RESEARCH AND EDUCATION SFCTION EDITOR

JOHN J. SHARRY

Relation of Gothic arch apex to dentist-assisted centric relation

Michael Myers, D.M.D.,* Robert Dziejma, D.D.S.,** Joel Goldberg, D.D.S.,*** Robert Ross, Ph.D.,**** and John Sharry, D.M.D.*****

Medical University of South Carolina, College of Dental Medicine, Charleston, S. C., and University of Texas Dental School, San Antonio, Texas

v b’l’ f aria 1 rty o mandibular position as indicated by the apex of the Gothic arch tracing from method to method and over periods of time has been described.‘-“’ Many clinicians have argued that a posterior position of the jaw located with firm pressure by the dentist’s thumb and forefinger on the patient’s mandible may be less variable and located more posteriorly than the Gothic arch apex and thus be more reliable as a reference position for occlu- sion.

The apex of the Gothic arch tracing recorded by a pin attached to one jaw which writes on a plate attached to the other, is considered a starting point from which protrusive and lateral jaw movements are made by patients. The reference point designated centric relation has been defined as the most retruded unstrained relation of the mandible to the maxillae at a given degree of jaw separation.

Several contending schools of thought have devel- oped regarding centric relation, due in part to the looseness and confusion of terminology and in part to misunderstandings related to varying anatomic and physiologic influences upon mandibular posi- tion. Unfortunately, each term has a specific detailed connotation to each listener. Sicher” considered centric relation the ideal position which coincides with the “median occlusal position.” Meyers” defined centric relation as “the position of the mandible as determined by the neuromuscular reflex

*.4sslstant Professor, Department of Crown and Bridge Prosthe- tics, Medical University of South Carolina, College of Dental Medicine

**Worcester, Mass. ***Newport Richey, Fla ****.\ssociate Professor, Office of Educational Resources, Medi-

cal Umversity of South Carolma. *****Dean. Umversity of Texas Dental School at San Antonio.

78 JULY 1980 VOLUME 44 NUMBER 1

first learned for controlling the mandibular position when the primary teeth were in occlusion.” Lucia” and McCollum and Stuart” wrote that the mandi- ble is in centric relation when the centers of vertical and lateral motion are in their terminal hinge position. Sheppard” felt that the relationship of the mandible to the maxillae when the mandible is braced during swallowing demonstrates centric rela- tion. Baer’” contended that centric relation and physiologic rest position are one and the same. Page” and Rader’” expressed doubt that there is any practical way of locating or using centric relation. Posselt” stated that mandibular border positional movements are controlled by the ligaments. Bouch- er”” wrote that the muscles determine the normal limits of border movements. Aprile .lnd Saizar” expressed the opinion that ligaments and capsules of the temporomandibular joints are the positioners of the mandible. Moses” did not accept fixed points of reference of soft tissues as life long factors in the registration of mandibular positions. Sears’” postu- lated that the soft tissues posterior to the ramus are also a controlling factor of mandibular position.

MATERIALS AND METHODS

A group of 25 subjects between 20 and 30 years of age were chosen for this study.* All had a complete or nearly complete healthy dentition. Alginate (ir- reversible hydrocolloid) impressions of both the upper and lower arches of each subject were made and stone casts poured.

.c\ppliances (clutches) of autopolymerizing acrylic resin were constructed on the maxillary and mandi- bular casts to cover the occlusal surfacer of all of the posterior teeth and to extend to the height of contour

*Three subjects were soon excluded from the study because the) did not record a clear Gothic arch tracing

CK’Z-3913/80/070078 + 04800.40/00 1980 The C. V. Mosby Co.

GOTHIC ARCH APEX vs DENTIST-ASSISTED CENTRIC RELATION

of each tooth. Sharpened steel central bearing points were.positioned near the center of the arch. On the lower cast, a milled steel plate was attached to the acrylic resin so as to be appropriately located for scribing a Gothic arch (needle point) tracing. The lower steel plate was an unequal trapezoid and could be seated in only one manner. Both of these tracing components were mounted at a specific vertical dimension of occlusion for each subject and, once fixed, this dimension did not change during the exheriment.

A micrometer table was constructed with two micrometers and a receptacle for the steel plates. The micrometers have pointed rods and are permanently attached to the table, each rod at a right angle to the other. The first rod recorded the position of the apex in a mediolateral direction, and the second rod recorded anteroposteriorly (Fig. 1). The pointed rods were placed as close to the plate as possible without making contact with it or with the ink on it. The receptacle in the micrometer table was accurately machined to accept the central bearing recording plates which were identical to each other. A hole was placed in the center of the receptacle so that the recording plates could be pushed out from beneath. The observation angle of the apex of the tracing was kept constant to diminish possible sources of error. To achieve this goal the right eyepiece of a binocular dissecting microscope was used; the left eyepiece was covered over.

All registrations were made with the subjects seated in a dental chair so as to orient the Frankfort horizontal plane parallel to the floor. Before placing the clutches into the subject’s mouth, the scribing plate was painted with a thin coat of draftsman’s layout fluid. When the fluid had dried, the clutches were placed in position. The subject then closed until the central bearing point made contact with the metal plate at which time the subject was instructed to move the mandible through all possible excursions maintaining light contact of the pen and plate. During this process, the subject was never touched by the investigator; only visual and verbal directions were given.

