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Automated Production of Multilayer Anterior Restorations with Digitally Produced Dentin Cores

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Abstract and Figures

The identical replication of adjacent teeth creates the illusion of “natural-identical” restorations. Single maxillary anterior teeth, especially the central incisor, cannot be successfully realized as monolithic crowns, as these cannot adequately mimic the individually layered structure of anterior teeth. Here the craftsmanship of an experienced dental technician will continue to be required. But even the most skillful expert will have to redo maxillary anterior crowns at times, as the desired esthetic result does not always materialize on the first try. In addition to the correct shape and surface, the shade also plays a significant role. In particular, the correct individual layering—or, in other words, the correct three-dimensional structure—of the crown is crucial for a perfect reproduction of a natural tooth. The internal structure of the crown—especially the dentin—will determine the esthetics of an anterior restoration to a considerable extent. Experienced dental technicians are able to mimic the dentin in its threedimensional manifestation but will generally not be able to provide any precise spatial definitions. The design of the dentin core is therefore based mainly on the training and the experience of the dental technician and often follows a “traditional” approach. Thus, almost all dental technicians leave a clearly discernible “signature” as they prepare their restorations, a restorative artefact that does not, strictly speaking, have any connection with the case at hand.
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207
QDT 2015
When producing digital dental restorations,
it is now possible to mirror the geometry
of teeth, to output the result as a data re-
cord, and to mill the resulting shape monolithically
from a tooth-colored blank. The result is acceptable
when restoring an entire anterior maxilla or mandible,
although the esthetic results achieved with layered
tooth build-ups will generally be more natural looking.
The identical replication of adjacent teeth creates the
illusion of “natural-identical” restorations.
Single maxillary anterior teeth, especially the cen-
tral incisor, cannot be successfully realized as mono-
lithic crowns, as these cannot adequately mimic the
individually layered structure of anterior teeth. Here
the craftsmanship of an experienced dental technician
will continue to be required. But even the most skillful
expert will have to redo maxillary anterior crowns at
times, as the desired esthetic result does not always
materialize on the rst try.
In addition to the correct shape and surface, the
shade also plays a signicant role. In particular, the
correct individual layering—or, in other words, the cor-
rect three-dimensional structure—of the crown is cru-
cial for a perfect reproduction of a natural tooth.
The internal structure of the crown—especially the
dentin—will determine the esthetics of an anterior res-
toration to a considerable extent. Experienced dental
technicians are able to mimic the dentin in its three-
dimensional manifestation but will generally not be
1
Dental Technician, Department of Prosthodontics, Dental School,
Ludwig-Maxmilians University, Munich, Germany.
2
Director and Chair, Department of Prosthodontics, Dental School,
Ludwig-Maxmilians University, Munich, Germany.
3
Assistant Professor, Department of Prosthodontics, Dental School,
Ludwig-Maxmilians University, Munich, Germany.
Correspondence to: Josef Schweiger, Department of Prosth-
odontics, Ludwig-Maxmilians University, Goethestrasse 70, 80336
Munich, Germany. Email: Josef.Schweiger@med.uni-muenchen.de
Josef Schweiger, CDT1
Daniel Edelhoff, CDT, Dr Med Dent, PhD2
Michael Stimmelmayr, Dr Med Dent3
Jan-Frederik Güth, Dr Med Dent3
Florian Beuer, DDS, Dr Med Dent, PhD3
Automated Production of Multilayer
Anterior Restorations with
Digitally Produced Dentin Cores
SCHWEIGER ET AL
QDT 2015
208
able to provide any precise spatial denitions. The de-
sign of the dentin core is therefore based mainly on
the training and the experience of the dental techni-
cian and often follows a “traditional” approach. Thus,
almost all dental technicians leave a clearly discernible
“signature” as they prepare their restorations, a restor-
ative artefact that does not, strictly speaking, have any
connection with the case at hand.
