be reported to alert the relevant clinicians. Early identification of caries lesions is key to preventing the irreversible demineralization of dental tissues and potential damage to the dental pulp.

Restorative materials frequently used in the oral cavity often contain components with a high atomic number, which can generate artifacts that reduce the quality of CBCT images.3, 4, 5, 6 The presence of these materials can also hinder the CBCT-based detection of dental caries, a topic that has been explored in the scientific literature.3, 4, 5, 7, 8 Several studies have investigated the impact of artifacts on the diagnosis of various conditions - such as vertical root fractures, internal and external root resorption, and proximal caries lesion - when these conditions are positioned adjacent to high-density materials, including dental implants, endodontic fillings, and restorative materials.3, 4, 5, 6, 9, 10, 11

Recent studies have indicated that metal restorations can impede the CBCT-based detection of caries lesions in adjacent teeth.4, 5 However, insufficient research has involved a substantial volume of high-density materials commonly present in the oral cavity alongside natural caries lesions, which are relatively challenging to detect due to their indistinct borders. Additionally, certain key factors warrant consideration in these studies, such as acquisition and reconstruction parameters. These include the tube current (milliamperage, or mA) and the implementation of metal artifact reduction (MAR), both of which have been shown to impact specific diagnostic tasks.4, 12, 13 Relevant factors also include the proximity of the zone of interest to the source of the artifact, which has been demonstrated to relate inversely to the severity of its detrimental effects.14 In light of these considerations, this study was conducted to assess the impact of artifacts produced by metal crowns on the detection of proximal caries lesions in teeth both near and distant to the artifact-generating object on CBCT images, and to determine the potential effects of the milliamperage and MAR settings on this process. The null hypothesis posited that the presence of a metal crown, the distance between the lesion and the artifact-generating metal, the mA setting, and the activation of MAR would not influence the diagnosis of caries lesions.

The intra-observer agreement ranged from fair (0.367) to substantial (0.661), while the interobserver agreement varied from fair (0.251) to moderate (0.489). Table 1 presents the mean values of AUC, sensitivity, and specificity in the detection of proximal caries using CBCT, categorized by the factors under investigation. The AUC values ranged from 0.40 to 0.60. These figures were consistently lower on surfaces adjacent to the first molar - where a crown was present - compared to more distant surfaces (P<0.0001), with the exception of the images taken at 12 mA without MAR in the control group. The presence of a metal crown, the mA level, and the activation of MAR did not significantly influence the AUC (P>0.05). The sensitivity ranged from 0.28 to 0.45 and was not significantly affected by the studied factors (P>0.05). Specificity values ranged from 0.44 to 0.80, and (in general) were significantly lower on surfaces closer to the first molar compared to distant surfaces. However, for scans performed at 12 mA without MAR in the control group, the specificity was higher for the close surface. Additionally, for scans at 12 mA with MAR in the control group, and without MAR in the metal crown group, the proximity of the surface to the metal object did not significantly impact the results. The AUC, sensitivity, and specificity values suggest that the use of CBCT for detecting non-cavitated caries lesions is of limited applicability.17

Table 1
Area under the receiver operating characteristic curve (AUC), sensitivity, and specificity organized by tube current (mA), use of metal artifact reduction (MAR), and positioning of the evaluated teeth in the cone-beam computed tomography-based detection of proximal caries

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Research regarding the impact of artifacts on diagnostic procedures is widely available in the dental literature.3, 4, 5, 6, 7, 8, 9, 10, 11 These studies have primarily focused on the influence of intracanal materials and the presence of dental implants in the oral cavity.9, 10, 11 Regarding the assessment of artifacts on the identification of carious lesions, previous studies have typically utilized direct restorations3, 4 or orthodontic materials8 as the artifact sources or have simulated artificial caries lesions.5 The present study introduced a more complex scenario by simulating robust metal crowns alongside natural proximal caries lesions, which were confirmed using micro-CT. Impacts were also examined of CBCT acquisition parameters, such as the tube current (mA) level, and reconstruction techniques, including MAR activation, on the detection of caries. The effect of the distance between the lesion and the source of the metal artifact was similarly investigated.

Contrary to expectations, the presence of a metal crown did not result in poorer caries detection. This surprising outcome may be due to the substantial amount of enamel in the first molar, which can also produce artifacts that complicate the diagnostic process. Furthermore, the small nature of the caries lesions and the type of high-density material used may have impacted caries detection. The present findings stand in contrast to several prior studies that have documented the detrimental effects of artifacts on the identification of caries.3, 4, 5 Factors such as the size of the caries lesion, the dimensions and type of high-density material used, and the quality of the CBCT scans may influence the detection of caries.

