CBCT in orthodontics: assessment of treatment outcomes and indications for its use

Abstract

Since its introduction into dentistry in 1998, CBCT has become increasingly utilized for orthodontic diagnosis, treatment planning and research. The utilization of CBCT for these purposes has been facilitated by the relative advantages of three-dimensional (3D) over two-dimensional radiography. Despite many suggested indications of CBCT, scientific evidence that its utilization improves diagnosis and treatment plans or outcomes has only recently begun to emerge for some of these applications. This article provides a comprehensive and current review of key studies on the applications of CBCT in orthodontic therapy and for research to decipher treatment outcomes and 3D craniofacial anatomy. The current diagnostic and treatment planning indications for CBCT include impacted teeth, cleft lip and palate and skeletal discrepancies requiring surgical intervention. The use of CBCT in these and other situations such as root resorption, supernumerary teeth, temporomandibular joint (TMJ) pathology, asymmetries and alveolar boundary conditions should be justified on the basis of the merits relative to risks of imaging. CBCT has also been used to assess 3D craniofacial anatomy in health and disease and of treatment outcomes including that of root morphology and angulation; alveolar boundary conditions; maxillary transverse dimensions and maxillary expansion; airway morphology, vertical malocclusion and obstructive sleep apnoea; TMJ morphology and pathology contributing to malocclusion; and temporary anchorage devices. Finally, this article utilizes findings of these studies and current voids in knowledge to provide ideas for future research that could be beneficial for further optimizing the use of CBCT in research and the clinical practice of orthodontics.

Introduction

CBCT was introduced to dentistry in 1998¹ in Europe and approved for use in the USA in 2001.² An early assessment of the likely future impact of this technology to dentistry, and more specifically to orthodontics, was first discussed in a symposium on “Craniofacial Imaging in the 21st Century” held in 2002 in Pacific Grove, CA, and documented in its proceedings.³ Since then, CBCT technology has undergone a rapid evolution, driven largely by the demands of each speciality for accurate, reproducible and safe three-dimensional (3D) images. In orthodontics, 3D imaging can help unravel the complexity of dental and skeletal malocclusions and improve diagnosis and treatment planning in specific case types.^4,5

The varied utilization of CBCT by clinicians for orthodontic purposes exists within the context of research evidence, published case reports or anecdotal observations on a broad spectrum of cases ranging from impacted teeth to temporomandibular joint (TMJ) morphology. Several of these studies show that CBCT provides clinically relevant information and novel 3D research data. Nevertheless, scientific evidence that the utilization of CBCT alters diagnosis and improves treatment plans or outcomes has only recently begun to emerge for some of its suggested applications. Also, for several of these recommendations in which CBCT use is logical and/or supported by scientific evidence, the specific indications for acquiring CBCT images and protocols for imaging and extracting appropriate information have not been resolved fully. This article provides a review of literature on 3D craniofacial anatomy and orthodontic treatment outcomes determined by CBCT as well as evidence-based indications for the use of CBCT in clinical orthodontics. These include studies on impacted and supernumerary teeth, root resorption and angulations, cleft lip and palate (CL/P), alveolar boundary conditions, temporary anchorage devices (TADs), maxillary transverse deficiency, airway analyses, obstructive sleep apnoea (OSA), TMJ disorders and orthognathic and craniofacial surgery. It is likely that as the field advances and more evidence of the efficacy of CBCT emerges, its applications in orthodontics will increase or be modified, which will enable clinicians to realize the ultimate goal of increased treatment efficiency or outcomes or both in many more clinical scenarios than currently possible.

Radiology guidelines in orthodontics

Both European and US radiography guidelines specify that routine radiographs are not indicated for any patient in any dental discipline,^6–10including orthodontics,^11,12 because the risk of unnecessary exposure to ionizing radiation may outweigh its benefits. In fact, three systematic reviews concur that two-dimensional (2D) and 3D radiographs are not routinely needed for diagnosis or treatment planning of orthodontic patients.^13–15 Based on the radiography guidelines in the USA and Europe, it is now universally advised that a thorough clinical examination and adherence to the “as low as reasonably achievable” radiography principle should guide clinicians in their justification for and choice of radiographs needed to optimize diagnosis, treatment planning and outcomes assessment for their patients. It is even more critical to adhere to this principle when radiographing children who comprise the majority of orthodontic patients owing to the attributable lifetime radiation risk.^16,17 When required, CBCT should be performed using the smallest possible field of view needed for the specific clinical scenario.⁵ Finally, the comparison of CBCT radiation exposure with the combined exposure of cephalograms and panoramic radiographs to justify the routine use of CBCT on orthodontic patients is based on a paradigm that assumes that all orthodontic patients should be subjected to routine 2D radiography. Indeed the findings of the systematic reviews^13–15 suggest that these 2D radiographs should also only be taken when they are expected to provide additional information that could aid in diagnosis or treatment planning or for assessing progress or complications during treatment.

Orthodontic research and findings using CBCT

A large proportion of published original CBCT studies have focused on craniofacial and airway morphometric analyses in health and disease; CBCT use in analysing treatment outcomes; and evidence-based indications, uses and efficacy of CBCT in diagnosis and treatment planning, that are summarised below.

Three-dimensional craniofacial morphometric analyses and superimpositions

CBCT-based 3D craniofacial and dental morphometrics is important for defining normal and abnormal 3D anatomy of structures with a potential for longer-term utility in diagnosis and treatment planning. Much work to date on this topic has focused on quantitative and qualitative determinations of the morphology of craniofacial structures, airway, TMJ, roots and alveolar boundary conditions as discussed below.