STATE OF THE ART
Ingots with Plane-Parallel Layers
Various approaches have been used to imitate the
layered structure of natural teeth using digital meth-
ods. For example, various manufacturers offer ingots
for computer-aided design/computer-assisted manu-
facture (CAD/CAM) processing that consist of several
plane-parallel layers, with the individual layers hav-
ing different shades. Examples include the Vitablocs
TriLuxe forte ingot (Vita Zahnfabrik), the CEREC Bloc
C PC (Sirona), and the Noritake Katana Zirconia ML
disc (Kuraray Noritake). These ingots attempt to mimic
the shade gradient of the natural tooth, from cemen-
tum and dentin to enamel, by presenting differently
colored layers within the material. The software can
modify the vertical alignment of the restoration within
the ingot, allowing the chroma of the restoration to
be modied. The esthetic results of restorations from
these polychromatic blocks are certainly better than
the esthetics of restorations made from monochromat-
ic blanks. Nevertheless, they cannot be used to create
customized, patient-specic layers.
Three-Dimensional Ingot Structure with
Dentin Core and Enamel Coating
A second approach to imitating the layered structure
of natural teeth is a millable ingot that has a three-
dimensional block structure with dentin core and
enamel coating and an arched gradient between the
dentin and incisal (VITA RealLife Block, Sirona CEREC
Blocs C In). The software can relocate the virtual de-
sign within the ingot such that the proportion of dentin
and enamel is modied. This is supposed to give users
the opportunity to imitate the appearance of natural
teeth as closely as possible. But even these ingots can-
not be used to produce customized, patient-specic
layers.
Semi-nished Crowns
A third approach is that of the so-called semi-nished
crowns, such as the priti crown (pritidenta), which al-
ready features the anatomical outer geometry of the
clinical crown and a standardized layered structure of
the dentin and incisal areas. The only thing left to do
is use the CAD/CAM system to remove a volume cor-
responding to the prepared tooth in shape and form
of the basal aspect of the crown. The disadvantage
is that only subtractive processing is possible, so that
slightly larger blanks are generally used that are re-
duced by milling to match the CAD design. Milling can
never add material!
Tooth Databases
Systems for computer-aided manufacturing of dental
restorations include tooth databases based on data
from scanned natural teeth, scanned prefabricated
teeth, or scanned manually waxed-up tooth shapes.
However, these databases invariably refer only to the
external tooth geometry.
Biogeneric Occlusal Surfaces
Sirona’s CEREC system uses so-called “biogenic oc-
clusal surfaces” (developed by Professor Dr Albert
Mehl of the University of Zürich, Switzerland), which
operates on the basis of several thousand scanned
natural teeth. The system determines the closest
match in the tooth database to the remaining tooth
structure, “adds” the missing portions, and thereby
obtains a very natural partial-crown (or inlay, or onlay)
geometry. But even the “biogeneric tooth model” is
conned exclusively to the external tooth geometry.1
In the biogeneric tooth model, missing parts of the
external tooth surface are added in by adapting a ge-
neric record of the desired tooth to the residual tooth
structures and/or antagonists and/or adjacent teeth
Automated Production of Multilayer Anterior Restorations with Digitally Produced Dentin Cores
QDT 2015 209
situation and/or bite registration. Furthermore, Sirona
provides a database for the CEREC system in which
the user is presented with a static mamelon structure,
generated according to geometric design guidelines,
which is then customized by CAD and produced by
CAM in the milling unit.
In his doctoral thesis, Probst2 described the mor-
phology of maxillary anterior teeth and the determi-
nation of similarity metrics of identical anterior tooth
types in the left and right maxilla. However, he did not
address the layered internal three-dimensional struc-
ture of anterior teeth.
Looking at the current state of the art as just pre-
sented, it can be summarized that no database is
currently available for the layered internal three-
dimensional tooth structures in the anterior and pos-
terior regions. The term “tooth-structure database” as
used below denotes a database/library that includes
internal three-dimensional tooth structures and the
corresponding surfaces of the respective specic teeth
in digital and/or physical form. Neither has a method
been described for the automated generation of the
layered internal tooth structure, especially the dentin.
