REVIEWS
Adrian Strzecki
1, A–D, Sławomir Miechowicz
2, C, E, F, Elżbieta Pawłowska
1, A, E, F3D Diagnostics in Orthodontics and Orthognathic
Surgery – Achievements, Limitations, Expectations
Diagnostyka 3D w ortodoncji i chirurgii ortognatycznej
– osiągnięcia, ograniczenia, wyzwania
1 Department of Orthodontics, Medical University of Lodz, Łódź, Poland
2 Chair of Machine Design, Mechanical Engineering and Aeronautics Department, University of Technology,
Rzeszow University of Technology, Rzeszów, Poland
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 concern-ing 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 man-agement 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, zdo-bywa coraz większe znaczenie w ortodoncji oraz chirurgii ortognatycznej. Nowoczesne metody wizualizacji szkiele-tu twarzoczaszki, łuków zębowych oraz konszkiele-turu tkanek miękkich odpowiadają potrzebom ortodontycznej diagno-styki 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 konieczno-ścią 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
tech-nik obrazowania.
Dent. Med. Probl. 2014, 51, 2, 231–246
ISSN 1644-387X © Copyright by Wroclaw Medical University and Polish Dental Society
Recent developments concerning digital im-aging diagnostics and the further strengthening of the bonds between modern dentistry and dig-ital engineering in the course of the last decade have contributed to major changes in the
treat-ment process of orthodontic patients. Both den-tal and skeleden-tal defects should be analyzed by cli-nicians on the proper level of complexity – in all three dimensions. Traditionally, most cases are managed on the basis of plaster casts of patients’
dentition and lateral cephalograms. Such a da-taset could only be described as incomplete. Al-though 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 assess-ing 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 represent-ing details of patients’ morphology remains limit-ed. Moreover, these “conventional” methods seem to overlook the importance of soft tissue profile which also should be assessed in 3D. Re-discover-ing the role of soft tissues in the treatment out-come forced clinicians to search for alternative di-agnostic techniques. Rapid Maxillary Expansion, during which the shape of the nasal basis could change [1], can be mentioned as an important ex-ample; 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 en-gineering and material analysis [1]. Routine acqui-sition 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 ex-amine 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, neces-sary to fuse the results of at least two of them in order to create a complete composite skull mod-el that could improve diagnostics and treatment planning especially in cases requiring orthogna-thic surgery [3]. Such “virtual head” can under-go a detailed examination and countless modifica-tions 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 con-sistent 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-ing methods combined with specialized software allow for an accurate representation of both hard and soft tissues, which complements the short-comings of current diagnostic standards.
In order to review contemporary literature concerning the topic discussed, an electronic da-tabase search was conducted in several databas-es (EBSCO HOST, Scopus, Science Direct). Key words/phrases used for the article search were: “3D orthodontic diagnostics”, “Orthodontic CT”, “3D digital imaging”, “Rapid Prototyping orthodon-tics” 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 Nucle-ar 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 or-thodontists in this method is largely due to con-stantly diminishing costs and reasonably lim-ited dose of radiation. Clinical data provided by CT cannot by matched by the conventional ra-diographs used in the treatment planning process (Fig. 1–3). Apart from the above-mentioned ad-vantages, the other reasons for increasing the role of CT are the developments in personal computers technology resulting in computing power suffi-cient 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 rotat-ed and evaluatrotat-ed from any point of view (Fig. 4). Although such an approach opens many diagnos-tic possibilities it can also be considered a chal-lenge – it requires a new skills set that needs to be mastered; skills that have more in common with computer engineering than regular clinical prac-tice [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 suit-able or intuitive approach in the complete assess-ment 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 im-age resolution (with the dimensions of isotropic
Fig. 1. Panoramic picture
of a patient suffering from cleido-cranial syndrome. Numerous impacted and supernumerary teeth make assessment of anatomic con-ditions virtually impossible
Ryc. 1. Zdjęcie
pantomogra-ficzne 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
anatomicz-nych 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-dimen-sional model
Ryc. 3. Cyfrowy model 3D szczęki pacjentki
cierpiącej na dysplazję obojczykowo-czasz-kową. Złożone stosunki anatomiczne stają się łatwiejsze do analizy w zbliżonej do rzeczy-wistej formie przestrzennej
voxel ranging from 0.4 to 0.076 mm [10]) and lim-ited radiation dose [11, 12] (Fig. 5). Increasing ac-cessibility to CBCT equipment is also caused by a constantly diminishing price and its relative-ly 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 di-minishing 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 dur-ing orthodontic planndur-ing [14]. Other advantage of CBCT over MSCT is the possibility of scanning in the sitting position resulting in natural head po-sition [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
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
points are not accessible to the analysis (Fig. 6). However, the FOV (field of view) of recently in-troduced 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 ar-eas 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 pre-viously 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 ob-tain volumetric data of a scanned object [20]. In
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
Fig. 6. Comparison between field of view of MSCT (blue
background) and CBCT scanning devices (black back-ground). 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 przy-padku pole skanowania skanera CBCT wynosi 65 mm
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 an-other. 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 abbrevia-tion from Digital Imaging and Communicaabbrevia-tions in Medicine) has greatly improved the standards of communication between clinicians all over the world and allowed for easier development of spe-cialized software. Dicom files folder additionally contains the Dicomdir file that stores all the nec-essary information concerning the conditions and examination settings, calibration data and the se-quence 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 cali-brated prior to taking measurements.
