THE ACCURACY OF AN EAR FACE-BOW

WALTER R. TETERUCK, BSc., D.D.S.,M.S.D.," AND

HARRY C. LUNDEEN, B.A., B.S., D.D.S"" University of Kentucky, College of Dentistry, Lexington, Ky.

T HE PROPER USE of an anatomic articulator is dependent upon an accurate face-bow transfer.l The terminal hinge axis is generally accepted as the most

stable reproducible point from which maxillary casts can be mounted.’ In 1961 Lau- ritzen” described a relatively simple method of locating the true hinge axis. How- ever, the arbitrary location is still the most common method of determining the axis. The most popular arbitrary method employs a point approximately 13 mm. anterior to the tragus of the ear on a line from the tragus to the outer canthus of the eye. The chief advantage of the arbitrary method is its ease of application to all situa- tions requiring the mounting of casts. Results from several stud:es comparing both methods are conflicting.S-6

1 t was the purpose of this study to compare the accuracy of a new arbitrary method of mounting maxillary casts to both a true hinge axis and the conventional 13 mm. arbitrary axis method. The new method (ear face-bow)+ employed the external auditory meatus and nasion as reference points.

In addition to the primary objective of the investigation, measurements of the vertical position of the cast between the maxillary and mandibular members of the articulator as dictated by the nosepiece were recorded.

ADJUSTING THE FACE-BOW FORK

Accurately fabricated maxillary casts were obtained from 47 subjects with good complements of firm teeth. All three methods of locating the axis and trans- ferring it to the articulator were employed on every subject.

The same face-bow fork was used in all transfers to standardize the three methods of mounting. Black modeling compoundf was placed on the face-bow fork because of the dimensional stability and accuracy of the compound. The fork was centered on the upper teeth and the subjects were instructed to close gently into the softened modeling compound to register light imprints of the lower teeth. When the modeling compound had sufficiently hardened, the face-bow fork was

*Assistant Professor, Department of Restorative Dentistry. **Associate Professor and Director of Occlusion. Department of Restorative Dentistry. tWhip-Mix Corp., Louisville, Ky. dKerr Mfg. Co., Detroit, Mich.

1040 TETERUCK AND LUNDEEN November-De!;:;::; fs%

removed from the mouth and chilled under cold water. Then the compound was trimmed to a level so that only light imprints of the cusps remained. All compound in contact with soft tissue was removed. Only those records into which the casts would fit accurately were accepted.

THE EAR FACE-BOW TECHNIQUE

The face-bow fork was fitted onto the upper teeth, and the subject was asked to close the lower teeth into the imprints in the modeling compound to hold the fork rigidly. Then the patient was handed the caliper-like face-bow and instructed to place the plastic tips into his ears in a manner similar to using a stethoscope. At the same time, the universal toggle of the apparatus was attached to the face-bow fork stem. The patient held the plastic ear tips in the most stable bony part of the external auditory meatus by exerting an inward, forward, and upward pressure on the sidearms of the bow while the adjustment screws were tightened (Fig. 1).

The ear face-bow is designed to indicate the intercondylar width of the patient as small, medium, or large. This width was recorded and used later to adjust the intercondylar width of the articulator.

A third plane of reference which corresponds to the axis-orbital plane was established by means of a nosepiece spacer. The plastic tip of the nosepiece was raised to the level of the nasion and the toggle of the vertical support rod was tightened. The face-bow was removed by loosening the center screw and instructing the patient to remove the plastic ear tips while simultaneously opening the mouth to release the face-bow fork.

The face-bow apparatus was secured to the articulator by fitting the small holes in the ear plugs over metal projections located on the sides of the condylar guidance housing. The center screw was tightened again, and the maxillary member of the articulator was placed on the cross member of the face-bow. The horizontal condylar guidance inclination was set at 35 degrees for all subjects. The maxillary cast was placed in the imprints in the modeling compound on the face-bow fork and attached to the mounting plate of the articulator with a minimum amount of fast-setting artificial stone (Fig. 1).

