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Roentgenographic Technics Made to Order
R O EN T G EN O G R A PH IC T E C H N IC S M ADE TO ORDER Albert G. Richards, B.S. (Ch.E.), M.S., Ann Arbor, Mich.
technic is a pro cedure involving a number of factors which, when properly combined, yield a desired diagnostic roentgenogram. For a long period very little change occurred in either the factors utilized or the man ner in which these factors were combined in dental roentgenographic technics. To day it is possible to devise roentgeno graphic technics that are made to order. Since roentgenograms are essentially shadow pictures, the production and properties of shadows should be re viewed. A source of radiation, an object or structure the image of which is de sired, and a screen or film for the recep tion of the shadow image are the three basic requirements for the production of shadows of any type. The x-ray tube is the source of radiation used in dental roentgenography, while the teeth and contiguous structures are the objects of which shadow images are desired. The images can be rendered temporarily vis ible by means of a fluorescent screen or can be permanently recorded on an x-ray film. As the x-ray beam leaves the cone of the x-ray machine, the rays diverge. The intensity of the x-ray beam decreases in versely as the square of the distance from the x-ray tube. This relation is commonly called the inverse-square law. Shadows consist of two parts: the umbra, which is the interior portion of the image, and the penumbra, which is the blurred region at the periphery of the image (Fig. i, left). The distinctness or clarity of outline with which the image of a given object will register on a film
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or screen is called the definition of the image. An image possessing a minimum amount of penumbra is considered to have good or “ high” definition. Defini tion is improved by the use of a small stationary source of radiation. The source or focal spot in the usual dental x-ray unit measures about 0.05 or 0.06 inches square, but if the machine is allowed tcrvibrate during the exposure, the effective dimensions of the source are increased by the extent of the vibration. Definition can be improved either by de creasing the distance between the object and the film or by increasing the distance between the source and the film. Nature imposes an anatomic limit on the approx imation of the film to the teeth, and the inverse-square law determines the maxi mum practical distance that the source can be removed from the film. The ideal x-ray beam would be com posed of parallel rays which would per mit the formation of shadow images having exactly the same dimensions as the object (Fig. 1, right, A ). Actually, however, the rays of the x-ray beam are not parallel but divergent. This diver gence results in magnified images (Fig. 1, right, B ). The amount of enlargement can be minimized by either decreasing the object-film distance or increasing the source-film distance. How close the film can be placed to the teeth is determined by the anatomy of the mouth, and the inverse-square law determines how much it is practical to increase the source-film distance. Assistant professor in dentistry, School of Dentistry, University of Michigan.
J.A.D.A., Vol. 39, O c to b e r 1949 . . . 397
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F ig . I .— L e ft : S h a d o w prod u ction . R ig h t : E ffe c t of d iverg en ce of x -r a y beam on size of im age
Certain improvements in dental roentgenographic technics were developed years ago, but only recently have these improvements achieved any degree of popularity. These changes w ere mainly in the current and voltage values, the method of positioning the film packet relative to the teeth, and the source-film distance em ployed.1"7 When the conventional technic is used, the film packet is placed in the mouth with some part of the packet touching either the incisal edges or the lingual cusps of the teeth and the rest of the film inclined to form an angle with the long axes of the teeth. T h en, in order to pro duce an im age h aving the same length as the tooth itself, the central ray of the x-ray beam is directed at the apex of the tooth and perpendicular to a plane which bisects the angle form ed by the plane of the film packet and the long axis of the tooth (Fig. 2, A ) .2»8 Even though this
method reproduces approxim ately the true length from the lingual apex to the lingual cusps and from the buccal apexes to the buccal cusps, there still is present a type o f distortion which makes it diffi cult to ju dge the relative lengths of the 1. Price, W . A. The Science of Dental Radiography. C om pt. rend.. Acad. d. sc. 2:345 (A ug.) 1900. 2. Price, W . A ., T he Technique Necessary fo r Mak ing G ood Dental Skiagraphs. D . Items Interest 2 6 :161 (M arch) 1904. 3. LeMaster, C. A ., A Modification of Technic for Radiographing U pper Molars. J. Nat. D . A . 8:328 (April) 1921. 4. M cCorm ack, D . W ., Dental Roentgenology: A Technical Procedure for Furthering the Advancement Toward Anatomical Accuracy. J. California D . A. 13:89 (May-June) 1937. 5. Fitzgerald, G . M ., Dental Roentgenography. I. An Investigation in Adumbration, or the Factors That Control Geom etric Unsharpness. J .A .D .A . 3 4 :1 (Jan. 1) 1947 6. F itzgerald, G . M ., D ental Roentgenography. I I . V ertical A ngulation, F ilm Placem ent and Increased O bject-Film Distance. J.A .D .A . 3 4 : 160 (Feb. 1) 1947. 7. Frank, Leonard, A Long Distance and L ow Pen etration Technique for Dental X -R ay Units. D . Digest 4 4 : i i 4 (Jan.) 1938. 8. Cieszynski, A ., T he Position o f the Dental Axis in the Jaws and the Adjustment o f the Chief Ray in the Intraoral M ethod W ith Regard to Maxillary Ir regularities. Internat. J. Orthodont. 11:742 (A ug.)
I925-
F ig . s . — A an d B , bisecting p rin ciple. C , elongated profile im age of tooth. D an d E , prod uction of profile im age o f tooth
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E, =
KNOWN EXPOSURE CIVEN FILM AT DISTANCE UNKNOWN EXPOSURE REQUIRED FOR AT NEW DISTANCE
.
D,
SAME T Y P E OF FILM
D,
0 = SOU«Ce-F!t.M DISTANCE IN INCHES
= e.x-Í2í\
P - SOURCE WIDTH X-~r
EQUATION I
f t)
F ig .
