A cephalometric method to determine the angulation of the occlusal plane in edentulous patients

DENTURE

12.

13. 14.

PLAQUE

AND

INFLAMMATION

routines and the development of a new denture cleanser. ix;1 Pratt 19:83. 1968. Bloem TJ, Razzoog i%E, Chamberlain BB, Lang BR: The efficacy of tissue hrushing as assessed by the prosthodontic tissue index. Spec Care Dent 4:70, 1984. Bloem T, Razzoog ME: An index for assessment of oral health in the edentulous population. Spec Care Dent 2:121, 1982. Nyquisi (G. Influence of denture hygiene and the bacterial flora

on the condition of the oral mucosa Odontol Stand 11:24. 1953. Kqmnt DR.

in full denture

cases. ;\cta

request.! Ill.. B. <:H.~MBERI.,AIN

BARBARA

UNIVERWY

OF ~'~ICHK\N

SCHOOLOF DENTISTRY ,,NN ARBOR, IdI 48109-1078

A cephalometric method to determine the angulation of the occlusal plane in edentulous patients Brian D. Monteith, Medical

University

M.Ch.D.*

of Southern

Africa,

Faculty

of Dentistry,

Medunsa,

Republic

of South

M

ost prosthodontic text books advise that artificial teeth be placed in the positions previously occupied by the natural teeth.’ This is particularly true of the plane of occlusion: a feature that plays an important role in fulfilling the criteria of both function and esthetics. From the functional viewpoint, the occlusal table is a milling surface, strategically placed so that the tongue on the lingual side and the buccinator muscle on the buccal side are able to position the food bolus onto it and hold it there while mastication takes place. Faulty orientation of the occlusal plane in fixed or removable prostheses will jeopardize this interaction between tongue and buccinator muscle (Fig. 1) and result at one extreme in food collection in the sulcus, and at the other extreme in biting of the cheek or tongue. The correct orientation of the occlusal plane plays a vital role in optimal esthetic achievement. In the natural smile, the incisal tips follow the curve of the lower lip (Fig. 2). This effect is an expression of a correctly oriented occlusal plane; if the plane hangs posteriorly, the lip-line viewed from the front will appear straight (Fig. 3) and contribute more than any other factor to the so-called “denture look.” With the occlusal plane correctly oriented, however, the natural anterior curve will be achieved almost automatically (Fig. 4) and contribute a proper sense of perspective to the dental composition.

OCCLUSAL

THE

JOURNAL

and Chairman,

Department

OF PROSTHETIC

of Prosthetic

DENTISTRY

Dentistry

TABLE

B

A OCCLUSAL

OCCLUSiTABLE IN CORRECT RELATIONSHIP

TABLE

TOTONGUEAND BUCCINATOR

Fig. 1. Importance of correct and harmonious functional relationship between occlusaltable and related structures.

Fig. 2. Natural *Professor

Africa

curve

of lower

smile lip.

shows

how

incisal

tips

follow

81

Fig. 3. Incorrectly oriented allowed to hang posteriorly, totally out of harmony with

occlusal plane, which is results in straight lip line curve of lower lip.

Fig. 4. Correctly oriented occlusal produces lip line in harmony with

plane automatically curve of lower lip.

Fig. 5. Effect that anteroposterior length of maxillary base has on orientation of occlusal plane; angulation varies inversely with distance between hamular notch and anterior nasal spine.2

Many theories have been postulated over the years as to how orientation of the occlusal plane might best be

established.Sloane and Cook2 showedthat the plane of occlusionis related to the length of a line connecting the anterior nasal spine (ANS) and the hamular notch on dry skulls. This line, called Cook’s plane, forms an angle with the occlusal plane that varies inversely with the distance separating the two reference points (Fig. 5). Thus the greater the distance between ANS and the hamular notch, the more acute the angulation of the occlusalplane, and, conversely, the smaller the distance, the more obtuse the angle will be. This tendency has been confirmed by the cephalometric studies of L’Estrange and Vig3 and representsa phenomenonthat may be explained by the “denture glasseffect” depicted in Fig. 6. The portion of Cook’s plane that lies between the two reference points represents the skeletal base of the maxillary arch and is analogous to the diameter of a glass used to soak an upper denture. The greater the diameter of the glass, the more horizontal will be the attitude that the denture is able to assume;whereasin a glassof lesserdiameter, the denture will be obliged to assumemore of a tilt. The analogy is that the length of the maxillary skeletal baseacts in much the sameway to determine the tilt of the dental arch that it supports. It is probable, however, that this is a simplistic view and that other factors are-involved. It is immediately apparent in a typical mammalian skull (Fig. 7) that, while endowed with formidable 82