DEJ AND OES
By far the largest part of the human tooth consists of
dentin, which forms the inner “protective coating” for
the pulp cavity in its center. The pulp consists mainly
of loosely packed connective tissue with numerous
cells, intercellular basic substance, reticular and colla-
gen bers, and—not least—nerves and blood vessels.3
The dentin in turn is covered by enamel in the clinical
crown area and by cementum in the root area. Togeth-
er, enamel, dentin, and cementum represent the hard
tissue of the human tooth. The enamel is the hardest
substance in the human body, with a Vickers hardness
of 250 to 550 and a compressive strength of 300 to
450 MPa. Its modulus of elasticity is 50,000 to 85,000
MPa.4 The dentin, by contrast, is much more elastic
(Young’s modulus of 15,000 to 20,000 MPa), because
it contains a signicantly higher percentage of organic
matter. The Vickers hardness of the dentin is 60 to 70,
and its compressive strength is 200 to 350 MPa.4 The
cementum is similar to human bone in both structure
and hardness but differs from bone in that it is not vas-
cularized. The cementum is already considered part
of the attachment apparatus, or periodontium. This is
where the periodontal bers are attached that keep
the teeth in their bony sockets, or alveoli.3
The dentinoenamel junction (DEJ) and the outer
enamel surface (OES) (Fig 1) are essential features
of the three-dimensional structure of the tooth and
signicantly affect its visual appearance. Some study
results indicate that the DEJ provides considerable
information about the OES.5–25 It is known that the
shape of the DEJ closely resembles the shape that the
OES reects6,7,26 and that, unlike the OES, the DEJ is
preserved intact in abraded teeth.
There are different ways to represent the DEJ in
three dimensions, as described as follows.
Fig 1 Sagittal sections of anterior crowns. The
dentinoenamel junction (DEJ) and the outer
enamel surface (OES) are clearly discernible.
DEJ
OES
SCHWEIGER ET AL
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210
Chemical Removal of the Enamel Layer
Chemical removal of the enamel layer is a destruc-
tive method for preparing the DEJ. The entire enamel
layer can be removed with 37% phosphoric acid.5,15–18
Since the enamel layer is destroyed in the process, it is
necessary to preserve the OES. This can be done in an
analog manner, by taking an impression of the tooth
crown and subsequent pouring of a cast, or digitally,
by scanning the tooth crown. The scan operation can
be performed mechanically (eg, Procera Forte, Nobel
Biocare) or by means of an optical scanner (eg, BEGO
3Shape D 700, Bego Medical).
Computed Tomography
The three-dimensional geometry of the OES and DEJ
can be acquired by standard computed tomography
(CT) or cone beam computed tomography (CBCT).
The resolution and accuracy of the data vary greatly
depending on the manufacturer. Therefore, it is often
difcult to obtain 3D data sufciently accurate for
further processing from CT or CBCT data. The InVesa-
lius software (CTI Renato Archer) can convert two-
dimensional data from CT scans to three-dimensional
DICOM (Digital Imaging and Communications in Med-
icine) data. These DICOM data are then converted to
STL (Standard Tessellation Language) data.27–29
Microcomputed Tomography
The best way to acquire three-dimensional OES and
DEJ data is by microcomputed tomography (microCT).
In the study presented here, the extracted teeth were
scanned with the exaCT S S60 HRE desktop CT unit
(Wenzel Volumetrik). The voxel size was 45 µm. The
exaCT Analysis software (Wenzel) was used for data
acquisition and output. The data for the enamel (with
the OES on the outside and the DEJ on the inside)
and the root with dentin core (DEJ) and pulp chamber
were converted to the STL format and output.