3 .— L e f t : R ela tio n betw een exposure an d sou rce-film distance. R ig h t : R elatio n betw een p en u m b ra w idth and sou rce-film distance
roots, because the lingual root extends higher in the roentgenogram than the buccal roots (Fig. 2, B ) . Sim ilarly, the lingual cusps appear above the buccal cusps. W ith the conventional film packet position it is possible to produce a sil houette or profile im age of the tooth by directing the central ray perpendicular to the long axis of the tooth. However, another type of distortion, called elonga tion, w ill usually accom pany the desired profile im age (Fig. 2, G ) . Profile images of the teeth, free of elongation, can be produced by placing the film packet parallel to the long axes of the teeth and directing the central ray perpendicular to the plane of the film and the long axes of the teeth (Fig. 2, D and E ) . In order to have the packet parallel to the long axes of the teeth, it usually must be positioned some distance from the teeth at a place where the oral cavity is more spacious. Th is positioning of the packet involves an increase in the object-film distance, which in turn re sults in poor definition and increased enlargement. Both the definition and en largem ent difficulties can be minimized by properly increasing the source-film distance. Since the intensity of the x-ray beam decreases in a certain m anner with in creased source-film distance, the length of exposure must correspondingly be in creased to compensate for the reduced
intensity of the rays (Fig. 3, le ft). By using the conventional 8 inch source-film distance as a standard and assigning it a time-distance factor of unity, time-distance factors for other source-film dis tances can be calculated, using equation I. Som e results are listed in T ab le 1. F o r exam ple, an operator would cus tom arily expose the m andibular m olar region fo r 6 seconds when operating with an 8 inch source-film distance. I f he wished to use a source-film distance of 16 inches, he would have to use an ex posure four times as long, because 4 is the new time-distance factor. T h e new ex posure fo r the same teeth would now be 6 X 4 or 24 seconds. T h e m erit of various source-film dis tances can be quantitatively com pared (T a b le 1) by using the theoretic width of penum bra (Fig. 3, right) as a measure of the definition of an image. Equation I I is derived fo r the case where the object is 154 inches from the film. F o r the 8,
Table I
Source-film distance, inches 8 16
20 36
W idth of Tim edistance penumbra, Percentage factor inches enlargement I 4
6.25 20.25
0.011 0.005 0.004 0.004
18.5 8-5
6.7 3-6
J.A.D.A., Vol. 39, O c to b e r 1949 . . . 399
Richards
L = LE N G T H OF IM AGE IN IN CHES %
EN LAR G EM EN T
%
F ig .
100
X 100
EN LAR G EM EN T = p _ lf | 5
e q u a t io n
It
4 .— R elation b etw een p e r cent enlarge m ent a n d source-film distance
1 6 and 20 inch source-film distances a dental x-ray unit with its 0.06 inch square source is used in the calculations, whereas a medical x-ray unit with a o. 1 1 inch square source is used in the calculations for the 36 inch source-film distance tech nic. T h e 16 inch technic reduces the width of the penum bra to less than half the value obtained with the conventional 8 inch technic. A dding another 4 inches to the source-film distance reduces the width of the penum bra one-thousandth of an inch, but simultaneously requires more than a 50 per cent increase in ex posure time. T h e 36 inch technic with its larger source offers no better definition than the 20 inch technic and does require very lengthy exposures. Equation I I I affords a means for com paring the enlargement that a 1 inch tooth, located 1 inches from the film, experiences with various source-film dis tances (Fig. 4 ) . Considerable enlarge ment accompanies the conventional 8 inch technic. This amount is reduced more than 50 per cent with the 16 inch technic and decreased still further with the longer distances (T ab le 1 ) . A ll particles of a patient’s head be come sources of secondary radiation when irradiated by the prim ary x-ray beam of the unit.® These secondary rays differ from the prim ary rays in that they are less intense and travel in all direc tions. Secondary rays produce an unde
sirable fog or gray veil over the entire area of the film, which tends to reduce the contrast of the image. T o decrease the amount of fog from secondary radiation on a film, the film packet m anufacturers have incorpo rated a thin sheet of lead in the back of each packet. This sheet of lead absorbs the secondary rays which normally would reach the film through the back of the packet. T o limit the amount of secondary radiation which reaches the film through the front of the packet requires that the number o f sources of secondary rays be limited. T h is limitation is accom plished by using a lead diaphragm with a small aperture, mounted in the cone of the unit, to limit the diameter of the pri m ary x-ray beam. Irradiatin g a smaller area of the patient’s face reduces the number of sources of secondary rad ia tion, which in turn reduces the fogging of the film. Figure 5, left, was produced without a lead diaphragm in the pointed cone. T h e beam, when measured 7 inches from its source, covered a circular area 4'/8 inches in diam eter on the patient’s face. A ll factors used in the production of F ig ure 5, right, were the same as for Figure 5, left, with the exception o f a d ia phragm. This diaphragm limited the di-
9. Muncheryan, H. M., M odern Physics of Roent genology, ed. 2. Los Angeles: Wetzel Publishing Com pany, 1940.
F ig . 5 .— E ffe c t of d ia ph ragm on fo g caused by secondary radiation. L e f t : N o diaph ragm used. R ig h t : D ia p h ra g m used
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ameter of the x-ray beam to 2/2 inches when measured 7 inches from the x-ray source. The reduction in the diameter of the x-ray beam produced a decrease of 63 per cent in the area irradiated and an appreciable decrease in secondary radiation. Only in Figure 5, left, do the large restorations have a gray veil over them, caused by the fogging of secondary radiation. This fogging is most notice able in the thin or clear areas on the roentgenogram, but it is present in the darker areas as well. It is this fogging which causes Figure 5, left, to appear darker and to have less contrast than Figure 5, right. The voltage applied to an x-ray tube determines the penetrating ability of the x-rays produced.10 Increased voltages generate more penetrating rays, which subsequently produce less contrast in the roentgenogram. Conversely, decreased voltages generate less pene trating rays, which produce greater contrast in the roentgenogram. Accom panying the increased or decreased pene tration is the need for shorter or longer exposures, respectively. The conventional dental x-ray unit is operated at n o volts as indicated by the voltmeter. With certain dental x-ray ma chines,11 when n o volts are supplied to the step-up transformer in the head of the x-ray unit, a potential of 63,000 volts or 63 K.V.P.12 is applied across the x-ray Table 2
Voltage applied to x-ray tube, K .V .P .f
Corresponding voltmeter reading for Ritter Model B x-ray unit
I
70 63
3/2 5/2
50
120 110 100 90
Time-voltage factor* 3 /4
55
*Caution! Time-voltage factors do not apply to the use of intensifying screens in cassettes. For such equip ment the factor is higher. fK .V.P. is the abbreviation for kilovolt peak. Most dental x-ray units can be changed in this manner. One should consult his x-ray serviceman.
