Detection of Caries under Fixed Prosthodontic Restorations Using Cone-beam CT: A Meta-analysis

INTRODUCTION

Dental cone-beam computed tomography (CBCT) is used when intraoral and periapical dental X-rays are inconspicuous in representing the exact picture of a three-dimensional dentoalveolar structure. It is an evolution from conventional computed tomography with the advantage of low radiation exposure, higher contrast images, rapid scan time, and lower cost.¹ Although digital intraoral imaging has been a breakthrough, image geometry has always been a concern, including that in panoramic technology.² Historically, the use of CBCT has been primarily limited to the temporomandibular joint, implant site examination, and other maxillofacial applications with previous studies reporting no consistency in the system used, technical device properties, setting, and other parameters of the CBCT system.³ The 10-year survival rate of fixed prosthodontic restorations has been reported to be around 85-95% depending on the material (metal, ceramic) and the type of restoration (crowns, veneers, bridges, inlays, onlays),^4-7 and secondary caries has been identified as the most common cause of failure.⁸ The two-dimensional radiographic methods have demonstrated low sensitivity, higher specificity, and high intraoperator variability for the detection of secondary caries under restorations. In addition to this, metal restorations are radiopaque making it further challenging.¹ Early detection of caries under these restorations could possibly help in initiating strategies that can prevent tooth loss. Considering the disadvantages of twodimensional intraoral radiographs, CBCT has been tried in various studies to detect caries underneath fixed restorations. This metaanalysis is aimed to address the use of CBCT to detect secondary caries under fixed prosthodontic restorations.

MATERIALS AND METHODS

RESULTS

DISCUSSION

The present review is an attempt to evaluate the available evidence on the use of CBCT to detect caries under fixed prosthodontic restorations. The key results from the review indicate significant differences in the mean gray values between dentin and caries, which suggests that CBCT could accurately detect secondary caries under these restorations.

The broad category of fixed prosthodontic restorative materials includes metal and ceramic which are used as crowns, bridges, veneers, inlays, and onlays, and their modifications.¹⁵ The most common cause of failure of these restorations is secondary caries⁸ which leads to loss of tooth and the restoration as well. Early detection of caries under these restorations using radiographs could serve as a guide to initiate preventive strategies and avoid adverse outcomes to both the tooth and the restoration. Intraoral radiographs are accessible, economical, less radiation exposure, and offer high specificity. However, they lack sensitivity with greater inter- and intraoperator variability.^1-3 Careful clinical examination can be used for occlusal, facial, and lingual surface caries. However, for proximal and secondary caries, radiographs are the only available tool for detection. A study by Terry et al. in 2016¹⁶ indicate that the percentage of non-readable proximal caries was 4.1, 18.3, and 51.5% with the use of intraoral bitewings (BWs), extraoral panoramic BWs, and standard panoramic images, respectively. This indicates the disadvantages of panoramic radiographic techniques as well. The differences may be attributed to various factors such as depth of caries, tooth position, and restoration if any, superimposition of adjacent structures, artifacts, X-ray beam saturation and angulation, and other patient factors.¹⁷

As regards caries under fixed restorations, the presence of metal makes it almost impossible to detect caries using the conventional intraoral radiographic technique because of the radiopacity. It has been reported that as the number of metal restorations increase, metal artifacts and image degradation also increases¹⁸ A study conducted by Murat et al.¹⁹ on the use of CBCT to detect caries imitating lesions on the tooth under restorations identified CBCT as a better diagnostic tool when compared with intraoral radiographic technique. A higher interobserver agreement was also obtained with CBCT.

Earlier attempts at three-dimensional (3D) imaging used variations of tomosynthesis, the most notable one is the turned aperture computed tomography (TACT). This was followed by volume tomographic machines which are based on the statistical inversion principle. A stack of 256 cross-sectional images was produced within a limited volume of 6 × 6 cm.²⁰ However, by the end of the 20th-century CBCT apparently became the most accepted 3D imaging technique with radiation exposure paralleling a panoramic radiograph or a full mouth intraoral radiographic technique.¹

Dental CBCT rotates around the patient, capturing data using a cone-shaped X-ray beam. These data are used to reconstruct a 3D image of the structure studied. The advantages are it is fast, noninvasive, and provides 3D information, rather than the twodimensional (2D) information provided by a conventional X-ray image. The image is produced by absorption of X-ray photon energy by the materials located between the X-ray source and the detector and represented as attenuation value. This value depends on the density of the material. Denser materials absorb more energy, resulting in greater attenuation values. These attenuation values are then converted into mean gray values or voxel values in a digital image during slice reconstruction. It is also to note that these attenuation values are reported as Hounsfield Units in a CT machine.²¹ Studies in the past comparing Hounsfield Units and voxel values from CBCT and conventional CT showed a linear relationship suggesting that CBCT could be used with predictable results.²²

Although the radiation doses from these devices are lower than conventional CT, dental CBCT typically delivers more radiation than conventional dental intraoral X-rays. Concerns about radiation exposure are greater for patients more sensitive to radiation like pregnant women and children. It is advised that the rationale for use is well discussed with the patient and/or parent to ensure a clear understanding of benefits and risks.¹²

Considering the advantages of CBCT, studies evaluated the reliability of CBCT in diagnosing caries under fixed prosthodontic restorations fabricated with ceramic, metal ceramic, all-metal, zirconia, and metal acrylic.^4,5,7 Three studies were identified from electronic databases and all studies showed a statistically significant difference between caries and dentin under zirconia, lithium disilicate, and metal-ceramic restorations. All metal and metal acrylic was studied only in one study.⁵ The results also suggest that when the FOV is smaller, CBCT is a better alternative. Collimation of the X-ray beam by adjustment of the FOV limits the radiation to the region of interest which helps in yielding better images and avoiding unnecessary exposure. This depends upon the detector size and shape, beam projection geometry, and the ability to collimate or not. It is always desirable to limit the field size to the smallest volume that can accommodate the region of interest.¹³

Though results from individual studies showed CBCT to be promising in the diagnosis of caries under all metal and metal acrylic restorations, it is still inconclusive because of lack of sufficient evidence in the form of randomized controlled trials with a larger sample size. All studies included were in vivo, with a small sample size, which is a limitation. Metal artifact reducing softwares like metal deletion technique (MDT) or metal artifact reduction (MAR) are available which can reduce the metal artifacts that are commonly seen during crown imaging. However, studies indicate that although softwares decreases metal artifacts and increases diagnostic confidence, there is a greater tendency that the software introduces new artifacts that can obscure pertinent structures, interfering with the diagnostic accuracy.²³ However, these softwa res were not used in the included studies. Future studies should include the effect of using these softwares in reducing metal artifacts in CBCT. The study by Vedpathak et al. suggests the use of the smallest FOV. However, they used a FOV of 8 × 8. A 4 × 4 FOV could have been preferred.

CONCLUSION

Though there is no conclusive evidence from available literature, this review suggests that CBCT could be used as an alternative caries detection tool in patients with high caries risk and those with multiple restorations. This review is a basis on which future randomized controlled trials can be planned with the factors that are mentioned above.

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