Diagnostics and Epidemiology: 6 Radiographic and Other Additional Diagnostic Methods
Many sites that have a predilection for caries naturally lie in regions of the dentition that are difficult to view. Consequently, for diagnostic purposes, a visual–tactile examination of the approximal regions, deep fissures and grooves, and the margins of restorations is frequently insufficient to detect and evaluate early caries lesions. Other methods can be used to provide additional, valuable diagnostic information.
The most widely used additional diagnostic aid is bite-wing radiographs. Caries lesions are visualized by means of the increased radiotranslucency of the dental hard substance associated with mineral loss. Bitewing radiographs provide useful additional diagnostic information on approximal and occlusal lesions or enable them to be detected for the first time.
Other diagnostic methods are based on the transillumination of teeth using visible light (fiber-optic transillumination), the fluorescence of healthy or carious dentin and enamel (DIAGNOdent, camera systems), or the electrical conductivity of caries (measurement of electrical resistance, impedance spectroscopy). This chapter addresses the various additional diagnostic methods and describes their clinical use as well as their advantages and disadvantages.
In particular, the following will be discussed:
- How to use bitewing x-ray radiography
- Analogue and digital radiographic systems
- Evaluation and relevance of radiographic findings
- Bitewing radiograph diagnostics and follow-up intervals
- Diagnostic methods based on visible light
- Caries diagnostics using electrical current
Radiographic Caries Diagnostics
Context of Radiographic Caries Detection
Primary caries lesions arise with greater frequency in children and adolescents.1 The time window for the elevated risk of developing occlusal caries lesions in the permanent dentition starts with the eruption of the teeth, whereas approximal defects arise at a greater rate beginning at 13 years of age. Today, the number of initial caries lesions that have not penetrated the surface is substantially greater than the number of established defects or filled tooth surfaces,2 and this makes it significantly more difficult to identify and evaluate caries lesions.3
Whereas the discovery of a carious lesion used to be an indication for restorative therapy, the present goal of therapy is to stop the progress of caries by attacking the underlying cause, such as by controlling the bacterial biofilm or providing nutritional education, etc. Only when the biofilm directly contacts the dentin there is a sufficiently potent bacterial invasion to cause demineralization and destruction of the organic dentin components. Consequently, at this stage restorative measures are generally indicated. However, reactive changes in the dentin to a carious process in the tooth surface are an unsuitable indication for invasive therapy.4
The primary goal of the causal therapy of caries is to stop or at least slow the progress of the lesion by controlling the biofilm.
The quality of the surface is currently held to be the decisive criterion for restorative treatment, since it can be directly influenced by hygiene; that is, it is the primary factor in the ability of the cariogenic biofilm to be removed through hygiene. When a cavity exists, the entire metabolism of the biofilm changes.5
When there is surface cavitation, the options for noninvasive preventive measures are generally exhausted, and restorative treatment is indicated.
The primary goal of radiographic diagnostic methods is to detect and evaluate caries lesions in areas that are clinically inaccessible, or are only accessible with difficulty. This holds true particularly for approximal surfaces but also for occlusal surfaces and caries in restorations. Radiographic methods therefore enhance diagnostic sensitivity (detection). Furthermore, radiographic findings can also refine clinical findings through additional details on the extent of caries (assessment). Series of radiographs taken over an extended period also provide a longitudinal view of caries lesions and provide information about their progression ( Fig. 6.1 ).
Radiographic diagnostic methods primarily focus on the detection and assessment of caries lesions in approximal surfaces that cannot be clinically visualized as well as occlusal surfaces and existing restorations.
Radiographic images visualize caries lesions due to a decreased radiopacity of demineralized dentin and enamel ( Fig. 6.2 ). When x-rays are used to detect caries, intraoral images should be used in the bitewing projection.6,7 Bitewings were introduced in 1925 and have only been modified slightly since that time.8 They have a film holder and bite tab that reduces proximal overlap and projection errors.9
Validity and Reliability of Radiographic Caries Detection
Which Gold Standard?
The radiographic detection of caries is only an additional tool in the overall effort to diagnose caries. It does not enable the dentist to determine whether surface cavitation exists. This clinically relevant criterion for therapy can only be estimated from the demineralization revealed in the bitewing radiographs. Consequently, when considering how scientific reports on the quality of radiographic caries diagnoses relate to the choice of therapy, one always needs to consider which validation method or “gold standard” was employed in the study. In most investigations of the radiographic detection of caries, the depth of lesion penetration is used as the reference criterion, which is determined histologically or clinically.10 These may or may not be relevant to practice, however (see Chapter 9).11 Rather, the gold standard is relative to the specific questions posed in the study.12 When assessing the informational quality of the radiographic detection as it relates to potential restorative treatment, the (indirect) evaluation of potential surface cavitation is the primary determinant of validity. It is true that data from in-vitro studies reveal a moderate correlation between the clinical appearance and the radiographic extension of approximal and occlusal lesions.13 However, the radiographic extension of caries only allows one to indirectly predict the probability of surface cavitation.
