The future of dentistry is heading towards customized dental treatment of each patient. This can be easily accomplished through the computer based technology and the virtual reality which helps a dentist to simulate the real life condition. This is very effective through virtual articulator, face-bow transfer and mandibular movement which analyse the static as well as dynamic occlusion and eliminates unwanted occlusal problem prior to designing of the prosthesis. With the advent of these tools, dentistry has become accurate, easier and effective.
Key words: articulator, face-bow, virtual Prosthodontics, CAD CAM
In the recent era, a lot of advancements are seen
in the field of dentistry, especially in materials
and manufacturing techniques. With the advent of
computers, the life of a dentist has become easier
and less complicated. The cumbersome laboratory
procedures and extensive casting method have
become simple and easy to do with the introduction
of digital technology in dentistry. The credit to this
goes to Duret et al who were the first to express
computer-assisted crowns and Mormann et al who
developed the CEREC System. Correspondingly,
Anderson and Oden developed the PROCERA
System and worked on nickel-chromium alloys. Whatever be the system followed, the process is
almost the same. The computer aided process
is broadly divided into three phases namely;
(1) the scanning phase, (2) the designing phase
(CAD software) and (3) the machining phase.
The scanning phase is undergoing development
with different type and forms of scanner while the
machining sector is also presently going through
lot of changes. However the design software has
undergone a drastic change with the creation of
virtual articulators which simulates the mandibular
movement and generates the dynamic occlusal
surface.
Need for virtual articulator can be explained
in two ways, First of all problems related to the
mechanical articulator can be reduced, secondly,
is the problems related to dental materials. The
difficulty in proper orientation of cast, the stability
of articulator, the difficulty in repositioning of
cast, difficulty in the bite registration without
leaving space, the deformation of registration
material and the use of rigid expanded plaster
materials which leads to reduced reliability in
reproduction of dynamic, excursive contacts can
be avoided. Moreover, it is possible to visualize
three dimensional TMJ movements and the path
followed by condyle. It helps in detailed analysis of static and dynamic occlusion. It is also applied for
orthodontic treatment plan and also for positioning
implant with a virtual set-up of suprastructure1.
The main advantage of virtual articulators is that
it helps in accurate repositioning of master cast
without use of interocclusal material, to quantify
the effect of resilience of the soft tissue on time
dependant basis during mastication, for better
communication between dentist and dental
technician, and to help dental students understand
the functional dynamics of dental articulator.
However, it does not simulate the biologic system
like complexity of movement, changes in TMJ
during loading and mobility of teeth.
Virtual articulator (software articulator) is an
articulator which exist as computer program
with virtual condylar and incisal guide plane.
They simulate mandibular movement by moving
digitalized occlusal surface against each other
providing a smooth movement. There are 2 types
of virtual articulator 1) Completely adjustable
virtual articulator and 2) Mathematically simulated
virtual articulator1. Completely adjustable virtual
articulator was designed by Kordass and Gaertner
of Greifswaid University of Germany in 2003. This
articulators records the mandibular movements
using an ultrasonic measurement system called
zebra jaw motion analyser (Zebris, Germany)
which is comprised of many ultrasound emitters
attached to labial surface of mandibular teeth
using an autopolymerising jig and sensor located
on a head frame. The two condylar points and
an infraorbital point are used to locate a plane
to which jaw movements are related and is saved
as ASCH file.
To program this articulator, it is necessary to
digitalize the tooth surface. This done by either
through a direct digitalization where tooth surface is scanned intraorally or indirect digitalization
where the cast is scanned. To visualize the image
the University of Greifwald, Germany developed
virtual reality rent CAM which operates on four
windows on the computer screen2. (1) The rendering
window displays both jaw during dynamic
occlusion like chewing (2) the occlusion window
which display static and dynamic contacts with
sliding movements during function and (3) the
smaller window which displays the movement
of TMJ in sagital and horizontal view (4) the slice
window which displays frontal slice throughout
the dental arch which helps to analyse the degree
of intercuspidation, and the height as well as
functional angles of the cusps. It helps to analyse
the guidance and to balance easily. It is used in
complex static and dynamic occlusal evaluations,
to improve the design of virtual prosthesis, adding
Kinematic analysis to design process, for occlusal
construction in CAD, CAM by kinematic method,
and dynamic visualization of occlusal surface.
