WHAT IS CENWC RELA~ONP

GEORGE A. HUGHES, D.D.S.,* AND CARL P. REGLI, D.D.S.*”

University of California, School of Dentistry, San Francisco, Calif.

T HE IMPORTANCE OF CENTRIC in complete denture prosthodontics has

RELATION
been well known for many years. Some dentists claim that a correct centtic
relation is the most important single measurementmade in the construction of
complete dentures, This opinion is clearly set forth by Tench,l Tuckfield, and
Block.3
Unfortunately, however, no common agreement has been reached as to what
constitutescentric relation or how it is best determinedand recorded for all patients.
Various terms have been proposed, someacceptedfor a time and then modified or
rejected. The old phrase “taking the bite” gave way to the more nearly correct
term “central occlusion.” The term “central occlusion” was rectified and two more
explicit terms were accepted,namely, “centric relation” and “centric occlusion.”
Perhaps the term “centric relation” should be rephrased as “centric maxillomandi-
bular relation” so that it would be more definite.4

METHODS OF REGISTERING CENTRIC RELATION

Centric relation is generally defined as the “most retruded relation of the

mandible to the maxillae when the condylesare in their most posterior unstrained
positions in the glenoid fossaefrom which lateral movementcan be made, at any
given degreeof jaw separation.“6Many prosthodontists are not in completeaccord
with this definition, Hence, modifying terms suchas “true centric, functional centric,
acquired centric, and habitual centric” are used. Some prosthodontists believe that
vertical dimension is a part of centric relation617while some believe that centric
relation should be recorded with zero pressures318 Others g~lorecommendedthat
light pressure be used. Silverman4insists that heavy stress be exerted while reg-
istering centric relation. The proponents of the nonpressure technique invariably
incorporate remounting and equilibrating the occlusion after dentures are con-
structed to compensatefor resiliency of tissue under masticating pressure. The
proponents of the functional or pressure techniquesexpect to do little or no cor-
rection of the occlusionafter delivery of the dentures.
The most commonly recommendedmethod of determining centric relation is
the Gothic arch (needle point) tracing proposedby Gysi in 1910.11Since that time,
the tracing method has been both used and abused.Tenchl acceptedthis method as
the only reliable way of determining centric relation. Hanau* recommendedthe
--
Read before the Amerkan Dental Association, Dtilas, Texas.
*Professor of Denture Prbsthesis, chairman of $he Wvtsfon of Prosthodontics.
**Professor of Denture ProstheS, Chairman of Den&@ Prosthesis.
?lg; ‘1’
u
WHAT IS CENTRIC RELATION? 17

use of the Gothic arch tracing but warned that it was a means of checking mandib-
ular position and should be neither overrated nor underrated. Granger12 insists
that needle point tracing is not a reliable means of determining centric relation,
since it is recorded in a horizontal plane only. He believes that centric relation
should be considered as a vertical rotational relationship related to the hinge axis.
However, most prosthodontists seem to agree with Sears13 “that some form of
tracing device is desirable if we hope to arrange posterior teeth with the required
accuracy.”
The prosthodontists who use some type of needle point tracing are not in
accord regarding the relationship of the stylus to the tracing for determining centric
relation. SwaggarP4 wants some freedom of movement. Brill15 claims that the
retruded position of the mandible (stylus at the apex of the tracing) does not
coincide with the maximum intercuspation in all persons. Trapozzanoia insists
that the retruded unstrained relation is the only proper position and that this
position is constant throughout life. Boos l7 claims that 35 per cent of 400 subjects
had their “best” centric position 1 to 7 mm. distal to the apex of the Gothic arch
tracing. Brown I8 believes that the needle point tracing is unreliable and recommends
repeated closures into wax under close observation. Jamiesonle recommends an
interocclusal record made with the tongue held at the posterior part of the upper
baseplate. Mandibular placement with manual pressure by the dentist is recom-
mended by some, but this method is questioned from a physiologic basis by Brill.lG
Why are there so many opinions and why so much confusion? The calm,
reflective attitude by Kingery? may be the solution. He states, “The position of
the horizontal relationship of the mandible to the maxillae is a positional relation-
ship inherent to each individual.” Boos20 probably comes close to explaining the
situation when he states that “in normal cases the occlusion, the temporomandibular
joints, the bone, the soft tissue, and the musculature all produce the same relation
to each other and any one of the many registration technics can be used.”
Unfortunately, no one method of registering centric relation will meet the
needs of both normal and abnormal patients. If all patients were normal, we would
consider “that centric relation is unique in that its regulation is the one and only
mechanical step in denture construction.“21 If this were true, uniformly satisfactory
results would be obtained by the use of a Gothic arch tracing with the stylus always
set at the apex of the tracing. Unfortunately, some patients do not fit this pattern
and must be considered abnormal. Hence, the abnormal must be differentiated from
the normal.