PRINCIPLE OF THE DENTIN-CORE
CROWN
As early as 1945, Weidenreich had noted that the sur-
face relief of the dentin (the DEJ) could not be a purely
accidental feature without any morphologic impor-
tance.19,30 The basic principle of the digital dentin-core
crown/digital dentin-core bridge according to Sch-
weiger31 is as follows: “There is a clear correlation be-
tween the three-dimensional tooth surface (OES) and
the layered internal structure of a tooth (the dentin
core and DEJ)” (Fig 2). As used here, the term “corre-
lation” signies the association of a record describing
the structure of the internal layer (ie, the DEJ) with a
record dening the external geometry of the tooth
(ie, the OES).
Based on this axiom,32 a tooth-structure database
can be compiled that allows, for the rst time, crown
or bridge restorations to be produced accurately and
with an esthetic appearance that replicates the natural
model (Fig 2).
Tooth-Structure Database
The idea on which the invention is based calls for ac-
quiring not only the outer structures of the tooth but
also the layered internal tooth geometry and to use
it in conjunction with the external geometry, for ex-
ample by storing them in a database (Figs 3 to 9).31,33,34
If the external and layered internal tooth structures
can be connected with each other dynamically, this
is a particular advantage, as a virtual modication of
the external geometry of the tooth can then be re-
ected by corresponding changes in the internal struc-
ture. Another advantage is that the digital acquisition
of a large number of three-dimensional external and
internal tooth geometries allows the establishment
of a well-dened relationship between the layered
structure of the inner tooth and its outer shape. Fur-
thermore, once a suitable external geometry has been
selected, the database can propose an internal tooth
geometry that, with great probability, will correspond
to the internal geometry of the natural tooth.
Automated Production of Multilayer Anterior Restorations with Digitally Produced Dentin Cores
QDT 2015 211
Dentin
core
OES
DEJ
Fig 2 Principle of the digital dentin-core crown according
to Schweiger (“inward biogenerics”). OES = outer enamel
surface. DEJ = dentinoenamel junction.
Fig 3 Natural tooth, metal-ceramic crown, and dentin-core crown (longitudinal sections).
Natural tooth Metal-ceramic crown Digital dentin-core crown
1 = Enamel
2 = Dentin
3 = Dental pulp
4 = Crown
5 = Root
1 = Ceramic material (incisal, enamel,
or transparent)
2 = Ceramic material (dentin)
3 = Framework (metal, ceramic,
acrylic)
4 = Prepared tooth
5 = Articial clinical crown
6 = Root
1 = Ceramic material (incisal,
enamel, or transparent)
2 = Ceramic material (dentin)
3 = Prepared tooth
4 = Articial clinical crown
5 = Root
4
5
1
2
3
1
2
3
4
5
6
5
1
2
3
4
SCHWEIGER ET AL
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212
Correlations between the external and internal
tooth geometries are recognized, for instance when
certain types of tooth shapes (eg, oval, square, trian-
gular) are associated with characteristic internal tooth
structures (eg, pronounced mamelons in the case of
projecting triangular teeth). Tooth types can be further
subdivided into different shape groups by looking at
tooth-specic surface and shape characteristics, in-
cluding:
• Mesiodistal curvature
• Incisocervical curvature
• Rounding of the distal incisal edge
• Rounding of the mesial incisal edge
• Angle characteristics
• Incisal edge contours
Surface-structure components such as longitudinal
grooves or elevations
Data records of scans can be assigned to shape groups
either by visual inspection or digitally using the best-
t alignment method. Either way, the result will be a
database of teeth that subdivides the different tooth
types into multiple shape groups.
Similarly, it is possible to assign the acquired inter-
nal tooth structures (especially the dentin cores) of the
various tooth types (central incisors, lateral incisors, ca-
nines, rst and second premolars, rst to third molars)
to different shape groups by using the same method.
Here, again, the assignment can be made by visual
inspection or using the best-t alignment method. The
result is a database of internal tooth structures, again
subdivided into multiple shape groups. In addition, it
is possible to establish a correlation between the in-
ternal and external tooth geometries. Using the data-
base data, an internal tooth geometry is proposed for
a given external tooth shape. It is highly probable that
this proposal corresponds to the “real” internal tooth
geometry (Figs 8 and 9)—the more so, the more re-
cords are included in the tooth database. BEGO Medi-
cal already realized this in its Dentaldesigner software
from 3Shape. This software stored, for the rst time,
tooth geometries that are created on the basis of the
real tooth.