masticatory machinery, it is patently short of cerebral accommodation. This is in complete contrast to the human skull, in which the brain accommodationis far more generous, albeit at the expense of the chewing apparatus (Fig. 8). This disparity is evidence of a phylogenetic process,whereby the frontal cerebral lobes thrust forward and over the nasomaxillary complex and causethe olfactory bulbs to becomedisplacedthrough 90 degrees,with a commensurateshortening of the nasal process

(Fig.

9). At

the same time,

the downward

development of the maxillary sinusespermits the occlusal plane to rotate in a way that has led to the establishment of its present position in humans.“ An important feature of this processis the posterior maxillary plane, which projects downward from the posterior extremity of the anterior cranial baseand representsan anatomic boundary between the face and cranium. It passesalmost exactly along the posterior surface of the maxillary tuberosity and there effectively bars distal encroachmentby the maxillary process. The three types of profile, orthognathic retrognathic, and prognathic, coincide roughly with the skeletal ClassesI, II, and III, respectively (Fig. 10). Therefore, it seemsthat the presenceof the posterior maxi&try barrier would have a significant effect on the orientation of the maxillary arch and, consequently, that of the occlusal plane.

This

is because

the maxillary

procas

has to

accommodateitself asbestit can within the boundsset by the profile line in front of it and the posterior maxillary plane behind it. In terms of the denture glasseffect (Fig. JULY

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IN EDENTULOUS

PATIENTS

Fig. 6. Denture-glass effect. Anteroposterior dimension of maxillary base governs angulation of occlusal plane in same way as diameter of glass of water determines tilt that upper denture immersed therein assumes.A, Glass of greater diameter permits flatter angle than that permitted by B, narrower glass.

ll), the lengthening of the anterior cranial base in responseto cerebral advancement would be represented by an increase in the rim diameter of the glass. The retrognathic effect that this would appear to have on the profile line presupposesa glass that tapers toward its base,the result being that the denture contained therein is obliged to assumean even steeper tilt than before. This line of speculation has prompted the hypothesis that the anterior cranial base acts as a template that governs the anteroposterior dimensionof the nasomaxillary complex in such a way that an increase in the uppermost dimensionis accompaniedby a corresponding, decreasein the lower, with a resultant steepening in attitude of the occlusal plane (Fig. 12). To test this hypothesis, the cephalometric angle encompassedby porion, nasion, and the anterior nasal spine (the PoNANS angle) was acceptedas representive of the angular relationship between the cranial baseand the vertical axis of the nasomaxillary complex. This might seem to be contrary to accepted cephalometric practice; however, becausethe sella turcica is such a variable structure5 and becausethe A point is of no further use once the maxillary teeth are lost, the more commonly usedsella-nasion-A point (SNA) angle would have little practical value when applied to the edentulous condition. The purpose of the study was to investigate the possibility of a correlation between the PoNANS angle and the occlusalangle formed by the intersection of the occlusal and Frankfort planes (Fig. 13). MATERIAL

AND METHODS

Lateral cephalograms of 32 white women 20 to 30 years of agewere obtained and tracings were made. The THE

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Fig. 7. Typical mammalian skull shows how limited cranial capacity is associatedwith formidable masticatory apparatus. (Adapted from Enlow.“)

