Evaluation of a cephalometric method of occlusal plane orientation for complete dentures

Evaluation of a cephalometric method of occlusal plane orientation for complete dentures Brian D. Monteith,

M.Ch.D*

Medical University of Southern Africa, Faculty of IIentistry,

Medunsa. Republic. of South Africa

C

cphalometric analysis has served for many years as a valuable adjunct to dental research and diagnosis. Although its clinical application has been directed largely toward orthodontics, cephalometrics is of special value to prosthodontics in that it can be used to reestablish the spatial position of lost structures such as the teeth. This is achieved by identifying predictable relationships between the teeth and other cranial landmarks that are not subject to postextraction changes. Recent studies’.* of dentate subjects have revealed that there is an intimate correlation between the angle encompassed by the cephalometric points porion, nasion, and the anterior nasal spine (the PoNANS angle) on the one hand, and the angulation of the occlusal plane relative to the Frankfort plane on the other (Fig. 1). This correlation is such that should the latter variable be absent as in an edentulous mouth, its best computed value may be mathematically derived from the PoNANS angle. This is of immediate clinical value in that it provides a scientific method of occlusal plane orientation for patients who require complete dentures: the commonly used ala-tragus line method having been shown to be not only anomalous but also unreliable.‘-’ As illustrated in Fig. 2, the patient’s PoNANS angle value (ABC) is measured cephalometrically and substitution of this value for X in the equation I”

= 83 4.707

-

(II ‘N(l7

sI

will provide the best computed value for the angle of occlusal plane orientation (DEF). This value, in turn, is used to program an adjustable jig (XP-117 Experimental Adjustable Plane, Teledyne Hanau, Buffalo, N.Y.) permitting the maxillary denture teeth to be set up directly in their correct occlusal plane orientation (Fig. 3). It will be immediately apparent, however, that the clinical feasibility of this concept rests entirely on the accuracy with which the Frankfort plane can be transferred from the patient to the articulator. Such a transfer is achieved through the use of an earbow with the prime objective of having the horizontal reference planes of both subject and articulator be made to share a common alignment. This is illustrated in Fig. 2, where the cephalometric Frankfort plane (passing through the ear-rod shadow and bony orbitale) is shown to be coincident with the Frankfort plane of the articulator 64

Fig. 1. Cephalometric basis of PoNANS angle analysis. Angle between occlusal and Frankfort planes varies inversely but predictably with PoNANS angle formed by the porion, nasion, and anterior nasal spine.

Fig. 2. PoNANS analysis applied to edentulous subject. PoNANS angle (A-B-C) is measured cephalometricaliy. Substitution of this value of X in equation Y’ = 83.4307 - (0.9907 . X) will provide best computed value of occlusa1 plane orientation (D-E-F).

(which passesthrough the earbow mounting lugs and the inferior surface of the orbitale plate and is parallel with the articulator’s upper member). A study by Bailey and Nowlir? has cast some doubt on JANUARY

1986

VOLUME

55

NUMBER

1

CEPHALOMETRICS

FOR OCCLUSAL

PLANE

ORIENTATION

Fig. 3. Adjustable jig in position on articulator. Denture teeth are set up directly according to cephalometritally derived occlusal plane orientation. a face-bow’s ability to effect such a transfer with anything approaching the required degree of accuracy. Comparing a cephalometrically determined Frankfort plane orientation with that achieved by a facebow transfer to an articulator, they demonstrated a mean orientation error of 7.5 degrees. Admittedly, the criteria used were not quite the same (a face-bow aligned to Beyron’s point); nevertheless, the magnitude of their error was sufficient to give cause for concern. In view of this error, the accuracy of an earbow as a means of effecting a similar transfer must be seen as a major criterion when evaluating the clinical practicability of PoNANS angle analysis. In addition, and far more difficult to demonstrate objectively, there is also the esthetic factor. How reliably will a PoNANS angleproduced occlusal plane orientation be consistent with a natural-looking dental composition?

