Radiology of Dental Caries

Dental caries is a dynamic, preventable, reversible, complex biofilm–mediated, multifactorial disease that involves a series of demineralization/neutrality/remineralization of dental hard tissue in primary and permanent dentition. An imbalance in the continuum with a net demineralization over time results in the initiation of caries lesions. Visual inspection and intraoral radiographs are vital in caries detection, although they are of suboptimal sensitivity for early caries lesions. Shifting toward a conservative, noninvasive approach to caries management has resulted in the development of innovative-sensitive technologies. These newer techniques may serve as adjunct for the dental practitioner in detecting earliest changes in tooth structure.

Key points

  • Dental caries is a preventable, reversible, multifactorial, complex biofilm disease that progresses with time.

  • Dental caries is a dynamic continuum of tooth demineralization/neutrality/remineralization with a net demineralization initiating caries lesion.

  • Visual examination and intraoral radiographs are still vital in diagnosis of dental caries.

  • Early caries detection is paramount to effective chemotherapeutic, noninvasive management.

  • Sensitive caries detectors serve as adjuncts for early caries detection that help to shift the dental practitioner toward minimal intervention dentistry.

Dental caries

Dental caries is a complex biofilm disease that creates prolonged periods of low pH in the mouth, resulting in a net mineral loss from the teeth. Dental caries forms through a complex interaction over time between acid-producing bacteria and fermentable carbohydrate, and many host factors, including teeth and saliva. The disease develops in the crowns and roots of teeth, and it can arise in early childhood as an aggressive tooth decay that affects the primary teeth of infants and toddlers.

Caries process and current concepts

Cariogenic bacteria are essential to the disease process. At least 2 major groups of bacteria, namely, the streptococci species (chiefly Streptococcus mutans ) and the Iactobacilli species (chiefly Lactobacillus fermentum , Lactobacillus casei/paracasei , and Lactobacillus salivarius ), can produce organic acids during metabolism of fermentable carbohydrates and are known as acidogenic. Acids produced by these bacteria include lactic, acetic, formic, and propionic acid, all of which readily dissolve the mineral content of enamel and dentin.

From historical definition to current evidence

The definition of dental caries has expanded to a more complex discussion of the caries process that represents a continuum of tooth demineralization/remineralization. This “modern” view started with the definition by Miller 1890 of a 2-step process whereby bacteria on the tooth, exposed to fermentable carbohydrates, produce acid, and in a second step, dissolve the surface of the tooth. Stephan demonstrated that this production of acid after exposure to fermentable carbohydrate resulted in a localized drop in pH within the plaque followed by a subsequent return to the baseline pH over time hence establishing the concept of caries being a cyclic event of demineralization/neutrality/remineralization. Englander and colleagues demonstrated the role of saliva in neutralizing the decrease in plaque pH after exposure to fermentable carbohydrate. Although considerable attention has been placed on a few bacterial species as the cause of dental caries (eg, S mutans , Lactobacillus ), there is general agreement today that the dental biofilm exists as a complex ecosystem that can shift from a neutral pH to a more acidic ecosystem. Today, dental caries is accepted as an imbalance in the biofilm-induced cyclic process of demineralization and remineralization of tooth structure, by the acidic by-products resulting in a pH maximal drop followed by the return of the pH to initial pH modulated by saliva. The mixed ecology of the biofilm may be naturally shifted in composition to a more acidic ecology by repeated exposure to fermentable carbohydrate. , , Saliva helps modulate both the composition of the biofilm and the recovery of the biofilm pH after sugar challenge. The susceptibility of tooth to demineralization may be modulated by the incorporation of fluoride in the tooth structure. Dental caries is a continuum of demineralization and remineralization with a net demineralization resulting in tooth surface alterations that eventually result in cavitation. The caries process is influenced by several factors, such as increase in frequency of sugar consumption and increase in sugar retention time, which directly relates to an increase in demineralization of teeth resulting in cavitation, measured clinically as decay/missing/filled/teeth. However, it must be remembered that in this balance, frequent/longer acid cycles result in a shift in the biofilm flora in favor of acidogenic bacteria. Importantly, an increase in the quantity of sugar consumed alone is not a predictor of increased caries, as many communities around the world have increased caries preventive strategies over the same time period as sugar consumption has increased. Decreases in saliva flow can also favor a shift in the caries balance toward demineralization, resulting in an increase in caries progression. The caries balance is affected by multiple social determinants favoring demineralization. Research has demonstrated that acid is not the only product of the mixed ecology. Alkali production has potential in changing the pH of the oral biofilm, which impacts demineralization.

