Brief Description

Radiographic examination is a necessary complementary means for oral diagnosis. Tooth cavities may be visible and easy to access, which may lead to fast diagnosis, but commonly lesions located at proximal surfaces are not very often detected by clinical examination, requiring radiographic examination to confirm the diagnosis. Occlusal cavities, due to their positioning, may be hidden or just present microcavities, and that also would difficult a correct diagnosis. The dental radiograph also enables the dental profession to evaluate the extent and severity of a carious lesion. The intraoral radiographic technique is by far the most indicated, allowing finding of two times more cavities at proximal surfaces on posterior teeth than clinical inspection. (Penteado, Almeida, Boscolo, Neto et al., 2005)

Demineralization and destruction of hard tooth structure (i.e due to caries) result in a loss of tooth density in the area of the lesion. The decreased density allows a greater penetration of x-rays in the carious area, and as a result, the carious lesion appears as radiolucency on a dental radiograph. Radiolucent structures permit the passage of the x-ray beam and appear dark or black on the dental radiograph. The degree of radiolucency seen on a dental radiograph is determined by the extent and severity of the destruction seen as a result of the caries process. (Haring, Lind, p.106)

Conventional radiography vs Digital radiography

A digital radiograph is comprised of a number of pixels. Each pixel carries a value between 0 and 255, with 0 being black and 255 being white. The values in between represent shades of grey, and it can be quickly appreciated that a digital radiograph, with a potential of 256 grey levels has a siginificantly lower resolution than a conventional radiograph that contains millions of grey levels. This would suggest that digital radiographs would have a lower diagnostic yield than a conventional radiograph than that of conventional radiographs. When assessing smal proximal lesions, research has shown that the sensitivites and specificities of digital radiographs were significantly lower than that of conventional radiographs. However, digital radiographs offer the potential of image enhancement by applying a range of algorithms, some of which enhance the white end of the grey scale (such as Rayleigh and hyperbolic logarithmic probability) and others the black end (hyperbolic cube root function). When these enhanced radiographs are assessed, their diagnostic performance is at least as good as conventional radiographs, with reported values of 0.95 sensitivity and 0.83 specificity for approximal lesions.
Digital images can also be archived and replicated with ease. (Pretty et al., 2006, p. 731)

Digital radiography also allows for subtraction radiography.
Subtraction radiography is the comparison of the pixel values of two radiographs of the same object. If the images have been taken using a geometry stabilising system (i.e bitewing holder) or software has been employed to register the images together, then any difference in the pixel values must be due to change in the object. The values of the pixels from the first object are subtracted from the second image. If there is no change, the resultant pixel will be scored 0. Any value that is not 0, must be attributable to either the onset or progression of demineralization or regression. Therefore, it is imperative that the radiographs that are being compared must be perfectly, or as close to perfect as possible, aligned. It is presumably because of the need for well aligned images that the uptake of subtraction radiography has been low. However, with recent advances in software that have enabled two images to be correctly aligned and subtracted, subtraction radiography is likely be more welcomed in mainstream practice. (Pretty et al., 2006, p. 732)

Types of radiographs

Common radiographs used to detect caries are:

  • Bitewing radiographs - A radiograph that shows both the upper and lower teeth on the same film (radiograph of choice for evaluation of dental caries)
  • Periapical radiographs - A radiograph demonstrating tooth apices (the tip/terminal end of the root of the tooth) and surrounding structures in a particular intraoral area.

Bitewing radiographs

The bitewing technique is a method used to examine the interproximal surfaces of teeth. The bite-wing film is a type of film used in the interproximal examination. As the term (bite-wing film) suggests, this film has a "wing" or tab. The patient bites on the "wing" to stabiliize the film, hence the term bite-wing.
Bite-wing film, in addition to permitting better visualization of proximal caries, has a decided advantage over the standard dental radiographic examination in revealing recurrent caries that takes place under restorations and near the gingival margins of artificial crowns. The exposure is made at right angles to the long axis of the tooth, and this is most often parallel to the base of the restoration, thereby providing a view of more than the tooth structure beneath it. If a crown is present, there is less superimposition of metal on the cervical and root portion of the tooth when the exposure is made at right angles to the long axis of the tooth.