One of the key advantages of CBCT over 2D radiography is its ability to provide 3D volumetric, surface and sectional information about the craniofacial structures. This has enabled orthodontists and researchers in the field to overcome the substantial limitations of 2D radiographs, including magnification, geometric distortion, superimposed structures and inconsistent head position. In the earlier stages of incorporating CBCT for orthodontic purposes, there was a tendency to collapse the 3D data set to a 2D image since analyses of the images in this format were the only methods known to the profession for assessing relationships of the dental and skeletal structures.^18,19 This is clearly not an optimal approach to utilizing and extracting all the information contained in the 3D data set. Since then, researchers have taken up the challenge of developing new software to measure 3D distances, angles and volumes and to superimpose 3D craniofacial images,^20,21 although the development of software for these purposes lags behind that of the hardware.¹⁴

Currently, three main methods are used for analysing 3D craniofacial anatomy and changes due to treatment. The first method draws heavily from 2D cephalometric measurement methods to derive linear and angular measurements from 3D images.^22–24 However, extracting 2D measurements from a 3D image results in loss of critical 3D information and diminishes the overall value of the 3D data set. The second method, called iterative closest point analysis determines the shortest distances between structures in two superimposed 3D images.^25–29 These changes can then be represented as a colour map that depicts inward or outward or no displacements between the two time points (Figures 1 and 2). Although iterative closest point cannot be used to assess changes in shape, an iterative closest point-based algorithm coupled with CBCT images has been developed to simulate orthodontic tooth movement with the goal of developing software to aid in orthodontic treatment planning.³⁰ The third method is shape correspondence (Figures 1 and 2), which determines the displacement of a given landmark between two time points and represents these as vectors and colour-coded maps to depict the directionality and amount of movements, respectively.^31,32 In the future, it is likely that similar approaches will replace or complement linear and angular measurements made from 3D or planar reconstructions for determining treatment changes from CBCT images.

Figure 1.

Distinctions between iterative closest point (ICP) and shape correspondence in determining growth and treatment changes in craniofacial structures. Diagrammatic representation of ICP (a) and shape correspondence (b) used to compute surface distances to quantify longitudinal changes for example shown here for condylar displacement. The closest points are linear distances (shown as lines), while the shape correspondence measurements are vectors (shown as lines with direction represented by arrows). Note that the closest surface points fail to quantify the displacement when large translational changes occur. Reproduced from Hajati et al²⁶, Copyright © 2014, John Wiley and Sons.

Figure 2.

Depiction of post-surgical three-dimensional changes in the mandible using cranial base superimposition and either iterative closest point or shape correspondence for the same patient to demonstrate how these methods result in different visual representation of treatment outcomes. (a) Semi-transparency superimposition provides a visual assessment of treatment outcomes, but the changes cannot be quantified. (b) Colour-coded map of the closest point method where blue represents inward and red outward changes with the magnitude of change shown in the accompanying colour scale. (c) Outcomes assessment with shape correspondence method depicts a colour-coded map and vectors that provide the direction and magnitude of the displacement. (d) A zoomed-in image of the chin to demonstrate the vectors from (c) in greater detail. Reproduced from Kim et al²⁷, Copyright © 2014, John Wiley and Sons. For colour images see online: www.birpublications.org/doi/pdf/10.1259/dmfr.20140282.

While 3D CBCT images are most often used to assess skeletal contributions to malocclusion, researchers are now investigating the use of these images to assess dental relationships in orthodontic patients.^33,34 When compared with OrthoCAD^® (Carlstadt, NJ) or InVivoDental™ (San Jose, CA) 3D digital models, linear measurements taken from CBCT-generated 3D digital models are sufficiently accurate to make initial diagnoses and treatment plans. Moreover, superimposing the CBCT- and OrthoCAD-generated digital models demonstrates clinically insignificant differences between the images. These findings may be surprising to those who considered the resolution of CBCT scans to be too low for accurately assessing dental relationships. While direct intraoral 3D scanners and software, such as OrthoCAD or InVivoDental, are state of the art for digital dental models, evidence suggests that clinicians can rely on current CBCT-generated dental models for accurate diagnosis and treatment planning.

Utilization of CBCT for orthodontic treatment outcomes, diagnosis and treatment planning

The justification for using CBCT in orthodontics is linked intricately to its diagnostic and therapeutic efficacies, for which research supporting its use has been performed in a relatively small subset of clinical problems that include impacted teeth, CL/P and orthognathic surgery. In the absence of such proof, a clinician still may choose to utilize the technology if there is adequate reason to believe that it likely would enhance the diagnosis and/or alter the treatment plan. This research or clinical evidence is important particularly in justifying the use of a technology, such as CBCT, which has associated risks of radiation exposure and increased costs of imaging and interpretation as compared with the technologies it is intended to replace. When used in any of these scenarios, and based on current literature, we would anticipate that CBCT likely will provide information that could result in one or more of the following outcomes: (1) enhanced diagnosis, such as precisely localizing impacted and supernumerary teeth; (2) quantifying the magnitude of a defect or deformity, such as in patients with craniofacial anomalies; (3) improving differential diagnosis of skeletal, dental or combined malocclusions, including identifying the jaw(s) contributing to malocclusion and determining whether the discrepancy is bilateral or unilateral, such as in orthognathic surgery, asymmetry, craniofacial anomaly and open bite cases; and (4) helping to identify possible causes of malocclusions, such as the contribution of TMJ abnormalities to an open bite or asymmetry. The expected outcome of the 3D information derived from CBCT relative to that obtained from traditional 2D radiographs ultimately may span from a refinement of treatment to a total modification in the treatment rendered.