EXPLANATION OF GOTHIC ARCH TRACING

A Gothic arch tracing with a sharp apex can only be obtained by a pivoting
of the mandible on first one condyle and then the other as right and left lateral
movements are made. If the condyles do not pivot or do not have centers from
which lateral movements are made, a faulty tracing or one with a rounded apex will
be obtained.
Most of the proponents of the needle point tracing method assume that a
correct tracing with a sharp apex is indicative of condyles properly situated in
their glenoid fossae. Unfortunately, the tracing merely indicates pivoting condyles,
 J. Pros. Lkri
18 HUGHES AND REGLI Jan.*Feb., 1961

not whether the pivoting occurs in normal, retruded, intruded, or protruded posr-
tions in the glenoid fossae. It is hoped that obtaining a tracing under unstrained
conditions will prevent an improper location of the condyles.
Many patients have developed faulty condylar positions because of malocclu
sions, either natural or the result of orthodontic treatment or faulty prosthesis.
Because some patients can accommodate the muscular positioning of the mandible
by tactile sensations from the teeth and periodontal membranes, the muscles ap-
parently are conditioned away from their normal action. ,qfter the natural teeth
are extracted, the condyles can assume a faulty position without the patient being
conscious of any strain. The Gothic arch tracing certainly can he misleading for
these patients unless much care and judgment are used.

Fig. l.-The posterior tracing is made from a free closure position. The anterior tracing is
made from the generally accepted retruded position of the mandible.

DIFFERENTIATING NORMAL AND ABNORMAL PATlENTS

In order to differentiate the abnormal from the normal patients, some type oi
an extraoral tracing device is essential. Gothic arch tracing is used as an indicator
or aid in observing horizontal mandibular action.

PROCEDURE

The vertical dimension of occlusion is established within close limits as a

separate and prerequisite step to determining centric relation. The tracing devices
are attached to the occlusion rims and the rims are placed in the mouth. The patient
opens and closes the mouth a number of times, and the relation of the stylus to the
table which has been coated with a thin layer of black wax is noted. The patient is
instructed to make the maximum anteroposterior movement of the mandible to es-
tablish the protrusive range. Next, the patient moves the mandibie forward, then
backward. This procedure is best demonstrated first by the dentist. When the patient
has the mandible in a retruded position, he is instructed to move the jaw laterally to
either right or left and to stop. The stylus is elevated and the patient is instructed to
bring the mandible back to the retruded position. Then, the patient moves the jaw
to the opposite side. The relationship at the initial point of contact to the apex of
the tracing is observed. It may be necessary to repeat the procedure several times
until a sharp, w&-defined tracing is achieved.
?2E ‘1’ WHAT IS CENTRIC RELATIQN? 19

The stylus is removed and the patient opens the mouth, closes it, swallows
several times, and then holds the mandible steady. The stylus is replaced, and the
relationship of the point to the apex of the retruded tracing is noted again. Many
times the stylus will rest at the apex, and these may be concluded to be normal
patients. Sometimes, the stylus will rest distal to the apex of the retruded tracing.
W’hen this occurs, the patient is instructed to hold the occlusion rims lightly
together and to move the mandible first to one lateral position and then to the
other. Frequently, a different Gothic arch tracing can be made. This second tracing
may have a greater included angle than the first tracing (the retruded tracing) or
it may have the same included angle, i.e., the lateral movements are parallel but
two distinct tracings have been recorded (Fig. 1). Both tracings are examined,
and if the more distal tracing has a sharp apex, the tracing made with the jaw
protruded rather than the conventional tracing with the jaw retruded is used as
the preferred registration.
Cephalometric roentgenograms of the temporomandibular joints indicate that
during the tracing made with the jaw retruded, many patients of this type have
the condyles in the posterior part of the glenoid fossae. When the mandible is
positioned with the jaw protruded (distal tracing), the condyles are in the anterior
part of the glenoid fossae, which is their normal position.22
If the patient rests with the stylus distal to the apex of the Gothic arch tracing
but cannot make a well-defined distal tracing with a sharp apex at the resting

Fig. 2.-.The patient was not comfortable at any position anterior to the erratic posterior tracing.

