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REVIEWS
Dent. Med. Probl. 2014, 51, 2, 231–246
ISSN 1644-387X
© Copyright by Wroclaw Medical University
and Polish Dental Society
Adrian Strzecki1, A–D, Sławomir Miechowicz2, C, E, F, Elżbieta Pawłowska1, A, E, F
3D Diagnostics in Orthodontics and Orthognathic
Surgery – Achievements, Limitations, Expectations
Diagnostyka 3D w ortodoncji i chirurgii ortognatycznej
– osiągnięcia, ograniczenia, wyzwania
Department of Orthodontics, Medical University of Lodz, Łódź, Poland
Chair of Machine Design, Mechanical Engineering and Aeronautics Department, University of Technology,
Rzeszow University of Technology, Rzeszów, Poland
1
2
A – research concept and design; B – collection and/or assembly of data; C – data analysis and interpretation;
D – writing the article; E – critical revision of the article; F – final approval of article
Abstract
3D diagnostic techniques based on 3-dimensional digital models of patients’ tissues are becoming an increasingly
important part of orthodontic and orthognathic treatment planning. Modern methods of visualization concerning dentofacial skeleton, soft tissues and dental arches can be considered an answer to the clinicians’ needs and
lead to the creation of 3D orthodontic diagnostics. Techniques of converting anatomic data into a 3-dimensional
model and fusing tissue models into one complete “virtual head” composite model could influence the case management improving both treatment planning and doctor-patient interaction. However, such new possibilities and
increasing amount of diagnostic data require clinicians to master new sets of skills, including operating specialized
software and complex interpretations of a vastly expanded amount of diagnostic data (Dent. Med. Probl. 2014,
51, 2, 231–246).
Key words: digital imaging, 3D orthodontic diagnostics, computed tomography, image fusion.
Streszczenie
Trójwymiarowa diagnostyka ortodontyczna wykorzystując do oceny przypadku modele 3D tkanek pacjenta, zdobywa coraz większe znaczenie w ortodoncji oraz chirurgii ortognatycznej. Nowoczesne metody wizualizacji szkieletu twarzoczaszki, łuków zębowych oraz konturu tkanek miękkich odpowiadają potrzebom ortodontycznej diagnostyki 3D, oferując możliwość łączenia poszczególnych zestawów danych w kompletną „wirtualną głowę” pacjenta.
Tak uzyskany złożony model 3D może znaleźć zastosowanie na etapie prezentacji przypadku oraz planowania
leczenia, dając klinicystom możliwość realistycznej oceny stosunków anatomicznych oraz zindywidualizowania
postępowania terapeutycznego. Zwiększająca się liczba danych z badań obrazowych wiąże się jednak z koniecznością opanowania nowych umiejętności ich właściwej interpretacji oraz obsługi specjalistycznego oprogramowania
(Dent. Med. Probl. 2014, 51, 2, 231–246).
Słowa kluczowe: cyfrowe badania obrazowe, diagnostyka ortodontyczna 3D, tomografia komputerowa, fuzja technik obrazowania.
Recent developments concerning digital imaging diagnostics and the further strengthening
of the bonds between modern dentistry and digital engineering in the course of the last decade
have contributed to major changes in the treat-
ment process of orthodontic patients. Both dental and skeletal defects should be analyzed by clinicians on the proper level of complexity – in all
three dimensions. Traditionally, most cases are
managed on the basis of plaster casts of patients’
Research was funded by scientific grant number 502-03/2-043-01/502-24-032 of Medical University of Lodz.
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dentition and lateral cephalograms. Such a dataset could only be described as incomplete. Although a plaster model provides clinicians with
anatomical details and the spatial orientation of
clinical crowns, it offers no information about
the angulation of roots. A lateral cephalogram
could prove insufficient when it comes to assessing the asymmetrical defects of a facial skeleton.
Although posterior-anterior cephalograms can to
a certain extent prove useful in the assessment of
craniofacial asymmetry, their accuracy representing details of patients’ morphology remains limited. Moreover, these “conventional” methods seem
to overlook the importance of soft tissue profile
which also should be assessed in 3D. Re-discovering the role of soft tissues in the treatment outcome forced clinicians to search for alternative diagnostic techniques. Rapid Maxillary Expansion,
during which the shape of the nasal basis could
change [1], can be mentioned as an important example; such change can hardly be observed and
measured by traditional methods of examination.