MATERIAL

AND METHODS

Ten subjects requiring complete dentures were selected. Four of these were men and six were women. Because the data base of PoNANS angle analysis was limited to caucasoids, all 10 subjects were white. The only other criterion for selection was a willingness to submit to the making of an additional lateral cephalometric radiograph immediately after the try-in stage of denture construction. Lateral cephalometric radiographs have formed part of our departmental diagnostic routine for some time, THE JOURNAL

OF PROSTHETIC

DENTISTRY

Fig. 4. Cephalometric radiograph showing radiopaque markers on try-in denture. Thick radiopaque lines are wire strengtheners in wax denture-base. being used primarily in the determination of interalveolar distance through gonial angle analysis.’ Radiographic material suitable for use in the present study was already available; therefore, the only additional criterion was that the midsagittal plane/film distance be noted in each patient to facilitate subsequent superimposition of tracings.

Occlusal plane angle prediction PoNANS analysis

by means of

A tracing was made of each subject in the usual way and the following salient points were identified: 1. Porion: plotted to coincide with the center point of the ear-rod shadow of the cephalostate; 2. Nusion: the most anterior point of the suture at the junction of the frontal and nasal bones; and 3. Anterior nasal spine (ANS) : determined according to Harvold’s criterion as “a point on the lower contour of the anterior nasal spine where the vertical thickness is three millimeters.“3~ 8 These points were joined (Fig. l), and the resultant PoNANS angle was measured to the nearest 0.25 of a degree. By substituting this value for X in the regression formula Y’ = 83.4307

- (0.9907

X)

65

Fig. 5. Patient No. 1 Table I.

--Computed

Patient No.

occlusal plane angle (degrees)

2 3 4 5 h 7 8 9 IO x *sn

16.5 13.5 15.5 13.5 13.0 9.0 20.0 h.O 19.5 13.5 14.00

Radiographic occlusal

plane angle (degrees) 16.25 12.23 14.25 11.75 13.00 8.75 19 50 5.00 19.25 13.25 13.32

Difference (degrees) -0.25 -1.25 -1.25 -1.75 0.00 -0.25 -0.50 - 1.no -0.25 -0.25 0.675’

= 0.3wu

the best computed value for the angle of occlusal plane orientation was obtained in each case. This computed occlusal angle, as postulated, will provide the best esthetic orientation of an individual subject’s prosthetic occlusal plane relative to his or her Frankfort plane. Earbow pickup relationship

of maxillae/Frankfort

plane

This relationship was achieved on completion of the jaw relation recording. An earbow was used to transfer the maxillae/Frankfort plane relationship of each subject to a semiadjustable articulator. The need for accuracy in recording both posterior and anterior Frankfort plane reference points prompted the following precautions. 1. Earbow earpieces were carefully checked for accurate engagement of the external auditory meatus on either side. 2. The operator’s left thumb was used to palpate the left orbital notch of each subject. The point of the orbital indicator rod was then carefully introduced at the

66

Fig. 6. Patient No. 2. interface of thumb-pad rotatory motion. When tion that indicator point level, the securing nut removed.

and facial skin using a gentle it could be ascertained by palpaand bony notch were at the same was tightened and the earboti

Transfer of the Frankfort articulator

plane to the

Fig. 2. reveals the importance of having the transferred Frankfort plane parallel to both upper and lower members of the articulator. The need for accurate transfer of both posterior and anterior reference points prompted the following precautions. 1. The earpieces of the earbow were made to engage the mounting lugs immediately posterior to the articulator hinge axis. 2. Use of the “long barrelled” orbitale indicator plate ensured that the anterior orbital reference and earpiece mounting lugs posteriorly would be in parallel alignment with the upper articulator member (Fig. 2). 3. Parallelism between upper and lower members of the articulator was ensured by setting the incisal pin on zero. Programming the adjustable occlusal plane angle

jig to computed

Precise details concerning the preparation of the adjustable jig have been described,3 with the device programmed to the required angle settings by reference to its integral callibrated degree scale. For purposes of the present study, however, and to minimize the possibility of systematic error, cardboard templates were cut to the indicated degree value and used to verify the accuracy of the jig’s adjustment.

