Читать книгу Orthodontic Treatment of Impacted Teeth - Adrian Becker - Страница 88

The clinical aim was to find aetiological evidence for the failure to erupt of the first mandibular molar on the left side. The first impression from looking into the 3D bony view (Figure 4.20a) was that the ID canal was embraced by both distal and mesial roots (Animation 4.5). Such a condition is usually caused by some obstruction in the way of the eruption path of the tooth, as a result of which the roots continue to develop and, in this case, to embrace the canal. The obstruction can be caused by a physical blockage or by ankylosis preventing the natural eruption of the tooth. While exploring the 3D view, a resorption lesion was observed in the mesial root next to the furcation. Figure 4.20(b) shows clipping of the buccal side all the way to the furcation, leaving the volume with only the lingual side of #36 (19), thereby enhancing the lesion area view (Animation 4.6). The 3D module is a wonderful tool for gaining a general impression, but the investigation is actually carried out on the MPR screen. Figure 4.21 is a cropped MPR screen where the arrows indicate the ICRR lesion. A scroll through all three planes of the MPR screen can be viewed in Animation 4.7.

Fig. 4.20 First mandibular molar embracing inferior dental canal. (a) 3D bony view using bone peeling, sculpting and clipping. (b) 3D transparent view, buccal side clipped up to molar furcation, leaving only the lingual side for visualization. (c) Panoramic view with defining cross‐sectional grid. (d) A series of cross‐sectional slices showing the neurovascular bundle on each slice.

It is important to note that the majority of ICRR lesions originate at the cemento‐enamel junction (CEJ). When searching for aetiological evidence for an impacted tooth eruption failure, it is important to check the CEJ carefully, while rotating the tooth 360° as explained in Case 3, for an early‐stage ICRR.

With the earlier introduction of spiral CT and subsequently of CBCT, much debate was generated in relation to the justification for their use in orthodontics in general, and their value in the diagnosis and treatment planning of impacted teeth in particular. For planar radiography to provide a comparable level of positional information, a number of different views of the impacted tooth would need to be taken and the accumulated level of radiation that these would generate is of the same order as that emitted by the new CBCT machines.

The many studies that have been undertaken, for patients with an impacted tooth, to compare the advantages of using CBCT over plain 2D radiography have not provided the conclusive evidence that one would expect. It is clearly an indisputable fact that 3D imaging has, at least in theory, provided an infinite number of possible angles from which to view the tooth, as compared with a 2D radiological representation. So why was there no conclusive result? Could it be that the range and depth of CBCT post‐processing techniques that were performed were not as sophisticated as they should have been? Or perhaps only a minimum/standard orthodontic service was offered by the imaging technicians for the individuals comprising the patient samples. It is our opinion that the technicians in many of the radiological institutes in most of the Westernized countries we have visited or with whom we have had professional communication have not succeeded in mastering the complexities involved in the sophisticated interpretation of the CBCT imaging modality.

Fig. 4.21 Multi‐planar reconstruction view. Arrows indicate the invasive cervical root resorption lesion in the first molar mesial root.

The greatest advantage that the cone beam volumetric machine has over conventional CT machines is that its radiation dosage is only a fraction of that emitted by the medical machine. As shown in Table 4.1, the CBCT machine irradiates the patient at approximately 8–23% of the regular CT machine, when 8% is compared to small FOV and 23% compared to craniofacial (extra‐large) FOV.

Table 4.1 is based on typical exposure protocols and is calculated from data collated from multiple published studies. The levels of standard 2D dental radiography, CT and CBCT patients’ median effective dose are compared and are shown as equivalent to daily background radiation. An average small FOV effective dose is 50 μSv, while that of a dental panoramic is about 20 μSv. A complete mouth series done with a round collimator ranges from 100 (CCD) to 200 (photostimulable phosphor plate, PSP) μSv.

What do these figures mean to the lay public? With our responsibility as dentists to convey information in a manner understandable to those seeking our treatment and in order to obtain informed consent, it is imperative to present the issue in its context, without blinding the patient with scientific data. Thus, it may be more pertinent to use the comparison that (a) the average person receives a dose of about 8 μSv per day or 2700 μSv per year from the environment [28]; and (b) flying from New York to Tokyo by the transpolar route exposes the passenger to ionizing (cosmic) X‐rays of approximately 150 μSv and from New York to Seattle approximately 60 μSv [29].CBCT represents state‐of‐the‐art technology, with direct relevance to the determination of macroscopic anatomy and accurate positional diagnosis of impacted teeth. The machinery is not beyond the financial means of most hospitals, radiology institutes, imaging centres, dental clinics and dental school radiology departments. Its advantages to the orthodontist and surgeon are manifest. Its level of emitted ionizing radiation is low and the cost to the patient affordable. It is a recommended procedure for many of the cases discussed within the context of this book.

Table 4.1 Typical effective dose from radiographic examination.

Source: Reproduced by kind permission of Dr S.M. Mallya and Elsevier Publishers.

Examination	Median Effective Dose	Equivalent Background Exposurea
Intra‐oral b
Rectangular collimation
Posterior bite‐wings: PSP or F‐speed film	5 μSv	0.6 day
Full‐mouth: PSP or F‐speed film	40 μSv	5 days
Full‐mouth: CCD sensor (estimated)	20 μSv	2.5 days
Round collimation
Full‐mouth: D‐speed film	400 μSv	48 days
Full‐mouth: PSP or F‐speed film	200 μSv	24 days
Full‐mouth: CCD sensor (estimated)	100 μSv	12 days
Extra‐oral
Panoramicb	20 μSv	2.5 days
Cephalometricb	5 μSv	0.6 day
Chestc	100 μSv	12 days
Cone beam CTb
Small field of view (<6 cm)	50 μSv	6 days
Medium field of view (dentoalveolar, full arch)	100 μSv	12 days
Large field of view (craniofacial)	120 μSv	15 days
Multidetector CT
Maxillofacialb	650 μSv	2 months
Headc	2 mSv	8 months
Chestc	7 mSv	2 years
Abdomen and pelvis, with and without contrastc	20 mSv	7 years

^a Approximate equivalent background exposure is calculated based on an estimated background radiation dose of 3.1 mSv/year. Exposures more than the equivalent of 3 days are rounded off to the nearest day, month or year.

^b Median dose from dento‐maxillofacial radiography with typical exposure protocols is calculated from data collated from multiple published studies. Doses in the range of 10–1000 μSv are rounded off to the nearest multiple of 10.

^c American College of Radiology, https://www.acr.org/Clinical‐Resources/Radiology‐Safety/Radiation‐Safety.

CCD, charge‐coupled device; CT, computed tomography; PSP, photostimulable phosphor.

Having said that, however, there is an inherent danger with this type of comprehensive imaging. The means of presentation of the results of the CBCT scan are very attractive to the layperson and several of the animations may be undertaken to impress the orthodontic patient, who may request a copy of the ‘before and after’ portfolio as a souvenir of the orthodontic treatment and outcome. In today’s world, this can easily become part of the ‘hard sell’ and a means of attracting new patients. Consequently, the danger is that the stage may be set for the production of animations for the sake of ‘completeness’, much of which may be superfluous to the clinical needs of the patient, resulting in excess exposure of the patient to a large overdose of ionizing radiation.