Method/ Radiographic technique:

  1. The desired holder is selected together with an appropriate sized image receptor - typically a 31 x 41mm film packet or phosphor plate or the equivalent sized solid-state sensor
  2. The patient is positioned with the head supported and with the occlusal plane horizontal
  3. The holder is inserted carefully into the lingual sulcus opposite the posterior teeth
  4. The anterior edge of the image receptor should be positioned opposite the distal aspect of the lower canine - in the position, the image receptor extends usually just beyond the mesial aspect of the lower third molar
  5. The patient is asked to close the teeth firmly together onto the bite platform (Note: Extra care needs to be taken of solid-state sensor cables)
  6. The X-ray tubehead is aligned accurately using the beam aiming device to achieve optimal horizontal and vertical angulations
  7. The exposure is made
  8. If required, the procedure is repeated for the premolar teeth with a new image receptor and X-ray tubehead position

Advantages (+) and Disadvantages/ Problems that might be encountered (-):

+ Relatively simple and straightforward
+ Image receptor is held firmly in position and cannot be displaced by the tongue
+ Position of the X-ray tubehead is determined by the beam-aiming device so assisting the operator in ensuring that the X-ray beam is always at right angles to the image receptor
+ Avoids coning off or cone cutting of the anterior part of the imge receptor
+ Holders are autoclavable or disposable

- Position of the holder in the mouth is operator-dependent, therefore images are not 100% reproducible, so not absolutely ideal for monitoring progression of caries
- Positioning of the film holder and the image receptor csn be uncomfortable for the patient, particularly when using solid-state digital sensors
- Some holders are relatively expensive
- Holders are not usually suitable for children (use a tab attached to the image receptor for children)
- Prone to certain radiographic technique errors such as:
  • Positioning the image receptor too far posteriorly in the mouth, thereby failing to image premolar teeth
  • Failure to insert the image receptor correctly into the lingual sulcus between the tongue and posterior teeth allowing the tongue to displace the image receptor
  • Failure to align the X-ray tubehead correctly in the horizontal plane, either too far posteriorly (cone cutting/ coning off), too far anteriorly (cone cutting or coning off) or not aimed through the contact areas at right angles to the line of the arch and the image receptor causing overlapping of the contact area
  • Failure to align the X-ray tubehead correctly in the vertical plane thereby not superimposing the buccal and lingual cusps

Periapical radiographs

There are two techniques for periapical radiographs:

  1. Paralleling technique
  2. Bisected angle technique

1. Paralleling technique

]Parellelling technique
The image receptor is placed in a holder and positioned in the mouth parallel to the long axis of the tooth under investigation. The X-ray tubehead is then aimed at right angles (vertically and horizontally) to both the tooth and the image receptor. By using a film or sensor holder with fixed image receptor and X-ray tubehead positions, the technique is reproducible.

Method/ Radiographic technique:

1. The patient is positioned with the head supported and with the occlusal plane horizontal

2. The holder and image receptor are placed in the mouth as follows:
  • Maxillary incisors and canines - the image receptor is positioned sufficiently posteriorlyto enable its height to be accomodated in the vault of the palate
  • Mandibular incisors and canines - the image receptor is positioned in the floor of the mouth, approximately in line with the lower canines or first premolars
  • Maxillary premolars and molars - the image receptor is placed in the midline of the palate, again to accomodate its height in the vault of the palate
  • Mandibular premolars and molars - the image receptor is placed in the lingual sulcus next to the appropriate teeth
3. The holder is rotated so that the teeth under investigation are touching the bite block

4. A cottonwool roll is placed on the reverse side of the bite block. This often helps to keep the tooth and image receptor parallel and may make the holder less uncomfortable

5. The patient is requested to bite gently together, to stabilize the holder in position

6. The locater ring is moved down the indicator rod until it is just in contact with the patient's face. This ensures the correct focal spot to film distance

7. The spacer cone is aligned with the locator ring. This automatically sets the vertical and horizontal angles and centres the X-ray beam on the image receptor

8. The exposure is made (Whaites, p.88)

2. Bisected angle technique
Bisecting angle technique

The image receptor is placed as close to the tooth under investigation as possible without bending the receptor. The angle formed between the long axis of the tooth and the long axis of the image receptor is assessed and mentally bisected. The X-ray tubehead is positioned at right angles to this bisecting line with the central ray of the X-ray beam and aimed through the tooth apex. Using the gepmetrical principle of similar triangles, the actual length of the tooth in the mouth will be equal to the length of the tooth on the image.