Acquisition of digital data precisely describing the
3D nature of the objects was already possible in
the 1980’s. However, it was exclusively used in engineering and material analysis [1]. Routine acquisition of data connected with the unique features
of skull anatomical relations remained a challenge
as the cost, duration and health precautions were
considered an immovable obstacle. Nowadays, it is
possible to obtain accurate 3D data of a skull with
the ease comparable to any other scanned object
[1]. In order to obtain a complete description of an
orthodontic case clinician needs to thoroughly examine a triad of anatomic structures: dentofacial
skeleton, relation of dental arches and soft tissues
profile. In this case “thorough” means a complex
3 dimensional set of anatomic data [2]. Despite the
advances in medical imaging, it is still impossible
to visualize all 3 elements of the triad by means of
a single imaging technique. It is, therefore, necessary to fuse the results of at least two of them in
order to create a complete composite skull model that could improve diagnostics and treatment
planning especially in cases requiring orthognathic surgery [3]. Such “virtual head” can undergo a detailed examination and countless modifications such as virtual osteotomies and preparation
of orthodontic setups of a case. It is important to
emphasize the fact that the “virtual head” concept
cannot be fulfilled by simple data acquisition from
different imaging techniques as the obtained data
should be truly fused into one anatomically consistent model. The aim of this article is to present
new, clinically applicable methods of diagnosing
orthodontic/orthognathic problems by means of
3-dimensional visual language. 3D-digital imag-
A. Strzecki, S. Miechowicz, E. Pawłowska
ing methods combined with specialized software
allow for an accurate representation of both hard
and soft tissues, which complements the shortcomings of current diagnostic standards.
In order to review contemporary literature
concerning the topic discussed, an electronic database search was conducted in several databases (EBSCO HOST, Scopus, Science Direct). Key
words/phrases used for the article search were: “3D
orthodontic diagnostics”, “Orthodontic CT”, “3D
digital imaging”, “Rapid Prototyping orthodontics” and “soft tissues visualization”. Search results
were limited to full-articles written in English.
Data Acquisition
– Dentofacial Skeleton and
Dental Arches
Visualization of skull hard tissues could be
achieved by means of three imaging techniques:
Multi-Slice Computed Tomography (MSCT) [4],
Cone-Beam Computed Tomography [3] and Nuclear Magnetic Resonance Imaging (NMRI) [2, 5, 6].
A computed tomography regardless of its type is
considered the method of choice in visualizing
morphology of a skull skeleton [7]. According to
Halazonetis [8] the growing interest of many orthodontists in this method is largely due to constantly diminishing costs and reasonably limited dose of radiation. Clinical data provided by
CT cannot by matched by the conventional radiographs used in the treatment planning process
(Fig. 1–3). Apart from the above-mentioned advantages, the other reasons for increasing the role
of CT are the developments in personal computers
technology resulting in computing power sufficient for the analysis of every case in 3 dimensions.
3D diagnostics are based on a 3D model rendered
from 2D layers of a scan, not the visual analysis of
the layers themselves. Such a model can be rotated and evaluated from any point of view (Fig. 4).
Although such an approach opens many diagnostic possibilities it can also be considered a challenge – it requires a new skills set that needs to be
mastered; skills that have more in common with
computer engineering than regular clinical practice [9]. Traditional 2D radiograms have taught
many clinicians to perceive a case from a “lateral”
point of view. It may not be either the most suitable or intuitive approach in the complete assessment of orthodontic defect [8]. Nowadays, Multi-Slice Computed Tomography is largely losing its
popularity in dentistry due to the introduction of
Cone-Beam CT scanners that offer improved image resolution (with the dimensions of isotropic
3D Diagnostics in Orthodontics and Orthognathic Surgery
233
Fig. 1. Panoramic picture
of a patient suffering from
cleido-cranial syndrome.
Numerous impacted and
supernumerary teeth make
assessment of anatomic conditions virtually impossible
Ryc. 1. Zdjęcie pantomograficzne pacjentki z dysplazją
obojczykowo-czaszkową.
Liczne zęby zatrzymane
i nadliczbowe czynią ocenę
warunków anatomicznych
praktycznie niemożliwą
Fig. 2. CBCT scan of a patient suffering from cleido-cranial syndrome. Case assessment and treatment planning is
possible due to the multi-planar evaluation of anatomic conditions
Ryc. 2. Skan CBCT szczęki pacjentki cierpiącej na dysplazję obojczykowo-czaszkową. Ocena warunków anatomicznych jednocześnie w trzech płaszczyznach umożliwia staranne planowanie leczenia
Fig. 3. D digital model of a patient’s maxilla.