Table 3
Current, Ma. 7-5
10 12 .5 15.0
Time-current factor 4 /3 I
4 /5 2/3
tube. If a time-voltage factor of unity is assigned to the 63 K.V.P. setting of the unit, then a factor of 3/2 must be used in the calculation of the new exposure when the unit is operated at 55 K.V.P. Some representative figures are con tained in Table 2. The factors were de termined experimentally. For example, an operator would customarily expose the mandibular molar region for 6 seconds when operating at a voltage setting equivalent to 63 K.V.P. If he wishes to use the voltage setting equivalent to 55 K.V.P., he would have to increase the exposure by a factor of 3/2, which is the new time-voltage fac tor. The new exposure for the same teeth would now be 6 X 3 / 2 or 9 sec onds. The amount of current which flows through the x-ray tube is measured by the milliammeter. The standard dental x-ray machine is adjusted to operate with 10 milliamperes of current. The density or blackening of a roent genogram is largely controlled by the amount of exposure the film receives. The exposure is measured in milliampere-seconds (abbreviated M a.S.), which is the product of the number of milliamperes (Ma.) of current flowing through the tube multiplied by the num ber of seconds (S.) that the x-rays are turned on. Theoretically it does not mat ter whether an exposure of 6 Ma. for 10
10. Hodge, H. C., and others, Factors Influencing the Quantitative Measurement of the Roentgen-Ray Absorption of Tooth Slabs. IV. Absorption Coefficient Factors. Am. J. Roentgenol. 34:8i7 (Dec.) 1935. 11. Ritter Model B. 12. Peak kilovolts are abbreviated K .V .P .
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seconds or io Ma. for 6 seconds be given a region which requires a 6o Ma.S. ex posure. However, it would be more prac tical to use the io Ma. for 6 seconds exposure, since there is more danger of movement by the patient if the longer time is involved. If a time-current factor of unity is assigned to the io Ma. setting of the unit, then a factor of 10 /15 or 2/3 must be used in the calculation of new exposures when the unit is operated at 15 Ma. Some factors are listed in Table 3. For example, an operator would custo marily expose the mandibular molar re gion for 6 seconds when operating at 10 Ma. If he wished to use 7.5 Ma., he would have to increase the exposure 4/3 times, which is the new time-current factor. The new exposure' for the same teeth would now be 6 X 4/3 or 8 sec onds. No current greater than 10 Ma. should be used with a dental x-ray tube oper ating at 63 K.V.P. The use of higher currents would in this case decrease the life expectancy of the tube. Currents as high as 15 Ma. can safely be used with 50 K.V.P. without harming the tube. Dental films are available in various speeds. The higher the speed of a film the less exposure is required to produce a roentgenogram of desired density. Ap proximate time-film speed factors can be assigned to the various types of films to facilitate conversion of exposures from
Table 4
Time-film speed factors I 4 /5 l/i l/Ö
1/2 1/2 1/4 1/8 1/12
Film types Kodak Regular Rinn Universal-BB DuPont S Kodak Radia-Tized Kodak Rapid-Processing Rinn Intermediate-DC DuPont D Kodak Ultra-Speed Rinn Extra-Fast
Table 5
Time-developing factor
Developing time at 65 F., minutes
i
3/2
3 /4
5
one film type to another. These factors and corresponding film types are listed in Table 4. For example, an operator would cus tomarily expose the mandibular molar re gion for 6 seconds when using Kodak Regular dental film packets. If he wishes to use Kodak Radia-Tized film, he would have to reduce the length of the ex posure to one-half, which is the new time-film speed factor. The new ex posure for the same teeth would now be 6 X 1/2 or 3 seconds. A disadvantage in using the higher speed types of films is that they do not have the wide latitude of exposure, pos sessed by the slow speed films. In other words, when using the faster films the operator must be more critical in his ap praisal of the size and type of the ana tomic structures in order to avoid over exposure or underexposure of the film. Fast developing solutions are avail able commercially which will completely develop a dental film in 3 /g minutes at 65 F. If the developing time at this temperature is prolonged to 5 minutes, a 25 per cent reduction in exposure time can be realized (Table 5). For example, an operator customarily would expose the mandibular molar re gion for 6 seconds for 3 ^ minute devel opment at 65 F. If he wished to prolong the development time to 5 minutes at 65 F., he would have to multiply the length of exposure by a factor of 3/4, which is the new time-developing fac tor, The new exposure time for the same teeth would now be 6 X 3/4 or 4/^ sec onds. Using the data contained in the tables, it is possible to devise a roentgenographic
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F ig . 6 .— T e c h n ic designed fo r in creased contrast. L e ft : 63 K .V .P . , 10 M a ., K o d a k R e g u la r film an d 6 second exposure. R ig h t : 50 K .V .P . , 12 .5 M a ., K o d a k R a d ia -T iz e d film an d 6 second exposure
tcchnic to meet every need. C hanging from one technic to another requires that the exposure-time values of the original technic be modified, for use in the new technic, in accordance with the new conditions. Assigned to each part of a roentgenographic technic (sourcefilm distance, voltage, current, film speed, developing time) is an exposure time factor (Tables 1- 5 ) . T h e appropriate exposure-time factor involved in a change of only one part of a given technic is determined by dividing the exposure-time factor of the new part by that of the original part. An exam ple of a change of only one part of a given technic would be a change from using D uPont S film, which has a factor o f i / 2 , to D uPont D film, with a factor o f 1/ 4 (T ab le 4 ) . T h e new factor, 1/ 4 , divided by the original factor, 1/ 2 , indicates 1 /2 to be the exposure-time fac tor involved in this change of film types. I f more than one part of a roentgeno graphic technic is changed, the over-all exposure-time factor is the product of the factors for each change. As an ex ample, if in addition to the mentioned change of film type the value of the current were changed from 10 M a. (factor = 1) to 12 .5 M a. (fa c to r= 4 / 5 ) (T able 3 ) , the over-all factor would be: 1 /4