Virtual articulator requires a virtual facebow to
transfer the planes. Hence a virtual face bow
recording is done.
A mathematically simulator virtual articulator3,4
by Szentoetery of Martin Luther University of
Halle in 1999 is a fully adjustable 3D virtual
articulator which reproduces movements similar
to a mechanical articulator. It has a curved
Bennett movement but behaves as an average
value articulator, hence it is difficult to obtain
individualized condylar path. At the same time,
there are different movements possible in identical
setting, making it a very versatile instrument. Egstratos 200, Szentpetery’s articulator.
Although virtual articulators are of good use, one
problem which exists is the transfer of the digitized
cast to this virtual articulator. This is solved by the
introduction of digital facebow which locates the
maxillary cast to the cranial coordinate system thereby making the transfer of the exact position
of the maxillary cast to the articulator. This is
done in two steps. The horizontal reference plane
is scanned first followed by scanning the dental
arches of the patient to obtain digital casts using
an extraoral or intraoral dental scanner. To obtain
horizontal reference point, place 3 skin –adhesive
targets on the head; one on the infraorbital point
and two next to TMJ (Point 1,2 and 3). Now the
scanner is attached the fixed part secured on the
head. These three points are scanned as point
A,B,C by touching the tip of the scanner to the 3
adhesive targets placed on the head. This gives
horizontal reference plane of the patient. Further,
maxillary occlusal plane should be related to this
horizontal reference plane. This done by identifying
3 most prominent cusps on the occlusal surface
of the maxillary arch. To do this, first place the
articulating paper in a flat metal facebow fork and
introduce the facebow fork into the patient’s mouth.
Position the facebow fork against the maxillary
cast and determine the 3 most prominent cusps on
the occlusal surface which is marked three points
(points 4, 5, and 6). These three points when joined
together determine the occlusal plane. Place
the pointer on these three points and transfer the
position of the pointer into the same coordinate
system with the COM professional software, which
coincides with the fixed part of each scan. With
reverse engineering software, blending these 6
points create patients co-ordinate system from the
first three points and occlusal plane form the last
three point. Now with rapid form software, align
the virtual model of maxillary cast to the cranial
co-ordinate system by applying the method of
least squares. It further transfers the maxillary
cast to the virtual articulator software and align
in such a way that the cranial co-ordinate system
coincides with the virtual articulator’s co-ordinate
system. Scan the mandibular arch and further
both arches scanned together with an intraoral
scanner in centric occlusion from 3 directions.
This is further transferred to the virtual articulator software5.
Further advancement occurred in this field with
the ability to record mandibular movement and
transfer this to the articulator. This is done in a
sequential manner.
First step is to construct the mechanical articulator.
Hanau H2 was the first articulator that has been
modeled as it has simple geometrical bodies. This
is done by taking measurements of this articulator
followed by scanning the parts. The scanning is
done using ATOS 1 3D Scanner which is followed
by designing3.
In the second step of design process, the drafts
are located on the correct position in space.
Hence the sections of the scanned point cloud
are fused using a Rapid form XOR software to get
the parts of the articulator. The whole articulator
has been constructed combining both measured
and scanned parts. Once the Virtual Articulator
is constructed, all the measures are verified.
The third step is to record the dental arches and
bite registration using a 3D scanner “scan BD”
by Willy tech. It scan’s 8000 to 14,000 points per
second with a reproducibility of 2 μm and accuracy
of matching of 10μm.
The fourth step deals with locating the models
on the articulator. For this purpose, the relative
position of the upper model is scanned using
facebow. The location of the lower model is made
using an electronic bite which was taken in centric
Relation.
The fifth step is to use the CATIA CAD system for
kinematic simulation on a virtual articulator6.
Similarly, Stratos 200 and the lvoclar stators 200
have been modeled using a solid Edge CAD
system. Some parts were modeled directly after measuring the mechanical dental articulator.
However, the Handyscan 3D scanner has been
used, due to its mobility and versatility as almost all
the articulator can be scanned. Using Geomagic
point cloud edition software, the useful data has
been taken from the millions of points that has
been scanned. The parts of the articulator modeled
in the CAD system and the scanned points are
converted to solid by means of the Rapid form
software. Further, they are assembled adding the
necessary constraints. Mechanical joints will have
to be created either by automatically converting
the existing assembly constraints or by manually
selecting different joints between parts. Then,
after adding the commands on articulators, the
user will be able to control the DOFs.