The patient is suffering from cleido-cranial
syndrome. Complex anatomic conditions
analysis is facilitated by a realistic 3-dimensional model
Ryc. 3. Cyfrowy model 3D szczęki pacjentki
cierpiącej na dysplazję obojczykowo-czaszkową. Złożone stosunki anatomiczne stają się
łatwiejsze do analizy w zbliżonej do rzeczywistej formie przestrzennej
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A. Strzecki, S. Miechowicz, E. Pawłowska
Fig. 4. 3D digital model of maxilla and mandible presented in basic projections; model can be freely positioned
according to clinician’s wish
Ryc. 4. Cyfrowy model 3D szczęki i żuchwy przedstawiony w podstawowych projekcjach; możliwe jest ustawienie
modelu w dowolnej pozycji
voxel ranging from 0.4 to 0.076 mm [10]) and limited radiation dose [11, 12] (Fig. 5). Increasing accessibility to CBCT equipment is also caused by
a constantly diminishing price and its relatively compact size allowing it to fit in almost any
dental practice. However, the technique of object
scanning with cone shaped X-ray beam has been
known for almost 30 years [10, 13]. The radiation
dose to which the patient is exposed during head
CBCT scan is estimated at 50 microsiverts [14].
The regular MSCT head scan resulted in a dose of
radiation of 2000 microsiverts [14]. Other studies
mention that the dose of radiation during CBCT
examination is 8 to 10 times smaller than during
a MSCT examination [15]. A further decrease in
the radiation dose in CBCT can be achieved by diminishing the signal to noise ratio [16] even to the
1/60th of MSCT scan.
According to certain researchers CBCT can
replace all other radiograms routinely taken during orthodontic planning [14]. Other advantage of
CBCT over MSCT is the possibility of scanning in
the sitting position resulting in natural head position [17]. Oftentimes the scanner’s and sensor’s
field of vision is purposely limited in order to lower
the radiation dose. In such cases certain reference
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3D Diagnostics in Orthodontics and Orthognathic Surgery
Fig. 5. Comparison between 3D digital models created on the basis of MSCT (blue background) and CBCT scan
(black background). Differences in visualization accuracy in the favor of CBCT model are apparent
Ryc. 5. Porównanie cyfrowych modeli 3D powstałych na podstawie skanu MSCT (niebieskie tło) oraz CBCT (czarne
tło). Większa szczegółowość wizualizacji na podstawie skanu CBCT jest łatwo dostrzegalna
points are not accessible to the analysis (Fig. 6).
However, the FOV (field of view) of recently introduced CBCT scanning systems is significantly
increased and reaches 24 cm in the vertical aspect
being sufficient for a complete skull examination
in a single detector rotation. The most exposed areas of the skull are usually the corpus of mandible
in the molar area, cervical section of dental spine,
salivary glands [16] and thyroid gland [14]. As previously mentioned, in CBCT X-rays are grouped
in cone-shaped beam whereas in MSCT the beam
is fan-shaped [18, 19]. Such altered, 3-dimensional
beam shape results in smaller number of detector’s
turns and diminished radiation dose. Cone shaped
beam allows for data acquisition in couple planes
simultaneously and as a result it is possible to obtain volumetric data of a scanned object [20]. In
Fig. 6. Comparison between field of view of MSCT (blue
background) and CBCT scanning devices (black background). Field of view of shown CBCT scan is 65 mm
Ryc. 6. Porównanie pola skanowania skanerów MSCT
(niebieskie tło) oraz CBCT (czarne tło). W tym przypadku pole skanowania skanera CBCT wynosi 65 mm
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A. Strzecki, S. Miechowicz, E. Pawłowska
Fig. 7. Three-dimensional digital model created
on the basis of CBCT scan without data segmentation. Anatomic features are indiscernible
Ryc. 7. Trójwymiarowy model stworzony na
podstawie skanu CBCT twarzoczaszki pacjenta
bez dokonywania segmentacji danych. Zwraca
uwagę brak wyraźnych struktur anatomicznych
other words, the 3-dimensional cone shaped X-ray
beam is rotating along with the round or square
detector around the patient’s head thus capturing
all the necessary data. MSCT uses a fan-shaped
2-dimensional X-ray beam that needs to rotate
many times to acquire data one layer after another. 3-dimensional data is a result of the proper
layer sequence organization [21]. Ever since 1993,
the file format of either CT or MRI scan files is
the same [22]. DICOM file format (the abbreviation from Digital Imaging and Communications
in Medicine) has greatly improved the standards
of communication between clinicians all over the
world and allowed for easier development of specialized software. Dicom files folder additionally
contains the Dicomdir file that stores all the necessary information concerning the conditions and
examination settings, calibration data and the sequence of images – enabling the creation of the
image stack [23]. Due to the facts mentioned above
the CT or MRI Dicom files do not need to be calibrated prior to taking measurements.