4 /5
2
1/2
1
5
I f the original technic with D uPont S film and 10 M a. required a 5 second exposure fo r a certain region, then the new technic with D uPont D film and 12 .5 M a. would require the original exposure
time (5 seconds) multiplied by the over all factor ( 2 / 5 ) , or a 2, second exposure. E x a m p le 1 . —
Design a technic for increased con trast (Fig. 6 ). In place of a technic employing 63 K .V .P ., 10 Ma. and Kodak Regular film, the operator would change to a technic employing 50 K .V .P ., 12.5 M a. and K odak Radia-Tized film. Time-voltage factor (Table 2) : 50 K .V .P ................................................ 5 /2 63 K .V .P ................................................ i Tim e-current factor (Table 3 ) : 12.5 M a .. ............................................... 4 /5 10.0 M a ....................... : ......................... i Tim e-film speed factor (Table 4 ) : K odak Radia-Tized film ..................... 1/2 K odak Regular f ilm ............................ 1 S/ 2 4 /S 1 /2 ---- X ----- X ----- l= 1, the over-all factor.
1
1
1
The new exposure time for a given region would be determined by m ultiplying the for mer exposure time by the over-all time factor. If the former exposure time for the m axillary bicuspids and first molar region had been 6 seconds when operating with 63 K .V .P ., 10 Ma. and K odak Regular film, then the new exposure time for this same region would be 6 X 1 or 6 seconds if 50 K .V .P ., 12.5 M a. and K odak Radia-Tized film were used. In this example no change of exposure time would be necessary. E x a m p le 2 . —
Design a technic for high defini tion. In place of a technic employing an 8 inch source-film distance and R in n IntermediateD C film, the operator would change to a tech
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F ig . y.— T e c h n ic designed fo r in creased contrast an d definition. L e f t : 6s K .V .P . , 8 inch s o u rc e -flm distance K o d a k R e g u la r film , i o M a . a n d 6 second exposure. R ig h t : 50 K . V .P . , 16 inch s o u rc e -flm distance, K o d a k U ltra S p e e d film , 12 .5 M a . an d 6 second exposure
nic employing a 20 inch source-film distance and R in n Extra-Fast film. Time-distance factor (Table 1 ) : 20 inches.............................. (approx.) 6 8 in c h e s......................................................... 1 Time-film speed factor (Table 4 ) : R in n Extra-Fast film .............................. 1/12 R in n Interm ediate-DG film ...................1/2
6
1/12
1
1/2
- X ------= 1, the over-all factor.
Since the over-all time factor in this exam ple is unity, the new technic would utilize the same exposure time values as the old. Example 4 .— Design a technic for greater pene tration and short exposure time. In place of a technic employing 63 K .V .P ., 10 M a., K odak Regular film and 3J/2 minutes’ developing time at 65 F., the operator would change to a technic employing 70 K .V .P ., 7.5 M a., K odak Radia-Tized film and 5 minutes’ developing time at 65 F.
Since the over-all time factor in this exam ple is unity, the new technic would utilize the same exposure time values as the old.
Time-voltage factor (Table 2) : 70 K .V .P ................................................ 3 /4 63 K .V .P ................................................ i
Example 3 .— Design a technic for increased con trast and definition (Fig. 7 ). In place of a technic employing 63 K .V .P ., 8 inch source-film distance, K odak Regular film and 10 M a., the operator would change to a technic employing 50 K .V .P ., 16 inch source-film distance, K odak Ultra-Speed film and 12.5 Ma.
Tim e-current factor (Table 3 ) : 7-5 M a................................................. 4 /3 10.o M a ................................................. i
Time-voltage factor (Table 2) : 50 K .V .P ........................................ .. 5/2 63 K .V .P ................................................. i Time-distance factor (Table 1 ) : 16 in c h e s ............................................... 8 in c h e s...............................................
4 1
Time-film speed factor (Table 4 ) : K odak Ultra-Speed film ..................... 1/8 K odak Regular film .............................. 1 Tim e-current factor (Table 3 ) : 12.5 M a...................................................4 /5 10.0 M a................................................... i 5 /2
4
1 /8
4 /5
■— X - X ---- X — 1
1
1
.
= 1, 1
the over-all time factor.
Tim e-film speed factor (Table 4 ) : K odak Radia-Tized film ................. .... 1/2 K odak Regular film .............................. 1 Time-developing factor (Table 5 ) : 5 minutes at 65 F ..................................3 /4 3 / ¡ minutes at 65 F ............................. 1 3 /4
4 /3
1 /2
3 /4
---- X ----- X — X — = 3/ 8 , 1 1 1 1 the over-all time factor. The new exposure time for a region which formerly required 4 seconds of exposure, would be 4 X 3 /8 or i / s seconds. Summary
T h e production and properties of shadow images are discussed with spe cial emphasis on how definition is im proved and enlargement is minimized. T h e role that source-film distance,
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diaphragms, voltage, current, film speed and developing time play in the produc tion of dental roentgenograms is indi cated. Each of these items is carefully evaluated and assigned a conversion
factor which facilitates a change from one technic to another. Examples are given to show how a roentgenographic technic may be de vised to fulfill specific requirements.