Based on a synthesis of current scientific evidence, case reports and other available information, the sections below and Figure 3 summarise clinical scenarios where CBCT may be beneficial and ways to use this imaging modality under specific circumstances. For several of the situations, the studies aim to test the utility of CBCT in aiding diagnosis and treatment planning (e.g. impacted teeth), while for other situations, it has primarily been applied for assessing treatment outcomes [e.g. rapid maxillary expansion (RME)] or to define normal and abnormal morphology (e.g. airway). Thus it is important to recognize that the discussion below does not imply that CBCT has proven clinical utility in all case types discussed. Moreover, since orthognathic surgery and craniofacial surgery are outside the scope of this review, these topics are not discussed. Nevertheless, it is now well accepted that CBCT in combination with computerized treatment planning and 3D printed wafers (splints) is a powerful tool for treating these cases.^35–37

Figure 3.

Clinical scenarios in which the use of CBCT may be indicated on the basis of research evidence or case- or clinical judgment-based determination of the need for imaging. All three levels of indicators require a careful consideration of the benefit-to-risk analyses prior to undertaking CBCT. Reproduced from Kapila⁵, Copyright © 2014, John Wiley and Sons. dx, diagnosis; TAD, temporary anchorage device; TMJ, temporomandibular joint; tx, treatment.

Impacted teeth

After third molars, maxillary canines are the second most commonly impacted teeth^38,39 and are probably the most common indications for CBCT imaging in orthodontics. Indeed, of the many types of clinical situations being presented to the orthodontist, impacted teeth are ones in which CBCT has been most shown to improve diagnosis and contribute to modifications in treatment planning in a significant number of subjects.^15,40–43 CBCT enhances the ability to localize impacted canines accurately, evaluate their proximity to other teeth and structures, determine the follicle size and the presence of pathology, estimate space conditions, assess resorption of adjacent teeth, assist in planning surgical access and bond placement, and aid in defining optimal direction for extrusion of these teeth into the oral cavity^43–48 (Figures 4 and 5). The findings of these studies imply that CBCT facilitates accurate 3D depiction of the entire impacted tooth relative to neighbouring structures and teeth and assists in planning their surgical access and bond placement and in defining optimal direction for extrusion of these teeth into the oral cavity. For root resorption associated with impacted teeth, CBCT scans provide substantially superior visualization of roots compared with conventional 2D radiographs by eliminating superimposition artefacts and capturing 3D root structures from all possible directions^44–49 as discussed later (Figure 5). In fact, a recent study suggests that small volume field of view CBCT may be indicated for impacted maxillary canines if the canine inclination on a conventional 2D panoramic radiograph exceeds 30° relative to a perpendicular midline, when adjacent root resorption is suspected, and/or when canine root dilaceration is suspected on conventional panoramic radiographs.⁵⁰ The detection of abnormal anatomy of the root by CBCT, including dilacerated roots—particularly, in the buccolingual direction not seen in 2D radiographs—also may help determine the amount and direction that a dilacerated tooth can be moved or aid in the decision to extract it.⁵⁰

Figure 4.

Pre-treatment images derived from CBCT of a patient with retained mandibular primary second molars and impacted second premolars. (a) Reconstructed panoramic radiograph shows a distally impacted mandibular right second premolar and mesially impacted mandibular left second premolar. The precise spatial positions of the mandibular second premolars and their relationships to neighbouring structures can be determined from axial (b, c), sagittal (d, e) and three-dimensional volumetric (f) reconstructions to develop a virtual treatment and biomechanical plan. Reproduced from Kapila and Nervina⁴⁸, Copyright © 2014, John Wiley and Sons.

Figure 5.

Utility of CBCT in diagnosis of localization of impacted teeth and identification of associated root resorption. Two-dimensional images are prone to superimposition and other limitations, which may be overcome with CBCT that can be useful in identifying the precise location of the impacted tooth, its relationship with other structures and any associated root resorption. In this case, pre-treatment panoramic (a) and periapical (b) radiographs are not adequate for precise location of the impacted tooth or discerning if root resorption truly is present. Approximately a year into treatment and failure of tooth to erupt, a CBCT scan was taken revealing the proximity of the impacted tooth to the lateral incisor and substantial root resorption on the lateral incisor as seen here in sagittal (c), axial (d), coronal (e) and lateral (f) volumetric representations. Given the lack of pre-treatment CBCT, it is not possible to determine the extent of pre-treatment root resorption or the contributions of treatment to the current root damage. However, because of the position of the bond on the tooth cingulum and despite desirable force vectors, it is likely that the cusp tip has continued to move along the root of the lateral incisor contributing to root resorption and difficulty in retrieval. Reproduced from Kapila and Nervina⁴⁸, Copyright © 2014, John Wiley and Sons.

Besides aiding in tooth localization, CBCT is also valuable in determining the optimal site for surgical access to an impacted tooth and more importantly contributes to significantly higher confidence in a clinician's diagnosis and treatment planning than does the combination of panoramic, periapical and occlusal radiographs that traditionally have been used for this purpose.^40–42 Findings from these studies also demonstrate that the original treatment plans derived from 2D radiographs are changed for >25% of the impacted teeth when orthodontists viewed these teeth in CBCT images as opposed to the 2D radiographs typically used for this purpose. Thus the scientific evidence for the utility of CBCT both in refining diagnosis and modifying treatment plans for significant numbers of impacted teeth validate its use for most impacted teeth. Overall, it can be expected that the optimal and accurate utilization of information derived from CBCT to customize treatment and biomechanics for impacted teeth should result in increased efficiency and enhanced success rates for tooth retrieval.