Image slices obtained by means of MSCT
composed in a stack can be converted into 3D digital model [9]. CBCT data has a completely different characteristic – it does not comprise of 2D slices but the acquisition process that uses larger detector enables us to capture volumetric data. The
most basic element of any 2D radiograph is a pixel i.e. a very small square of a specified grey scale
value. Any digital image consists of rows and columns of pixels. Volumetric data consists of voxels – small cubes – the counterparts of pixels in 3
dimensions. Similarly to the pixels, the brightness
of voxels represents the tissue density. Voxels in
CBCT are of isotropic characteristic [21] i.e. they
have uniform size in all three dimension, thus enabling data assessment in any arbitrarily chosen
plane. CBCT scan can be evaluated in at least two
ways. One would be the basic view of the acquired
data in the software provided by the scanner’s pro-
ducer. The second option is to open the CBCT files
in one of specialized applications used for more advanced processing of medical images. The first solution allows for a thorough examination of a patient’s anatomy in any desired plane, making simple measurements and image brightness/contrast
adjustment. Actual 3D assessment of a patient’s
hard tissues is rather simplified as the software’s
rendering options are limited to creating the 3D
volumetric model of the skull without the possibility of any advanced manipulation or processing
of the model. Furthermore, according to scientific literature, the accuracy of such 3D models is not
suitable for making any reliable measurements [8].
However, the usefulness of a CBCT scan, even in
the case of using this software, is very high as obtaining projections similar to panoramic picture
and lateral cephalogram is possible. The latter can
undergo similar analysis as the regular cephalograms [20, 24–27]. As mentioned before, CBCT
scan can vastly improve the diagnosing process
even without the “true 3D” diagnostic approach.
“True 3D” would in this case describe the diagnostic techniques based on a 3D digital model of any
desired part of patients’ tissues. In order to create
such a model, the clinician has to combat many
technical obstacles. Firstly, the volumetric data acquired in the scanning process contains information describing not only the patient’s tissues that
are the focal point of the clinician’s interest but also the air surrounding the patient’s head. Without any selection of acquired data and straightforward conversion to 3D file format, one would obtain a 3D model resembling a torus (Fig. 7). For
this reason the process of data selection – data segmentation needs to be performed. Data segmentation means marking the voxels that represent tissues important for clinical assessment while excluding the voxels of no clinical significance. To
improve this otherwise tedious process, voxel
are marked on the basis of their greyscale values
3D Diagnostics in Orthodontics and Orthognathic Surgery
237
Fig. 8. Data segmentation process. Anatomic structures of density larger than 350 Hounsfield units marked with red
color
Ryc. 8. Proces segmentacji danych. Struktury anatomiczne o gęstości większej niż 350 jednostek Hounsfielda zaznaczone na czerwono
(“brightness”). After setting such a threshold, only the voxels which grey value level lies within the
borderline values are marked and processed [9].
Similarly, voxels could be differentiated on the basis of their Hounsfield Units values (HU values)
(Fig. 8, 9). Hounsfield scale reflects the relation
between tissues’ density and their brightness level
on MSCT and to certain extent, CBCT scans [28].
It was not possible to either assess tissues density
or perform data segmentation by means of Hounsfield units on the scans of first generation CBCT
scanners, as the HU values of specific objects were
not constant (distilled water should always be described by 0HU and air by – 1000 HU). When assessing the CBCT scan one has to bear in mind that
the density of tissues represented by HU values or
grayscale is not entirely accurate as it is affected
by the scanned object’s capacity and patients’ position during the examination [29]. A partial solution to the problem of objective HU measuring
was the calibrating procedure described by Langravere et al. [28]. Such process had to be repeated
for every scanning device and scan settings. Currently, the majority of scanning devices are rou-
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A. Strzecki, S. Miechowicz, E. Pawłowska
Fig. 9. 3D digital models created on the basis of CBCT scan. Setting proper thresholds in segmentation process
enabled creation of separate models of soft tissues, bone and teeth roots and enamel of anatomic crowns. Relation
between these structures can be visualized due to the superimposition of semi-transparent digital models
Ryc. 9. Modele 3D powstałe na skutek segmentacji danych ze skanu CBCT. Dzięki dobraniu określonych progów
segmentacji było możliwe stworzenie modeli opisujących tkanki miękkie, kość zbitą, gąbczastą i korzenie zębów oraz
szkliwo koron zębów. Wzajemny stosunek tkanek miękkich i twardych może zostać uwidoczniony przez zestawienie
ze sobą modeli oraz dostosowanie poziomu ich przezroczystości
tinely calibrated and optimized for measurements
in Hounsfield Units and the measurements themselves are reliable for overall bone density evaluation. However, the above-mentioned algorithms of
calibration require further improvement [29, 30].