FUNCTIONAL ANALYSIS OF OCCLUSION John R. Thom pson,* M.S., D.D.S., M.S.D., C hicago , and F. W inston C ra d d o ck,f Cert.Dent.(N.Z), M.S.D., Dunedin, N ew Zealand
term “ functional analysis” is used to describe a method of diagnosis of occlusal disharmony or frank mal occlusion which places proper emphasis on the biologic and functional aims of orthodontic, prosthetic, or restorative treatment rather than on merely ana tomic or cosmetic aims. Functional anal ysis aims to produce an occlusion which will not only possess the traditionally accepted characteristics of “normality” in the formal or anatomic sense, but which will be efficient in the functional or physiologic sense as well. To fulfil this condition an occlusion must be developed which is in functional harmony with the facial skeleton sup porting it, with the muscles of expres sion and mastication and with the tem poromandibular joints. The concept is thus a dynamic or kinematic one rather than a static one and looks beyond the mutual occlusal relations of the teeth to the masticating mechanism as a whole. It has important practical consequences for the description of the normal, for diagnosis and for treatment and method. he
T
to classify individuals of the same race and same general physical build; but they were even more apparent when an at tempt was made to apply them to differ ent races. It is now realized that occlusal harmony and biologic efficiency can exist over a wide range of anatomic variation; and conversely, an occlusion which has a superficial appearance of normality may be in fact abnormal or even patho logic when subjected to functional anal ysis. Diagnosis
It is also realized that merely by oc cluding upper and lower casts in the hand and looking at individual tooth relations a correct diagnosis is almost im possible except in the simplest cases. Even these may be deceptive because an apparently normal occlusion, judged in the static manner, may not function nor mally. The inclusion of functional analy sis as a supplement to traditional methods of diagnosis will go far toward substitu tion of rationalism for empiricism in treatment and toward the elimination of
Description of the Norm al
The inadequacies of the older norms established on a static consideration of occlusion became evident in any attempt
♦Professor of orthodontics, Northwestern University Dental School, Chicago. fSenior lecturer in prosthetic dentistry. University of Otago Dental School, Dunedin, New Zealand.
technic is a pro cedure involving a number of factors which, when properly combined, yield a desired diagnostic roentgenogram. For a long period very little change occurred in either the factors utilized or the man ner in which these factors were combined in dental roentgenographic technics. To day it is possible to devise roentgeno graphic technics that are made to order. Since roentgenograms are essentially shadow pictures, the production and properties of shadows should be re viewed. A source of radiation, an object or structure the image of which is de sired, and a screen or film for the recep tion of the shadow image are the three basic requirements for the production of shadows of any type. The x-ray tube is the source of radiation used in dental roentgenography, while the teeth and contiguous structures are the objects of which shadow images are desired. The images can be rendered temporarily vis ible by means of a fluorescent screen or can be permanently recorded on an x-ray film. As the x-ray beam leaves the cone of the x-ray machine, the rays diverge. The intensity of the x-ray beam decreases in versely as the square of the distance from the x-ray tube. This relation is commonly called the inverse-square law. Shadows consist of two parts: the umbra, which is the interior portion of the image, and the penumbra, which is the blurred region at the periphery of the image (Fig. i, left). The distinctness or clarity of outline with which the image of a given object will register on a film
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or screen is called the definition of the image. An image possessing a minimum amount of penumbra is considered to have good or “ high” definition. Defini tion is improved by the use of a small stationary source of radiation. The source or focal spot in the usual dental x-ray unit measures about 0.05 or 0.06 inches square, but if the machine is allowed tcrvibrate during the exposure, the effective dimensions of the source are increased by the extent of the vibration. Definition can be improved either by de creasing the distance between the object and the film or by increasing the distance between the source and the film. Nature imposes an anatomic limit on the approx imation of the film to the teeth, and the inverse-square law determines the maxi mum practical distance that the source can be removed from the film. The ideal x-ray beam would be com posed of parallel rays which would per mit the formation of shadow images having exactly the same dimensions as the object (Fig. 1, right, A ). Actually, however, the rays of the x-ray beam are not parallel but divergent. This diver gence results in magnified images (Fig. 1, right, B ). The amount of enlargement can be minimized by either decreasing the object-film distance or increasing the source-film distance. How close the film can be placed to the teeth is determined by the anatomy of the mouth, and the inverse-square law determines how much it is practical to increase the source-film distance. Assistant professor in dentistry, School of Dentistry, University of Michigan.
J.A.D.A., Vol. 39, O c to b e r 1949 . . . 397
Richards
F ig . I .— L e ft : S h a d o w prod u ction . R ig h t : E ffe c t of d iverg en ce of x -r a y beam on size of im age
Certain improvements in dental roentgenographic technics were developed years ago, but only recently have these improvements achieved any degree of popularity. These changes w ere mainly in the current and voltage values, the method of positioning the film packet relative to the teeth, and the source-film distance em ployed.1"7 When the conventional technic is used, the film packet is placed in the mouth with some part of the packet touching either the incisal edges or the lingual cusps of the teeth and the rest of the film inclined to form an angle with the long axes of the teeth. T h en, in order to pro duce an im age h aving the same length as the tooth itself, the central ray of the x-ray beam is directed at the apex of the tooth and perpendicular to a plane which bisects the angle form ed by the plane of the film packet and the long axis of the tooth (Fig. 2, A ) .2»8 Even though this
method reproduces approxim ately the true length from the lingual apex to the lingual cusps and from the buccal apexes to the buccal cusps, there still is present a type o f distortion which makes it diffi cult to ju dge the relative lengths of the 1. Price, W . A. The Science of Dental Radiography. C om pt. rend.. Acad. d. sc. 2:345 (A ug.) 1900. 2. Price, W . A ., T he Technique Necessary fo r Mak ing G ood Dental Skiagraphs. D . Items Interest 2 6 :161 (M arch) 1904. 3. LeMaster, C. A ., A Modification of Technic for Radiographing U pper Molars. J. Nat. D . A . 8:328 (April) 1921. 4. M cCorm ack, D . W ., Dental Roentgenology: A Technical Procedure for Furthering the Advancement Toward Anatomical Accuracy. J. California D . A. 13:89 (May-June) 1937. 5. Fitzgerald, G . M ., Dental Roentgenography. I. An Investigation in Adumbration, or the Factors That Control Geom etric Unsharpness. J .A .D .A . 3 4 :1 (Jan. 1) 1947 6. F itzgerald, G . M ., D ental Roentgenography. I I . V ertical A ngulation, F ilm Placem ent and Increased O bject-Film Distance. J.A .D .A . 3 4 : 160 (Feb. 1) 1947. 7. Frank, Leonard, A Long Distance and L ow Pen etration Technique for Dental X -R ay Units. D . Digest 4 4 : i i 4 (Jan.) 1938. 8. Cieszynski, A ., T he Position o f the Dental Axis in the Jaws and the Adjustment o f the Chief Ray in the Intraoral M ethod W ith Regard to Maxillary Ir regularities. Internat. J. Orthodont. 11:742 (A ug.)