IANUARY

1986

VOLUME

55

NUMBEK

1

CEPHALOMETRICS

FOR OCCLUSAL

PLANE

ORIENTATION

Fig. 7. Patient No. 3.

Fig. 10. Patient No. 5. Mouth

in close up.

Fig. 8. Patient No. 4.

Fig. 11. Patient No. 5. Close-up three-quarter

Fig. 9. Patient No. 5.

Preparation of the try-in denture and cephalometric evaluation Maxillary denture teeth were arranged directly against the jig (Fig. 3) The mandibular arrangement was completed later in the usual way. The in&al edge of the left upper central incisor and the mesiopalatal cusp of the left maxillary first molar were selected as key contacts. Special care was taken to ensure that both were in proper contact with the jig’s adjustable plane. Having provided each contact with a radiopaque marker, the

THE JOURNAL

OF PROSTHETIC

DENTISTRY

view.

second cephalometric radiograph was made with the maxillary try-in denture in the mouth (Fig. 4). The same midsagittal plane/film distance was used for both exposures, thus minimizing distortion and making it possible to superimpose the previously obtained tracing on the second radiograph. The prosthetic occlusal plane, radiographically visible through the radiopaque markers, was indicated on the tracing; and its angle relative to the cephalometric Frankfort plane was measured to the nearest 0.25 of a degree. The radiographic occlusal plane angle thus obtained was compared with the computed occlusal angle used earlier to program the jig, and differences between them were noted (Table I).

67

MONTEITH

Fig. 12. Patient No. 6. Fig. 15. Patient No. 9

Fig. 13. Patient No. 7.

Fig. 14. Patient No. 8.

Efsthetic evaluation Postinsertion photographs were made of each subject (Figs. 5-16). However, because esthetics is a highly subjective concept and cannot be objectively measured, its assessment was limited largely to each patient’s individual reaction.

RESULTS AND DISCUSSION The results appear in Table I. With a mean error of less than 1 degree between the intended angles and those actually obtained, the disquiet occasioned by Bailey and NowlinV findings has been largely dispelled. Use of the external ear opening as a posterior reference common to both face-bow and cephalostat was probably a significant factor in the improved result. Of interest is the 68

Fig. 16. Patient No. 10. range of the error: from 0 to -1.75 degrees. The difference between the radiographic and predicted occlusal plane angles was such that the final angle obtained was in no case greater than that originally intended. This flattening effect could be ascribed to an idiosyncracy manifesting as a tendency to locate the orbitale reference on the patient’s face at a point slightly higher than its corresponding bony level. Alternatively, the weight of the face-bow might have been sufficient to cause the ear-rods to sag slightly, thus lowering the posterior reference point relative to the maxillary occlusion rim inside the patient’s mouth. Notwithstanding the implications for the practicability of the PoNANS angle technique, these results appear to vindicate the role of the earbow as an acceptably accurate means of transferring the horizontal plane of orientation from patient to articulator.

Esthetic evaluation In view of its bearing on the esthetic qualities involved, the range of occlusal plane angles obtained within the sample is also worthy of comment. Spanning 14.5 degrees, its outer limits are represented by the S-degree angle of patient No. 8 at one extreme and the 19.5-degree angle of patient No. 7 at the other. Comparing these with their respective photographs indicates that such extremes are not inconsistent with individual facial harmony. It has been mentioned that the esthetic quality of prosthetic restoration defies objective analysis; and the JANUARY