Supernumerary teeth

The scientific evidence on the superiority of CBCT over 2D radiography for diagnosis and treatment planning of impacted teeth could be applicable to supernumerary teeth. Supernumerary teeth are extra teeth that develop anywhere in the dentition, although they are most commonly found in the anterior maxilla and are often difficult to distinguish from normal teeth.⁴² There are two imaging goals in these cases. The first goal is to precisely localize all supernumerary teeth, many of which are unerupted or may be impacted. The second goal is to detail the morphology of the supernumerary teeth. Information derived from CBCT images of unerupted supernumerary teeth could facilitate decisions on which of the teeth to retain, determination of the retrievability of those teeth and mapping the optimal surgical access to the teeth,⁵¹ as illustrated in the example in Figure 6.

Figure 6.

CBCT offers important information and finer details in treatment planning of supernumerary teeth. (a) The panoramic view extracted from the CBCT scan shows the presence of a supernumerary tooth in the upper right lateral incisor area with delayed eruption of the maxillary right central incisor. It is difficult to discern from the panoramic view (or even from periapical radiographs, not shown) which of the two teeth, marked with an asterisk and arrowhead would be optimal morphologically to serve as the lateral incisor. Since the contralateral lateral incisor has not erupted yet, it cannot be examined clinically for size and form for comparison. (b–d) Various three-dimensional views from CBCT scans allow the comparison of the two teeth on the right lateral incisor area with the unerupted left lateral incisor. An analysis of the mesiodistal measurements of these unerupted teeth revealed that the tooth marked with an asterisk most closely matches the contralateral lateral incisor morphologically and dimensionally, while the tooth marked with an arrowhead is almost 1 mm larger mesiodistally than the contralateral lateral incisor. Reproduced from Kapila et al⁴.

Root angulation, morphology and resorption

Since root parallelism is an important goal of orthodontic treatment, its accurate determination may provide valuable information in assessing the quality of treatment outcomes and, possibly, of post-treatment stability. Root parallelism and relationships customarily are determined with panoramic radiographs that often demonstrate inaccuracies in root angulation, especially that of maxillary and mandibular anterior teeth.^52–54 By contrast, CBCT provides more accurate root angular measurements relative to those derived from 2D radiographs.^55–57 Nevertheless, given that panoramic radiographs provide adequate though not accurate information on root parallelism and the low benefit to risk of a progress CBCT, using CBCT for this purpose is not indicated.

Root resorption is the occasional and undesirable sequelae of orthodontic treatment that may compromise the longevity of teeth. Root length, form and resorption have traditionally been assessed via periapical radiographs. CBCT has been shown to be at least as good as periapical radiography for determining tooth and root length.^58,59 Furthermore, because CBCT can generate precise images of small root defects, it provides more accurate insights into root resorption and has greater sensitivity and specificity than do panoramic or other 2D radiographs in detecting these lesions.^47,60–64 Also, relative to CBCT, panoramic radiographs underestimate the presence of external apical root resorption (EARR).^65,66 Finally, while 2D radiographs only provide visualization of the apex and the mesial and distal root surfaces, CBCT imaging enables the visualization of buccal and lingual root surfaces. This has led to the discovery that root loss is not only present at the root apex but often presents as a slanting root loss on surfaces adjacent to the direction of tooth movement. This finding highlights the efficacy of the 3D rendering capacity of CBCT for accurate diagnosis of both EARR and other previously uncharacterized types of root resorption. Thus, in addition to the previously accepted diagnosis of EARR that is observed in 2D radiographs, high-resolution CBCT may usher in a new diagnostic criteria of root resorption affecting root surfaces visualizable by 3D but not by 2D radiographs.

Currently, there is no evidence that detection of moderate-to-severe EARR differs between 2D and CBCT radiography or that its discovery by CBCT during treatment would lead to a different treatment decision—typically entailing stopping the treatment at least temporarily—than if detected by 2D radiography. However, identifying buccal or lingual root resorption, which is not visualized by 2D radiography but is detectable by CBCT, could contribute to differences in pre- or in-treatment decisions. The question that remains to be answered in this scenario is how and when a clinician would decide that a patient has undergone such buccal and/or lingual root resorption to justify taking CBCT scan.

By minimizing superimposition artefacts and enabling the visualization of roots in 3D, CBCT provides superior visualization of roots compared with 2D radiographs for root resorption associated with impacted teeth^46,47,60,67 (Figure 5). This enhanced information derived from CBCT scans compared with 2D images may be critical in changing treatment plans, including the option to extract a resorbed lateral incisor rather than a premolar in an extraction case. While such treatment decisions are a logical clinical outcome arising from the utilization of CBCT, the effects of the superior information on root resorption derived from CBCT images and the threshold of root resorption at which a clinician opts to extract a tooth with a resorbed root rather than a healthy premolar remain to be determined.

Alveolar boundary conditions

Alveolar boundary conditions are the depth, height and morphology of alveolar bone relative to tooth root dimensions, angulation and spatial position.⁴ Alveolar boundary conditions are determined not only by dentoalveolar anatomy prior to treatment but also by the bone's adaptability during tooth movement and its morphology following the final positioning of teeth. Thus, in the context of orthodontic tooth movement, alveolar boundary conditions can be considered to be dynamic and dependent on the patient's pre-treatment bone and gingival biotype as well as bone physiology. Compromised or inadequate pre-treatment boundary conditions as well as limited ability to adapt to tooth movement may restrict or interfere with the planned or potential tooth movement, as well as the final desired spatial position and angulation of the teeth⁶⁸ (Figure 7).

Figure 7.