I925-
F ig . s . — A an d B , bisecting p rin ciple. C , elongated profile im age of tooth. D an d E , prod uction of profile im age o f tooth
398
The Journal of the A m erican Dental A ssociation
E, =
KNOWN EXPOSURE CIVEN FILM AT DISTANCE UNKNOWN EXPOSURE REQUIRED FOR AT NEW DISTANCE
.
D,
SAME T Y P E OF FILM
D,
0 = SOU«Ce-F!t.M DISTANCE IN INCHES
= e.x-Í2í\
P - SOURCE WIDTH X-~r
EQUATION I
f t)
F ig .
3 .— L e f t : R ela tio n betw een exposure an d sou rce-film distance. R ig h t : R elatio n betw een p en u m b ra w idth and sou rce-film distance
roots, because the lingual root extends higher in the roentgenogram than the buccal roots (Fig. 2, B ) . Sim ilarly, the lingual cusps appear above the buccal cusps. W ith the conventional film packet position it is possible to produce a sil houette or profile im age of the tooth by directing the central ray perpendicular to the long axis of the tooth. However, another type of distortion, called elonga tion, w ill usually accom pany the desired profile im age (Fig. 2, G ) . Profile images of the teeth, free of elongation, can be produced by placing the film packet parallel to the long axes of the teeth and directing the central ray perpendicular to the plane of the film and the long axes of the teeth (Fig. 2, D and E ) . In order to have the packet parallel to the long axes of the teeth, it usually must be positioned some distance from the teeth at a place where the oral cavity is more spacious. Th is positioning of the packet involves an increase in the object-film distance, which in turn re sults in poor definition and increased enlargement. Both the definition and en largem ent difficulties can be minimized by properly increasing the source-film distance. Since the intensity of the x-ray beam decreases in a certain m anner with in creased source-film distance, the length of exposure must correspondingly be in creased to compensate for the reduced
intensity of the rays (Fig. 3, le ft). By using the conventional 8 inch source-film distance as a standard and assigning it a time-distance factor of unity, time-distance factors for other source-film dis tances can be calculated, using equation I. Som e results are listed in T ab le 1. F o r exam ple, an operator would cus tom arily expose the m andibular m olar region fo r 6 seconds when operating with an 8 inch source-film distance. I f he wished to use a source-film distance of 16 inches, he would have to use an ex posure four times as long, because 4 is the new time-distance factor. T h e new ex posure fo r the same teeth would now be 6 X 4 or 24 seconds. T h e m erit of various source-film dis tances can be quantitatively com pared (T a b le 1) by using the theoretic width of penum bra (Fig. 3, right) as a measure of the definition of an image. Equation I I is derived fo r the case where the object is 154 inches from the film. F o r the 8,
Table I
Source-film distance, inches 8 16
20 36
W idth of Tim edistance penumbra, Percentage factor inches enlargement I 4
6.25 20.25
0.011 0.005 0.004 0.004
18.5 8-5
6.7 3-6
J.A.D.A., Vol. 39, O c to b e r 1949 . . . 399
Richards
L = LE N G T H OF IM AGE IN IN CHES %
EN LAR G EM EN T
%
F ig .
100
X 100
EN LAR G EM EN T = p _ lf | 5
e q u a t io n
It
4 .— R elation b etw een p e r cent enlarge m ent a n d source-film distance
1 6 and 20 inch source-film distances a dental x-ray unit with its 0.06 inch square source is used in the calculations, whereas a medical x-ray unit with a o. 1 1 inch square source is used in the calculations for the 36 inch source-film distance tech nic. T h e 16 inch technic reduces the width of the penum bra to less than half the value obtained with the conventional 8 inch technic. A dding another 4 inches to the source-film distance reduces the width of the penum bra one-thousandth of an inch, but simultaneously requires more than a 50 per cent increase in ex posure time. T h e 36 inch technic with its larger source offers no better definition than the 20 inch technic and does require very lengthy exposures. Equation I I I affords a means for com paring the enlargement that a 1 inch tooth, located 1 inches from the film, experiences with various source-film dis tances (Fig. 4 ) . Considerable enlarge ment accompanies the conventional 8 inch technic. This amount is reduced more than 50 per cent with the 16 inch technic and decreased still further with the longer distances (T ab le 1 ) . A ll particles of a patient’s head be come sources of secondary radiation when irradiated by the prim ary x-ray beam of the unit.® These secondary rays differ from the prim ary rays in that they are less intense and travel in all direc tions. Secondary rays produce an unde
sirable fog or gray veil over the entire area of the film, which tends to reduce the contrast of the image. T o decrease the amount of fog from secondary radiation on a film, the film packet m anufacturers have incorpo rated a thin sheet of lead in the back of each packet. This sheet of lead absorbs the secondary rays which normally would reach the film through the back of the packet. T o limit the amount of secondary radiation which reaches the film through the front of the packet requires that the number o f sources of secondary rays be limited. T h is limitation is accom plished by using a lead diaphragm with a small aperture, mounted in the cone of the unit, to limit the diameter of the pri m ary x-ray beam. Irradiatin g a smaller area of the patient’s face reduces the number of sources of secondary rad ia tion, which in turn reduces the fogging of the film. Figure 5, left, was produced without a lead diaphragm in the pointed cone. T h e beam, when measured 7 inches from its source, covered a circular area 4'/8 inches in diam eter on the patient’s face. A ll factors used in the production of F ig ure 5, right, were the same as for Figure 5, left, with the exception o f a d ia phragm. This diaphragm limited the di-
