Luca Martinelli
Basic aspects of the face bow
Publication no. 24 - 3 June 2013
1 The face bow
1.1 Foreword
This is a complex field
that's quite difficult to discuss unless a certain amount of basic knowledge is
obtained beforehand, which in itself would require much time and effort.
We will nevertheless attempt
to simplify the argument by providing a brief overview of some of the main types of face bows and other
systems for adjusting the movements of the mandible, which are used for
programming the dental articulator.
Publication no.
6, entitled Basic aspects of the
articulator, showed how the articulator is used to replicate the movements
of the mandible.
Regardless of the movements
that a dental articulator is capable of replicating, the models must be
positioned using a face bow as this allows us to replicate the position of the
maxilla: a necessary position that allows the articulator to subsequently
perform the correct movements.
A face bow is a device that's used to measure the positional
relationship of the teeth and the maxillary crest with respect to the centre of
the glenoid fossa in three-dimensions: sagittal, vertical and
anterior-posterior (J.S. Landa 1954).
This means that it provides
us with a static position, or rather tells us how to position the models, but
does not tell us the movements they must make in order to replicate the actual
movement of the mandible.
In fact, the data for
programming the articulator's movements are recorded on the patient using other
tools, which also have varying degrees of accuracy, like the pantograph for
example: a tool that's capable of recording all the movements that the mandible
can perform.
Depending on the technique
used to record the mandible's movements, we will naturally use a certain type
of articulator that's capable of being programmed with the data recorded by the
dentist, or rather one that's capable of replicating the position and the
movements of the patient's mouth.
There are many systems available
on the market for recording the position and the movements of the mouth, the
structure and operating concepts of which are often quite different.
2 Readings
The masticatory organ is the
most complex motor system in the human body. In fact, we are able to perform a
large number of movements with our mandible in three different directions. So
how can these movements be recorded in such a way so as to allow the
articulator to replicate them?
There are basically two types
of readings that can be taken in the mouth:
-Static readings;
-Dynamic readings;
Static readings allow us to
pinpoint the location of the maxilla, which is often referred to as the
position of the skull.
Dynamic readings allow us to
pinpoint all the movements that the mandible might make during chewing and
during other physiological activities.
The face bow serves to
perform static readings, which are fundamental to properly positioning the
model within the dental articulator, or rather to replicating the exact
location of the patient's maxilla.
2.1 Static readings – Face bow
A static reading is a reading
that provides the position of the maxilla in relation to the hinge axis, or
rather the ideal axis that joins the two condyles of the mandible.
How can this reading be
obtained?
By using a tool called a face
bow,: an instrument invented by George B.
Snow in 1899.
There are different types
face bows:
- the kinematic face bow;
- and the arbitrary face bow;
there are two type of the latter:
the Facia type;
the Ear type.
2.2 The kinematic face bow
The kinematic face bow allows
for the precise determination of the patient's hinge axis (terminal hinge
axis), and therefore the spatial reference of the skull will be consistent with
the patient's actual hinge axis.
As we can see in Figure 1,
this bow is positioned upon the mandible's articulation point, after having
identified it both manually and by means of various movements performed by the
patient. Thanks to the insertion and the correct reading of a graph (Figs. 2
and 3), upon which extremities of the face bow write, the exact position of the
maxilla can be identified.
Fig. 1
Example of a kinematic face bow
Fig 2
Graph placed on the kinematic face bow
Fig 3
Graph with the determination of the position of
the terminal hinge axis
2.3 The arbitrary face bow
As indicated by the name,
this face bow is used to pinpoint the arbitrary hinge axis, rather than the
actual one. Therefore the spatial reference of the maxilla will not be 100%
accurate. Let's see why.
As we have seen, the
kinematic face bow takes readings that allow for the precise determination of
the position of the patient's terminal hinge axis.
The arbitrary face bow, on
the other hand, takes a reading of the maxilla that's based on a reference for
an approximate hinge axis.
2.4 The "Facia bow" type face bow
This face bow takes its name
from the fact that it rests upon the face, like the kinematic bow (Fig. 4).
It rests upon the arbitrarily
identified temporomandibular joint, or rather the hinge axis as identified by
touch. In this manner, the bow is applied without additional procedures,
proceeding directly with the recording of the maxilla's position.
Fig. 4
Example of a "Facia bow" type
arbitrary face bow
2.5 The "Ear bow" type arbitrary face
bow
In this case, once again, the
name is indicative of the device's operating principle.
The bow rests upon the
external auditory meatus (Fig. 5). The bow's earplugs (ends) are basically
inserted into the ears, and the position is subsequently taken making reference
to an axis that is actually farther back than the patient's true terminal hinge
axis.
Fig. 5
Example of an "Ear bow" type
arbitrary face bow
Articulators (or transfer
systems) whose programming make use of arbitrary Ear bow type face bows feature an artificial correction whereby the
position of the bow, which should be placed behind the actual hinge axis, is
moved forward by about 11/13 mm, or rather by the average distance between the
acoustic meatus and the terminal hinge axis (Photo 6).
(Weimberg
1961)
Fig. 6
The terminal hinge axis reference point with
respect to the acoustic meatus
To briefly recap:
In the short sequence below
(Sequence 1), we can schematically observe a face bow being inserted into the
patient's mouth, with the wax positioned on the fork (a), the face bow being
removed from the patient's mouth after the reference for the upper teeth has
been taken (b), the face bow showing the exact location of the patient's
maxilla on the articulator (c), the upper model being positioned on the imprint
left by the teeth in the wax on the fork (d), the upper model being cast on the
articulator (e), the articulator complete with the antagonist (the lower model
antagonistic to the upper model) (f), and finally we can see the skull next to
the articulator, which shows how the models have the same spatial positioning
as that of the patient's mouth.