Sensitivity and Specificity
With in-vitro investigations, the sensitivity (fraction of correctly detected carious teeth) of dental lesion detection ranges from 50% to 70%, whereas specificity (fraction of correctly detected sound teeth) fluctuates from 70% to 97%.14 In contrast, it is much more difficult to detect caries lesions in the enamel.15,16 There is unanimous agreement that more caries lesions can be identified by combining radiographic information with clinical findings than by visual inspection alone.17–19 When bitewing x-ray images are used, it has been demonstrated that the number of identified caries lesions can be increased 827% over visually detected caries lesions.20 In other words, this means that only 10.8% of carious defects are discovered through visual inspection in comparison to combined visual and radiographic findings.
The additional information provided by a radiographic image in combination with the clinical examination significantly increases the number of correctly detected caries lesions.
The reproducibility of detections from bitewing radiographs assumes a moderate-to-good inter observer agreement.20,21 This means, however, that the subjective element involved in analyzing radiographs influences the conclusion just as much as the basic ability to identify visual information in an x-ray photo.
Conventional and Digital Bitewing X-ray Imaging
Over the years, numerous digital systems have become available in addition to conventional x-ray equipment that use intraoral film.22,23 In addition to digitizing conventional radiographs, both direct and indirect semiconductor sensors or luminescence films are used to create digital intraoral images. At the beginning of the digital era, the analog/digital conversion of intraoral x-ray films was extensively investigated to determine digital processing options.24–26 Over time, only direct techniques gained currency in digital intraoral radiology.27–29 Semiconductor sensors on CCDs (charge coupled devices) or CMOS (complementary metal oxide semiconductors) use a luminescent film to convert the x-rays into visible light that is detected by the back-illuminated CCD chips, or the x-ray is converted directly into an electrical signal in the silicon circuits without an intermediate carrier. In comparison to a CCD sensor, a CMOS sensor has a somewhat higher sensitivity with a slightly greater level of noise, and requires less space and power. At the moment, however, both systems are apparently considered equivalent. Imaging plates first need to be read by a scanner, but they are easier to position than sensors.30
Advantages of Digital Technologies
In comparison to conventional techniques, digital intraoral radiographs possess numerous advantages. Due to the higher exposure tolerance, fewer imaging errors arise, and technical and environmental problems associated with wet chemistry no longer exist. In addition, the exposure time and dose can be reduced, and digital images can be processed electronically.6 However, each new system that appears on the market must demonstrate that the images possess a level of validity and reproducibility at least similar to conventional techniques for the depiction of caries.14,19 This has been demonstrated for numerous available digital systems and dental films,31–34 so it can therefore be assumed that digital and analog imaging systems are at least equivalent. In addition, digital systems allow computer-supported image processing.35 The goal of digital image manipulation is to more clearly depict information relevant to the detection of caries and suppress irrelevant details.24 Software programs with their own image processing features are available for all digital systems.36 Although these types of manipulation can be described with the specific terminology used in the field, generally individual descriptions or buttons with icons are employed which makes the programs difficult to compare. It is unclear, however, whether programs that offer the same types of modifications and filters produce the same results, or whether the options can be freely combined with each other.36 Consequently, further investigations are required to determine the effectiveness of image modifications relative to various diagnostic issues and especially the detection of caries.
Digital radiography possesses advantages over conventional radiography with regard to clarity, exposure tolerance, environmental compatibility, the required time, and radiation dose.
Tuned-Aperture Computed Radiography and Digital Volume Tomography
Since there is no perfect correlation between the detectable demineralization front and surface quality,37 the search continues for a way to obtain this essential diagnostic information for visually inaccessible tooth surfaces. Three-dimensional (3-D) radiographic techniques could be particularly suitable, since the summation effect of classic intraoral x-ray photos is eliminated by the relevant information obtained from the structures in the beam path. Furthermore, the position of the slice from the 3-D volumetric record is freely determinable, which frees one from the bucco-oral projection and the associated image obtained from conventional intraoral radiographs. The conventional form of radiographic imaging, also termed transmission radiography, is characterized by a dot-shaped beam source and a flat beam receptor. The radiograph accordingly results from the attenuation of the transmission beam in the tissue. Two-dimensional reproductions of a 3-D configuration of tissues with overlapping anatomical structures can produce false negatives or false positives when (for example) buccal or oral caries lesions appear as a single lesion on the approximal surfaces in the radiograph.38
One approach for accommodating radiographic data in a 3-D matrix is tuned-aperture computed tomography (TACT).39 At least three different projected digital radiographs are overlapped using a reference point, and a 3-D record is generated that can be portrayed in the form of slices or a 3-D animation40 ( Fig. 6.3 ).