The second problem encountered during segmentation is the relative similarity of density of neighboring tissues e.g. teeth roots and surrounding alveolus. In certain cases thin bone fragments have
grayscale value similar to soft tissues [8]. The most
advanced software offers a wide range of filters for
differentiating between soft and hard tissues making the segmentation a partially or entirely automated process. The final effect of segmentation
is always a result of the clinician’s knowledge of
anatomy and his or her ability to identify the borders of processed object. Software algorithms, image contrast and digital noise can also affect the
accuracy of segmentation [9]. This all contributes
to the fact that not all digital 3D models are suit-
3D Diagnostics in Orthodontics and Orthognathic Surgery
able for quantitative evaluation [31, 32]. The third
challenge encountered during segmentation is the
conversion of voxels to vector graphic 3D model.
Such a process requires a considerable amount of
computing power [8]. The result of the conversion
is a digital 3D model that can be modified in numerous ways and thoroughly examined from any
angle and in any plane. Furthermore, such a model could be exported into one of open-source file
formats such as .stl and “printed” in one of many
rapid prototyping systems [9]. The list of possible applications of 3D orthodontic diagnostics as
mentioned by Halazonetis et al. [8] could serve
as an interesting conclusion of this paragraph.
Among others, 3D digital models can be used as
a tool for assessing alveolar processes in terms of
bone quality and quantity, pathological bone resorption, crown inclination and root anatomy, localization of impacted teeth (Fig. 10) and the visualization of surrounding tissues, root resorption during the course of orthodontic treatment,
tongue size and position at rest, morphology of
upper airways and the analysis of complex asymmetrical skeletal defects. Furthermore, it can improve the planning of orthognathic and plastic
surgery procedures. What is more, the role of 3D
cephalometry is bound to increase within the next
few years. For example, the rules of superimposition of lateral cephalograms as proposed by Bjork
and Skieller [33, 34] also apply to 3D models – thus
patient’s growth, ageing and orthodontic defect relapse could be monitored in 3 dimensions [9].
CBCT/MSCT imaging has certain limitations
that also need to be discussed. Scanning time,
which depending on the settings, can last from
few to even 70 s [7]; scanning can result in the occurrence of artifacts due to the patient’s movement (breathing, swallowing) distorting the representation of soft tissues. However, in most modern systems, the scanning time is limited to less
than 20 seconds [14] and is similar to the exposition time of a panoramic picture. The other factor
affecting image quality is the deformation of facial
tissues caused by head positioners. Oftentimes, the
anatomical structures such as the tip of the nose
and the chin are not entirely or properly represented on a CT scan [7]. However, the field of view
of most modern CBCT scanners can be adjusted to the needs of the clinician in any particular
case [21]. Digital noise is another factor impairing
the image contrast especially the soft tissue representation [20]. Artifacts present on the scan may be
the result of the beam hardening effect responsible
for creating image distortions of amalgam fillings,
brackets or implants visualization. Black stripes
visible between objects of high radiological density are also “created” due to the beam hardening ef-
239
Fig. 10. Digital 3D models created from CBCT scan
allow for an accurate (and easily interpretable) visualization of the impacted teeth position in relation with
the surrounding anatomic structures
Ryc. 10. Cyfrowe modele 3D otrzymane ze skanu
CBCT pozwalają na dokładne (i łatwiejsze w interpretacji niż w przypadku samego skanu CBCT) określenie
położenia zatrzymanych zębów oraz ich relacji względem sąsiednich struktur anatomicznych
fect [10]. Another problem during CBCT scanning
is the partial volume effect causing the approximation of voxel brightness when the size of the voxel
itself is larger than the scanned structure. A similar limitation also applies to the MSCT. Adversely, cone beam effect resulting in a less precise visualization of the structures scanned with the most
marginal X-rays of X-ray beam is a phenomenon
encountered only in CBCT [10]. It is probable that
CBCT scan may become the routinely performed
part of orthodontic planning [6]. What is more, it
may replace the panoramic X-ray as the examination performed at the very beginning of every dental treatment. Currently, the exposition to radiation from a panoramic radiograph is 3 to 7 times
smaller than during CBCT scan [35, 36]. This ratio
is probably going to change with the constant development of new, less invasive devices.
Nuclear Magnetic Resonance Imaging (NMRI)
is by far the less important method of routine examinations of hard tissues in clinical dentistry.
However, it is considered a method of choice in
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TMJ and masticatory muscles assessment [6, 37].
Numerous in vitro studies have also attempted to
visualize hard tissues by means of NMRI [38, 39].
Due to the application of a specific imaging protocol, it was possible to limit the exposition time to
6 and a half minutes with the voxel size of 0.7 mm.