CBCT images of incisors with thin alveolar boundary condition biotype before (a–e) and after (f–j) non-extraction orthodontic treatment involving anterior expansion and flaring of the incisors. Sagittal sections along the long axis of the mandibular left lateral (a, f), left central (b, g), right central (c, h) and right lateral (d, i) incisors show considerable loss of alveolar buccal bone following treatment. Pre- and post-treatment axial slices (e, j) demonstrate protrusion of mandibular incisors at the end of orthodontic treatment. To make valid comparisons, the sections in post-treatment images were taken as close as possible to those in pre-treatment images. Reproduced from Kapila and Nervina⁶⁸, Copyright © 2014, John Wiley and Sons.

The effect of orthodontic treatment and various appliances on bone morphology and boundary conditions in three planes of space can be assessed relatively well with CBCT, although not perfectly owing to some of its technological limitations.^69–71 Although CBCT provides accurate assessment of alveolar bone height, caution must be exercised in evaluating fenestrations owing to the high number of false positives in the determination of these defects.^69,71 Despite these limitations, CBCT has been used to discern the potential effects of treatment- or patient-specific variables on the integrity and morphology of bone around tooth roots in full-fixed appliance therapy and with both rapid and slow expansion.^72–80

Routine orthodontic therapy using full-fixed appliances is accompanied by significant changes in bone width even with small amounts of buccal movement of posterior teeth.^75,80 This finding raises the question of whether biologically compatible expansion is possible using specific combinations of fixed appliances and wires as claimed by some manufacturers. A recent study tested such claims by evaluating the effects of active (In-Ovation^® R; Dentsply GAC, Islandia, NY) and passive (Damon 3 MX; Ormco Corporation, Orange, CA) self-ligating brackets on alveolar boundary conditions.⁷² Buccal cortical bone thickness on the second premolars decreased significantly with both types of appliances, even though the change in buccolingual tip of the teeth was the same with both systems. These findings, thus, do not support the claims that specific appliances generate biologically compatible forces in which the bone remodels to maintain its integrity despite substantial arch expansion during treatment. Similarly, it is known that buccal crown tipping during RME (also referred to as rapid palatal expansion) is accompanied by a concomitant decrease both in buccal bone thickness and buccal marginal bone height.⁷⁶ Finally, slow palatal expansion using quadhelix or Schwarz appliances decrease and increase, respectively, buccal and lingual bone thicknesses.^73,77–79

Bimaxillary protrusion in which orthodontic treatment aims to reduce the dentoalveloar prominence is an example of a patient plus treatment-specific variable that can contribute to compromised alveolar boundary resulting in dehiscences following incisor retraction.⁸¹ Similarly, post-orthodontic increase in incisal proclination is known to be a risk factor for dehiscences.⁸² Such incisor proclination-related recession may be worse in patients with thin initial symphysis bone width.^83,84 In general, it appears that both patient- and treatment-specific variables, such as pre-treatment boundary conditions, the magnitude of expected dental movements and the potential adaptability of the bone to remodel adequately with these movements may be important to the quality and quantity of bone retained following orthodontic treatment.

While it is clear that CBCT scans can accurately capture the dentoalveolar complex in 3D, it is best to be selective about what cases may benefit from CBCT scans for assessing boundary conditions.^85,86 These include cases presenting with clinically noticeable thin alveolar bone phenotypes that may not tolerate significant labio- or buccolingual displacements (Figure 7); cases with pre-existing periodontal disease; cases requiring orthodontic tooth movements that extend beyond pre-treatment alveolar boundaries; and cases where a tooth may need to be translocated past another tooth or obstruction (Figure 8). Precise information on alveolar boundary conditions may also be helpful in treatment planning cases that require moving teeth close to the alveolar boundaries such as in borderline non-extraction cases or in situations where teeth are being decompensated, such as commonly is performed in orthognathic surgery cases.

Figure 8.

Pre-treatment CBCT images of a patient with bilateral complete transpositions of the maxillary canines and first premolars demonstrate the possible restrictions placed by boundary conditions on treatment options. Three-dimensional volumetric reconstructions of the buccal aspect of the right (a) and left (b) sides showing the complete transpositions of the canines and first premolars and detailing the spatial positions of the transposed teeth and their relationships to each other and neighbouring structures. (c) Coronal section demonstrating the proximity of the roots of the translocated teeth to each other and to alveolar boundaries. Lines d, e and f in this panel represent locations at which axial cross-sections of the images, depicted in d, e and f, respectively, were reconstructed to visualize the relationships of the crowns and roots of the transposed teeth to each other. Axial section at mid-crown (d), cemento-enamel junction (e) and mid-root (f) of the transposed teeth demonstrate details of tooth–tooth and tooth–bone relationships. These images can be used to establish the treatment decisions on extractions if needed, and in non-extraction cases, whether to retain the transposed teeth closest to the current locations or move them into their correct locations in the arch. The images are also useful for biomechanics planning in any of the latter two treatment options that may include proactively moving tooth roots out of the path to be used to relocate the transposed tooth or root, determining if the boundary conditions will permit such movements, planning the force systems, and vector(s) of movements. These considerations taken together can help define the prognosis of moving transposed teeth or roots past each other to arrive at the optimal treatment choice. Reproduced from Kapila and Nervina⁴⁸, Copyright © 2014, John Wiley and Sons.