Image slices obtained by means of MSCT composed in a stack can be converted into 3D dig-ital model [9]. CBCT data has a completely differ-ent characteristic – it does not comprise of 2D slic-es but the acquisition procslic-ess that usslic-es larger de-tector enables us to capture volumetric data. The most basic element of any 2D radiograph is a pix-el i.e. a very small square of a specified grey scale value. Any digital image consists of rows and col-umns of pixels. Volumetric data consists of vox-els – small cubes – the counterparts of pixvox-els 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 en-abling 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 ad-vanced processing of medical images. The first so-lution allows for a thorough examination of a pa-tient’s anatomy in any desired plane, making sim-ple 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 possi-bility of any advanced manipulation or processing of the model. Furthermore, according to scientif-ic 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 ob-taining projections similar to panoramic picture and lateral cephalogram is possible. The latter can undergo similar analysis as the regular cephalo-grams [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 diagnos-tic 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 ac-quired in the scanning process contains informa-tion describing not only the patient’s tissues that are the focal point of the clinician’s interest but al-so the air surrounding the patient’s head. With-out any selection of acquired data and straightfor-ward conversion to 3D file format, one would ob-tain a 3D model resembling a torus (Fig. 7). For this reason the process of data selection – data seg-mentation needs to be performed. Data segmenta-tion means marking the voxels that represent tis-sues important for clinical assessment while ex-cluding the voxels of no clinical significance. To improve this otherwise tedious process, voxel are marked on the basis of their greyscale values
Fig. 7. Three-dimensional digital model created
on the basis of CBCT scan without data seg-mentation. 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
(“brightness”). After setting such a threshold, on-ly the voxels which grey value level lies within the borderline values are marked and processed [9]. Similarly, voxels could be differentiated on the ba-sis 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 Houn-sfield units on the scans of first generation CBCT scanners, as the HU values of specific objects were
not constant (distilled water should always be de-scribed by 0HU and air by – 1000 HU). When as-sessing 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’ po-sition during the examination [29]. A partial so-lution to the problem of objective HU measuring was the calibrating procedure described by Lan-gravere et al. [28]. Such process had to be repeated for every scanning device and scan settings. Cur-rently, the majority of scanning devices are
rou-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
tinely calibrated and optimized for measurements in Hounsfield Units and the measurements them-selves are reliable for overall bone density evalua-tion. However, the above-mentioned algorithms of calibration require further improvement [29, 30]. The second problem encountered during segmen-tation is the relative similarity of density of neigh-boring tissues e.g. teeth roots and surrounding al-veolus. 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 mak-ing the segmentation a partially or entirely auto-mated 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 bor-ders of processed object. Software algorithms, im-age 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-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
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 nu-merous ways and thoroughly examined from any angle and in any plane. Furthermore, such a mod-el 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 possi-ble 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 re-sorption, crown inclination and root anatomy, lo-calization of impacted teeth (Fig. 10) and the vi-sualization of surrounding tissues, root resorp-tion during the course of orthodontic treatment, tongue size and position at rest, morphology of upper airways and the analysis of complex asym-metrical skeletal defects. Furthermore, it can im-prove 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 superimposi-tion of lateral cephalograms as proposed by Bjork and Skieller [33, 34] also apply to 3D models – thus patient’s growth, ageing and orthodontic defect re-lapse 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 oc-currence of artifacts due to the patient’s move-ment (breathing, swallowing) distorting the rep-resentation of soft tissues. However, in most mod-ern systems, the scanning time is limited to less than 20 seconds [14] and is similar to the exposi-tion 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 repre-sented on a CT scan [7]. However, the field of view of most modern CBCT scanners can be adjust-ed to the neadjust-eds of the clinician in any particular case [21]. Digital noise is another factor impairing the image contrast especially the soft tissue repre-sentation [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 densi-ty are also “created” due to the beam hardening
ef-fect [10]. Another problem during CBCT scanning is the partial volume effect causing the approxima-tion of voxel brightness when the size of the voxel itself is larger than the scanned structure. A simi-lar limitation also applies to the MSCT. Adverse-ly, cone beam effect resulting in a less precise visu-alization 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 examina-tion performed at the very beginning of every den-tal treatment. Currently, the exposition to radia-tion 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 de-velopment of new, less invasive devices.
