HANDS ON

Dental Imaging—Point/CounterpointAdjunct cone-beam computed tomography in orthodontics

Background.—Selective use of cone-beam computedtomography (CBCT) for orthodontic patients can be basedon the guidelines prepared by the SEDENTEXCT projectof the European Union. These guidelines are the result ofa systematic review of current evidence and conclude thatlarge-volume CBCT is not appropriate for routine use inorthodontic diagnosis. These guidelines need to bepromulgated to the orthodontic community and are notcompulsory, but provide help to clinicians in the justifica-tion process.

CBCT Radiation Dose.—The effects of ionizing radia-tion (IR) are classified as stochastic events, which meansthe risk of the adverse condition that may develop dependson the dose of the exposure. The probability that an impor-tant stochastic event will develop is 7.3 � 10�2 Sv. The riskdoubles to about 0.15 Sv in patients age 10–20 years. A largefield-of-view CBCT provides a dose of 68–368 mSv, whereas acephalometric plus panoramic radiograph combinationproduces about 30 mSv. The CBCT risk is about 1 in170,000 to 1 in 20,000 over the current standard procedure.If each of the 1.6 million persons who begin orthodontictreatment each year underwent one CBCT image, 10–80additional cancer cases would be seen each year. Whetherthe risk is worth taking depends mainly on what benefitsCBCT offers the patient. CBCT should be consideredwhen the treatment outcome will be significantly betterthan with conventional approaches.

Efficacy and Incidental Findings.—Six levels of efficacyare used to assess the benefit of an imaging method for apatient and society in general. Large field-of-view protocolswere considered for the lower four levels of efficacy, sinceonly these protocols can provide reconstructed lateralcephalometric and panoramic views similar to those of con-ventional radiographs.

The quality of the image (technical efficacy) producedby CBCT is well established, but artifacts and noise arehigher than with multislice CT, yielding low contrast andpoor depiction of soft tissues. Even high-contrast objectssuch as teeth are associated with errors that limit clinicalusefulness.

The accuracy of diagnosis (diagnostic accuracy efficacy)analyzed using alveolar bone thickness and height plus thepresence of fenestrations and dehiscences has found thatthin structures are hard to detect and alveolar bonecovering the incisors may be underestimated. Fenestrationsand dehiscences are greatly overestimated. CBCT imagesshow better sensitivity and specificity compared to pano-ramic radiographs in depicting resorption of adjacent teeth.Patients who have skeletal asymmetry are better analyzedusing CBCT than posterior cephalograms. Periodontal as-sessments can be complemented by CBCT but are limitedby reduced resolution. The accuracy of measurements ofskull material is also questionable. As a result, the SEDEN-TEXCT guidelines state that CBCT is not indicated as aroutine method of assessing periodontal bone support,but may be useful in selected patients. The American Boardof Orthodontics gives priority to clinical examination andconventional radiography in periodontal evaluation.

Diagnostic thinking efficacy assesses whether the imag-ing method alters a diagnosis, and therapeutic efficacy eval-uateswhether the test changes treatment plans.With respectto impacted third molars and impacted canine teeth, CBCTcan be more useful than traditional radiographs and mayalter the treatment plan in about 30% of cases. No patientoutcome efficacy studies have yet confirmed this. Oftenimpactedmaxillary canine teeth can be clearly shownon con-ventional radiographs and clinical examination, requiring nofurther imaging.With respect to resorptionof adjacent teeth,diagnostic thinking efficacy and therapeutic efficacy studiesshow CBCT is better at depicting resorption defects. Howev-er, usually small or medium fields of view are used in theseinvestigations. No evidence indicates that CBCTwill providebetter treatment in cases of temporomandibular disorders.

Although CBCT provides many incidental findings,incidental findings are not a justification for radiographicexposure. Many represent normal anatomic variants or arebenign and require no intervention. Often the patientalready knows about the situation. Carotid artery calcifica-tion and periapical osteitis are themost commonpathologicfindings seen in CBCT images that could require interven-tion. However, panoramic radiographs can also find these.

