Soft- and hard-tissue facial anthropometry in three dimensions

26 Pages·2013·1.87 MB·English

JASs Invited Reviews
doi 10.4436/JASS.91007
Journal of Anthropological Sciences
Vol. 91 (2013), pp. 159-184
Soft- and hard-tissue facial anthropometry
in three dimensions: what’s new
Chiarella Sforza1, Marcio de Menezes2* & Virgilio F. Ferrario1
1) Functional Anatomy Research Center (FARC), Laboratorio di Anatomia Funzionale dell’Apparato
Stomatognatico (LAFAS), Dipartimento di Scienze Biomediche per la Salute, Facoltà di Medicina e
Chirurgia, Università degli Studi di Milano, Italy
e-mail: [email protected]
2) Department of Preventive and Social Dentistry, Universidade Federal do Rio Grande do Sul, Brazil.
School of Health Science, State University of Amazonas, Brazil
Summary - In the last few years, technology has provided new instruments for the three-dimensional
analysis of human facial morphology. Currently, quantitative assessments of dimensions, spatial positions
and relative proportions of distinctive facial features can be obtained for both soft- and hard- (skeletal and
dental) tissues. New mathematical tools allow to fuse digital data obtained from various image analyzers, thus
providing quantitative information for anatomical and anthropometric descriptions, medical evaluations
(clinical genetics, orthodontics, maxillo-facial and plastic surgery), and forensic medicine.
Keywords - Human face, Morphometrics, 3D analysis.
Introduction Additionally, the presence of a large number of
facial (subcutaneous) muscles makes facial appear-
The quantitative analysis of the human face ance instantaneously variable and dynamic, even
has always received a large attention from both producing problems for its correct representation
scientists and artists: the face allows to com- and measurement (Kovacs et al., 2006; Ferrario &
municate and interact with the environment, it Sforza, 2007; Maal et al., 2008, 2010; Schimmel
is used to identify the persons, and it can carry et al., 2010; Smeets et al., 2010; Sforza et al.,
information about the health state of an indi- 2010a, 2010d, 2011b, 2012d; Trotman 2011;
vidual (Hennessy et al., 2005; Tollefson & Sykes, Verzè et al., 2011b; Lubbers et al., 2012).
2007; Kochel et al., 2010; Sforza & Ferrario, In a previous review, we analyzed the state of
2010; Smeets et al., 2010; Mutsvangwa et al., the art for the assessment of the soft-tissue facial
2010, 2011; Fang et al., 2011; Ritz-Timme et structures of human beings in all three spatial
al., 2011; Verzè et al., 2011b). dimensions. Information about the instruments
This unique morphology is made from sepa- to be used for data collection, on the analytical
rate cartilaginous, osseous, dental and soft-tissue methods for data analysis, as well as on the main
elements, where their coordinated pattern of interdisciplinary applications, were provided
growth, development and aging produces a never (Sforza & Ferrario, 2006).
static outline that can be modeled and varied In the subsequent years, new applications of
by the combined action of internal (genetic and the instruments for data collections have been
epigenetic) and external (environmental) fac- proposed, together with new mathematical tools
tors (Breitsprecher et al., 1999; Hammond et al., that allow fusing the digital data obtained from
2008; Smeets et al., 2010; Aldridge et al., 2011; various image analyzers. A web search using the
Baynam et al., 2011; Hammond & Suttie 2012). key-words “3D, face, human” retrieved 11029
the JASs is published by the Istituto Italiano di Antropologia www.isita-org.com
160 3D facial anthropometry
Tab. 1 - Papers with the key words “3D”, “face”, et al., 2009; Ji et al., 2010; Fourie et al., 2011b;
“human” published between 1950 and 2012 Papagrigorakis et al., 2011; Wang et al., 2011;
divided into decades (research performed on
Aboul-Hosn Centenero & Hernandez-Alfaro,
May, 16th 2012).
2012; Bechtold et al., 2012; Hammond & Suttie,
2012; Lee et al., 2012). These volumetric scan-
yEARS nO. OF PAPERS
ners can image both the internal body structures
and the external cutaneous covering, allowing a
1950-1959 2
complete assessment of facial morphology. Other
1960-1969 1
scanners (namely, laser scanners and stereopho-
1970-1979 8 togrammetric systems) can record and reproduce
only the external body surface, permitting 3D
1980-1989 10
measurements of the external (soft tissues, in the
1990-1999 930 living persons) structures (Gwilliam et al., 2006;
Heike et al., 2010; Friess, 2012).