1986

VOLUME

55

NUMBER

1

CEPHALOMETRICS

FOR OCCLUSAL

PLANE

ORIENTATION

individual illustrations provided are intended to serve merely as an indication of the PoNANS method’s esthetic potential. Admittedly, the occlusal plane is not the sole arbiter of esthetic excellence, but it represents a compositional perspective line about which artistic variation in the arrangement of the six maxillary anterior teeth may take place. It is contended, however, that the success of the latter is primarily dependent on the correctness of the former in which the same way as the impact of artistic detail in a painting is dependent on consistency with the laws of perspective. Space does not permit the number of views necessary to illustrate this point in all the subjects represented; therefore, one patient has been provided with multiple coverage (Figs. 9-11). In gauging personal reaction to the appearance of their dentures, all 10 subjects expressed satisfaction with the esthetic outcome of their treatment, and in none did it become necessary to alter the occlusal plane orientation as determined by the use of PoNANS angle analysis. REFERENCES

2.

3.

4.

5. 6.

Monteith BD: Cephalometric analysis of occlusal plane angulation: A comparison of regression in male and female samples. J Dent Assoc S Afr (in press). Monteith BD: A cephalometrically programmed adjustable plane: A new concept in occlusal plane orientation for complete denture patients. J PROSTHET DENT 54~388, 1985. Abrahams R, Carey PD: The use of the ala-tragus line for occlusal plane determination in complete dentures. J Dent 7:339, 1979. Ismail YH, Bowman JF: Position of the occlusal plane in natural and artificial teeth. J PROSTHET DENT 20~407, 1968. Bailey JO, Nowlin TP: Evaluation of the third point of reference for mounting maxillary casts on the Hanau articulator. J PROSTHET DENT 51:199,

1984.

7.

Potgieter PJ, Monteith BD, Kemp PL: The determination of free-way space in edentulous patients: A cephalometric approach. J Oral Rehabil 10~283, 1983. 8. Harvold EP: The Activator in Interceptive Orthodontics. St. Louis, 1974, The CV Mosby Co, p 41.

Ke,&inl requests to: DR. BRIAN D. MONTEITH MEDICAL UNIVERSITV OF SOUTHERN AFRICA FACULTY OF DENTLSTRY P.O. MEDUNSA 02040 SOUTH AFRICA

1. Monteith BD: A cephalometric method to determine the angulation of the occlusal plane in edentulous patients. J PROSTHET DENT 54:81,

1985.

Movement of three removable partial denture clasp assemblies under occlusal loading J. D. Browning, D.M.D., * L. W. Meadors, D.M.D.,** and J. D. Eick, Ph.D.*** Oral Roberts University,

Michael

Cardone,

Sr., School of Dentistry,

I

n the distal-extension removable partial denture, where support is obtained partly from unyielding teeth and partly from yielding mucosa, restoring function may defeat a key objective of preserving these structures. Often forces transmitted from the prosthesis to abutment teeth are greater than they can tolerate, and the teeth may become mobile.

Presented at the International Association for Dental Research, New Orleans, La. Supported in part by the National Institute of Dental Research grant No. DE 05952. *Professor, Department of Reconstructive Dentistry. **Private practice, Randolph, Neb. ***Professor, Director of Dental Biomaterials, Department of Reconstructive Dentistry. THE JOURNAL

OF PROSTHETIC

DENTISTRY

Tulsa, Okla.

LITERATURE REVIEW Many suggestions have been made for resolving the dilemma of dental clasp assemblies that move under occlusal loading. Several impression materials and impression techniques have been suggested.‘” Various types of stress-breakers such as hinges,4 special designs of major and minor connectors,5 and other components6 have been recommended. Differing approaches to denture base extension, from broad’ to restricted: and several occlusal schemes9-‘2have also been advocated. Perhaps the greatest amount of discussion and controversy has centered on various direct retainers designed to minimize movement of abutment teeth during function.‘3-‘7 Research efforts have been directed toward determining which clasp assembly caused the least movement of 69