Whereas artificial lesions are more discernible in comparison with conventional x-ray film and CCD sensors,41 there was no discernible improvement on the detection of occlusal and approximal caries.42,43
Another approach is cone-beam computer tomography or digital volume tomography (DVT). Similar to conventional computed tomography, an x-ray beam executes a maximum 360° orbit as it scans the object. Different than conventional computer tomography, in which the object is scanned using a fan-shaped x-ray beam and a ring-shaped detector, a conical x-ray beam and flat detector are employed ( Fig. 6.4 ).
Current publications have demonstrated experimentally that the detection of caries can be improved using local slice images.44 The question therefore arises of whether cavitation from approximal caries can be detected better with cone-beam computer tomograms than with conventional intraoral x-rays. It was revealed that 80% sensitivity was achievable with a specificity of 96% in cone beam images, whereas only 29% of caries lesions with cavitation were correctly identified in bite-wing x-ray images.45
These results indicate that cone-beam technology can be helpful in differentiating between intact and cavitated tooth surfaces in the approximal region. It was also demonstrated in isolated cases that occlusal lesions extending into the dentin are easier to identify by this method than in classic radiographs ( Fig. 6.5 ). One problem that needs to be overcome before the method can be used in practice is the formation of artifacts in radiopaque materials, which always arise in tomographic imaging and can mask defects. It remains to be demonstrated whether the substantial investment that this technique requires will actually allow it to be employed in the evaluation of caries.
In comparison with two-dimensional bitewing x-ray images, the use of three-dimensional tomographic imaging yields a higher detection rate of approximal cavitation, although it is not yet ready for widespread use.
Traditionally, it is recommended to image a region extending from the distal surface of the canine to the distal surface of the terminal molar,19 so that generally two size 2 films are required on each side of the mouth. If a long bitewing film (size 3) is used instead, the radiation exposure can be reduced, but this yields increased overlapping, projection errors and undisplayed approximal surfaces.46 Since the probability of developing caries lesions in canines and first premolars is rather low in most patients today, it can be assumed that lesions will be rarely overlooked when only one x-ray is taken per side.47 In addition, the approximal surfaces between the canine and premolar can frequently be visually evaluated.
There are a few basic rules regarding patient positioning, the positioning of the film or sensor, and the alignment of the central beam when creating bitewing images. Beforehand, the patient should be asked about any previous medical exposure to ionizing radiation. Pregnancy should be excluded in women of childbearing age, radioprotective shielding should be draped over the patient, and removable dentures should be removed as well as jewelry, glasses, etc.
Taking Bitewing X-ray Photos
As mentioned above, digital radiography offers distinct advantages, especially in regard to radiation exposure, environmental considerations, and the avoidance of imaging errors. Consequently, the following clinical images were exclusively generated by a digital system (Xios, Sirona, Bensheim), but other systems can be used as well. Special bitewing holders are provided with a tab for technical reasons to which the sensor is mechanically anchored ( Fig. 6.6 ). Attached to the centering rod extending out of the mouth is an aiming ring as a positioning tool for the tube that allows the orthoradial alignment of the central beam. The patient′s head should be stabilized with a headrest, and the bipupillary line as well as the occlusal plane should be aligned approximately parallel to the floor. This is followed by the intraoral positioning of the sensor. The bite plate of the holder is fixed between the occlusal surfaces of the upper and lower posterior teeth, and the sensor is placed as close as possible to the oral surfaces of the relevant teeth ( Fig. 6.7 ). Due to the aforementioned epidemiological situation, it is frequently sufficient to create one bitewing per jaw half. If two x-ray images are indicated for anatomical reasons, however, it is recommendable to start with the anterior projection since it is more difficult due to the bulkiness of the sensor. Errors in imaging generally impair quality and decrease the available information in the radiographs or render them useless in case of doubt.
Using special bitewing holders, including an aiming ring and centering rod, and correct patient positioning enable an orthoradial and hence overlap-free projection of the approximal areas.
If the radiographic progression or arrest of a caries lesion is to be assessed in longitudinal monitoring, the imaging parameters should ideally be the same each time. The projection can only be reproducible if the intraoral position of the tab, the position of the film (sensor), and path of the central beam remain the same. Special film holders ( Fig. 6.8 ) enable the projections to be reproducible and also allow special x-ray software to be used for digital subtraction radiography. By subtracting the pixels of two images recorded under identical conditions, a new image is generated that visualizes the differences between the radiographs.