Fig 4 Natural tooth with
corresponding dentin
core.
Figs 5a and 5b STL records from a tooth structure database of the outer enamel surface and
the dentinoenamel junction.
Fig 6 CAD/CAM dentin cores, manufactured based on STL
data of the dentinoenamel junction.
Fig 7 Virtual rendering of the outer enamel surface and the
CAD/CAM dentin cores.
Automated Production of Multilayer Anterior Restorations with Digitally Produced Dentin Cores
QDT 2015 213
A novel application—the biogeneric tooth model—
is also supported. This model calculates, based on the
vast number of different tooth records in the database,
an internal tooth geometry (eg, a dentin core) that has
all the features characteristic of the respective tooth
type. This is not achieved by merely averaging or su-
perimposing the individual data points (xn, yn, zn) that
describe the internal tooth geometry, as this would re-
sult in noisy, unstructured data that do not correspond
in any way to the typical geometry. Rather, the den-
tin core is segmented into individual building blocks
(mamelons, incisal grooves, incisal contours of the
dentin, etc) to uncover correspondences and to com-
pare like with like. This prevents essential structures of
the dentin core from being averaged out in calculating
the geometry, as happens, for example, with mam-
elons during conventional alignment calculations, for
example with regard to mamelons. This method pro-
duces an average internal tooth geometry with aver-
aged values for the characteristic building blocks, such
as mamelons, incisal grooves, incisal edges, etc.
Fig 8 Palatal view of tooth structure data of maxillary an-
terior teeth, showing the dental pulp, the dentinoenamel
junction, and the outer enamel surface.
Figs 9a to 9c STL records of dentin cores and enamel coat-
ings of maxillary anterior teeth.
SCHWEIGER ET AL
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214
In the next step, the deviation of the individual in-
ternal geometries from the respective average geom-
etry is calculated by a principal-axis transformation.
If the goal is to reconstruct a layered internal tooth
structure, the biogeneric tooth model must be cor-
related with the external tooth geometry. Here, the
layered internal tooth structure—especially the dentin
core—corresponds to the missing hard tissue of the
tooth substance in a biogeneric inlay reconstruction.
A certain spatial distribution of a few design points on
the external tooth surface requires a certain morphol-
ogy of the dentin core. The combination of an average
dentin core with the biogeneric model of the external
tooth geometry makes it possible to assign the most
probable dentin core to a given external tooth geom-
etry. The morphologic relationship between the ex-
ternal tooth geometry and the layered internal tooth
structure is essentially based on a genetic blueprint.
The probability is high that a specic external tooth
geometry can be correlated with a specic layered
internal tooth structure, especially with regard to the
dentin core, and vice versa. It should be pointed out
in this context that one of the lead structures during
odontogenesis is the preformative membrane, which
eventually forms the DEJ.
This membrane is an anatomical structure that forms
during tooth development. As a basement membrane,
it constitutes the interface between the mesenchymal
connective tissue (the mesodermal papilla) and the
ectodermal enamel organ. Shortly before the dentin
begins to form, this basement membrane thickens
and henceforth separates the dentin from the enamel.
Here, odontoblasts and ameloblasts are initially locat-
ed back-to-back as the pre-dentin/pre-enamel is con-
verted, gradually moving away from each other while
the hard tissues of the tooth they have formed are left
behind.
Once the external tooth geometry has been digital-
ly linked to the internal tooth geometry, a correlation
is formed between the two records, a correlation that
can be either dynamic or static. In a static correlation,
the internal geometry is not changed by a modication
of the outer geometry, which among other things im-
plies that the dentin core always retains its shape. In a
dynamic correlation, however, the internal tooth struc-
ture is modied in response to any modications of
the external tooth surface. On modifying the external
tooth geometry, all X/Y/Z values of the internal tooth
geometry change proportionately to the X/Y/Z values
of the external tooth geometry (scaling). Rotations will
be performed with the same angle, and translations
with the same X/Y/Z values will be performed by add-
ing the translation values.
