Basic aspects of the face bow
Publication no. 24 - 3 June 2013
1 The face bow
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.
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 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.
Example of a kinematic face bow
Graph placed on the kinematic face bow
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.
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.
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).
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.
c d e
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.
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:
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).
Example of a Gothic Arch and models in an articulator
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).
Example of a pantograph
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.
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)
- Simple (little experience required);
- Provides for excellent results;
- By far the most commonly used worldwide.
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).
Example of the use of an arbitrary Ear bow type face bow
some of these photos are photos posted on the web, anyone who believes they have the right to object to the use he gives notice, the photos will be removed immediately.