Orthodontic Treatment of Impacted Teeth. Adrian Becker

or labial aspect of the root of an incisor may be impossible to determine using plane 2D radiography. As a result, an undiagnosed and severely resorbed tooth, with a poor long‐term prognosis, may be mistakenly included as an integral but ‘weak link’ in the final scheme of the dentition in a projected treatment plan [17].

      The relative accuracy of positional diagnosis using planar radiography is, therefore, inadequate in many instances. While this is so, there can be no question that a good number of cases continue to be successfully treated, despite a lack of adequate imaging documentation that would be needed to make even an approximate positional diagnosis. In some cases, no serious attempt at definitive diagnosis of the position of the impacted tooth is made until the unsuspecting and potentially unfortunate patient is on the operating table.

Computerized tomography

      It was in the late 1980s [18, 19] that the use of computerized tomography (CT) scanning was first proposed as a tool for the identification of the exact position of the palatally impacted canine, particularly when root resorption of the lateral incisor is suspected [20]. At that time, while its excellent potential was recognized for diagnosis of the position of impacted and supernumerary teeth, the large dosage of radiation that routine CT imaging required was difficult to justify for all but the most complex and exceptional cases. The previously common use of plain 2D radiography often failed to disclose the exceptional and difficult nature of the particular case – a matter that would later be abundantly clear on a CT scan.

      In the years following the first edition of this book, CT has found and established an important place in planning the treatment of impacted teeth. Accurate 3D localization of the impacted tooth is immediately available. In this way, the exact relationships between the impacted teeth and their adjacent teeth could be seen along the entire lengths of the crowns and roots of each.

      Using this modality, it has become possible to improve the overall assessment of cases in which the impaction could best be resolved with orthodontic treatment and to sufficiently separate them from those where the tooth was in an intractable position. Trial and error slowly became a practice of the past [21, 22], since it became possible to present a 3D radiographic image of what the surgical field would look like when an impacted tooth was uncovered by the oral surgeon. This helped to eliminate positional misdiagnosis and the consequent undertaking of treatment for those relatively few cases in which the position and proximity of other teeth made it impossible to arrive at a successful conclusion to the treatment.

Similarly, the axial (horizontal) and cross‐sectional (vertical) ‘slices’ as selected provided information in the bucco‐lingual plane, which had been generally impossible to discern with routine plane radiography. These views contributed materially to the evaluation of the prognosis of the intended treatment. Thus, the bucco‐lingual proximity of teeth and the existence and extent of oblique root resorption all become assessable, and these were and are important factors in determining choice of teeth for extraction or indeed whether to undertake treatment at all.

      In a study performed in 1988 [19], the prevalence of resorption of the roots of incisor teeth, as associated with an impacted canine, was investigated by plain 2D radiography and found to affect 12% of the individuals in the sample. When the same investigators repeated their study 12 years later using spiral CT scanning [23], the number of affected individuals increased to 48%! There can be little doubt that this was due to this vastly improved diagnostic tool and to the fact that resorption of the buccal or palatal aspects of the roots of the incisor teeth cannot be seen on regular radiograph. It is only when the buccal or palatal resorption has become sufficiently extensive to cause a change in the shape of the mesio‐distal profile of the root that it may be identified by plain 2D radiography and this type of resorption would go undiagnosed.

      CT offers advantages in assessing the proximity of the impacted tooth to an adjacent pathological entity. It also provides valuable assistance in evaluating aberration in the shape and appearance of the crowns and roots of teeth that were suspected of having been damaged or have suffered from abnormal development due to past trauma [24].

      Conventional spiral CT machines, as used in routine hospital practice for imaging various parts of the body, expose the body to an X‐ray beam in the form of a progressive spiral, encircling the body over a specific, defined area, with continuous radiation during the whole scanning time. This submits the patient to a high dose of ionizing radiation and has been a subject of concern when considering its use in the dental context. The dosage was evaluated by Dula et al. [25, 26] using what they called a ‘hypothetical mortality risk’. In this assessment, the mortality risk associated with routine dental radiographs ranged between 0.05 and 0.3 × 10⁻⁶ units, depending on the type and number of radiographs performed, while a CT scan of the dental area alone was assessed at 28.2 × 10⁻⁶ for the maxilla and 18.2 × 10⁻⁶ for the mandible.

Cone beam computerized tomography

      Hounsfield conceived the idea of CT in 1967 and, together with Cormack, invented the first commercially viable CT scanner in 1972. He and Cormack were later awarded the Nobel Prize for their contributions to physiology and medicine. However, the use of CT in dentistry only lasted 24 years until, in 1996, the QRsrl Company from Verona, Italy introduced CBCT with the ‘NewTom 9000’. Radiation was 90% less than for a routine medical CT. This new technology was referred to as digital volume tomography (DVT) and it has revolutionized the world of dental and maxillofacial imaging. In comparison with CT, CBCT has made imaging simpler, more accessible and cheaper. The NewTom 9000 and its successor the NewTom 3G, which was launched in 2004, employed an image intensifier connected to a charge‐coupled device (CCD) camera‐type detector. This was not new technology and other manufacturers, such as Morita (3D Accuitomo), Hitachi (CB MercuRay) and Sirona (Galileos), chose similar technology. All machines employed an image intensifier‐type detector, reconstructed to a sphere‐shaped volume.

      In the first years of the new millennium, the newer and superior flat panel detector (FPD) technology was introduced to the dental market by Imaging Sciences International, with its first CBCT machine, the iCAT, employing an amorphous silicon FPD. Within a short time thereafter, all new CBCT machines were fitted with an FPD and today there are over 20 CBCT manufacturers worldwide.

      The FPD is superior to its predecessors in all characteristics including its size and weight, but also because it prevents information loss due to the peripheral truncation from which image intensifiers suffer, with their spherically shaped volume. The FPD used in CBCT machines employs indirect conversion, in which the X‐ray energy is converted first into light energy and from there into a signal. The amplitude of the signal from each pixel in the detector is dependent on the amount of illumination indirectly converted. Indirect conversion FPDs have become standard detectors in all CBCT machines. Direct conversion technology from X‐ray energy straight to a signal is (at the time of writing this chapter) the latest expected CBCT developmental stage, which has so far only reached panoramic radiography. It produces high‐resolution quality images with better signal‐to‐noise ratios and dose efficiency.

      The detailed workings of a CBCT machine are beyond the scope of this book and will only be discussed here insofar as they relate directly to the context of impacted teeth. For a comprehensive description of the manner in which CBCT works, the reader is referred to a supplement article that appeared in the Australian Dental Journal in 2017 [27].

       Cone beam computed tomography technology

The X‐ray source, emitting a pyramidal‐shaped beam, is mounted on a gantry facing a detector. Unlike the rotation centre in a panoramic machine, which slides during its rotation, the CBCT gantry axis is fixed. The patient is positioned with the centre of the region of interest (ROI) in close proximity to the gantry's rotation axis. The gantry performs 180–360° rotation around the patient’s head, during which, in most machines, multiple exposures (150 to about 1000) are taken. The number of exposures taken depends on the protocol chosen in the specific machine, but certain machines radiate continuously during the exposure. The 2D images taken in this single rotation of the