This database with correlations between the in-
ternal and external tooth geometries (correlations
database) can be used in different ways in the produc-
tion of dental restorations. Using computer-assisted
output devices (computer numerical control [CNC];
rapid prototyping [RP]), restorations can be produced
that mimic the layered internal structure of a natural
tooth. The internal structure of the restoration is pro-
duced based on a record from the database, where
the external surface corresponds exactly to the inter-
nal tooth structure of a record selected from the data-
base. Suitable materials for creating the internal core
include materials with a toothlike esthetic appearance
in terms of shade and translucency, especially resin,
glass ceramics, feldspar ceramics, lithium disilicate ce-
ramics, and oxidic high-performance ceramics such as
zirconia and alumina. Once this computer-generated
internal aspect of the restoration has been produced,
the incisal aspects can be added. This can be per-
formed manually using a ceramic layering technique
or a wax-up technique with subsequent overpressing.
Alternatively, this incisal area can be designed by sub-
tracting the internal tooth structure from the external
tooth surface, creating a differential record that can
be transformed into a real-world object using a CAM
procedure. In a subsequent additive step, this incisal
area is then connected to the dentin core by sintering
(using a ceramic connector mass), by a polymerization
process, or adhesively.
In the context of the method described here, there
are several ways to design and manufacture dental
restorations digitally (Fig 10), as shown below.
Best-t alignment
The arch situation comprising the teeth to be replaced
as well as the adjacent teeth is acquired by three-
dimensional scanning (intraoral or extraoral). If a “mir-
ror tooth” is present, its three-dimensional structure
is mirrored. A study2 has shown that mirror-image re-
placements of anterior teeth are satisfactory with re-
spect to interproximal, occlusal, and esthetic aspects.
Using an iterative procedure, the external structure of
the mirror-image tooth is compared to and correlated
Automated Production of Multilayer Anterior Restorations with Digitally Produced Dentin Cores
QDT 2015 215
with the natural or manually designed teeth in the cor-
relations database until the most appropriate record is
found. To determine the appropriate record by way of
an iterative procedure, it is possible to devise a simi-
larity metric based on the standard deviation of the
smallest distances of points on the surface of the mir-
rored tooth from the respective closest points of each
tooth record in the database.
(SD = standard deviation over the shortest
distance = similarity metric)
This method is also called “best-t alignment.” To
achieve a best-t alignment, the tooth is mirrored and
then superimposed on a reference tooth from the data-
base in the optimal position by rotation, translation,
and possibly also scaling. Image analysis software (eg,
Geomagic Qualify, Geomagic GmbH) can be used for
this. As a layered internal tooth structure exists for the
best-tting record, this structure can be used for de-
signing the restoration, and specically its dentin core,
using computer-assisted methods. Once the dentin
core has been created, the incisal aspect can be built
up manually; alternatively, a CAM-created incisal seg-
ment can be connected to the dentin core by sintering
or adhesively.
Completed manually by coating
or overpressing or by connecting
the different layers (eg, dentin and
enamel) adhesively or by sintering
Mirror image of the missing
tooth present
No mirror image of the missing
tooth present
No mirror image of the missing
tooth present
The records of external tooth
surfaces of the same tooth types
(same arch segment, eg, maxillary
anterior) that are most similar to
the residual dentition are selected
from the database (eg, by using
the best-t alignment method)
User selects most appropriate
record according to subjective
criteria
Record with the most appropriate
external tooth surface
Record of the correlating internal
layered structure, eg, dentin core,
enamel
Output of, eg, dentin core and/or
enamel surface data to CAD/CAM
or generative unit
Record of the jaw subsection denes
record for the missing tooth
Axisymmetric mirroring on the
vertical axis of the tooth
Dynamic/static/
biogeneric correlation
Dynamic/static correlation
Fig 10 Schematic representation of the fabrication of dental restorations with the aid of a tooth-structure database.