A series of 10 successive ear face-bow mountings was performed on one subject to serve as a control. Differences in the ear mounting method on the same subject ranged between 0.25 and 2.75 mm. for an average of 1 mm. variation.

THE HINGE AXIS TECHNIQUE

The Lauritzen method3 was used to locate the true hinge axis. After the axis was located, the subject was placed in an upright position with the head out of the headrest. The axis points were marked on the skin while the mandible was held in the most retruded position.

The hinge axis locating equipment was removed and the same face-bow fork was placed in the mouth. A hinge axis transfer bow was now securely attached to the face-bow fork stem and the condylar styli were adjusted to the true hinge axis markings with the patient again sitting erect (Fig. 2). The hinge-bow apparatus was removed from the patient and carefully repositioned onto the previously mounted maxillary cast on the articulator. The positions of the hinge axis styli

EAR FACE-BOW ACCURACY

Fig. l.-Above, The ear face-bow is in position on the subject. Below, The maxillary cast is oriented to the upper member of the articulator by means of the ear face-bow.

were then recorded on 1 mm. graph paper which was attached to a disc that ac- curately replaced the condylar housing mechanism of the articulator. The graph paper was positioned with the horizontal lines parallel to the upper member of the articulator.

The discs were extended laterally to just meet the tips of the terminal hinge axis styli. At this time, the location of the styli was compared with the center axis of the instrument which was indicated by the intersection of the lines at the center of the disc (Fig. 2). By mentally dividing each millimeter square into four areas, it was possible to locate the position of the stylus to within 0.25 mm. This measure- ment was recorded on a master graph sheet.

THE CONVENTIONAL TRANSFER METHOD

The arbitrary location of the hinge axis was performed by aligning a flexible ruler between the foot of the tragus of the ear and the outer canthus of the eye and measuring anteriorly 13 mm. from the tragus (Fig. 3). This point was marked on the skin.

1042 TETERUCK AND LUNDEEN 3. Pros. Den. November-December. 1966

POS I pig .2 .-Above, The true hinge axis transfer bow is in position on the subject.

liti .Oll of the true hmge axis is recorded in relation to the ear face-bow axis. !Z o*, The

Fig. 3.-The axis is located by measuring 13 mm. anteriorly on the tragus-canthus line.

v01u1nc 16 Number 6

EAR FACE-BOW ACCURACY 1043

The previously employed hinge axis transfer bow was reseated on the sub-, ject’s teeth and the condylar styli were adjusted to the arbitrary axis points. This measurement was recorded on the master graph sheet in a similar manner as de- scribed for the hinge axis location.

RESULTS

The results were transposed to show the variations of both arbitrary axis loca- tions as they corresponded to the true hinge axis (Fig. 4 and Table I). This was necessary because the design of the ear face-bow made it impossible to record the data in the described manner.

Measurements of the distance from the top of the upper mounting plate to the incisal edges of the maxillary central incisors were also recorded (Fig. 5). The distances as dictated by the nosepiece varied from 42.7 to 59.9 mm. for an average value of 51.3 mm. On this articulator, which measured 94 mm. between the upper and lower members on the average, the cast would be oriented slightly below the midpoint. For those dentists who desire to use a split cast technique, this location would seem to provide sufficient space for mounting the casts.

Fig. 4.-The records of the ear face-bow axis Fig. 4.-The records of the ear face-bow axis points (cross marks) and the conventional 13 mm. axis points (dots) are plotted in relation to the true hinge axis (center).

TABLE I. THE RELATIVE LCCATION (PER CENT) OF 94 ARBITRARY AXIS POINTS (BOTHLMETH- ens) AS COMPARED TO THE LOCATION OF THE TRUE HINX AXIS.

-.~--__ - - -.----___-. I I

DISTANCE FROM THE HINGE AXIS (MM.)

Ear axis (%) 1.1 24.5 56.4 85.1 93.6 95.7 100 13 mm. axis (%) 3.2 9.6 33.0 62.8 78.7 96.8 100

1044 TETERUCK AND LUNDEEN J. Pros. Den. November-December, 1966

Fig. K-The distance from the top Of the mounting plate to the incisal edges of the maxll- lary central incisors is measured by a vernier caliper.