Quantity and quality of bone and anatomical considerations in temporary anchorage device placement

TADs are often used to provide a stable anchor for the application of orthodontic forces. Because TADs can be placed nearly anywhere in the oral cavity, ensuring that they do not impinge on important structures, such as roots or nerves, is critical for safe treatment. While there is no evidence supporting the need for CBCT to treatment plan the placement of TADs, these images can prove helpful for macroanatomical analyses through visualization of neighbouring structures such as tooth roots, sinuses and nerves that can be valuable for avoiding damage or complications. CBCT can also be useful for microanatomical evaluation of the quantity and quality of cortical bone and quality of the underlying trabecular bone that may determine primary stability of TADs, which in turn, is relevant to their secondary stability over the longer term.^87,88 Thus, CBCT could have applications where a TAD needs to be placed in sites with complex anatomical structures or relationships, or where the quality and quantity is thought to be compromised.⁸⁹ These determinations can aid in identifying optimal sites for TAD placement, thereby enhancing the chances of success. For example, it has been shown that a location 4 mm palatal to the incisive foramen provides excellent bone volume for palatal bone screws.^90,91

Quantifying cleft lip and palate defects and outcomes of alveolar bone grafts

CL/P is the most common craniofacial anomaly in humans, and it has significant impacts on affected individuals.^92,93 Typically, orthodontists first perform rapid palatal expansion on CL/P patients at about 9 years of age prior to the placement of a bone graft at the defect site. This timing of graft placement allows the alveolar graft to heal in time for the canine to erupt into the arch. Orthodontists then align the teeth, open space for implants and/or prepare the patients for orthognathic surgery, as needed. Missing or dysmorphic incisors are common at the cleft site. Therefore, radiographs of the dentition at an early age are needed to examine the number and morphology of the patient's teeth, which provides the craniofacial team time to treatment plan possible extractions, restorative dentistry, orthodontics to open or close space and implants. While 2D radiographs have been used for this purpose, CBCT may provide more precise information on the numbers, quality and location of teeth in proximity of the cleft site,⁹⁴ eruption status and path of canines in grafted cleft sites,⁹⁵ and diagnosing for implant placement.⁹⁶

At a later stage of treatment pre-alveolar graft radiographs provide the orthodontist and the surgeon with information on how much expansion and graft material will be needed to provide sufficient space for the canine to erupt. Although conventional 2D radiographs have been used for these purposes, the ability to discern the precise volume of the post-expansion defect and therefore optimally plan the surgery and amount of donor tissue needed may provide relative advantages of using CBCT over 2D radiographs. CBCT images are valuable for determining the volume of the alveolar defect and, therefore, the amount of bone needed for grafting in patients with CL/P and for determining the success of bone fill following surgery^96–98 (Figure 9). In comparing CBCT with panoramic radiographs, it has been shown that while the panoramic radiograph enables the approximation of vertical bone height of the bone bridge, it does not permit determination of the buccal–palatal width of the bone both of which can be discerned with CBCT.⁹⁹ Additionally, the CBCT images enable the visualization of the 3D morphology of the bone bridge, the relationship between the bone bridge and roots of the neighbouring teeth and their periodontal condition. Finally, CBCT can be useful for diagnosis and treatment of impacted canines that are common in patients with CL/P and their paths of eruption through grafted bone sites.^{95,96,98–102}

Figure 9.

Volume rendering of CBCT scans of an individual with a unilateral cleft lip and palate (a) before and (b) after alveolar bone grafting. With CBCT imaging, assessing the morphology, locating the position and determining the developmental stage of the unerupted maxillary left canine (arrow) permit the orthodontist and surgeon to time the placement of the alveolar graft ahead of canine eruption. Sufficient lead time allows the graft to mature and gives the orthodontist sufficient time for arch development to better support the canine as it erupts into the arch. Reproduced from Oberoi et al⁹⁸, Copyright © 2014, John Wiley and Sons.

Temporomandibular joint morphology and pathology contributing to malocclusion

Limited opening or excursive movements, joint pain and joint sounds are indicators of various TMJ pathologies, including osteoarthritis, rheumatoid arthritis, idiopathic condylar resorption and other less common TMJ disorders. The progressive radiographic changes common to most of these TMJ diseases include irregular and/or thickened cortical outlines (sclerosis), erosions, osteophyte formation, subchondral cysts, and flattening and narrowing of the joint space.^103,104 Optimizing the visualization of these changes could be useful in discerning the magnitude of degenerative changes and distinguishing the finer details of joint pathology critical for accurate diagnosis and referring the patients to the appropriate specialists prior to commencing orthodontic treatment.

As expected, CBCT images provide clinicians with more accurate anatomic detail of the TMJ than do conventional 2D panoramic radiographs.^105,106 CBCT facilitates visualization of minor to overt osseous hard tissue changes and congruency of articulating surfaces resulting from pathology and adaptive processes and allows for accurate detection and evaluation of pathological changes.^107,108 CBCT has been shown to be more efficacious than conventional tomography and MRI in detecting osseous changes.^109,110 Finally, comparison of asymptomatic control and osteoarthritic TMJs by shape correspondence also has shown significant differences between the morphologies of healthy and degenerative condyles and significant correlations between the intensity of pain and local anatomic changes in the condyle¹¹⁰ (Figure 10). While these findings suggest the potential utility of CBCT as a diagnostic aid in TMJ osteoarthritis, it is important to understand that structural bony changes of the TMJ alone do not reveal whether or not the disease is active and no direct correlation between TMJ morphological changes and clinical findings in osteoarthritis or other arthritides exists. Although CBCT alone without an accurate history and clinical findings cannot distinguish between these disorders, a recent study confirms that clinicians are more likely to change their diagnosis of TMJ disease after viewing CBCT images of symptomatic subjects.¹¹¹

Figure 10.

Statistical significance maps of correlations between local morphological differences in condylar shape and pain intensity. Significance maps show statistically significant correlations between pain intensity and morphologic differences in the superior surface of the condyle (a) and the lateral and posterior surfaces of the condyle (b). The colour scale at the bottom represents correlation p-values between pain and morphological variance in the condyle relative to an average condyle. Reproduced from Majati et al²⁶, Copyright © 2014, John Wiley and Sons. For colour image please online: www.birpublications.org/doi/abs/10.1259/dmfr/20140282.