[(X1i–x2j) + (y1i – y2j) + (z1i – z2j)]2
n
n
i,j
Σ
SD =
SCHWEIGER ET AL
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216
Customizing the data according to user
preferences
The arch situation comprising the teeth to be re-
placed as well as the adjacent teeth is acquired by
three-dimensional scanning (intraoral or extraoral). If
no mirror-image tooth is present, a record presumed
to be appropriate is selected from the database and
can be three-dimensionally adapted to the actual situ-
ation by rotation, translation, and scaling. Due to the
dynamic correlation of the record of the three-dimen-
sional external tooth geometry with the record of the
three-dimensional layered internal tooth structure, a
design for a layered core, eg, a dentin core, will be
suggested. The suggested dentin core can then be
customized as required. The three-dimensional record
is implemented physically on a computer-assisted out-
put device such as a CNC or RP unit.
Best-t alignment after customizing the data
according to user preferences
The arch situation comprising the teeth to be re-
placed as well as the adjacent teeth is acquired by
three-dimensional scanning (intraoral or extraoral). If
no mirror-image tooth is present, the software com-
pares the residual dentition with the records from the
database of arch segments, and the record presumed
to be the most appropriate is selected using the best-
t alignment method. Since this record is assigned to
exactly one record of the missing tooth, it can serve
as a basis for the tooth to be replaced. Due to the
dynamic correlation of the three-dimensional external
tooth geometry with the three-dimensional layered
internal tooth structure, a design for a layered core,
eg, a dentin core, will be suggested. The suggested
dentin core can then be customized as required. The
three-dimensional record is implemented physically
on a computer-assisted output device such as a CNC
or RP unit (eg, Bego Medical).
Automated Manufacturing Process for
Individual Anterior Crowns
Using tooth-structure databases it is possible to
produce—using a partially or fully automated process—
highly esthetic restorations, especially for the anterior
region.
Let us assume a single anterior crown is to be pro-
vided for the maxillary left central incisor using a digital
process, where the natural right central incisor is used
as a template. Producing a single crown for a maxillary
central incisor is considered one of the most difcult
challenges in prosthodontics. The procedure consists
of the following steps:
1. Acquiring the external tooth surface
2. Identifying the matching dentin core
3. Mirroring the external tooth surface and dentin
core data
4. Digital manufacturing of the dentin core
5. Adding the incisal region
6. Digital nishing of the tooth surface
7. Finalizing the crown (glaze ring, polishing, etc)
Acquiring the external tooth surface
The external surface of the natural right central incisor
can be acquired by three-dimensional digital scanning
(Fig 11) with a mechanical or optical scanner or us-
ing a sonographic or radiologic procedure such as CT,
CBCT, or micro-CT.
Fig 11 Maxillary right central incisor and
the corresponding die of the left central
to be restored.
Figs 12a and 12b STL data of the mirrored dentinoenamel junction of the maxillary
right central.
Automated Production of Multilayer Anterior Restorations with Digitally Produced Dentin Cores
QDT 2015 217
Identifying the matching dentin core
The dentin core matching the external surface of the
tooth dentin can be determined based on the tooth
structure database. The acquired surface data are
compared with the records of tooth surfaces in the
tooth-structure database, and the record that is in
closest agreement with the newly acquired data is se-
lected. In the tooth-structure database, each external
tooth surface is associated with a unique dentin core.
Consequently, it is possible, on the basis of the ac-
quired data, to identify the matching dentin core.
Mirroring the external tooth surface and dentin
core data
Next, the two records are mirrored (Fig 12) to produce
the crown based on the mirrored external tooth sur-
face and dentin core data.
Digital manufacturing of the dentin core
Using dental CAD software, the record of the digital
dentin core can be placed on the prepared tooth. The
three-dimensional orientation of the dentin core data
set is determined by the external tooth surface data.