When the Gothic arch had been accomplished, both of the clutches were removed. The metal plate was then separated from the lower clutch and placed into the special receptacle in the micrometer table. Coordinates of the apical positions were recorded for both micrometers which were calibrated to read accurately to 0.01 mm. Three investigators recorded the measurements independently. An average value

Fig. 1. Most retruded point, viewed through the right side eyepiece of a dissecting microscope, was located by recording two micrometer coordinates.

for the three recordings was used for analysis. Each subject recorded consecutive Gothic a.rch tracings on the first and seventh days.

After the readings of the apex were made, the plate was replaced in the clutch, painted over with draftsman’s fluid, and both clutches were again placed in the subject’s mouth. Then, one of the investigators placed his thumb and fore-finger on the subject’s jaw and moved the jaw as far posteriorly as possible. This movement was repeated out and back, several times. The jaw was moved firmly by the operator. This record was removed, and the coordi- nates of the most posterior markings were measured on the micrometer table. The vertical jaw separation was identical for both the Gothic arch method and the dentist-assisted method, and it remained con- stant during the course of the experiment. Again, three investigators recorded independent measure- ments. An average for the three recorded values was used for the statistical analysis, and again each subject recorded consecutive Gothic a.rch tracings on the first and seventh days.

RESULTS

The data (Table I) reveal that: 1. The dentist-assisted position (DA) was located

posterior to the Gothic arch apex (GA) in 9 of 22 subjects on the first day.

THE JOURNAL OF PROSTHETIC DENTISTRY 79

MYERS ET AL

Table I

Subject No.

First day Seventh day

Gothic Dentist- Gothic Dentist- arch assisted arch assisted

2 Il.44 11.52* 11.37 1142

3 10.92 1106 10.74 10.73

4 9.72 962 9.46 9 35

5 10.23 10.16 10.25 10.12

6 7.13 7 00 7.05 6.78

7 8.49 8.39 8.29 8 34

8 13.45 13 29 13 44 13.35

9 13 47 13 26 13 51 13.29

10 7 51 7.29 7.29 7.29

11 12.00 12.35 12.35 12.35

12 10.02 9 92 9.95 9.75

13 12.15 12 19 12 17 12 07

14 11.36 11.41 11.30 11.51

15 10.71 10.81 10.60 10 93

16 10.35 10.35 10 38 10 55

17 1193 12 08 12.15 12.13

18 12.28 12.10 12.31 12 42

19 1167 11.60 11.66 11.77

20 11.50 1145 11.26 11 52

21 8 85 8 98 8 89 9.27

22 7 44 7 60 7.63 7.59

25 11.50 11.02 1140 11 23

*The larger number Indicates [he more posterior mandibular

positton.

2. DA was posterior to G.4 in 9 of 22 subjects on the seventh day.

3. Only four subjects, however, showed DA to be posterior to G.4 on both days.

4. The location of DA varied from first to seventh day in 20 of 22 subjects.

5. The location of GA varied from first to seventh day in all 22 subjects.

6. There appear to be no statistically significant differences in the reliability of one method over another.

If one accepts a variation greater than one tenth millimeter as clinically significant, it must be noted that GA position varied from first to seventh day, greater than 0.10 mm in 10 subjects; DA varied greater than 0.10 mm from first to seventh day in 14 subjects.

DISCUSSION

It has been commonly accepted by most clinicians that the most retruded position of the mandible (centric relation) was the most duplicable position of the jaw. Furthermore, in practice, many clinicians exert thumb pressure against the jaw (pressure which varied from gentle to forceful backward pressure) believing that it will bring about a jaw position

which will be reproducible. There is, however, grow- ing argument about the accuracy of the most poste- rior position hypothesis.

The evidence suggested by this study suggests that the most posterior dentist-assisted position of the jaw is not more reproducible over several visits than the apex of a Gothic arch tracing. The data indicate that, in some instances, the Gothic arch tracing was actually more posterior than the dentist-assisted position at the same visit in any particular subject. One wonders whether some patienl:s react to the force from the thumb by a conscious or unconscious resistance to posterior movement of the jaw. The duplicability of jaw records during one patient visit (which may last up to an hour and a half) is reasonably predictable. However. when the position obtained at any one sitting is compared with a similar position taken days or weeks later, the variability may be considerable.

The critical question to be resolved in regard to jaw position is not whether the position varies from one method to another during a particular patient visit; it is unlikely that a dentist at any one visit will use more than one particular method. Rather, it is to determine whether the variation using a particular method is greater or lesser from ‘3ne sitting to another. Certainly, the determination of the position at one time during the treatment phase is made to establish an occlusion which will be tested in func- tion at a later time.

Obviously, if there is a great deal of variability in a particular method over a period of time, some other means to incorporate the variability must be chosen for the final restoration. The immediate temptation is to establish a “zero-degree” occlusion as is done in

treatment of edentulous patients. This, however, is not necessarily an optimal or effective occlusion for all patients. Rather, it might be prudent to use a cusp form with a flat area of contact around “centric relation” (perhaps a millimeter) so that minor varia- tions occurring from one time to another will be accommodated in this “free centric area” and cusp effectiveness can still be maintained.

CONCLUSIONS

These data suggest that the widely held belief that thumb pressure can position the mandible consis- tently more posterior than the position indicated by the Gothic arch apex is unfounded.

Furthermore, this study provides no evidence to support the contention that the dentist-assisted jaw relation is more reproducible than the relation indi- cated by the Gothic arch apex.

80 IULt’ 1980 VOLUME 44 NUMBER I

GOTHIC ARCH APEX vs DENTIST-ASSISTED CENTRIC RELATION

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THE jOURNAL OF PROSTHETIC DENTISTRY 81