While relatively infrequent, some patients who have TMJ pathologies, including degenerative joint disease, or developmental disorders, such as condylar hyperplasia, hypoplasia or aplasia, undergo adverse morphological and functional changes that include progressive bite changes, dental and skeletal compensations and limitation or deviation of jaw movements^{103,112–114} that contribute to unpredictable orthodontic outcomes. When these conditions occur during development, they can result in perturbed growth of the condyle on the affected joint, decrease in ipsilateral mandibular growth and contribute to compensations in the maxilla, tooth position, occlusion and cranial base.^112–115 Bilateral degenerative changes in the TMJ also may alter the facial growth pattern resulting in adverse skeletal and dental changes in the vertical, horizontal and transverse directions and contributing to mandibular retrusion, anterior open bite and Class II malocclusion.^{103,112–114} Because of the large numbers of structures involved and the inherent limitations of 2D radiography, changes resulting from these disorders are difficult to characterize accurately with conventional 2D radiography. By contrast, CBCT images by allowing the concurrent visualization of the TMJs and assessment of the maxillo-mandibular-spatial relationships and occlusion provide the opportunity to visualize and quantify the local and regional effects associated with the TMJ abnormalities.

Airway morphology, vertical malocclusion and obstructive sleep apnoea

A constricted pharyngeal airway is considered a potential contributor to vertical malocclusion in children who develop a mouth breathing habit.¹¹⁶ In addition, there is a growing interest among orthodontists in airway morphology, its relationship to OSA and the effects of orthodontic treatment on OSA.^117–119 A constricted airway, especially in children with enlarged adenoids and tonsils, is often diagnosed clinically. Conventional 2D lateral cephalographs are also sufficient to diagnose airway constriction in the sagittal plane and have been used for some of the most recent studies on airway changes following orthodontic treatment.^120,121 However, the possibility that volume or cross-sectional area may be a better measure of airway contriction has been proposed, which requires CBCT, rather than conventional images.¹²²

While CBCT is generally used to image mineralized tissues, it can also be used to accurately image the airway, which allows clinicians to measure cross-sectional area, minimum cross section and total volume of the patient's airway^123–125 (Figure 11). Initial investigations on airway patency, function and disorders utilizing CBCT have provided preliminary answers, including dimensions of normal airway anatomy in adults,^122,126 relationship of 2D to 3D measurements,¹²⁷ differences in airway morphology in subjects with OSA and non-OSA,^128–130 the effects of extractions on 3D pharyngeal volume and structure,¹³¹ and the consequences of RME^132–135 and orthognathic surgery on airway dimensions.^136,137 Several of these studies show no relationships between 2D linear dimensions and 3D cross-sectional areas of the airway, which suggests that the use of 2D data may not be valid for assessing airway patency.

Figure 11.

Three-dimensional (3D) airway visualization in the lateral (a), three-quarter (b) and frontal (c) views. Both qualitative and quantitative assessments of the airway can be made by thresholding-specific tissue density either through features built into the software program as performed here, or by customized selection of a window of density to obtain refined and accurate 3D volumetric, cross-sectional area and linear measurements of the airway. Reproduced from Kapila⁵, Copyright © 2014, John Wiley and Sons.

Early thinking in orthodontics suggested that a constricted pharyngeal airway may contribute to mouth breathing in children, which then would lead to a steep mandibular plane angle and an anterior open bite tendency.¹¹⁶ Two recent studies using CBCT imaging to test this hypothesis have generated conflicting results with one study showing no relationship between facial pattern and airway volume, while the other study demonstrated the existence of such a relationship.^122,126 The discrepancies in the findings of the two studies highlight the need to use a standardized protocol for measuring airway volumes. Perhaps consistent with this lack of agreement is the fact that there are no studies demonstrating that qualitative or quantitative assessments of CBCT images are capable of predicting OSA accurately.

CBCT imaging has been used to investigate the efficacy of RME and surgery as treatment options for a constricted airway. Since many patients with OSA have a reduced transverse airway dimension pointing to the potential contribution of this finding to airway patency, the effects of RME treatment on nasal cavity and upper pharyngeal airway dimensions has been studied in 3D.^118,132,138 RME is generally shown to increase maxillary and nasal widths but not pharyngeal airway volume. This suggests that the effects of palatal expansion do not extend far enough posteriorly to significantly improve breathing in OSA patients but may be beneficial for patients with mild respiratory problems owing to nasal constriction. Likewise, orthognathic surgery effects on pharyngeal airway space has been evaluated with CBCT images taken on Class II¹³⁹ and Class III^140,141 patients. As would be predicted, surgical correction of Class II patients results in an increase in pharyngeal airway volume, while Class III correction reduces upper airway volume. The clinical effects that increased pharyngeal airway volume from these procedures has on respiratory function have not yet been determined.

Maxillary transverse dimension and maxillary expansion

Maxillary transverse deficiency is a common cause of malocclusions that are notable for posterior crossbites and are often accompanied by crowding and/or increased overjet. Correction of these occlusal and maxillary arch anomalies with RME is indicated in growing patients to widen the maxillary transverse dimension primarily through widening the mid-palatal suture. This goal of RME treatment in these cases is to re-establish the correct posterior transverse occlusion and increase the arch length to relieve crowding through skeletal expansion and/or dental tipping.