The obvious advantage of NMRI is its non-invasive character (with only few counter-indications),
excellent visualization of mandibular canal and
the lack of artifacts caused by amalgam restorations [6]. However, in the current stage of imaging
technology, NMRI imaging quality is not sufficient for the purposes of routine orthodontic planning and orthognathic surgery [30]. As it was mentioned in the previous paragraphs, CBCT/MSCT
imaging of dentition can, in certain cases, prove
problematic due to the prevalence of metallic objects in the patient’s teeth. From the orthodontic
point of view, brackets, wires and bands contribute to even more image artifacts. In the best case
scenario it makes the segmentation process very
time-consuming and difficult; in the worst cases, it prevents the clinicians from obtaining an accurate morphology of the clinical crowns. There
are, however, few different ways of digitizing patients dentition including the CT scan of plaster
casts of patients dentition [38] or dental impressions [39] laser surface scan of plaster cast and intra-oral scanning by means of laser or light related
with the technique of digital impression.
Data Acquisition
– Soft Tissues Profile
The central role of lateral cephalogram in ortodontic planning was largely due to its feature of
presenting the relation between the patient’s skeleton and dentition. Such an approach, although sufficient for creating perfectly aligned dental arches, often resulted in worse than desired facial esthetics [40]. The re-discovery of the role of soft
tissues that emerged in the 1980s altered the focal point of clinicians’ interest. Methods of quantitative evaluation of soft tissue began to gain attention. Contemporary, yet currently perceived as
a “traditional”, approach in visualizing facial soft
tissues requires taking digital photographs upfront and from the each side. This method is significantly limited as it tries to describe 3D object
by means of a 2D technique. Attempts to make
this technique repeatable often failed despite implementing rigorous protocol [1] Even more obstacles were encountered by clinicians who wanted to
superimpose lateral cephalogram with digital photograph; achieving satisfactory accuracy was virtually impossible due to the number of variables
A. Strzecki, S. Miechowicz, E. Pawłowska
which could impair the whole task [1]. According
to Sarver [41] the above-discussed process is burdened with countless and hardly predictable inaccuracies (the geometry of one or both pictures
needs to be profoundly altered) creating superimposed picture that cannot undergo a quantitative
analysis. The importance of a thorough examination of facial soft tissues is strongly emphasized by
Holdoway [42] with his view suggesting that information gathered on the basis of patients soft tissues
covering the bony structures are more important
than the evaluation of the patient’s facial skeleton.
The surface of a patient’s face (soft tissue profile) can be visualized in 3 dimensions by means
of 5 methods: 2D digital photograph imposed on
a 3D model acquired in other technique, NMRI,
3D ultrasonography and most importantly stereophotography (stereophotogrammetry) and surface
scanning with laser beam [1, 2, 43] or structured
lighting [7]. The first technique is not truly 3 dimensional, as it requires taking from 2 to 6 digital
photographs (projections: upfront, from the each
side, from under the chin, from each side with the
45 degree angle). In the second step, pictures are
used as textures imposed on 3D digital model from
CBCT scan [44, 45]. This method is considered accurate and inexpensive as it does not require sophisticated equipment (e.g. 3D camera) but is burdened with inevitable image distortions. Nuclear
Magnetic Resonance Imaging despite its previously
discussed advantages such as lack of radiation and
ability to precisely visualize all of skull’s soft tissues
cannot be labeled a perfect diagnostic tool in this
aspect. Accessibility to NMRI scanning procedures
still remains limited due to the size and very significant cost of the device itself. Furthermore, the
patient needs to remain in a horizontal position,
which may affect the soft tissues’ profile and mandible position [2]. The acquisition time is much longer than in other scanning techniques, which may
lead to inaccuracies caused by movement [2]. 3D ultrasonography is limited in too many aspects to become a routine diagnostic tool visualizing soft tissues profile of the face [2]. A long examination time,
lack of repeatability and the risk of tissue deformation while examining them with USG probe, are
the main drawbacks of the described method [46].
3D stereophotogrammetry is a spatial analysis of
an object based on the principles similar to those of
human vision i.e. the parallax effect. With the use
of two (or more; usually the number varies from
2 to 6) registering devices positioned in a known
distance from each other that have their optical axis aligned and identical lens focal lengths, one can
obtain two images on which point of interest will
have slightly different localization. These two images subsequently undergo an analyzing process
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3D Diagnostics in Orthodontics and Orthognathic Surgery
that consists of searching and matching of the corresponding points of the scanned object. Thus, the
3D surface model is created.
Among the stereophotogrammetry methods, passive and active methods can be differentiated [1]. Passive methods use the subtle details of
skin anatomy such as freckles, scars, skin pores etc.
as reference points. Active stereophotogrammetry
uses the combination of facial anatomic reference
points with the specific unstructured lighting cast
on the patients face. Active stereophotogrammetry
systems take control over the lighting of the scene.