In the present study, the mean accuracy value was approximately 0.50 for both groups (that is, with and without a metal crown). This low accuracy value, observed even in the absence of metal artifacts, underscores the inherent difficulty of this diagnostic task when using CBCT images. Previous research assessing the detection of caries lesions with CBCT has also reported low accuracy values even when metal artifacts were not present, which corroborates the findings of the current study.2, 18, 19, 20 The present authors propose that the presence of adjacent teeth - specifically, the enamel in contact at the interproximal surfaces - may contribute to the low accuracy values observed, even in the absence of high-density objects.

Crucially, CBCT is not the primary imaging modality for diagnosing dental caries. However, a comprehensive assessment of the oral volume, including both the teeth and the supporting structures, remains essential for preserving the patient’s oral health. Should an image suggestive of a carious lesion be detected, the present research indicates that caution is warranted due to the limitations of CBCT in identifying such lesions. This cautionary stance is supported by several previous studies, despite variations in their methodological designs.3, 4, 5, 8

Furthermore, the proximity to the mandibular first molar, where either a dental or metal crown was present, affected the diagnostic outcomes. Despite the generally low diagnostic values for both surfaces, a trend was observed of diminishing AUC and specificity values nearer the first molar (with the mean AUC value declining from 0.56 to 0.47 and the mean specificity value from 0.71 to 0.59). This reduction in diagnostic performance is likely due to the more pronounced nature of artifacts near high-density materials, as demonstrated in previous investigations.14

The composition of the present sample, which included more surfaces with carious lesions compared to sound surfaces, likely contributed to the increased sensitivity observed. However, it is important to note that the sensitivity values remained lower than the specificity values, a finding that aligns with results from previous studies.3, 4 This suggests that the sample composition alone does not account for this effect. Although hypodense artifacts could theoretically resemble caries lesions, potentially leading to higher sensitivity and lower specificity, such an effect was not observed. It seems plausible that the artifacts introduced a degree of uncertainty, prompting evaluators to err on the side of caution and favor a negative diagnosis when the presence of a lesion was unclear.

To minimize the impact of metal artifacts in diagnostic tasks, various studies have investigated factors that may affect their manifestation, including changes in current and peak kilovoltage settings as well as the use of MAR. In the present study, variations in both the current (mA) setting and the activation of MAR were tested; however, these adjustments did not enhance the detection of dental caries. This finding aligns with the work of Isman et al.,8 who observed no significant effect of MAR on caries detection in the presence of orthodontic materials. Conversely, Cebe et al.4 reported that MAR was effective in certain study groups. The ineffectiveness of MAR in the present study may stem from the fact that a metal crown can produce more artifacts than other restorative materials. This could account for the discrepancies between the present findings and those of previous studies. Another consideration is that MAR technology may perform differently across various CBCT units. The influence of tube current (mA) on diagnosis remains a topic of debate within the dental literature. While some studies have found no significant effect on the identification of secondary caries beneath amalgam fillings in primary teeth,21 others have noted diagnostic improvements in the detection of root fractures, especially when artifact-producing materials are present.12, 13

Comparative studies examining caries detection have consistently shown bitewing radiography to be the more accurate and practical method when compared to CBCT. This preference stems from the lower radiation dose and increased accessibility that bitewing radiography provides.22, 23, 24 Additionally, CBCT is particularly susceptible to interference from metal artifacts. Although high-density materials can alter gray values in digital radiography due to automatic exposure compensation, this effect does not significantly compromise the diagnosis of carious lesions.25 Consequently, bitewing radiography remains the preferred imaging modality for detecting caries lesions, even in the presence of high-density materials.

Notably, the present study was conducted ex vivo, limiting its direct applicability to clinical settings. Experiments involving ionizing radiation on humans would pose ethical issues. However, this study was designed to mimic a clinical environment during CBCT acquisition, except for patient movement. In spite of this constraint, rigorous standardization was applied to the sample, phantom, and examination protocols, which were both blinded and independent. The low levels of inter- and intra-examiner agreement observed highlight the inherent difficulty in detecting carious lesions on CBCT images.

In summary, while emphasizing the importance of evaluating the entire CBCT examination, it is essential to recognize the complexities involved in assessing dental crowns. Despite these challenges, clinical examination remains the cornerstone of routine clinical practice, particularly for the detection of caries.

In conclusion, the presence of a metal crown did not adversely affect the detection of incipient caries lesions using CBCT. However, the overall diagnostic utility of CBCT in caries detection was determined to be low, with a further reduction for the surfaces closest to dental or metal crowns. Within this framework, none of the investigated factors enhanced detection capabilities, as neither the application of MAR nor the adjustment of tube current led to improved diagnostic outcomes. Consequently, CBCT is not recommended for the detection of incipient caries lesions.