CBCT has enabled more in-depth dissection of responses of bone and teeth to maxillary expansion than was possible through 2D radiography or study models. Studies to date on RME have primarily focused on determining treatment outcomes rather than the utility of CBCT in diagnosis and treatment planning of transverse discrepancies. Specifically, CBCT has been used to address two questions related to RME treatment, namely how expansion forces affect different regions of the maxilla and the effect of age on the relative magnitude of skeletal expansion vs dental tipping.^{74,76,142,143} These studies show that tooth-borne RME treatment in growing children result in separation of several circum-maxillary sutures contributing to an increase in not only the transverse dimension but also in the sagittal and vertical dimension.^142,143 Not surprisingly, younger children (6–8 years old) demonstrate greater skeletal expansion than older children (9–11 years old) who show greater dental tipping following RME treatment.^144,145 Treatment with four-banded maxillary expanders also reveal that although the first premolar, second premolar and first molar all have similar magnitudes of total overall expansion (which includes skeletal expansion, dental tipping and alveolar bone bending), the skeletal expansion is greater in the anterior than in the posterior maxilla.^74,76,146 These differences in findings between skeletal and overall expansion result from increasingly greater alveolar bone bending and buccal crown tip going back from the first premolar to the first molar. Additionally, there is an associated increase in nasal width and decrease in maxillary sinus width.⁷⁴ Taken together, these findings confirm that besides sutural expansion, RME produces both dental and alveolar tipping, and suggests that much of post-RME relapse may occur owing to “rebound” from the alveolar bending and dental tipping, since these two modalities of expansion are difficult to retain. These findings also imply that RME treatment has the potential for moving teeth through cortical bone particularly if it is accompanied by dental movements as in older patients, or if the roots are initially positioned too close to the alveolar boundary.¹⁴⁷ In contrast to expansion using the fixed RME appliance, a removable Schwarz appliance achieves expansion in both arches through alveolar buccal tipping.^77–79

From the perspective of utility of CBCT as a diagnostic and treatment planning tool for transverse corrections, recent attempts have been made to understand if sutural maturation could help predict the relative magnitudes of skeletal vs dental expansion expected in patients undergoing RME with inconclusive findings.¹⁴⁸ Also, early attempts have been made to quantify pre-treatment transverse dental inclinations from CBCT coronal segment reconstructions to discern the relative magnitudes of dental and skeletal movements needed to correct transverse discrepancies, which might prove valuable if optimal biomechanics can then be applied to achieve the desired plans.¹⁴⁹

Areas requiring further study

Despite promising studies and anecdotal support for the use of CBCT scans for specific clinical applications, the ultimate question to be answered is what true measurable quantitative and/or qualitative impact data from 3D scans have in modifying or enhancing diagnosis and altering or refining treatment plans when compared with decisions made with 2D radiographs. In this context, studies confirm that CBCT contributes to enhanced diagnosis, changes in treatments and greater confidence in treatment decisions by clinicians in cases with impactions, CL/P or severe skeletal malocclusions.^{40–42,95–102,150} It is likely that future research will provide evidence for efficacy of CBCT in additional clinical situations further addressing the question of when CBCT should be used, and also will help for enhanced diagnosis and detailed treatment planning of specific types of cases. For example, while CBCT scans are valuable for accurate 3D localization of impacted teeth, it is becoming increasingly clear that this information is equally valuable for optimizing the biomechanics plan to ensure that the direction of traction travels the shortest possible path while minimizing damage to adjacent teeth.⁴⁸

CBCT research is also beginning to provide critical information on potential new diagnostic categories such as alveolar bone boundary conditions and root integrity in dimensions not observed in that of 2D radiographs. Current orthodontic treatment entails moving teeth as efficiently as possible within the constraints of the bone encasing the roots without damaging the roots or adjacent structures. Determining the dynamic limits of the alveolar boundary conditions and root remodelling during treatment for each patient would be a major step towards providing individualized orthodontic treatment. This would, of course, require significant inroads in determining the genetic contributions to each patient's risk for inadequate alveolar boundary remodelling and root resorption. Nonetheless, as further scientific evidence on the limitations imposed by boundary conditions and risks for root resorption on orthodontic treatment become available, clinicians may be able to answer important treatment planning questions such as (1) can the desired tooth movement be accomplished within the existing boundary conditions without damaging the roots or adjacent structures? (2) Will boundary conditions or root morphology be affected positively or negatively by skeletal or dentoalveolar expansion or tooth retraction? (3) What are the effects of compromised bone as in periodontal disease on the ability to move teeth in the sagittal or transverse planes? (4) How are alveolar bone adaptability and risks for root resorption affected by age, periodontal health and the pre-treatment bone and anatomic phenotype of the patient? Additionally, it is possible that high-resolution CBCT scans might offer definitive diagnostic information on tooth ankylosis, which is an important question in need of investigation. Finally, CBCT imaging could help answer numerous questions within the broad thematic areas of orthognathic surgery, CL/P, craniofacial anomalies, developing asymmetries, as well as airway morphology, OSA and TMJ disorders.¹⁵⁰

Conclusions

Since its introduction into dentistry in 1998, CBCT has become an increasingly important source of 3D volumetric information in clinical orthodontics. Over this period, valuable CBCT data have been gathered on 3D craniofacial morphology in health and disease, treatment outcomes and the efficacy of CBCT in diagnosis and treatment planning. Although CBCT continues to gain popularity, its use currently is recommended in cases in which clinical examination supplemented with conventional radiography cannot supply satisfactory diagnostic information. To date, this applies to impacted teeth, CL/P and orthognathic or craniofacial surgery patients. CBCT on other types of cases can also be performed where there is likely to be a positive benefit-to-risk outcome such as supernumerary teeth, identification of root resorption caused by unerupted teeth, evaluating boundary conditions, TMJ degeneration and progressive bite changes and for placement of TADs in complex situations. Based on research evidence, orthodontists are advised to use their best clinical judgment when prescribing radiographs, including CBCT scans, to obtain the most relevant data using the least ionizing radiation possible.