Cone-beam computed tomography is not the imaging technique of choice for comprehensive orthodontic assessment

It was a pleasure to see that Dr Larson did not take the extreme view of proposing cone-beam computed tomography (CBCT) as a routine diagnostic modality—ie, for every patient, irrespective of malocclusion or other patient-specific factors—as some orthodontic postgraduate programs in the United States seem to do. Even so, he does recommend CBCT as the standard procedure, stating in his conclusions that “CBCT has replaced conventional lateral cephalograms and panoramic images as the most commonly ordered imaging for comprehensive orthodontic patients.” In my Counterpoint , I will try to present arguments against CBCT as the imaging technique of choice for comprehensive orthodontic assessment.

Assuming that use for every patient is not advocated, what are the patient selection criteria? The answer should stem from a comprehensive assessment of the benefits and burdens to each patient. This assessment cannot be completely objective, but our decision making should be based on current evidence, which could also serve as the basis to develop general guidelines. Such guidelines already exist. The SEDENTEXCT project of the European Union had as its primary goal “to acquire key information necessary for sound and scientifically based clinical use of CBCT” and “to use this information to develop evidence-based guidelines dealing with justification, optimization and referral criteria for users of dental CBCT.” The guidelines section dealing with orthodontic diagnosis concludes that “large volume CBCT should not be used routinely for orthodontic diagnosis.”

The British Orthodontic Society guidelines give a similar recommendation: “routine use of CBCT even for most cases of impaction of teeth . . . cannot yet be recommended.”

A similar conclusion was adopted by the American Association of Orthodontists in 2010: “the AAO recognizes that while there may be clinical situations where a cone-beam computed tomography (CBCT) radiograph may be of value, the use of such technology is not routinely required for orthodontic radiography.”

If guidelines already exist, what is the purpose of this debate? First, it is an opportunity to make these guidelines well known to the orthodontic community at a time when CBCT use is increasing. The SEDENTEXCT guidelines are based on a systematic review of the literature, thus representing current evidence-based knowledge at a confidence level much higher than this debate can achieve. Most importantly, however, is that these guidelines are not compulsory. The use of ionizing radiation is governed by law in most countries, but all the law requires is clinical justification. The guidelines are designed to assist the clinician in the justification process. I hope that this debate will convince clinicians to follow the guidelines’ recommendations.

Radiation burden

The effects of ionizing radiation are considered stochastic events. This signifies that the risk, not the severity, of the condition (eg, cancer) depends on the dose. Using a low-dosage vs a high-dosage CBCT machine will not result in cancers that are easier to treat, only fewer of them. The probability of an important stochastic effect (cancer and severe hereditary effect) is 7.3 × 10 −2 Sv. For patients aged 10 to 20 years, this doubles to approximately 0.15 Sv. Since a large field-of-view CBCT will provide a dose of 68 to 368 μSv compared with approximately 30 μSv for the cephalometric and panoramic combination, this translates to a risk of about 1 in 170,000 to 1 in 20,000 above the current customary procedure. In the United States, more than 1.6 million orthodontic patients start treatment every year. If each patient had 1 CBCT image, this would result in 10 to 80 additional cancer cases per year. Is this a risk worth taking? This is not an easy question and depends mainly on the benefit to the patient. What significant improvements in patient outcomes does CBCT offer? To answer, we should not confuse the benefits to the patient with the technical capabilities of CBCT technology. The fact that CBCT images are 3-dimensional is not directly relevant. Justification for CBCT images can only be considered when the treatment outcome will not only be better because of them, but also significantly better to outweigh the above risks.

Assuming that use for every patient is not advocated, what are the patient selection criteria? The answer should stem from a comprehensive assessment of the benefits and burdens to each patient.

Efficacy

The following terms are used to evaluate the efficacy of diagnostic imaging procedures : technical efficacy, diagnostic accuracy efficacy, diagnostic thinking efficacy, therapeutic efficacy, patient outcome efficacy, and societal efficacy. These efficacies constitute a hierarchy of levels of increasing importance. The top 2 levels evaluate whether the imaging method produces a net benefit to the patient and society in general, and should dictate our imaging policy. Regarding CBCT and its use in orthodontics, no such studies have been conducted. We will consider the relevant evidence for each of the lower 4 levels, focusing on large field-of-view protocols, since only these can provide reconstructed lateral cephalometric and panoramic views, similar to conventional radiographs.

Technical efficacy is related to the quality of the image. The dimensional accuracy of CBCT images has been well established. Voxel size is typically 0.3 to 0.4 mm, corresponding to a lower resolution than that of conventional intraoral radiographic imaging. Artefacts and noise are higher than those observed in multi-slice computed tomography, making it difficult, if not impossible, to obtain consistent density values and resulting in low contrast and poor depiction of soft tissues. Segmentation is problematic, and even high-contrast objects, such as teeth, are measured with errors that can exceed 1 mm, limiting clinical usefulness.