2000-2009 7844
To overcome problems related to facial illu-
2010-2012 2234
mination, near infrared light can be used to scan
facial surface (Li et al., 2007). A new kind of
instruments are those using the terahertz radiation
full text papers published between 1950 and of the electromagnetic field. These scanners can
2012 (http://search.proquest.com/, accessed on image several millimeters of tissue with low water
May, 16th 2012) (Tab. 1). Among these papers, content (e.g., fatty tissue), detecting differences in
10078 were published in the current Century, tissue density. Another promising field of applica-
about 3580 before our previous review (2000- tion is the 3D imaging of teeth (Jalil et al., 2012).
2005), and about 6498 after it (2006-currently).
Investigations and relevant literature on this Computerized tomography
topic are therefore increasing very fast, and a CT provides 3D digital reconstruction of
revision of the most recent instruments, findings the entire craniofacial skeleton from axial slices
and fields of application seems necessary (Fig. 1). allowing to evaluate all internal structures. CT
In the current review, some information can be efficiently used also to assess, archive and
about the new trends in soft- and hard-tissue measure archaeological specimens (Badawi-Fayad
facial analysis are provided, along with their & Cabanis, 2007; Papagrigorakis et al., 2011;
principal fields of anatomical, anthropometric, Kullmer, 2008; Friess, 2012). Additionally, CT
medical and dental application. data can be shared among research laboratories
all over the world, permitting a widespread use
of archaeological collections without moving the
The instruments and their use investigators or the specimens (Abel et al., 2011).
Both MR and CT can be used for special
Three-dimensional (3D) images are becom- medical applications: virtual endoscopy, surgical
ing a daily reality in several clinical and research planning and medical training. Virtual endos-
contexts all over the world. Currently, two image copy uses the clinical data, primarily CT, visual-
analyzers can provide combined 3D reconstruc- ized in real-time for on-screen simulation of the
tions of the soft tissue structures together with interior of viscera (eg, virtual bronchoscopy and
the craniofacial skeleton: computed tomography colonoscopy) and vessels (virtual angioscopy),
(CT) and magnetic resonance (MR) imaging helping in diagnosis and surgical planning. 3-D
(Adams et al., 2004; Papadopoulos et al., 2002; visualization can provide simulation of complex
Hajeer et al., 2004; Katsumata et al., 2005; Maal surgical procedures, such as organ transplanta-
et al., 2008; Keller & Roberts, 2009; Swennen tion (McGhee, 2010).
C. Sforza et al. 161
Fig. 1 - Papers with the key words “3D”, “face”, “human” published between 2000 and 2012 (research
performed on May, 16th 2012). The colour version of this figure is available at the JASs website.
Additionally, research can make use of CT brackets. The fusion of dental surface images
archival images, selected from the existing data- obtained from a 3D measuring device into
bases in health care units. Indeed, CT scans are maxillofacial CT images has been proposed to
usually made to patients for traumas, fractures overcome this problem (Nakasima et al., 2005;
or neoplasias, but the databases can be screened Bechtold et al., 2012).
according to well defined inclusion criteria, To reduce patient’s exposure to X-rays, the
selecting only normal individuals. A similar pro- European Academy of DentoMaxilloFacial
cedure was followed by Wang et al. (2011) who Radiology (EADMFR) recognized an urgent
assessed the 3D quantitative morphology of the need to set standards for CBCT use, and devel-
external ear in normal Han Chinese adults. oped a set of “Basic Principles” using existing EU
However, CT has some limitations: apart Directives and Guidelines on Radiation Protection
from cost, the devices expose patients to high (www.sedentexct.eu/content/basic-principles
amounts of unnecessary radiation. The most -use-dental-cone-beam-ct, accessed on August,
recent modifications of CT, namely the conical 10th 2012). These statements recommend that
x-ray approach or cone beam CT (CBCT), now CBCT should not be repeated ‘routinely’ on a
can offer affordable 3D craniofacial reconstruc- patient without a new risk/benefit assessment
tions, with a reduced radiation exposure (Adams having been performed. CBCT examinations
et al., 2004; Hwang et al., 2012). must be justified for each patient to demonstrate
CBCT systems have been developed specifi- that the benefits outweigh the risks.