Due to this fact, they are not vulnerable to distortions caused by additional light sources or ambient light. 3DMD active sterophotogrammery system consists of a 3D camera that takes photographs
from all necessary projections within 2 ms [1, 7].
The only requirement for the patient is to make facial expressions according to the scanning protocol.
The final result of such “photogrammetric” scan
is a very realistic, esthetic and accurate 3D digital
surface model of a patient’s head [47]. Apart from
many advantages discussed above, certain drawbacks need to be mentioned: majority of scanning
systems require daily calibrations, visualization of
hair, shining surfaces such as teeth and undercut
surfaces of subnasal and submental regions is also problematic [2]. The last of the previously mentioned phenomena enabling the reconstruction of
a 3D contour of facial soft tissues is the light scatter
pattern on the object’s surface. This method is also burdened with few limitations among which the
lack of uniform light scatter pattern of most reallife objects is the most important.
Methods that are based on a structured lighting scanning are even more accurate and offer
improved quality. The most basic scanning setup consists of a light beam moving on the object’s
surface and the static digital camera registration
of the aforementioned process. Further improvement of setup design is the use of laser beam which
scans the object in the form of a moving strip. Another solution is to cast on the whole surface of
the object a “net” of parallel and perpendicular
light stripes and points of various colors. The implementation of a parallel scanning method allows for real-time capturing of surface data. Laserbeam based methods were the first commercially
applied techniques that enabled a 3D surface model to be obtained [1] and their relevance was validated clinically in many studies [48, 49]. However,
they possess certain flaws which limit their application in human face scanning. The first one that
needs to be mentioned is the relatively long duration of capturing data (with mean scanning time
of 20 seconds; in modern devices it was limited to
less than 10 seconds; any motion during the expo-
sition time results in image distortion) [1, 50]. Undercut surfaces, dark-colored areas are also difficult to visualize [1, 42]. A laser beam is generally
considered as hazardous to eye; however, in more
contemporary devices scanning process is entirely safe for human vision [49]. Laser scanning can
also be disturbed by additional light sources and
the presence of metallic surfaces [51]. In order to
obtain information concerning the object’s color,
additional laser beam needs to be used. The accuracy of laser scanning reaches approximately
0.5 mm [43, 50] and is sufficient for the purpose
of scanning a patient’s face [44]. This method also allows for repeatable measurements [49] in both
short-term (3 min) and longer (3 days) observation
time. According to Kusnoto and Evans [48], the accuracy of laser scanning is 0.5 mm in vertical and
0.3 mm in horizontal aspect. Methods based on
stereophotogrammetry are considered as equally precise [52]. However, data acquisition time is
shorter and the amount of data is significantly
larger [53] as the colors and details of facial anatomy are also captured. It is especially important for
practitioner-patient communication. Studies comparing measurements taken on 3D models and patients’ faces [54] indicate that 3D models accuracy is sufficient for clinical purposes and the landmark identification is not problematic. It is worth
mentioning that small differences could be the result of not entirely objective process which is reference points identification [52, 55]. Furthermore,
measurements of longer sections prove to be less
repeatable; on the other hand, very short sections
are more prone to the fault due to the matrix resolution, software measuring and calibrating algorithms. Properly applied and technically advanced surface scanning device proves to be very
useful complementation of CBCT data. As it is
purely non-invasive, it enables constant monitoring of treatment process and, if needed, treatment
plan modification at clinician’s will [48, 56, 57].
Not to mention the fact that 3D digital model
greatly improves dentist – patients’ interaction as
it enables thorough and esthetically pleasing case
presentation. Other implementation of 3D surface
scanning is a non-invasive creation of databases of
facial profiles specific for a given population [58].
Fusion of the Anatomical
Triad 3D Models
As discussed above, there is no imaging technique enabling the accurate visualization all three
elements of anatomical triad at the same time. In
order to create a “virtual patient” dataset at least
two imaging techniques need to be combined [2].
242
CBCT derived 3D model is precise enough in representing skeletal structures; however, the model
of dentition is burdened with artifacts and soft tissues representation is lacking either texture or color. The fusion of CBCT skeletal data with dataset created by CT scanning of plaster cast models
of dentition is considered a method of choice in
hard tissue 3-dimensional visualization [59–62].
This process is time consuming and requires significant computing power but the final result is
accurate enough for the purposes of othodontic/
orthognathic diagnostics [2]. The fusion process
requires a certain commentary as few methods
are presently used. To match model of dentition
with model of the skull one can use occlusal splint
with radiopaque markers and point-matching algorithm [60, 62–66], surface matching algorithm
without any markers [67] or voxel matching algorithm and PVS bite registration [67]. Models are
aligned on the basis of not only points or surfaces but whole corresponding regions. A 3D model of a skull from CBCT is most favorably fused
with a 3D digital stereophotographic model [68].