Diagnostic accuracy efficacy measures the accuracy of diagnosis by using CBCT in comparison with a reference standard—in our case, a cephalogram or panoramic radiograph. Alveolar bone thickness and height, and the presence of fenestrations and dehiscences, have been compared between CBCT images and direct measurements. Due to the relatively large voxel size, thin structures are difficult to detect, and alveolar bone covering the incisors might be underestimated, although the results are conflicting. Errors in measuring bone thickness can exceed 1.4 mm for a 0.4-mm voxel size. Fenestrations and dehiscences are overestimated to a large degree.

At present, there are no diagnostic accuracy studies regarding the localization of impacted canines, and none are expected because this question is not seriously debated. Regarding resorption of adjacent teeth, CBCT images show improved sensitivity and specificity over panoramic radiography. CBCT has been shown to have increased diagnostic accuracy over posteroanterior cephalograms in patients with skeletal asymmetry.

Concerning periodontal assessment, although it has a definite 3-dimensional advantage, CBCT complements but cannot replace intraoral radiography, mainly because of reduced resolution. Studies on skull material have shown that CBCT images provide better diagnostic information, but there is no consensus regarding the accuracy of these measurements. The SEDENTEXCT guidelines conclude that “CBCT is not indicated as a routine method of imaging periodontal bone support,” although it might be indicated in selected patients, but preferably not with a large field of view. The American Board of Orthodontics includes CBCT images as an option to document periodontal status but does not consider radiographic images, in general, as compulsory data and gives priority to clinical examination and conventional radiography.

Diagnostic thinking efficacy evaluates whether the imaging method changes the diagnosis from the pretest situation. Therapeutic efficacy assesses whether the test produces changes to the treatment plan. These efficacies have been evaluated for impacted third molars and impacted canines. CBCT images are perceived to be more useful than traditional radiographs for such cases and might change the recommended treatment plan in approximately 30% of them. However, no patient outcome efficacy studies have been conducted, and CBCT is recommended only when “the information cannot be obtained adequately by lower dose conventional (traditional) radiography.” Dr Larson referred to the study of Becker et al of 28 failed cases of impacted canines, but the main reason for failure was inadequate anchorage rather than improper localization. The authors acknowledged that the initial clinical and radiographic signs were sometimes sufficient to diagnose properly but were misinterpreted by the clinician. There are numerous cases when an impacted maxillary canine can be clearly localized based on conventional radiographs and clinical examination (eg, palpation, position, and inclination of adjacent teeth), and no further imaging is justified.

Regarding resorption of adjacent teeth, diagnostic thinking efficacy and therapeutic efficacy studies showed that resorption defects can be identified better with CBCT images, but these studies mostly used a medium or small field of view.

Dr Larson also referred to the temporomandibular joint, but asymptomatic patients surely do not need temporomandibular joint imaging. Condylar position in the fossa can certainly be seen on CBCT images, but this information should not affect our diagnosis and treatment plan. The value of temporomandibular joint imaging even for patients with temporomandibular disorders is a debatable subject, and there is no evidence to show that CBCT images will provide better treatment.

It seems, therefore, that CBCT might benefit some patients with the conditions mentioned above, but no evidence exists for the remaining majority of our patients. The application of 3-dimensional cephalometrics, or increased measurement accuracy, could be an indication. However, currently, there are no established 3-dimensional cephalometric analyses and no 3-dimensional normative data. CBCT images are used to simulate old technology—ie, reconstruct 2-dimensional lateral cephalometric views. In this transitory, backward step, we should not carry with us the misconceptions of the early cephalometric era: strict adherence to cephalometric standards and blind faith in numbers.

Cephalometric analyses have significant, well-recognized deficiencies, and increased accuracy of measurements does not address them. There is, as yet, no evidence that increased accuracy from CBCT contributes to a change of treatment plan or better treatment. Even though such a notion might seem self-evident, one should consider that our treatment modalities are not so fine tuned to specific craniofacial patterns that a conventional cephalometric radiograph is inadequate to serve. Furthermore, identifying landmarks on CBCT images introduces significant errors that might mitigate the advantage of increased accuracy. Lastly, most of our diagnostic information is gained from clinical evaluations. The cephalogram serves as an adjunctive tool and has been shown to be superfluous in some circumstances, affecting treatment-planning decisions in some patients and to a limited degree.

Efficacy

The following terms are used to evaluate the efficacy of diagnostic imaging procedures : technical efficacy, diagnostic accuracy efficacy, diagnostic thinking efficacy, therapeutic efficacy, patient outcome efficacy, and societal efficacy. These efficacies constitute a hierarchy of levels of increasing importance. The top 2 levels evaluate whether the imaging method produces a net benefit to the patient and society in general, and should dictate our imaging policy. Regarding CBCT and its use in orthodontics, no such studies have been conducted. We will consider the relevant evidence for each of the lower 4 levels, focusing on large field-of-view protocols, since only these can provide reconstructed lateral cephalometric and panoramic views, similar to conventional radiographs.