cally for the maxillofacial region, and their field Considering these radioprotection norms,
of view allows an efficient imaging of the skull Nakasima et al. (2005) proposed to create a stand-
including most of the landmarks used in cepha- ard skeletal and facial model from CT images of
lometric analysis, together with a 3D volumetric subjects of a well-defined ethnic group, and to
rendering of the external facial surface (Maal et al., obtain individual models by fitting the stand-
2008; Moro et al., 2009; Swennen et al., 2009; ard model to each patient by using his or her
Fourie et al., 2011a; Bechtold et al., 2012) (Fig. 2). cephalograms and facial photographs. 3D digital
CT craniofacial scans do not allow determin- dental models can be fused with the individual
ing dental morphology accurately because of arti- model, thus obtaining a complete 3D image
facts from metallic restorations or orthodontic with a low biological price. Although interesting,
www.isita-org.com
162 3D facial anthropometry
Fig. 2 - Three-dimensional reconstruction of craniofacial hard and soft tissues in a 24 years old
woman. The images were obtained with cone beam computerized tomography (White Fox, De
Goetzen, Olgiate Olona, Varese, Italy; X-ray tube voltage 105 KV, X-ray tube current 8 mA). A cepha-
lometric, expanded field of view was used (diameter 200 mm, height 170 mm). A: soft tissue recon-
struction. A notable facial asymmetry can be observed (the left labial commissura is more cranial
than the right one, the right nasal ala is bigger than the left one), together with alterations in the
right orbital area. B: hard tissue reconstruction. The occlusal plane is asymmetric, and the right
orbital cavity altered. The colour version of this figure is available at the JASs website.
the method would require a set of reference CT persons (See et al., 2007, 2008). Ferrario et al.
scans selected for sex and ethnicity, posing ethi- (2009) introduced a method that fused the 3D
cal problems for the individuation of the normal stone models of the teeth and of the lips, obtain-
individuals to be scanned. Also, the deformations ing 3D virtual reproductions of both mucosal
required to modify the 3D scan according to the and skin labial surfaces. Labial thickness, ver-
2D cephalograms change the skeletal structures milion area, volume of the upper and lower lips,
with an isotropic model that probably does not and relevant dental positions were measured
actually represent the true shape variations. from the digital reconstructions, thus including
a complete assessment of the anatomical region
Magnetic resonance and of its sex- and age-related characteristics (De
Among the other applications, magnetic res- Menezes et al., 2011; Rosati et al., 2012a).
onance imaging has recently been used for the
3D assessment of labial dimensions in healthy Stereophotogrammetry
subjects. In particular, lip thickness was meas- Stereophotogrammetry is safe, non-invasive,
ured, with a good accord between 3D age-related fast (typical scan time 2 ms), does not require
changes and classic histological findings (Iblher a physical contact between the instrument and
et al., 2008; Penna et al., 2009). Unfortunately, the face, and it provides superior quality ‘exter-
the method could not efficaciously record the nal surface’ photographs, coupling a color facial
anatomy of the underlying supporting hard tis- image (texture) with a 3D mesh of the ana-
sues, impeding the assessment of the soft- and lyzed surface. In stereophotogrammetry a light
hard-tissues relationships. Also, MR should be source illuminates the face, and two or more
performed in a supine position, with a significant coordinated cameras (or set of cameras) record
alteration in the normal relationships between the images from different points of view (Fig.
the facial soft tissues, especially in the aged 3). The different views/ images of the face are
C. Sforza et al. 163
merged into a 3D point cloud to represent the
surface of the subject’s face. Using a previous cal-
ibration of the instrument, a computerized ste-
reoscopic reconstruction of the face is finally pro-
duced (Hammond et al., 2008; de Menezes et al.,
2010; Heike et al., 2010; Schimmel et al., 2010;
Friess, 2012). Two additional three-quarter color
pictures are mapped onto the mesh formed by
the point cloud to reproduce facial appearance.