Next, the CAM software calculates the milling paths
and corresponding NC le based on the CAD record
of the dentin core. In the example shown, the dentin
core was produced using the Everest unit (KaVo) and
was prepared using the “counter-bed” procedure. In
this procedure, once the cavity side has been milled,
the cavity is lled with polyurethane resin. Once the
resin has hardened, the surface of the dentin core (the
DEJ) is milled. The material used was lithium disilicate
(IPS e.max CAD LT, Ivoclar Vivadent). A side benet of
the counter-bed procedure is that it produces a copy
of the die in polyurethane that is precisely positioned
within the CNC unit in relation to the machine zero
and workpiece zero points, facilitating precise repo-
sitioning of the crown within the CNC unit. The CAM
process uses diamond grinding points. The IPS e.max
CAD material is present in the metasilicate phase be-
cause it is easier to mill in this phase. After milling, the
dentin core is crystallized at 840°C; then the lithium
metasilicate is converted into lithium disilicate, attain-
ing the target tooth shade and the nal strength of
360 MPa (Figs 13 and 14).
Adding the incisal region
The ceramic veneer was made of IPS e.max Ceram
(Ivoclar Vivadent). Before the actual application of
the ceramic in the incisal area, a wash ring was per-
formed at 760°C. Experience has shown that a mixture
of Transpa Incisal and Opal Effect 1 at a ratio of 1:1
achieves a good result when using the incisal single-
layer technique. Material is applied generously to the
incisal area to provide enough bulk for the subsequent
subtractive process. Once applied, the ceramic mate-
rial is red at 750°C (Figs 15 and 16).
Fig 14 Dentin core from
lithium disilicate after crystal-
lization at 840°C.
Figs 13a and 13b CNC-milled dentin core made of lithium metasilicate to
restore the maxillary left central.
SCHWEIGER ET AL
QDT 2015
218
Digital nishing of the tooth surface
After ceramic ring, the entire crown is repositioned
on the polyurethane die that was produced by the
counter-bed process. Next, the tooth surface is ma-
chined based on the three-dimensional record of the
outer enamel surface (Fig 17). The result is a two-layer
restoration in which both the inner dentin core and the
outer enamel surface were obtained in a digital pro-
cedure.
Finalizing the crown (glaze ring, polishing, etc)
The manufacturing process is nalized with a stain-
and-glaze ring and nal polishing of the restoration
(Fig 18).
15 16a
18
Fig 15 Application of incisal veneering material (IPS e.max Ceram, 50% Transpa Incisal 2, 50% Opal Effect 1).
Figs 16a and 16b The ceramic material is red at 750°C.
Figs 17a to 17d The tooth surface is machined based on the three-dimensional record of the outer enamel surface.
Repositioning is done with the aid of the polyurethane “copy.”
Fig 18 The restoration is completed with a stain and glaze ring.
16b
17b 17c17a
17d
Automated Production of Multilayer Anterior Restorations with Digitally Produced Dentin Cores
QDT 2015 219
CONCLUSIONS
The process described in this article allows, for the
rst time, fabrication of highly esthetic anterior resto-
rations based on tooth structure records in a digital,
and therefore reproducible, procedure. The result is
predictable, and good outcomes can be achieved
even by users who are experienced in these technical
matters. The digital dentin core is the key to digital
anterior esthetics. It is important to create tooth struc-
ture databases that contain data both for the external
geometry of the teeth and the corresponding dentin
cores. The data for natural teeth especially will open
up an entire new dimension of natural anterior esthet-
ics. Production options include both subtractive and
additive manufacturing processes (Fig 19).
Users have the ability to access a wide variety of
tooth shapes and their structural designs to achieve
reproducible results. Ultimately, it should be possible
to use the digitally acquired external geometry of a
tooth to identify the corresponding dentin core from
a database in a highly predictable manner. Future re-
search projects will have to demonstrate the statistical
relationship between the external tooth shapes and
dentin cores.
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