Fig. 8. Cerebral development in human has led to vastly increased cranial capacity at expense of masticatory apparatus. (Adapted from Enlow.4)

various cephalographic points were determined according to accepted criteria with the exception of porion, which was plotted to coincide with the ear-rods of the cephalostat rather than with its more correct but less frequently visible bony counterpart. Although contrary to the directives of Ricketts,’ this measurewas adopted for prosthetic reasons.The Frankfort plane that results from the use of this compromiseporion is coincidental with the plane of orientation associatedwith the use of an ear-bow and a semiadjustablearticulator (Fig. 14). The ANS point was determined according to Harvold’s7criterion, who definesit as “a point on the lower contour of the anterior nasal spine where the vertical thickness is 3 mm”’ (Fig. 13). The choice of an occlusal plane extending from the midincisal point of the upper central incisors to the mesiolingual cusp of the maxillary first molar was also prompted by prosthetic reasons.The occlusal plane is 83

A u BIj c\ Fig. 10. Three types of profile. A, Retrognathic. R, Orthognathic. C, Prognathic. Forward thrust of frontal lobes appears to exert progressively retrognathic effect on profile line. Phylogenetically, therefore, retrognathic profile might be regarded as more advanced than prognathic profile.

C

Fig. 9. Phylogenetic process has thrust human frontal cerebral lobes forward and over nasomaxillary complex and caused vertically positioned olfactory bulbs (A) to rotate through 90 degrees, and assume present horizontal alignment (C). Development of maxillary sinuses (stippled area) permitted occlusal plane to assume more horizontal position; however, restraint is placed on distai extremity of occlusal plane by posterior maxillary plane (PM). (Redrawn from Enlow.4)

that on which arranged during

prosthetic teeth are denture construction.

most

commonly

RESULTS AND DISCUSSION The PoNANS angle value obtained for each subject was plotted against its respective occlusal angle value, which resulted in a “scatter diagram” made up of a number of points (Fig. 15). The arrangement of these points reveals a linear tendency with a negative slope; the implication is that an increase in the one value is accompanied by a decrease in the other. The strength of the correlation is indicated by how closely the plotted 84

Fig. 11. Retrognathic response of profile line to lengthening of anterior cranial base affects orientation of both maxillary base and occlusal plane. These structures are obliged to undergo geometric accommodation within limits set by PM line behind and profile line in front.

pattern approximates to a straight line: a criterion that can be expressed statistically in terms of Pearson’s correlation coefficient. When subjected to analysis in this way, the paired results (Table I) revealed a correlation coeficient with a value of r = -0.8758. This indicates the presence of a strong linear relationship between the two variables;

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Fig. 12. Interaction between anteroposterior dimension of nasomaxillary complex and profile line in presence of inflexible PM plane appears to have predictable effect on angulation of plane of occlusion. Thus increase in upper dimension is accompanied by decrease in lower and leads to steepening in attitude of occlusal plane.

testing for t = -9.9399,

Fig. 13. Cephalometric reference points and used in study. Possible correlation between encompassed by porion, nasion, and anterior spine and angle between Frankfort and occlusal was investigated.

planes angle nasal planes

true correlation resulted in a value of which with p < ,005 is highly statistically

significant. While

the r value is a useful gauge of correlation,

the

coefficient of determination is a more useful statistic in that it indicates the proportion of the total variation in one variable that can be attributed to its relationship with the other variable. It is obtained by squaring the r value, so that in the present study it would have a value of ? = 0.767. Expressed as a percentage (r* X loo), the inference is that 76% of the variation in the occlusal angles measured

can be directly

attributed

to the rela-

tionship with the varying PoNANS angles within the sample. This does not necessarily imply a cause and effect relationship but might suggestthat each is subject to a separate causecommon to both.

porion to coincide with ear-rods of Fig. 14. Plotting cephalostat results in cephalometric Frankfort plane that corresponds to plane of orientation achieved when using ear-bow in combination with orbital indicator.

CONCLUSIONS

regression.This involves the application to the data of the “least squarescriterion,” and results in a “line of closestfit” to the points depicted on the scatter diagram (Fig. 15). The mathematic expression of such a line is

The results obtained

appear

to uphold

the hypothesis:

an increasein the PoNANS angle has a flattening effect

on the orientation of the occlusalplane, while a narrow-

given by the formula

ing of the angle appears to force the occlusal plane into assuming a steeper attitude. Arising out of the close interrelationship of these two variables is the important benefit that if one of them is absent, its best value can be predicted from the measured value of the other. Such a

where a indicates the value of y when x = 0 (the point at which the regression line crosses the y axis) and b

prediction is achieved through the process of linear

indicates the gradient of the line itself.