According to the literature, such a procedure is
considered “golden standard” of modern facial
skeleton 3D assessment [2]. It is possible that such
a fused model could become in the future more
routinely used in orthodontic and orthognathic
procedure planning as it offers a unique simulation of soft tissue position in correlation with underlying bony structures. Another expected benefit could be qualitative and quantitative retrospective analysis of changes in soft tissue profile after
surgery and correlating such findings with the type
and range of the procedure. However, transferring
the results from a virtual operating room to a real
one still remains a challenge [2]. Rapid Prototyping techniques, such as stereolithography and 3D
printing that have become more widespread recently, are one of the solutions, as they allow for
a precise fabrication of occlusal splints reflecting
any virtual set-up. Obviously, the previously mentioned fused model of facial skeleton and soft tissue profile needs to be “upgraded” with the third
element of the anatomic triad – dentition [2]. Superimposition of 2D digital photographs on 3D
model from CBCT, although not a purely 3D technique, can prove useful in doctor-patient communication. 3D stereophotography and 2D cephalogram fusion is also possible [69], but its clinical
relevance is rather doubtful as there is no volumetric data describing the facial skeleton. Adversely,
the synthesis of 3D stereophotography and 3D
model of dentition could prove useful as non-invasive method of 3 dimensional monitoring of cranial growth [39]. To sum up this paragraph, it is important to emphasize that the “fusion of choice”
A. Strzecki, S. Miechowicz, E. Pawłowska
of the anatomic triad would be a synthesis of the
skeletal component model from CBCT, soft tissue
profile as represented by 3D stereophotography
and 3D dentition model created by means of CT
scan of plaster models/PVS impressions or various
“digital impression” intra-oral scanners [2].
Summary
Contemporary 3D diagnostics appear to be the
answer to both patients’ and clinicians’ needs. Orthodontists and orthognathic surgeons can finally
assess the case in 3 dimensions without any simplifications and image distortions. Due to the facts
mentioned above, 3-dimensional diagnostic methods are used in more complex cases and are not
a part of a clinical routine. Obtaining either a 3D
model of a skull or a fusion of 3D anatomic models
is, despite constantly increasing computing power
of personal computers and user-friendly interface
of software, a time-consuming, arduous task that
requires the clinician to spend a lot of additional time preparing a case. A fully automated process of creation and processing of 3D digital models could probably vastly improve the situation.
It is important to remember that digitizing a patient’s anatomic data could enhance the treatment
process by establishing good communication with
a patient and the possibility of consulting the case
with clinicians worldwide. A treatment plan taking the form of 3D head model “before” and “after” treatment could set patients expectations on
a proper, realistic level and positively influence his
or her cooperation. Obviously, the clinician needs
to strongly emphasize the orientation character of
such models. The focal point of modern dentistry
seems to shift towards patient’s individual needs
and 3D diagnostics seem to follow that trend. One
of the factors that can contribute to the popularity of 3D diagnostics is the further development
of CBCT scanners. With reduced cost and radiation dose and increased field of view they can vastly influence the role of 3D cephalometry. 3D diagnostic possibilities in the field of orthognathic
surgery and complex skeletal defects diagnostics
need to be treated in a slightly different way. Computed Tomography has been considered a method
of choice for decades and further achievements in
digital imaging seem to perfectly fit the clinicians
expectations. Simulation of surgery in virtual operating room, assessment of virtual final outcome
in terms of esthetic and occlusal function have
long been postulated [68, 70]. Such pre-clinical
data could contribute to creating new procedures
and improve the efficacy and predictability of the
currently used surgery protocols [71]. An in-depth
3D Diagnostics in Orthodontics and Orthognathic Surgery
understanding of the relation between facial skeleton and soft tissue profile can also be possible.
One can safely assume that the effects of surgery
supported by 3D planning and diagnostic procedures is bound to be improved in comparison with
243
conventional methods [51, 68, 73–76]. Technology
is no longer a significant problem, rather a challenge; one of the biggest challenges for orthodontist seems to be taking advantage of treatment possibilities offered by 3D diagnostics.
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Address for correspondence:
Adrian Strzecki
Department of Orthodontics
Medical University of Lodz
Pomorska Street 251
92-216 Łódź
Poland
Tel.: 42 675 7516
E-mail: [email protected]
Conflict of interest: None declared
Received: 7.01.2014
Revised: 25.02.2014
Accepted: 11.04.2014
Praca wpłynęła do Redakcji: 7.01.2014 r.
Po recenzji: 25.02.2014 r.
Zaakceptowano do druku: 11.04.2014 r.