Nuclear Magnetic Resonance Imaging (NMRI) is by far the less important method of routine ex-aminations of hard tissues in clinical dentistry. However, it is considered a method of choice in
Fig. 10. Digital 3D models created from CBCT scan
allow for an accurate (and easily interpretable) visual-ization 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 interpre-tacji niż w przypadku samego skanu CBCT) określenie położenia zatrzymanych zębów oraz ich relacji wzglę-dem sąsiednich struktur anatomicznych
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 proto-col, 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-inva-sive character (with only few counter-indications), excellent visualization of mandibular canal and the lack of artifacts caused by amalgam restora-tions [6]. However, in the current stage of imaging technology, NMRI imaging quality is not suffi-cient for the purposes of routine orthodontic plan-ning and orthognathic surgery [30]. As it was men-tioned in the previous paragraphs, CBCT/MSCT imaging of dentition can, in certain cases, prove problematic due to the prevalence of metallic ob-jects in the patient’s teeth. From the orthodontic point of view, brackets, wires and bands contrib-ute to even more image artifacts. In the best case scenario it makes the segmentation process very time-consuming and difficult; in the worst cas-es, it prevents the clinicians from obtaining an ac-curate morphology of the clinical crowns. There are, however, few different ways of digitizing pa-tients dentition including the CT scan of plaster casts of patients dentition [38] or dental impres-sions [39] laser surface scan of plaster cast and in-tra-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 ort-odontic planning was largely due to its feature of presenting the relation between the patient’s skele-ton and dentition. Such an approach, although suf-ficient for creating perfectly aligned dental arch-es, often resulted in worse than desired facial es-thetics [40]. The re-discovery of the role of soft tissues that emerged in the 1980s altered the fo-cal point of clinicians’ interest. Methods of quan-titative evaluation of soft tissue began to gain at-tention. Contemporary, yet currently perceived as a “traditional”, approach in visualizing facial soft tissues requires taking digital photographs up-front and from the each side. This method is sig-nificantly limited as it tries to describe 3D object by means of a 2D technique. Attempts to make this technique repeatable often failed despite im-plementing rigorous protocol [1] Even more obsta-cles were encountered by clinicians who wanted to superimpose lateral cephalogram with digital pho-tograph; achieving satisfactory accuracy was vir-tually impossible due to the number of variables
which could impair the whole task [1]. According to Sarver [41] the above-discussed process is bur-dened with countless and hardly predictable in-accuracies (the geometry of one or both pictures needs to be profoundly altered) creating superim-posed picture that cannot undergo a quantitative analysis. The importance of a thorough examina-tion of facial soft tissues is strongly emphasized by Holdoway [42]with his view suggesting that infor-mation 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 pro-file) 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 stereo-photography (stereophotogrammetry) and surface scanning with laser beam [1, 2, 43] or structured lighting [7]. The first technique is not truly 3 di-mensional, 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 ac-curate and inexpensive as it does not require so-phisticated equipment (e.g. 3D camera) but is bur-dened 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 sig-nificant cost of the device itself. Furthermore, the patient needs to remain in a horizontal position, which may affect the soft tissues’ profile and man-dible position [2]. The acquisition time is much lon-ger than in other scanning techniques, which may lead to inaccuracies caused by movement [2]. 3D ul-trasonography is limited in too many aspects to be-come a routine diagnostic tool visualizing soft tis-sues profile of the face [2]. A long examination time, lack of repeatability and the risk of tissue deforma-tion 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 ax-is aligned and identical lens focal lengths, one can obtain two images on which point of interest will have slightly different localization. These two im-ages subsequently undergo an analyzing process
that consists of searching and matching of the cor-responding points of the scanned object. Thus, the 3D surface model is created.
Among the stereophotogrammetry meth-ods, passive and active methods can be differen-tiated [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 distor-tions caused by additional light sources or ambi-ent light. 3DMD active sterophotogrammery sys-tem 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 fa-cial 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 draw-backs 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 al-so problematic [2]. The last of the previously men-tioned 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 al-so burdened with few limitations among which the lack of uniform light scatter pattern of most real-life objects is the most important.
