2010 RWISO Journal - Roth Williams International Society of ...

tionship between the teeth, the joints, and the neuromusculature.
This information provides a physiologic treatment goal
for the orthodontist, a summary of which can be made by analyzing
the system in loaded and unloaded conditions. When
loaded, eg, during a swallow, the condyles are fully seated
upward and forward in the fossae, the elevating muscles are
active, and the dentition is in full intercuspation. 41,55,62,63,65,66
When unloaded, the condyles remain in firm and constant
contact with the disc and eminence, elevating muscles are inactive
and positioning muscles (eg, lateral pterygoids) are active,
posterior teeth are out of contact, and the anterior teeth
play a major role in guiding mandibular movements. 67-75
Given a reliable perspective of optimal static and dynamic relationships
between the teeth, joints, and neuromusculature,
we can consider some additional principles regarding gnathic
function. There are at least three reasons why the intercuspal
position is important. First, the positions and the shapes of
the teeth determine mandibular movements at and near the
intercuspal position. 7,50,61,76-100 Second, when the mandible is
brought to full intercuspation in a functionally healthy system,
the powerful elevating muscles are active and the system
is heavily loaded; the bulk of the resultant force is absorbed
by posterior teeth. 32,50-52,101-103 Third, condylar position is determined
by the dentition at intercuspation. 61,83,104-106
An additional important factor well supported in the literature
is the clinical observation that the neuromusculature
is exquisitely programmed to guide the mandible to the intercuspal
position80,85-100,107 ; the intercuspal position is dominant
over condylar position. 61,83,103,105,106,108-110 Thus, asking
a patient to “bite down” provides no dependable information
as to where the condyle is positioned. Moreover, efforts
to identify the seated condylar position through clinical
maneuvers such as manipulating the mandible are not reliable.
28,111-116 To quote the master clinician Dr. Thomas Basta,
“Don’t believe what you see in the mouth.” 2 Thus the value
of using interocclusal devices such as cotton rolls, anterior
jigs, and splints to deprogram the neuromusculature.
If we are to apply these physiologic principles to the
practice of orthodontics, we need additional information
besides that which we have traditionally used; for example,
techniques that record the optimal or “seated” position of
the condyle. Currently there are numerous such techniques
employed in restorative dentistry. Many clinicians use a hard
stop at the incisor midline to separate the posterior teeth,
along with a soft posterior material that can be hardened
thermally or chemically. When the patient bites against the
hard anterior stop and the neuromusculature seats the condyles
superioranteriorly, the posterior material is hardened,
and the musculoskeletally stable position of the mandible is
recorded (Figure 1).
70 Girardot | Physiologic Treatment Goals in Orthodontics
Figure 1 The anterior stop is hard and flat; it separates the
posterior teeth to create appropriate space for a recording
medium. The patient is instructed to close firmly, which
seats the condyles to the musculoskeletally stable
position of the mandible.
The information then must be transferred from the patient
to a device that will allow study and treatment planning
of the gnathic system in three dimensions. Currently,
the articulator appears to be the best tool for this purpose,
although computer-generated three-dimensional technology
may replace the articulator in the near future. Casts mounted
on an articulator provide invaluable physiological information
for diagnosis and treatment planning. For example,
numerous studies show that there is nearly always vertical
distraction of the condyle when the patient closes to intercuspation.
33,113,117-122 It is all but impossible to record, analyze,
and treatment plan this vertical discrepancy without the
use of a device such as an articulator.
Joint images are another tool that can serve orthodontists
with regard to physiologic treatment. Tomograms, as
first advocated by Ricketts, have provided an effective way
to study the health of the temporomandibular joint and the
position of the condyle in the fossa. 123-125 At present, cone
beam CT is a more effective way to study the temporomandibular
joint, as it provides a more-lucid, three-dimensional
view of joint structures. 36
There are sound data to support the concept that optimal
gnathic function can be defined and used as an evidence-based
treatment goal. There is little doubt that this
would also aid communication between orthodontists and
other dental professionals. In addition, knowledge of gnathic
physiology is of substantial value to orthodontists in that it
helps them to recognize and avoid myriad problems that occur
in everyday practice. ■