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Clinical Significance.—A CBCT image in-cludes a cephalogram and a panoramic imagefor orthodontic assessment and added imagesfor any potential periodontal assessment, TMJevaluation, temporary anchorage device place-ment, airway analysis, and incidental findings.With more knowledge, it could be assumedthat the clinician could make a better diagnosisand provide better treatment. However, mostpatients don’t have the problems listed andwould simply receive additional exposure to ra-diation.With each diagnostic test comes the pos-sibility of more false-positive findings, andthese can cause patient anxiety, unneededfollow-up examinations, and more tests.

6 Dental Abstracts

Diagnostic evaluations should be focused andused to determine specific information, notjust see what pops up. The jury is still out onthe routine use of CBCT in orthodontics, but cur-rent evidence indicates it should be reserved foradjunctive use in selected patients only.

Halazonetis DJ: Cone-beam computed tomography is not the im-aging technique of choice for comprehensive orthodontic assess-ment. Am J Orthod Dentofacial Orthop 141:402-411, 2012

Reprints available from DJ Halazonetis, 6 Menandrou St., Kifissia GR-145 61, Greece; e-mail: dhal@dhal.com

Rely on cone-beam computed tomography in orthodontics

Background.—Cone-beam computed tomography(CBCT) has been used as an adjunctive imaging techniquein orthodontics for some time.Whether it should be consid-ered the imaging protocol of choice for comprehensiveorthodontic treatment has been debated. The advantagesof using CBCT for orthodontics were outlined.

Benefits.—CBCT offers the advantage of 1:1 geometry,so that accurate measurements can be obtained of objectsand dimensions. Among the things that can be accuratelyand reliably measured from CBCT images are uneruptedtooth sizes, bony dimensions in all three planes of space,and soft-tissue anthropometric measurements. All of theseare important in orthodontic diagnosis and treatment plan-ning. Landmarks can also be located reliably on cephalo-metric images generated from CBCT volumes.

CBCT is able to locate ectopic teeth and assess rootresorption, both of which are important in formulatingpatient-specific treatment plans.With this improved localiza-tion compared to conventional imagingmethods, changes indiagnosis and treatment recommendations have occurred.

An asymmetric malocclusion is a difficult problem to di-agnose and treat orthodontically. CBCT volume permitsdirect measurement of the transverse dimensions and therelative positions of the teeth in the skeletal componentswith results judged superior to those obtained with previ-ous methods. In addition, both the right and the left TMJsare included in CBCT volumes, allowing their routine re-view. Orthodontists can use these images to screen forbony changes and condylar position.

The three-dimensional views of the airway dimensionsand sinuses allow clear visualization and measurement.Incidental findings with CBCT range from minor sinusitisor polyps to complete opacification of themaxillary sinuses.Endodontic involvement may be detected, including apicalperiodontitis, apical radiolucency, internal or external rootresorption, and retained root tips. All should be consideredbefore final orthodontic planning because they could alterthe treatment plan. When patients have CBCTas part of theinitial record set, areas that may be considered as place-ment sites for temporary skeletal anchorage devices canbe assessed individually for bone quality without incurringcosts or time for added imaging.

The American Board of Orthodontics requires that aformal periodontal evaluation be done for all patientsover age 18 years or for those with signs of periodontal dis-ease. CBCT images permit the measurement of buccal andlingual defects, interproximal defects, and periodontalbone levels before orthodontic treatment.

Future benefits that may accompany CBCTusemay facil-itate risk management. CBCT may be able to detect bonechanges caused by biophosphonates or permit the fabrica-tion of custom lingual orthodontic appliances.

Radiation Exposures and Costs.—Dosimetry testingof the CBCT machine and conventional two-dimensionaldigital radiography equipment has discovered that theCBCT imaging normally used for comprehensive orthodon-tic patients has a radiation dose of about 65 mSv while thedigital approach dose is 26 mSv for a lateral cephalogram