Methods that are based on a structured light-ing scannlight-ing are even more accurate and offer improved quality. The most basic scanning set-up consists of a light beam moving on the object’s surface and the static digital camera registration of the aforementioned process. Further improve-ment of setup design is the use of laser beam which scans the object in the form of a moving strip. An-other 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 im-plementation of a parallel scanning method al-lows for real-time capturing of surface data. Laser-beam based methods were the first commercially applied techniques that enabled a 3D surface mod-el to be obtained [1] and their rmod-elevance was vali-dated clinically in many studies [48, 49]. However, they possess certain flaws which limit their appli-cation in human face scanning. The first one that needs to be mentioned is the relatively long dura-tion 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]. Un-dercut surfaces, dark-colored areas are also diffi-cult to visualize [1, 42]. A laser beam is generally considered as hazardous to eye; however, in more contemporary devices scanning process is entire-ly 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 ac-curacy 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 al-so allows for repeatable measurements [49] in both short-term (3 min) and longer (3 days) observation time. According to Kusnoto and Evans [48], the ac-curacy of laser scanning is 0.5 mm in vertical and 0.3 mm in horizontal aspect. Methods based on stereophotogrammetry are considered as equal-ly precise [52]. However, data acquisition time is shorter and the amount of data is significantly larger [53] as the colors and details of facial anato-my are also captured. It is especially important for practitioner-patient communication. Studies com-paring measurements taken on 3D models and pa-tients’ faces [54] indicate that 3D models accura-cy is sufficient for clinical purposes and the land-mark identification is not problematic. It is worth mentioning that small differences could be the re-sult of not entirely objective process which is ref-erence 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 res-olution, software measuring and calibrating al-gorithms. Properly applied and technically ad-vanced surface scanning device proves to be very useful complementation of CBCT data. As it is purely non-invasive, it enables constant monitor-ing 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 tech-nique 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].
CBCT derived 3D model is precise enough in rep-resenting skeletal structures; however, the model of dentition is burdened with artifacts and soft tis-sues representation is lacking either texture or col-or. The fusion of CBCT skeletal data with datas-et 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 sig-nificant 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 al-gorithm [60, 62–66], surface matching alal-gorithm without any markers [67] or voxel matching algo-rithm and PVS bite registration [67]. Models are aligned on the basis of not only points or surfac-es but whole corrsurfac-esponding regions. A 3D mod-el 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 simula-tion of soft tissue posisimula-tion in correlasimula-tion with un-derlying bony structures. Another expected bene-fit could be qualitative and quantitative retrospec-tive 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 Prototyp-ing techniques, such as stereolithography and 3D printing that have become more widespread re-cently, are one of the solutions, as they allow for a precise fabrication of occlusal splints reflecting any virtual set-up. Obviously, the previously men-tioned fused model of facial skeleton and soft tis-sue profile needs to be “upgraded” with the third element of the anatomic triad – dentition [2]. Su-perimposition of 2D digital photographs on 3D model from CBCT, although not a purely 3D tech-nique, can prove useful in doctor-patient commu-nication. 3D stereophotography and 2D cephalo-gram fusion is also possible [69], but its clinical relevance is rather doubtful as there is no volumet-ric data describing the facial skeleton. Adversely, the synthesis of 3D stereophotography and 3D model of dentition could prove useful as non-inva-sive method of 3 dimensional monitoring of crani-al growth [39]. To sum up this paragraph, it is im-portant to emphasize that the “fusion of choice”
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. Or-thodontists and orthognathic surgeons can finally assess the case in 3 dimensions without any sim-plifications and image distortions. Due to the facts mentioned above, 3-dimensional diagnostic meth-ods 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 addition-al time preparing a case. A fully automated pro-cess of creation and propro-cessing of 3D digital mod-els could probably vastly improve the situation. It is important to remember that digitizing a pa-tient’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 tak-ing the form of 3D head model “before” and “af-ter” 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 popular-ity of 3D diagnostics is the further development of CBCT scanners. With reduced cost and radia-tion dose and increased field of view they can vast-ly influence the role of 3D cephalometry. 3D di-agnostic possibilities in the field of orthognathic surgery and complex skeletal defects diagnostics need to be treated in a slightly different way. Com-puted 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 op-erating 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
understanding of the relation between facial skel-eton and soft tissue profile can also be possible. One can safely assume that the effects of surgery supported by 3D planning and diagnostic proce-dures is bound to be improved in comparison with
conventional methods [51, 68, 73–76]. Technology is no longer a significant problem, rather a chal-lenge; one of the biggest challenges for orthodon-tist seems to be taking advantage of treatment pos-sibilities 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: adrian.strzecki@gmail.com 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.