DISCUSSION

In similar studies3m6 comparing variations between the conventional arbitrary axis and the “kinematic axis,” results varied from 33 to 95 per cent of true hinge axis points falling within a 5 mm. radius of the arbitrary point. Our results dif- fered markedly with those of Beyron4 and Shallhorn but were much closer to those of Lauritzen and Bodner3 and Becks.s Our tests indicate that 33 per cent of the arbitrary axis points located 13 mm. anterior to the tragus fell within a 6 mm. radius of the true hinge axis. Here again the location chosen for the 13 mm. arbitrary point could have been slightly different from the points selected by other investigators.

On the other hand, 56.4 per cent of the arbitrary points achieved by the ear face-bow fell within a 6 mm. radius of the true hinge axis. By calculating the epicenter of the greatest concentration of hinge axis points from the original data and modifying the mounting holes in the ear plugs (Fig. 6) to this new center, it was estimated that the above figure could be increased to 75.5 per cent (Fig. 7).

The concept of the ear face-bow is not new. 7-10 Although not the ultimate answer for condylar axis location, the ear face-bow technique has clear advantages over the most widely used method of arbitrary axis location. The accuracy, speed of handling, and simplicity of orienting maxillary casts with the ear face-bow are recommendations for its use in many routine restorative procedures.

SUMMARY

A study was conducted comparing the accuracy of a new arbitrary face-bow method and a conventional arbitrary method with a true hinge axis technique. A standardized method of recording the location of the various points was described. Of the 94 different axis locations registered, 33 per cent of the conventional axis locations were within 6 mm. of the true axis, as compared to 56.4 per cent located by the ear face-bow.

EAR FACE-BOLT. ACCI:RACY 1045

Fig. 6.-A modification of the original ear plug shows the relocation of the mounting holes (arrow).

Fig. 7.-Deviations from the true hinge axis of both arbitrary face-bow methods (shown in millimeters with the amount of calculated deviation of the corrected ear face-bow with new ear plugs included).

With a simple modification of the ear face-bow, it was calculated that the above figure could be increased to 75.5 per cent. Therefore, this method of ar- bitrarily mounting maxillary casts deserves consideration for many routine restora- tive procedures.

REFERENCES

1. Weinberg, L. A.: An Evaluation of Basic Articulators and Their Concepts. Part I. Basic Concepts, J. PROS. DENT. 13:622-644, 1963.

2. Posselt, U. : Physiology of Occlusion and Rehabilitation, Oxford, 1964, Blackwell Scientific Publications.

3. Lauritzen, A. G., and Bodner, G. H.: Variations in Location of Arbitrary and True Hinge Axis Points, J. PROS. DENT. 11:224-229: 1961.

4. Beyron, H. : Orienterings Problem vid Protetisk Rekonstruktioner ock Bettstudier, Svensk. tandl. taskr. 35:37, 53-54, 1942.

5. Schallhorn, R. G.: A Study of the Arbitrary Center and the Kinematic Center of Rotation for Face-Bow Mountings, J. PROS. DENT. 7:162, 1957.

6. Beck, H. 0.: A Clinical Evaluation of the Arcon Concept of Articulation, J. PROS. DENT. 9:409-421, 19.59.

1046 TETERUCK AND LUNDEEN J. Pros. Den. November-December, 1966

7. Dalbey, W. C.: How to Obtain Accurate Measurements in Artificial Denture Making. Locating the Fundamental Lines Used in Anatomical Articulation, D. Cosmos 56: 269-275, 1914.

8. Brandrup-Wognsen, T.: The Face-Bow, Its Significance and Application, J. PROS. DENT. 3:618-630, 1953.

9. GoodfrA&di$4 J. : New Face-Bow for Dentist-Laboratory Cooperation, J.A.D.A. 68 :866-

10. Bergstrom, G. .’ On the Reproduction of Dental Articulation by Means of Articulators, a Kinematic Investigation, Acta odont. scandinav. 9:Supp. 4, 1950.

UNIVERSITY OF KENTUCKY COLLEGE OF DENTISTRY LEXINGTON, KY. 40506