The systems can be divided into passive,
where the cameras record the black and white
(finer resolution) and the color (lower resolu-
tion) images of the face that are combined to Fig. 3 - Scheme of a stereophotogrammetric
give a final 3D mesh covered by a color texture, device for the analysis of facial soft tissues. Two
or active. In these last instruments, the face is sets of TV cameras record the facial characteris-
tics from the right and the left sides. The work-
also lightened by structured light (usually in
ing volume (black area) represents the part of
the infrared field), whose interferences with the space seen by two or more cameras with non-
facial structures enhance the final 3D recon- parallel optical axes. After a calibration proce-
dure, the computer can obtain the metric 3D
struction. Precision (difference between repeated
coordinates of each point of the working volume.
measures of the same item) and repeatability
(precision relative to the actual biological differ-
ence among subjects) of stereophotogrammetry is ideal to collect the 3D data of faces, even in
have been reported to be very satisfactory, even children, babies or disabled persons, where
better than caliper measurements (Aldridge et al., acquisition time is going to be critical. In par-
2005; Gwilliam et al., 2006; Ghoddousi et al., ticular, Mutsvangwa et al. (2011) found that
2007; de Menezes et al., 2010; Schimmel et al., stereophotogrammetry can obtain the 3D coor-
2010; Aynechi et al., 2011; Fourie et al., 2011b). dinates of facial landmarks in infants with a high
Previous marking of the landmarks of inter- level of precision. The instrument can be used
est increases the instrument precision, without also for the digital analysis and reconstruction of
reducing the information content of the acquired other body regions, like the head and the neck
3D image, a topic already discussed in our pre- (Dirven et al., 2008; Schaaf et al., 2010).
vious review (Sforza & Ferrario 2006), but that In a recent review, Heike et al. (2010) detailed
had received greater attention in the last years the main technical issues related to the practi-
(Ghoddousi et al., 2007; de Menezes et al., 2010; cal use of stereophotogrammetry, including its
Aynechi et al., 2011). Indeed, some landmarks physical location, suggestions to reduce image
can be efficaciously identified only with palpa- artifacts and maximize facial surface coverage,
tion of the underlying bone surface, a procedure and hints for the analysis of children and persons
that cannot be performed on the facial scan. For with special needs.
instance, the error of gonion identification was Figure 4 shows an example of a 3D scan per-
2-4 times larger than that of the other facial land- formed using a stereophotogrammetric instrument.
marks. Other landmarks (tragus, menton, orbitale
superior) were of difficult identification because Laser scanners
the facial region was covered by hair, or because Laser scanners are another well-known class
the scan was not optimal (Gwilliam et al., 2006; of instruments that can be used for surface analy-
de Menezes et al., 2010; Heike et al., 2010). sis. The instrument shines a low-intensity laser
Due to its safety, the fine resolution images (below 0.00008 W) on the object and poses
and the acquisition time (2 ms), this instrument no risk to the patient’s vision. Digital cameras
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164 3D facial anthropometry
Fig. 4 - Three-dimensional reproduction of the facial soft tissues of a normal 20-y old woman
obtained by a stereophotogrammetric instrument (three-dimensional image with texture, and
polygonal mesh). Areas covered by hairs (eyelashes, head), and areas covered by other structures
(lateral part of the face, below the mandibular angle) cannot be completely identified by the system.
The colour version of this figure is available at the JASs website.
capture the images (Fig. 5); the depth informa- than for the traditional manual method. Their
tion is obtained by triangulation geometry (Kau main limitation may be the time necessary for a
et al., 2006; Ramieri et al., 2006, 2008; Primozic complete facial scan, which is significantly higher
et al., 2009; Friess, 2012; Joe et al., 2012). During than that necessary for stereophotogrammetry
data acquisition, either the face or the laser light (Kovacs et al., 2006; Germec-Cakan et al., 2010;
move to cover the entire surface. For example, Zhuang et al., 2010a; Fourie et al., 2011b).
the laser scan used by Ramieri et al. (2006) In a multicentric study, Kau et al. (2010)
moves 360 degrees around the subject, digitiz- used both laser scanning and stereophotogram-
ing 512 vertical profiles in approximately 17 s, metric acquisitions, showing that the two instru-
with a scanning precision of 0.65 mm. Repeated ments can be efficiently used sharing data among
scans of human subjects were reported to result laboratories. In contrast, Germec-Cakan et al.