In order to record the
movements of the mandible, we will naturally use instruments other than the
face bow, which, as previously mentioned, only performs readings of a static
type.
a
b
c
d
e
f
g
Sequence 1
The Video / Movie section of the blog martinelliluca.blogspot.it contains a video regarding the use of
the arbitrary Ear bow type face bow.
3 Dynamic readings - Joint wax - Gothic Arch -
Pantograph
Dynamic readings are used to
record the movements of the patient's mandible is such a way so that they can
be used to program the articulator, which in turn will be able to replicate the
mandible's movements.
As previously mentioned,
there are many tools for reading and recording the mandible's movements, many of
which are completely different from those that rely on the face bow and
pantograph theorum. For exemplary and entirely non-exhaustive purposes,
however, some of what might be considered to be the more well-known techniques
are described hereafter:
1 - Joint wax - used for
programming semi-individual articulators;
a simple yet not very precise
method;
2 - The Gothic Arch - used
for programming semi-individual articulators; this method is also not very
accurate;
3 - The Pantograph- used for
programming the individual articulator;
certainly a more precise
method than those listed above.
3.1 Joint wax
Performing readings using wax
is extremely simple. The wax is placed in the mouth of the patient, who is
required to perform the following movements:
1 - The centric position;
2 - The maximum protrusion
position;
3 - Right laterality;
4 - Left laterality;
3.2 The Gothic arch
The Gothic Arch (Fig. 7) is a
rather dated tool (Designed by Gisy ca. 1901) that fell into disuse, but has
recently been cited in various works of literature.
Fig. 7
Examples of Gothic Arches
The tip of the Gothic Arch
records the movements of the jaw by engraving them on the plate opposite a kind
of arrow (Fig. 8), the segments of which indicate the following:
Fig. 8
The incision of the mandible's movements upon
the Gothic arch
1 - protrusion and retrusion
movement;
2 - left lateral movement;
3 - right lateral movement;
4 - the vertex of the Gothic
arch, which corresponds to maximum spontaneous retrusion.
Once the reading has been
taken, it is placed in the articulator (Fig. 9).
Fig. 9
Example of a Gothic Arch and models in an
articulator
3.3 Pantograph
The pantograph basically
records all the mandible's movements, including protrusion-retrusion, laterality
and the bennet movement.
It's a particular tool that
dates back to the early 1900s (the Gysi axiograph of the early twentieth
century - the Stuart pantograph of 1934), and requires a certain amounts of
skill and experience.
At first glance the device
(Fig. 10) resembles a face bow, upon which the mandible's movements are
transcribed by instructing the patient to perform certain movements (Fig. 11).
Fig. 10
Example of a pantograph
Fig. 11
Section of the pantograph with the readings
shown on the charts
All the data is translated
into degrees and mm via the articulator's programming.
4 Advanced techniques - a brief introduction
It should be noted that today
there are more advanced techniques (Fig. 12) that make use of ultrasonic arcs,
connected to computers with graphical representations and parameter
calculations for programming the articulator, which record and analyse all the
movements of the patient's mandible.
Fig. 12
Example of an ultrasonic arc and management software
Finally, there are also a
large number of auxiliary tools like joint noise catchers and EMGs for
verifying muscle activity, as well as a range other instruments used for
different techniques/theories, like the kinesiograph and the myo-monitor, which
will naturally not be discussed here.
5 Combinations of Face bows (and other
recording devices) and Articulators
As we have already seen,
there is a wide range of theories, articulators, and devices for recording
patient data.
The current trend in the
field of dentistry is to be able to perform an acceptable job from an ethical
(and legal) standpoint, was well as a job that is aesthetically, functionally
and economically acceptable to both the patient and the dentist alike.
It is therefore always desirable
to find a quick, easy and inexpensive solution.
In order to make some
economic considerations, below we will take a general look at the combinations
of recording devices and articulators necessary for reproducing the data.
5.1 Kinematic face bow and pantograph - Individual Articulator
- Very expensive;
- Very complex (lots of
experience required);
- Results often
unsatisfactory due to the complexity of use in the clinic and the laboratory;
- Not very common.
5.2 Kinematic face bow - Semi-individual
Articulator
(It should be noted that all
semi-individual articulators currently make use of the arbitrary Ear Bow type face bow)
- Still rather expensive;
- Quite labour intensive
(lots of experience required);
- Satisfactory results on
average;
- More common than individual
Articulators, but not very widespread.
5.3 Arbitrary face bow (the Ear Bow type is much more commonly
used with respect to the Facia Bow
type) -
Average value articulator (fixed
or variable)
- Inexpensive;
- Simple (little experience
required);
- Provides for excellent
results;
- By far the most commonly used worldwide.
Fig. 13
Examples of Ear bow type arbitrary face bows
While our intention is not to
discuss what's right and what's not,
the current tools that are most commonly used due to their speed, cost and
ease of use are Ear bow type
face bows in conjunction with average value articulators.
Sequence 2 shows the
arbitrary Ear bow type face bow being
prepared for use (1); the fork with the wax placed upon it for recording (2);
the fork being placed in the patient's mouth (3); the subsequent placement of
the face bow on the patient (4) - in this case it is oriented with the orbital plane;
the adjustment of the nasion (5) and the locking of the fork to the face bow
(6); the adjusted face bow in position (7); the face bow being removed from the
patient's mouth (8); the fork retention block being removed from the face bow
(9) and cast on the transfer table (13); the fork retention block being removed
from the table once the plaster has hardened (15); the table being placed on
the articulator (18, 19, 20); and finally the casting of the model to the
articulator (21,22).
Sequence
2
Example of the use of an arbitrary Ear bow type
face bow
Related publications
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