Method/ Radiographic technique:

1. The image receptor is pushed securely into the chosen holder. Either a large or small size of image receptor is used so that the particular tooth being examined is in the middle of the receptor. When using a film packet - the white surface faces the X-ray tubehead and the film orientation dot is opposite the crown

2. The X-ray tubehead is positioned using the beam-aiming device if available OR the operator has to assess the vertical and horizontal angulations and then position the tubehead without a guide

3. The exposure is made (Whaites, p.99)

Common radiographic technique errors:

- Failure to remove dentures or orthodontic appliances
- Failure to position the image receptor correctly to capture the area of interest, thereby failing to image the apices and periapical tissues
- Failure to position the image receptor correctly causing it to bend (if flexible) creating geometrical distortion
- Failure to orientate the image receptor correctly and using it back-to-front
- Failure to align the X-ray tubehead correctly in the horizontal plane, either:
  • Too far anteriorly or posteriorly (coning off or cone cutting)
  • Not aimed through the contact areas at right angles to the teeth and the image receptor causing overlapping of the contact areas
- Failure to align the X-ray tubehead correctly in the vertical plane, either:
  • Too far superiorly or inferiorly (coning off or cone cutting)
  • Too steep an angle causing elongation and geometrical distortion
  • Too shallow an angle causing elongation and geometrical distortion
- Failure to instruct the patient to remain still during exposure with subsequent movement resulting in blurring
- Failure to set correct exposure settings
- Careless inadvertent use of image receptor twice (Whaites, p.114)

Radiographic classification of caries

Incipient interproximal caries: Extends less than half way through the thickness of enamel. The term incipient means beginning to exist or appear. An incipient lesion, Class 1, is seen in enamel only.

Incipient interproximal caries
An incipient carious lesion is seen on the distal of the mandibular second premolar (Haring, Lind, p. 111)

Moderate interproximal caries: Extends more than halfway through the thickness of enamel but does not involve the DEJ. A moderate lesion, class 2, is seen in enamel only.

moderate carious lesion
A moderate carious lesion is seen on the distal of the mandibular second premolar (Haring, Lind, p. 112)

Advanced interproximal caries: Extends to or through the DEJ and into dentin but does not extend through the dentin more than half the distance toward the pulp. An advanced, Class 3, lesion affects both enamel and dentin.

advanced carious lesion
An advanced carious lesion that extends through the DEJ and into dentin is seen on the distal of the mandibular first molar (Haring, Lind, p. 112)

Severe interproximal caries
: Extends through enamel, through the dentin, and more than half the distance toward the pulp. A severe, Class 4, lesion involves both enamel and dentin and may appear clinically as a cavitation in the tooth.

severe carious lesion
A severe carious lesion is seen on the distal of the mandibular first molar (Haring, Lind, p. 113)

Incipient occlusal caries: Cannot be seen on a dental radiograph and must be detected clinically with an explorer.

Moderate occlusal caries
: Extends into dentin and appears as a very thin radiolucent line. The radiolucency is located under the enamel of the occlusal surface of the tooth. Little if any radiographic change is noted on the enamel.

occlusal caries
Occlusal caries is seen as a tiny radiolucency just below the DEJ on the mandibular second premolar (Haring, Lind, p. 113)

Severe occlusal caries
: Extends into dentin and appears as a large radiolucency. The radiolucency extends under the enamel of the occlusal surface of the tooth. Severe occlusal caries is apparent clinically and appears as a cavitation in a tooth.

severe occlusal carious lesion
A severe occlusal carious lesion is seen as a large radiolucency in dentin on the mandibular first molar (Haring, Lind, p. 114)