in a mean scanning error of 1.0–1.2 mm and a (2010) compared 3D nasal dimensions obtained
recording error of 0.3–0.4 mm. Figure 6 shows by facial impressions (stone casts), laser scan-
the 3D facial reconstruction of the same subject ning and stereophotogrammetry, and reported
imaged in Figure 4 obtained by laser scanning. that laser scanning was not sensitive enough
Laser scans have been proved to be as pre- to visualize the deeper indentations such as
cise, if not more so, than the traditional caliper nostrils, while better results were obtained by
and steel tape method of measurement, provid- stereophotogrammetry.
ing a more consistent data acquisition process Handheld laser scanners combine optical
compared with the traditional manual methods. scanning of the face and electromagnetic detec-
In the experiment reported by Joe et al. (2012), tion of the position of the instrument, which is
80% of the analyzed facial measurements had manually swept over the object by the operator,
lower standard deviations for the digital method paralleling a kind of spray painting, They are
C. Sforza et al. 165
Fig. 5 - Principle of a laser triangulation. The laser dot, the camera and the laser emitter form a trian-
gle, the distance between the camera and laser emitter is known, as well as the angle of the emitted
laser. The angle of the camera can be determined through laser dot in the camera’s field of view deter-
mining the principle of triangulation. The colour version of this figure is available at the JASs website.
portable, and allow a sufficiently fast and accu- single or multiple teeth prostheses, where a laser
rate digitization of the face (Harrison et al., 2004; scan is automatically swept around the object
Hennessy et al., 2005; Schwenzer-Zimmerer et (or the object is moved inside the laser light).
al., 2008; Sforza et al., 2011c, 2012b, 2012d). Unfortunately, these instruments cannot be used
They can be a practical solution for laboratories for soft tissue data collection, and monetary limi-
or clinical facilities with a reduced budget, or in tations have prompted the researchers to alterna-
countries where the patients cannot easily reach tive solutions. For instance, Sforza et al. (2012a)
the health care units. The principal limitation is successfully used a stereophotogrammetric unit
the time required for a scan, with the possibility to digitize the palatal casts of children with cleft
of motion artifacts. lip and palate.
An additional use of handheld laser scanners
may be the digitization of objects, and in par- Video scanners and photography: a low cost
ticular of stone casts of dental arches and palate. alternative?
In this case, there is no risk of motion artifacts, Among the disadvantages of stereophoto-
and the main limitation is the presence of shad- grammetric systems and laser scans there are
owy areas that may be not completely imaged their cost and their dimensions. Low-cost photo-
by the instrument. Technology offers dedi- graphic alternatives have therefore been devised
cated instruments, that are usually employed in in several medical fields, especially for small, pri-
dentistry for the design and manufacturing of vate clinical practices, where a first screening of
www.isita-org.com
166 3D facial anthropometry
Fig. 6 - Three-dimensional reproduction of the facial soft tissues of a normal 20-y old woman
obtained by laser scanning (three-dimensional polygonal mesh, and homogenous surface render-
ing). Areas covered by hairs (eyelashes, head) cannot be completely identified by the system. The
colour version of this figure is available at the JASs website.
the facial appearance is made, followed if neces- in three-dimensional facial anthropometry
sary by more complex and complete analyses. (Weinberg et al., 2004; De Menezes et al., 2010).
A recent introduction in the market is the In several clinical applications a 2D profile
video scanner. This instrument is similar to a view can be sufficient for diagnosis and follow-
video camera which captures images up to 16 up, leaving 3D assessments only to selected
frames per second producing 3D images. These patients (Dimaggio et al., 2007; Tollefson &
frames are automatically aligned in real-time, Sykes 2007; Abed et al., 2009; de Menezes et
which makes scanning easy and fast. The digital al., 2009; Deli et al., 2010; Han et al., 2010).
cameras capture the images and the depth infor- In these applications, a set of separate 2D facial
mation is obtained using triangulation geometry. photographs is taken under standardized condi-
The advantages of these video scanners are their tions, the images are calibrated and merged, and
portability, relative fast acquisition time, colored commercial software allows to perform quanti-
texture and good scan resolution (3D point tative measurements in the three dimensions.