9. Muncheryan, H. M., M odern Physics of Roent genology, ed. 2. Los Angeles: Wetzel Publishing Com pany, 1940.
F ig . 5 .— E ffe c t of d ia ph ragm on fo g caused by secondary radiation. L e f t : N o diaph ragm used. R ig h t : D ia p h ra g m used
The Journal of the Am erican Dental A ssociation
400
ameter of the x-ray beam to 2/2 inches when measured 7 inches from the x-ray source. The reduction in the diameter of the x-ray beam produced a decrease of 63 per cent in the area irradiated and an appreciable decrease in secondary radiation. Only in Figure 5, left, do the large restorations have a gray veil over them, caused by the fogging of secondary radiation. This fogging is most notice able in the thin or clear areas on the roentgenogram, but it is present in the darker areas as well. It is this fogging which causes Figure 5, left, to appear darker and to have less contrast than Figure 5, right. The voltage applied to an x-ray tube determines the penetrating ability of the x-rays produced.10 Increased voltages generate more penetrating rays, which subsequently produce less contrast in the roentgenogram. Conversely, decreased voltages generate less pene trating rays, which produce greater contrast in the roentgenogram. Accom panying the increased or decreased pene tration is the need for shorter or longer exposures, respectively. The conventional dental x-ray unit is operated at n o volts as indicated by the voltmeter. With certain dental x-ray ma chines,11 when n o volts are supplied to the step-up transformer in the head of the x-ray unit, a potential of 63,000 volts or 63 K.V.P.12 is applied across the x-ray Table 2
Voltage applied to x-ray tube, K .V .P .f
Corresponding voltmeter reading for Ritter Model B x-ray unit
I
70 63
3/2 5/2
50
120 110 100 90
Time-voltage factor* 3 /4
55
*Caution! Time-voltage factors do not apply to the use of intensifying screens in cassettes. For such equip ment the factor is higher. fK .V.P. is the abbreviation for kilovolt peak. Most dental x-ray units can be changed in this manner. One should consult his x-ray serviceman.
Table 3
Current, Ma. 7-5
10 12 .5 15.0
Time-current factor 4 /3 I
4 /5 2/3
tube. If a time-voltage factor of unity is assigned to the 63 K.V.P. setting of the unit, then a factor of 3/2 must be used in the calculation of the new exposure when the unit is operated at 55 K.V.P. Some representative figures are con tained in Table 2. The factors were de termined experimentally. For example, an operator would customarily expose the mandibular molar region for 6 seconds when operating at a voltage setting equivalent to 63 K.V.P. If he wishes to use the voltage setting equivalent to 55 K.V.P., he would have to increase the exposure by a factor of 3/2, which is the new time-voltage fac tor. The new exposure for the same teeth would now be 6 X 3 / 2 or 9 sec onds. The amount of current which flows through the x-ray tube is measured by the milliammeter. The standard dental x-ray machine is adjusted to operate with 10 milliamperes of current. The density or blackening of a roent genogram is largely controlled by the amount of exposure the film receives. The exposure is measured in milliampere-seconds (abbreviated M a.S.), which is the product of the number of milliamperes (Ma.) of current flowing through the tube multiplied by the num ber of seconds (S.) that the x-rays are turned on. Theoretically it does not mat ter whether an exposure of 6 Ma. for 10
10. Hodge, H. C., and others, Factors Influencing the Quantitative Measurement of the Roentgen-Ray Absorption of Tooth Slabs. IV. Absorption Coefficient Factors. Am. J. Roentgenol. 34:8i7 (Dec.) 1935. 11. Ritter Model B. 12. Peak kilovolts are abbreviated K .V .P .