point, the case history should be reviewed. Was the patient without posterior teeth
for a period of time and did he learn bad habits of mastication, or did the patient
wear a nonfunctioning partial denture which caused the same result? If this has
occurred, the sharp tracing made with the jaw retruded is probably the correct
centric relation, and an attempt is made to redevelop proper function of the muscles
of mastication by one of several methods. We use the House chew-in method most
frequently.
Occasionally, the patient cannot make a sharp tracing starting from the point
where the stylus contacts the recording table, and it is definitely uncomfortable for
him to assume the retruded position. Some patients have to use enough muscular
effort to make a tracing with a sharp apex that muscular spasms of the infrahyoid
 HUGHES AND REGLI J. Pros. i~iii.
LO Jan.-Feb., iY(iI

muscles occur (Fig. 2). For these patients, the PattersotG3 grinding method !.li
developing occlusal curves is used and flat teeth are set to the curves thus developed.
Another type of patient encountered occasionally comes to rest with the stylus
not at the apex of the tracing but part way out on one of the lateral paths. Patients
of this nature have developed faulty muscular patterns because of cuspal interference
of either natural teeth or dentures which were not in proper centric relation. The
relationship of the condyles in the fossae is observed with cephalometric roentgeno-
grams. Roentgenograms of the joints at rest position, with the occlusion rims in
contact, with the stylus in the relation to the tracing that the patient wishes 10

Fig. X-The tracing needle is to the right oi’ the tracing apex in the free rlosure position because
of a temporomandibular joint irregularity.

Fig. 4.-The posterior tracing to the left of the anterior tracing was caused by a disharnion>
between centric occlusion and centric relation of the patient’s natural teeth.

assume, and with the stylus at the apex of the Gothic arch tracing, are made. If a
posterior relationship of the eondyle is noted on one side when the stylus is at the
apex of the tracing, an abnormal unilateral condylar position may be suspectecL
Treatment with splints may be indicated to reposition the condyles in a more
favorable location in the glenoid fossae. These patients usually have a history of
temporomarldibular joint dysfunction (Fig. 3). If an examination of the roentgeno-
grams indicates a favorable relationship of the condyles in the glmoid fossae when
the stylus is at the apex of the tracing and a posterior placement of the condyle
on the side where the stylus rests to one side of the apex of the tracing, a muscular
$rJlw$~K ‘11 WHAT IS CENTRIC RELATION? 21
u

imbalanc:e is suspected. These muscular imbalances are frequently caused by faulty

centric relations of either natural dentitions or dentures. The apex of the tracing
is used as an indication of centric relation for these patients (Fig. 4). The use of
splints may be indicated prior to denture construction.

REGISTRATION OF CENTRIC RELATION

After centric relation has been determined, the next problem is to register
this relationship so that the casts can be properly mounted on an articulator. There
is no method which can be used for all patients. The House chew-in method*
registers the maxillomandibular relation under chewing conditions and compensates
in part for tissue resiliency. If the ridge conditions will not permit the use of the
House chew-in, a central bearing technique with an extraoral tracer is used. The
Robinson Equilibratorz4 is recommended in place of the single central bearing stud
if denture space permits.
If the patient seems to be absolutely unable to accept the apex of any Gothic
arch tracing as centric relation, the Patterson chew-in technique is used to develop
occlusal curves. The rims are secured together in the position previously determined.
Nonanatomic teeth are set to the curve generated to achieve functional balanced
occlusion in all ranges and in almost any relationship of the mandible to the
maxillae. Anatomic teeth are recommended for other patients.

SUMMARY

1. The correct centric relation is essential in complete denture construction.

2. Prosthodontists disagree as to what constitutes centric relation and how it
is best registered.
3. The present accepted definition of centric relation is based upon the rela-
tionships of the mandibular condyles to the glenoid fossae under normal or ideal
conditions.
4. The Gothic arch tracing is the best visual device for ascertaining the
horizontal relationships of mandibular positions and two dimensional movements.
5. The Gothic arch tracing does not determine centric relation.
6. A sharp Gothic arch tracing may be obtained with the condyles in more
than one location in the glenoid fossae.
7. A method is advocated to utilize and analyze Gothic arch tracings in order
to determine centric relation.

CONCLUSION

There is no doubt of the importance of centric relation. However, the present

definition is misleading. Centric relation may better be defined as “that relation of
the mandible to the maxillae when the condyles are in the most protruded or
anterior position in the glenoid fossae from which a definite needle point tracing
can be made at the correct degree of jaw separation.”
*This method is used as a standard procedure in the University of California Denture Clinic.
22 HUGHES AND REGLI

REFERENCES

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UNIVERSITY OF CALIFORNIA
SAN FRANCISCO MEDICAL CENTER
SCHOOL OF DENTISTRY
SAN FRANCISCO 22, CAI.IF.