precision, up to 0.1 mm). Indeed, they do not The method has been found to be sufficiently
require markers or calibration, and use a flash precise and repeatable for clinical application
bulb as light source (no laser). The cost (around but with some errors in the labial, orbital and
US$13,000) is inferior compared with laser scan- auricular areas (de Menezes et al., 2009). Indeed,
ners (price range: US$25,000 to US$55,000) discrepancies in facial structures lower than 1.5
and stereophotogrammetry systems (price range: mm cannot be usually appreciated by the naked
US$30,000 to US$140,000). Otherwise, the eye, thus defining some kind of precision thresh-
main limitation may be the reduced texture old for clinical use (Fourie et al., 2011; Lubbers
resolution (1.3 Mpixel), which may impede a et al., 2012). While landmark digitization was
precise identification of dots/ landmarks used found to have an acceptable precision, errors due
C. Sforza et al. 167
to subject and camera relocation for multiple therefore be controlled to avoid deformation and
photographs were large, up to 5.3 mm for dis- misalignment.
tances and 5.6 degrees for angles (de Menezes et One step towards a possible solution of the
al., 2009). For reduction in measurement error problem is the definition of a good set of fiducial
due to head movements among separate poses, landmarks, that is points imaged with both tech-
photographs may be obtained simultaneously niques that act as reference for the subsequent
with the use of three cameras, as described by superimposition of the digital images (Baik &
Deli et al. (2011), but this would increase the Kim, 2010; Gupta et al., 2010; Rosati et al.,
monetary cost of the analysis. 2010). These landmarks should be sufficiently
Nonetheless, photographs continue to be distant one from the other to allow the individu-
widely used in clinical settings (Han et al., 2010), ation of a reference plane for image registration
even if they often do not include appropriate and surface matching. Boulanger et al. (2009)
scales. Driessen et al. (2011) devised a technique matched CBCT and stereophotogrammetric
for calibrating photographs that have no scale or scans using a set of titanium targets. Together
grid included, starting from iris diameter, which with the use of fiducial points, surface match-
has been found to be of stable dimension from ing between homologous areas of couples of 3D
the age of 5 years. For facial structures placed facial images has been found to be effective, with
outside the coronal plane of the iris, a linear cor- average deviations between repeated scans lower
rection factor can be calculated to compensate than 1 mm (Maal et al., 2010). A combined use
for the different distances from the camera. The of fiducial landmarks (manual selection and ini-
method makes it possible to perform life-size tial matching) and of facial areas that were not
measurements in a frontal view photograph. modified by the treatment (manual selection and
automatic fine matching) is usually employed for
longitudinal investigations (Baik & Kim, 2010).
Tools, applications and fields of use A more recent and refined method is the
“anthropometric mask”, where the classic set of
Fusion of images from different sources anthropometric landmarks is expanded in a spa-
While CT scans offer a detailed image of the tially dense way using around 10,000 quasi land-
skeletal surfaces and volumes, 3D facial images marks (Claes et al., 2012a,c). Superimpositions
obtained by an optical method can provide addi- using this new tool appear to produce more
tional information about color and surface tex- biologically plausible results. Maal et al. (2008)
ture, as well as higher resolution of soft-tissue devised a method to fuse 3D facial images
surfaces (Kovacs et al., 2006; Cevidanes et al., obtained using stereophotogrammetry (textured)
2010; Hammond & Suttie, 2012). The two and CBCT (untextured). After an initial posi-
methods may therefore be combined, providing tioning of the two surfaces by indicating fiducial
a more complete assessment of the patients. landmarks on both of them, the manual exclusion
The fusion can be made using landmarks or of regions with large registration errors allows the
surface-based methods that are currently pro- automatic transfer of texture from textured surface
vided by some software tools, but the soft-tissue to untextured surface using non-rigid registration.
structures may be not stable enough as refer- Kochel et al. (2010) combined two-dimen-
ences, thus including an unknown amount of sional lateral head radiographs with 3D stereo-
error (Cevidanes et al., 2010). Modifications in photogrammetric facial images, finding a set of
the activity of facial muscles (especially around significant correlations between hard and soft tis-
the eyes, the nose, the mouth), together with sue angles and distances. Incrapera et al. (2010)
alterations in head and neck position, were assessed pre- and post-treatment records, discov-
found to provoke significant errors in facial ering a good accord between 3D and 2D modifi-
reconstruction. The combined images should cations at selected soft-tissue landmarks.