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y=a+bx

85

Fig. 15. Scatter diagram obtained when respective PoNANS ang_le values and occiusal angle values (Table I) were plotted against each other. Regression line represents line of closest fit and conforms to formula y = 77.3484 - (0.9098 . x).

Fig. 16. Maxillary set-up of denture teeth against occlusal plane indicator. Analysis of adjustable PoNANS angle of particular case will indicate degree of angulation required.

In terms of the data obtained in this study, the regression line illustrated in Fig. 15 has a formula of y = 77.3484

- (0.9098

. x)

This indicates that for every l-degree increase in the PoNANS angle, the occlusal angle decreases by 0.9098

Fig. 17. Experimental used to clinically described.

prototype evaluate

of adjustable plane method of analysis

degree. The practical value of the formula emerges when the problem of determining the aagulation of the occlusal plane is addressed in edentulous patients, in whom it is absent. This can be achieved by obtaining a lateral cephalogram of the patient, measuring the PoNANS angle, and by substituting this value for x in .the regression formula, a value for y is obtained that, in effect, will represent the best computed occlusal plane JULY

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Table I. Paired results

PATIENTS

from 32 subjects

(in degrees)

X

Y

X

Y

X

Y

70.50 62.50 73.00 69.00 64.50 70.50 75.50 68.75

14.00 20.00 12.00 12.00 20.00 13.50 8.00 11.00

68.00 65.25 66.75 70.00 79.00 64.50 70.50 67.25

15.50 20.00 14.00 12.00 7.00 17.50 9.00 19.00

79.75 66.00 71.00 79.50 72.25 64.00 72.50 67.75

22.50 13.50 4.00 15.00 22.00 8.50 16.00

X = PoNANS

angle measurement;

Y = occlusal

REFERENCES

THE

L’Estrange PR, Murray lometry to prosthodontics.

JOURNAL

OF PROSTHETIC

CG: Application of lateral skull rephaAust Orthod J 4:146, 1976.

DENTISTRY

67.00 79.00 68.25 70.00 65.50 65.00 71.50 68.00

Y 16.00 5.00 17.00 12.00 13.50 18.50 10.00 18.00

angle measurement.

angle for that individual. More simply, a reading can be madedirectly from the graph. In either event, this value may be used to program an occlusal plane projector on the articulator and permit the denture teeth to be set up directly against the plane in accordancewith the orientation indicated (Fig. 16). The method described seems to provide a more rational basis for the orientation of the occlusal plane than any hitherto available. An experimental prototype (XP-117, Teledyne Hanau, Buffalo, N.Y.) of an occlusal plane projector has been made available to me (Fig. 17) and will be usedin a future study to test the validity of these findings in the clinical context.

1.

8.00

X

2. 3.

4. 5. 6. 7.

Kq/mn1

Sloane RN, Cook J: A guide to orientation of the occlusal plane. J PROSTHET DENT 3~33, 1953. L’Estrange PR, Vig PS: A comparative study of the occlusal plane in dentulous and edentulous subjects. J PROSTHC.I, DENT 33495, 1975. Enlow DH: Handbook of Facial Growth, ed 2. Philadelphia, 1982, WB Saunders Co, chap 4. Enlow DH: Handbook of Facial Growth, cd 2. Philadelphia, 1982, WB Saunders Co, p 76. Ricketts RM: Perspectives in the clinical application of cephalometrics. Angle Orthod 51:115, 1981. Harvold EP: The Activator in Interceptive Orthodontics. St Louis, 1974, The CV Mosby Co, p 41. recpL”Jt,

1Cl.’

DR. BRI.AN D. MONTEITH ML’IEIXCAL UNIVEKSIT~ OF SOUTHERN AFRICA FACULTY OF DENTISTRY MEDUNSA 0204 REPUBLIC ok SOIITH .~E’RIc.~

87