Intraoral radiography

Intraoral radiography has been an integral part of the diagnostic arsenal for more than 100 years. Intraoral radiographs typically consist of periapical and bitewing radiographs, and both have excellent spatial resolution. Periapical means surrounding the apex of the root of a tooth ( Fig.1 ); hence it captures the complete crown and root of a tooth with about 2mm beyond the root apex. On the other hand, bitewing radiographs capture the crown and a third of the root of the maxillary and mandibular teeth with its accompanying interalveolar bone ( Fig. 2 ) and are used in clinical practice to evaluate interproximal caries; crestal bone height in the interproximal region; calculus; and periodontal disease.

Fig. 1
Mandibular right molar periapical radiograph showing both the erupted molars and the partially visible impacted third molar.

Fig. 2
A vertical left premolar bitewing showing interproximal contacts and crestal bone levels.

Bitewing radiographs are obtained for examination of interproximal surfaces as well as crestal bone levels. Based on the orientation of the detector in the mouth, they can be either vertical (see Fig. 2 ) or horizontal. Technological innovations and advancement in radiography with a focus on minimizing the amount of radiation to the patient when acquiring radiographs led to a shift of radiographic image receptors from analog films to direct digital sensors. Analog films are still being used in clinical practice; however, it is recommended that no dental radiographic film with speeds lower than E- or F-speed shall be used for intraoral radiography, as the dose is essentially halved from the older D-speed to E-plus or F-speeds. Phosphor plates may be compared with analog films in terms of flexibility and is suitable for pediatric and special needs patients. One major disadvantage of the phosphor plates is that after extensive usage, they sustain irreversible damage because of their susceptibility to scratches, bite marks, and creasing. Solid-state sensors, also known as direct digital sensors, are of 2 types depending on how the image is captured: charge coupled device (CCD) and complementary metal oxide semiconductors. Alcaraz and colleagues showed dose reduction using direct digital sensors in comparison with the analog films. Digital radiographic sensors are an objective and reproducible technique; however, its sensitivity for detection of early and recurrent caries is suboptimal, , with reported sensitivity being as low as 0.30. , Caries detection can be affected by a variety of factors during acquisition or interpretation, such as variation in image capture, detector placement, status of the detector, focus to object distance, kilovolt or milliampere used for capture of the radiograph, ambient lighting for interpretation, or the experience of the clinician.

Extraoral bitewing radiography (using panoramic radiographs)

Initial ex vivo studies have shown that intraoral bitewing radiography (IOBWR) is superior to extraoral bitewing radiography (EOBWR). Another study compared the detection accuracy of proximal caries and crestal bone loss using EOBWR or IOBWR and concluded that although EOBWR has promise, clinicians should be aware of the false positive diagnoses of proximal caries and crestal bone loss when using EOBWR. Despite these diagnostic issues, during the COVID-19 pandemic, the use of EOBWR ( Fig. 3 ) was recommended as a guideline, because of the possibility of creation of aerosols during intraoral procedures, more so in exaggerated gag and cough reflex cases (Personal communication from Dr. David MacDonald, University of British Columbia (UBC), Vancouver, Canada – Oral and Maxillofacial Imaging guidelines during COVID-19 pandemic. Submitted to Oral Surg Oral Med Oral Pathol oral Radiol, 2020).

Fig. 3
Extraoral bitewing of a patient showing bilateral premolar and molar contact areas. There are other significant periapical findings in this radiograph, especially apical regions of maxillary right canine, premolars, and molars.

Caries detection: digital versus conventional radiography

Sensitivity and specificity values for direct digital radiography were 73% and 95% at the buccal and lingual line angles, and 29% and 90% at the midgingival floor, respectively. Corresponding values for conventional radiography were 63% and 93% at the buccal line angle, 61% and 93% at the lingual line angle, and 44% and 95% at the mid-gingival floor, respectively. The total sensitivity and specificity values were 58% and 93% for digital radiography and 56% and 93% for conventional radiography with no significant difference ( P = .104). The sensitivity and specificity of film, CCD, and photostimulable phosphors (PSP) for the detection of enamel caries were 38% and 98%; 15% and 96%; and 23% and 98%, respectively. The sensitivity and specificity of film, CCD and PSP for the detection of both dentin and enamel caries, were 55% and 100%; 45% and 100%; and 55% and 100%, respectively. Sensitivity of all 3 receptors (CCD, PSP, film) for detection of enamel lesions was low (5.5%–44.4%), but it was higher for dentin lesions (42.8%–62.8%); PSP with 70 kVp and 0.03-second exposure time had the highest sensitivity for enamel lesions, but the difference among receptors was not statistically significant ( P >.05). PSP with 60 kVp and 0.07-second exposure time had higher sensitivity and lower patient radiation dose for detection of cavitated and noncavitated lesions, but the difference was not significant ( P >.05).