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168 3D facial anthropometry
In an attempt to couple the benefits of sur- reliability of the data depends strongly on the
face stereophotogrammetric images and digital operator’s experience, because landmarks are
images of the dental arches, Rangel et al. (2008) usually located within relatively large and curved
and Rosati et al. (2010) devised and tested a areas, rather than in correspondence of discrete
fusion protocol that integrated a digital dental points (Gwilliam et al., 2006; Kovacs et al., 2006;
cast into a 3D facial picture. According to the de Menezes et al., 2010; Calignano & Vezzetti,
average distance between the matched areas 2011; Claes et al., 2012a). Previous marking of
(anterior teeth, incisors, canines and first premo- the landmarks on the patient’s skin reduces both
lars) obtained in a group of patients, the method the measurement error and the time required
was reported to be reliable, without systematic for image analysis, but even this process requires
errors and with reduced random errors (techni- expertise and effort, and it may not be feasible
cal error of measurement less than 1 mm, relative for all subjects (Mutsvangwa et al., 2011).
error of the mean up to 1.2%). Considering that each facial scan provides a
Because the fusion between the 3D face and 3D mesh of known coordinates, we can assess
the digital casts must use the anterior teeth as the the invariant geometric characteristics of the
reference, any displacement or inclination of the mesh, such as curvatures, and combine them
digital dental arch would add a position error in with anatomical knowledge to find the required
the posterior region, which is of difficult assess- set of landmarks and selected facial structures,
ment and quantification (Rosati et al., 2012b). namely, the eyes, nose, and mouth. Common
Bechtold et al. (2012) proposed the use of a geometric shapes such as peak, ridge, pit, and
dental face bow connected to an extra-oral refer- ravine are defined from a mathematical point of
ence frame: an individual dental splint allowed view, and coupled to the anatomical landmarks
a repeatable positioning of the reference frame on hard and soft tissues (Gupta et al., 2010; Deo
permitting a good fusion of the facial and dental & Sen, 2010; Calignano & Vezzetti, 2011; Fang
images. Unfortunately, the face bow increases the & Fang, 2011; Arca et al., 2012). Identification
vertical dimension of the lower part of the face, of the midsagittal plane and facial midline is
creating image modifications and skin artifacts obtained from the symmetry of a set of primary
(Heike et al., 2010). Also, the method needs 10 landmarks. From facial midline, associated land-
separate steps to provide the final integration of marks can be recognized by using local identi-
the dental arches inside the stereophotogram- fication algorithms (Deo & Sen, 2010; Fang &
metric facial scan, and each step can increase the Fang, 2011). 3D facial and head scans can also
error. Overall, the authors reported a good preci- be virtually sectioned along selected anatomical
sion in vertical distances, while the sagittal meas- planes, thus allowing a direct assessment of differ-
urements revealed more deviations (Bechtold et ent faces or the longitudinal analysis of growth,
al., 2012). aging or treatment effects. In particular, the use
of multisectional spline curves gives a global
Automatic landmark identification morphological evaluation of the soft tissues,
Together with the technological develop- and it seems to be the most efficient solution for
ments for image recording, research focused on maxillofacial surgical assessments (Ramieri et al.,
mathematical, geometrical and statistical tools 2008; Deo & Sen, 2010; Vezzetti et al., 2010).
that allow to extract the largest possible amount Using model-base segmentation techniques,
of information from the 3D facial reconstruc- Chakravarty et al. (2011) obtained an automatic
tions. One of the most promising fields is the identification of facial landmarks in magnetic
automatic extraction of landmark coordinates. resonance images of adolescents. The method
Currently, landmark assessment is a lengthy requires landmarks identification only on the
process that requires expert operators, and that model of the face, with subsequent customiza-
may hinder a widespread use of 3D scans. The tion of their position on each individual face.