Technical efficacy is related to the quality of the image. The dimensional accuracy of CBCT images has been well established. Voxel size is typically 0.3 to 0.4 mm, corresponding to a lower resolution than that of conventional intraoral radiographic imaging. Artefacts and noise are higher than those observed in multi-slice computed tomography, making it difficult, if not impossible, to obtain consistent density values and resulting in low contrast and poor depiction of soft tissues. Segmentation is problematic, and even high-contrast objects, such as teeth, are measured with errors that can exceed 1 mm, limiting clinical usefulness.

Diagnostic accuracy efficacy measures the accuracy of diagnosis by using CBCT in comparison with a reference standard—in our case, a cephalogram or panoramic radiograph. Alveolar bone thickness and height, and the presence of fenestrations and dehiscences, have been compared between CBCT images and direct measurements. Due to the relatively large voxel size, thin structures are difficult to detect, and alveolar bone covering the incisors might be underestimated, although the results are conflicting. Errors in measuring bone thickness can exceed 1.4 mm for a 0.4-mm voxel size. Fenestrations and dehiscences are overestimated to a large degree.

At present, there are no diagnostic accuracy studies regarding the localization of impacted canines, and none are expected because this question is not seriously debated. Regarding resorption of adjacent teeth, CBCT images show improved sensitivity and specificity over panoramic radiography. CBCT has been shown to have increased diagnostic accuracy over posteroanterior cephalograms in patients with skeletal asymmetry.

Concerning periodontal assessment, although it has a definite 3-dimensional advantage, CBCT complements but cannot replace intraoral radiography, mainly because of reduced resolution. Studies on skull material have shown that CBCT images provide better diagnostic information, but there is no consensus regarding the accuracy of these measurements. The SEDENTEXCT guidelines conclude that “CBCT is not indicated as a routine method of imaging periodontal bone support,” although it might be indicated in selected patients, but preferably not with a large field of view. The American Board of Orthodontics includes CBCT images as an option to document periodontal status but does not consider radiographic images, in general, as compulsory data and gives priority to clinical examination and conventional radiography.

Diagnostic thinking efficacy evaluates whether the imaging method changes the diagnosis from the pretest situation. Therapeutic efficacy assesses whether the test produces changes to the treatment plan. These efficacies have been evaluated for impacted third molars and impacted canines. CBCT images are perceived to be more useful than traditional radiographs for such cases and might change the recommended treatment plan in approximately 30% of them. However, no patient outcome efficacy studies have been conducted, and CBCT is recommended only when “the information cannot be obtained adequately by lower dose conventional (traditional) radiography.” Dr Larson referred to the study of Becker et al of 28 failed cases of impacted canines, but the main reason for failure was inadequate anchorage rather than improper localization. The authors acknowledged that the initial clinical and radiographic signs were sometimes sufficient to diagnose properly but were misinterpreted by the clinician. There are numerous cases when an impacted maxillary canine can be clearly localized based on conventional radiographs and clinical examination (eg, palpation, position, and inclination of adjacent teeth), and no further imaging is justified.

Regarding resorption of adjacent teeth, diagnostic thinking efficacy and therapeutic efficacy studies showed that resorption defects can be identified better with CBCT images, but these studies mostly used a medium or small field of view.

Dr Larson also referred to the temporomandibular joint, but asymptomatic patients surely do not need temporomandibular joint imaging. Condylar position in the fossa can certainly be seen on CBCT images, but this information should not affect our diagnosis and treatment plan. The value of temporomandibular joint imaging even for patients with temporomandibular disorders is a debatable subject, and there is no evidence to show that CBCT images will provide better treatment.

It seems, therefore, that CBCT might benefit some patients with the conditions mentioned above, but no evidence exists for the remaining majority of our patients. The application of 3-dimensional cephalometrics, or increased measurement accuracy, could be an indication. However, currently, there are no established 3-dimensional cephalometric analyses and no 3-dimensional normative data. CBCT images are used to simulate old technology—ie, reconstruct 2-dimensional lateral cephalometric views. In this transitory, backward step, we should not carry with us the misconceptions of the early cephalometric era: strict adherence to cephalometric standards and blind faith in numbers.

Cephalometric analyses have significant, well-recognized deficiencies, and increased accuracy of measurements does not address them. There is, as yet, no evidence that increased accuracy from CBCT contributes to a change of treatment plan or better treatment. Even though such a notion might seem self-evident, one should consider that our treatment modalities are not so fine tuned to specific craniofacial patterns that a conventional cephalometric radiograph is inadequate to serve. Furthermore, identifying landmarks on CBCT images introduces significant errors that might mitigate the advantage of increased accuracy. Lastly, most of our diagnostic information is gained from clinical evaluations. The cephalogram serves as an adjunctive tool and has been shown to be superfluous in some circumstances, affecting treatment-planning decisions in some patients and to a limited degree.

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Apr 8, 2017 | Posted by in Orthodontics | Comments Off on Cone-beam computed tomography is not the imaging technique of choice for comprehensive orthodontic assessment
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