Each CAD system has different possibilities and
capacities for simulation. The project started
using the solid EDGE V18, Modelling the lvoclar
stratus 200 mechanical dental articulator. There
were problem to import digitalized models using
STL-files. This problem was solved by using the
V20 of Solid Edge, so the virtual articulator was
able to simulate excursive movements correctly.
The SolidEdge CAD system is not able to calculate
laterotrusion movements due to the contact surface
change that occurs at the same time with these
movements. These limitations were corrected
using the Dynamic Designer software, based on
the MSC, ADAMS simulation engine. Hence the
CATA DMU-Kinematics module has been used
for the following work. This module offers more
options than solidEDGE CAD system. Hence the
movements of the Hanau H2 and Denar MarkII
have been simulated more accurately. On the hand,
the movement of protrusion has been simulated
and the trajectory of the first lower left molar has
been analysed. On the other hand, the lateral
movement has been simulated using different
values of the Bennett.
The final step is to record mandibular movement
which is accomplished by using the jaw motion analyzer. Three skin adhesive targets are placed,
one on skin of TMJ, infraorbital point to obtain
a hinge- axis infra-orbital plane and a special
digitizing sensor analysis this plane. The ultrasonic
measurement system jaw Motion Analyzer (Comp
Zebris Isny, Germany ) is used to record and
implement the sonic impulses emitted from three
transmitters attached to the lower sensor which
analyses the mandibular movement. Four receivers
are attached to a face bow opposite them. This
positioning enables the detection of all rotation
and translation components in all degrees of
freedom. Movement data finally can be calculated
in relation to the digitized points. Silicon-based
jaw relation registrations are used to reproduce
the occlusion and this remain attached to the
upper teeth during opening. This registration is
stabilized with impression plaster on a metal
carrier plate. The digitizing sensor is attached to
detect three main reference points on the rear of
this metal plate. These three points are used to
combine movement data and the digitized dental
arches. First, the impression of the upper teeth is
digitized and then the record material and the
plaster of the lower teeth are scanned. Both dental
arches are correctly related to each other. The
digitized impressions of the lower and upper jaw
can be combined with the scanned data from casts
without losing the predefined jaw relationship.
By defining and calculating the same reference,
both data sets, were matched and presented in
the virtual articulator.
For the detection of tooth wear, there is another
model that semi-auto-matically analyzes the
teeth for signs of wear or bruxism. The algorithm
searches for facets and separates them from the
surrounding surface using special segmentation
algorithms. A CAD module allows the improvement
of the functional occlusion by manipulating the
occlusal surface. The occlusal profile of the teeth
can be designed with increased or decreased
cusps to eliminate occlusal interferences in the
dynamic pattern and optimize the occlusion. The
data set of newly designed and improved occlusal surface can be transferred to a milling machine,
producing real crowns and fixed restorations with
that particular optimized functional occlusion7.
To produce the VR articulator as a marketable
software tool University, of Greifswald, Kettenbach
GmbH Co KG, and the Fraunhofer Institute for
computer Graphics (IGD) cooperated to establish
the virtual articulator software system. If the jaw
motions analyzer tool is available, the patient
–specific jaw motion can be recorded. The data
can be loaded into the virtual articulator software
and the three dimensional movements of the jaws
are simulated. If the jaw motion analyzer tool
is not available, different jaw motions can be
defined via parameters, just as with the mechanical
articulator protrusion (parameters: radius of the
condylar pathways, maximal protrusion distance,
horizontal condylar slope).
Retrusion (parameters:radius of the condylar
pathways, maximal retrusion distance),
laterotrusion (parameters: maximal protrusion
value, Bennett Angle, radius of condylar pathways
(left, right ) horizontal condylar slope (left, right),
shift angle, immediate side shift), opening/closing
movement (parameter: maximal opening angle
parameters are entered). The occlusion detection
engine is triggered. For occlusion detection, a
distance can be chosen corresponding to the
thickness of the occlusion paper used in the
mechanical articulator. The occlusion points are
calculated according to this defined distance.
Virtual dentistry is raised to a higher level by the introduction of virtual articulators. Added to it the
digital facebow recording along with the recording
of mandibular movement has made life of a dentist
very simple and provide a high quality of treatment
and care. The designing process using all these
tools will revolutionize dentistry and replace the
mechanical instrument soon.