Radiographic interpretation of caries

Imaging is an integral component of caries detection. Radiographically, dental caries is essentially a process of demineralization leading to density changes within the enamel or dentine and hence detectable using radiographic imaging. Radiographic detection of dental caries and the methods used for detection have changed over the years. In the early days, the focus of radiographic imaging was on the periapical areas of teeth, as the investigation was based on pain or infection, which was the late stage of dental caries that had led to cavitation and pulp exposure with tracking of bacteria through pulpal blood vessels to the periapical region causing an inflammatory process. Currently, there is a shift toward early detection and minimal intervention dentistry (MID).

Interpretation of caries

Accurate interpretation of carious lesions starts with accurate radiographic depiction of adjacent contact points of teeth using bitewing radiography. To obtain bitewing radiographs that are of optimum diagnostic values, contacts should be opened using appropriate horizontal angulation and XCP positioning device. Appropriate kilovolt and milliampere as well as standardized exposure time are essential for optimally exposed radiographs. A good contrast is essential for the diagnosis of dental caries, as both underexposures and overexposures will lead to erroneous interpretation of dental caries, as demonstrated in Fig. 4 .

Fig. 4
Mandibular central incisor periapical radiograph. Poor contrast ( A ); appropriate contrast ( B ).

Radiographically, dental caries appears as radiolucency leading to loss of normal homogeneity of the enamel, as the lesion extends further toward the dentino-enamel junction (DEJ), the DEJ line loses its continuity in the region. The inherent low-contrast resolution of plain radiographs makes it impossible to determine the full extent of dentin involvement. The line pair resolution of digital dental radiographs is about 20 line pairs per millimeter. Small occlusal lesions, buccal and lingual pit cavities, are better studied clinically, as radiography plays a small role in the detection of these lesions. Dental caries recurs if not completely excavated before restoration, and lesions appear as radiolucency adjacent to or beneath the restoration. Because radiographs are a 2-dimensional representation of a 3-dimensional tooth structure, it is not always possible to determine caries extension to the pulp chamber or pulp horn because of anatomic variations and presence of radiopaque restorations in the crowns. In the presence of caries, pulp is generally reactive and lays down new dentin called, “secondary dentin,” which functions to wall off the receding pulp from the carious attack. The only radiographically certain way of determining pulp exposure is the visualization of secondary caries and periapical changes in the alveolar process, such as widened periodontal ligament space or lack of continuity of lamina dura. Rarely, cavitated dental caries undergoes spontaneous arrest.

Radiation caries

Radiotherapy for head and neck cancers lead to decreased salivation especially if the salivary glands are in the “direct path of radiation.” It is known that the short-term loss of function in the salivary glands leads to clinical xerostomia, which further accentuates clinical caries, especially rapidly advancing root caries. The lack of lubrication and buffering action from saliva, increased salivary pH (acidic), and increased colonization of acidogenic bacteria (especially S mutans ) all lead to caries of smooth surfaces, and this is termed “radiation caries,” although it not directly caused by radiation. Advanced radiation-induced hyposalivation may lead to tooth fracture, dental abscess, tooth loss, or osteoradionecrosis. Direct-acting cholinergic parasympathomimetic agents, such as Pilocarpine hydrochloride , or muscarinic agonist, like Cevimeline, are used for treatment of xerostomia. A thiol-containing agent, like Amifostine, has been used for its radioprotective properties in the prevention of radiation-induced changes by scavenging free radicals.

Classification of caries activity and radiographic detection

Caries is a dynamic disease that requires a classification system that is sensitive enough to monitor the disease activity, the surface of involved teeth, and the depth of caries penetration. There are several caries classification systems, and the American Dental Association Caries Classification System is as follows:

  • Sound surface: Healthy sound enamel with no detectable lesion with normal glossy surface.

  • Initial caries lesion: Early lesions that demonstrate net mineral loss in enamel or exposed dentin that may only be visible when the tooth is dried by air or color change toward white.

  • Moderate caries lesion: Moderate mineral loss with loss of tooth surface integrity/anatomy with deeper demineralization. There may be shallow or microcavitation. There may be color changes in enamel with brown or gray shadows and/or translucency.

  • Advanced caries lesion: Advanced mineral loss with cavitation through enamel. Dentin is exposed.

Caries activity is defined as active or inactive/arrested. Active lesions are shiny/glossy and smooth to touch; inactive/arrested lesions are frosty/matte in luster with a roughened surface. Caries can also be detected radiographically by looking at the approximal surface of teeth. Lesions are classified based on the depth of demineralization detected on the approximal surface. The stage of the lesion is based on depth of penetration from the outer tooth surface, as follows:

Jul 11, 2021 | Posted by in General Dentistry | Comments Off on Radiology of Dental Caries
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