Buccal caries
: Involves the buccal tooth surface, whereas lingual caries involves the lingual tooth surface. Because of the superimposition of the densities of normal tooth structure, buccal or lingual caries are difficult to detect on a dental radiograph and are best detected clinically. When viewed on a dental radiograph, caries that involves the buccal or lingual surface appears as a small, circular radiolucent area. To determine the location of the lesion, a clinical examination with an explorer is necessary.

buccal caries
Buccal caries is seen as a small, circular radiolucency on the mandibular second molar (Haring, Lind, p. 114)

Root surface caries: Involves only the roots of the teeth. The cementum and dentin located just below the cervical region of the tooth is involved and no involvement of enamel occurs. Bone loss and corresponding gingival recession precede the caries process and result in exposed root surfaces. Clinically, root surface caries is easily detected on exposed root surfaces, The most common locations include the exposed roots of the mandibular premolar and molar areas. On a dental radiograph, root surface caries appears as a cupped-out or crater-shaped radiolucency just below the cementoenamel junction (CEJ). Early lesions may be difficult to detect on dental radiograph.

root caries
Root caries appears as a crater-shaped radiolucency just below the cemento-enamel junction (CEJ) on the mandibular second premolar (Haring, Lind, p. 115)

Secondary or recurrent caries
: occurs adjacent to a preexisting restoration. On a dental radiograph, recurrent caries appears as a radiolucent area just beneath a restoration. Recurrent caries occurs most often beneath the interproximal margins of a restoration. (Haring, Lind, p.109-115)

recurrent caries
Recurrent caries is seen as a radiolucency below a two-surface amalgam restoration on the mandibular second premolar (Haring, Lind, p. 115)

Effectiveness of radiographs in the detection and evaluation of dental caries

Research has shown that radiographic systems yielded AUC measurements ranging from 0.74 to 0.90, with the film based systems scoring generally higher. For bitewing radiographs, the sensitivity was moderate (54% +/- 14) and specificity was high (97% +/- 5). The AUC was 0.88. In a study appraising the performance of conventional radiography in detecting recurrent decay, 91% of noncarious restored teeth were detected, but only 53% of the failed restorations were found. An ROC value of 0.78 was calculated, and it was suggested that careful clinical assessment of existing restorations was required before a definitive diagnosis of recurrent decay could be made. To increase the effectiveness of radiographs in the detection of dental caries, due care must be given to ensure that the common radiographic technique errors (mentioned above) are avoided, such as ensuring that the bite wing film used to detect dental caries must be free of overlapped contacts and that the horizontal angulation is proper, so that overlapping of the contact areas of the crowns of the teeth is avoided.Technique variations in film and X-ray beam positions can considerably affect the image of the carious lesion. For example, varying the horizontal angulation can make a lesion confined to enamel appear to have progressed into dentine. Hence, there is the need for accurate, reproducible techniques, as described above. To ensure more effective radiogaphic interpretations of dental caries, it is also crucial that film is not exposed. Radiographic diagnosis of caries is also limited by problems such as that carious lesions are always larger clinically than they appear radiographically and very early lesions are often not evident at all. Often, a number of radiolucencies that involve the crowns and the roots of teeth are seen on a dental radiograph and may be confused with dental caries. Restorative materials, abrasion, attrition and cervical burnout may all resemble dental caries on a radiograph. Hence, the final diagnosis of caries should only be made after the clinical and radiographic findings are corroborated. Both the clinical examination and the interpretation of radiographs are mutually contributory aids in making the diagnosis of dental caries.

Sensitivity and specificity of some common caries diagnostic tests in dentistry:

Test Sensitivity Specificity ROC (AUC)
Clinical examination 0.13 0.94 -
Radiography 0.58 0.66 -
Bitewing radiography 0.73 0.97 -
Probe and look 0.58 0.94 -
Radiography of occlusal caries: film - - 0.82
Radiography of interproximal caries: film - - 0.87
Radiography of occlusal caries: digital - - 0.90
Radiography of interproximal caries: digital - - 0.87
Caries in primary teeth: intraoral radiography - - 0.70
Caries in primary teeth: panoramic radiography - - 0.64
Root or dentine caries - - 0.81

(Pretty, Mauporne et al., 2004, p. 389)

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