Richards
J.A.D.A., Vol. 39, O cto b e r 1949 . . . 401
seconds or io Ma. for 6 seconds be given a region which requires a 6o Ma.S. ex posure. However, it would be more prac tical to use the io Ma. for 6 seconds exposure, since there is more danger of movement by the patient if the longer time is involved. If a time-current factor of unity is assigned to the io Ma. setting of the unit, then a factor of 10 /15 or 2/3 must be used in the calculation of new exposures when the unit is operated at 15 Ma. Some factors are listed in Table 3. For example, an operator would custo marily expose the mandibular molar re gion for 6 seconds when operating at 10 Ma. If he wished to use 7.5 Ma., he would have to increase the exposure 4/3 times, which is the new time-current factor. The new exposure' for the same teeth would now be 6 X 4/3 or 8 sec onds. No current greater than 10 Ma. should be used with a dental x-ray tube oper ating at 63 K.V.P. The use of higher currents would in this case decrease the life expectancy of the tube. Currents as high as 15 Ma. can safely be used with 50 K.V.P. without harming the tube. Dental films are available in various speeds. The higher the speed of a film the less exposure is required to produce a roentgenogram of desired density. Ap proximate time-film speed factors can be assigned to the various types of films to facilitate conversion of exposures from
Table 4
Time-film speed factors I 4 /5 l/i l/Ö
1/2 1/2 1/4 1/8 1/12
Film types Kodak Regular Rinn Universal-BB DuPont S Kodak Radia-Tized Kodak Rapid-Processing Rinn Intermediate-DC DuPont D Kodak Ultra-Speed Rinn Extra-Fast
Table 5
Time-developing factor
Developing time at 65 F., minutes
i
3/2
3 /4
5
one film type to another. These factors and corresponding film types are listed in Table 4. For example, an operator would cus tomarily expose the mandibular molar re gion for 6 seconds when using Kodak Regular dental film packets. If he wishes to use Kodak Radia-Tized film, he would have to reduce the length of the ex posure to one-half, which is the new time-film speed factor. The new ex posure for the same teeth would now be 6 X 1/2 or 3 seconds. A disadvantage in using the higher speed types of films is that they do not have the wide latitude of exposure, pos sessed by the slow speed films. In other words, when using the faster films the operator must be more critical in his ap praisal of the size and type of the ana tomic structures in order to avoid over exposure or underexposure of the film. Fast developing solutions are avail able commercially which will completely develop a dental film in 3 /g minutes at 65 F. If the developing time at this temperature is prolonged to 5 minutes, a 25 per cent reduction in exposure time can be realized (Table 5). For example, an operator customarily would expose the mandibular molar re gion for 6 seconds for 3 ^ minute devel opment at 65 F. If he wished to prolong the development time to 5 minutes at 65 F., he would have to multiply the length of exposure by a factor of 3/4, which is the new time-developing fac tor, The new exposure time for the same teeth would now be 6 X 3/4 or 4/^ sec onds. Using the data contained in the tables, it is possible to devise a roentgenographic
The Journal o f the A m erican Dental A sso ciatio n
402
F ig . 6 .— T e c h n ic designed fo r in creased contrast. L e ft : 63 K .V .P . , 10 M a ., K o d a k R e g u la r film an d 6 second exposure. R ig h t : 50 K .V .P . , 12 .5 M a ., K o d a k R a d ia -T iz e d film an d 6 second exposure
tcchnic to meet every need. C hanging from one technic to another requires that the exposure-time values of the original technic be modified, for use in the new technic, in accordance with the new conditions. Assigned to each part of a roentgenographic technic (sourcefilm distance, voltage, current, film speed, developing time) is an exposure time factor (Tables 1- 5 ) . T h e appropriate exposure-time factor involved in a change of only one part of a given technic is determined by dividing the exposure-time factor of the new part by that of the original part. An exam ple of a change of only one part of a given technic would be a change from using D uPont S film, which has a factor o f i / 2 , to D uPont D film, with a factor o f 1/ 4 (T ab le 4 ) . T h e new factor, 1/ 4 , divided by the original factor, 1/ 2 , indicates 1 /2 to be the exposure-time fac tor involved in this change of film types. I f more than one part of a roentgeno graphic technic is changed, the over-all exposure-time factor is the product of the factors for each change. As an ex ample, if in addition to the mentioned change of film type the value of the current were changed from 10 M a. (factor = 1) to 12 .5 M a. (fa c to r= 4 / 5 ) (T able 3 ) , the over-all factor would be: 1 /4
4 /5
2
1/2
1
5
I f the original technic with D uPont S film and 10 M a. required a 5 second exposure fo r a certain region, then the new technic with D uPont D film and 12 .5 M a. would require the original exposure
time (5 seconds) multiplied by the over all factor ( 2 / 5 ) , or a 2, second exposure. E x a m p le 1 . —
Design a technic for increased con trast (Fig. 6 ). In place of a technic employing 63 K .V .P ., 10 Ma. and Kodak Regular film, the operator would change to a technic employing 50 K .V .P ., 12.5 M a. and K odak Radia-Tized film. Time-voltage factor (Table 2) : 50 K .V .P ................................................ 5 /2 63 K .V .P ................................................ i Tim e-current factor (Table 3 ) : 12.5 M a .. ............................................... 4 /5 10.0 M a ....................... : ......................... i Tim e-film speed factor (Table 4 ) : K odak Radia-Tized film ..................... 1/2 K odak Regular f ilm ............................ 1 S/ 2 4 /S 1 /2 ---- X ----- X ----- l= 1, the over-all factor.
1
1
1
The new exposure time for a given region would be determined by m ultiplying the for mer exposure time by the over-all time factor. If the former exposure time for the m axillary bicuspids and first molar region had been 6 seconds when operating with 63 K .V .P ., 10 Ma. and K odak Regular film, then the new exposure time for this same region would be 6 X 1 or 6 seconds if 50 K .V .P ., 12.5 M a. and K odak Radia-Tized film were used. In this example no change of exposure time would be necessary. E x a m p le 2 . —
Design a technic for high defini tion. In place of a technic employing an 8 inch source-film distance and R in n IntermediateD C film, the operator would change to a tech
J.A.D.A., Vol. 39, O c to b e r 1949 . . . 403
Richards
F ig . y.— T e c h n ic designed fo r in creased contrast an d definition. L e f t : 6s K .V .P . , 8 inch s o u rc e -flm distance K o d a k R e g u la r film , i o M a . a n d 6 second exposure. R ig h t : 50 K . V .P . , 16 inch s o u rc e -flm distance, K o d a k U ltra S p e e d film , 12 .5 M a . an d 6 second exposure
nic employing a 20 inch source-film distance and R in n Extra-Fast film. Time-distance factor (Table 1 ) : 20 inches.............................. (approx.) 6 8 in c h e s......................................................... 1 Time-film speed factor (Table 4 ) : R in n Extra-Fast film .............................. 1/12 R in n Interm ediate-DG film ...................1/2
6
1/12
1
1/2
- X ------= 1, the over-all factor.
Since the over-all time factor in this exam ple is unity, the new technic would utilize the same exposure time values as the old. Example 4 .— Design a technic for greater pene tration and short exposure time. In place of a technic employing 63 K .V .P ., 10 M a., K odak Regular film and 3J/2 minutes’ developing time at 65 F., the operator would change to a technic employing 70 K .V .P ., 7.5 M a., K odak Radia-Tized film and 5 minutes’ developing time at 65 F.
Since the over-all time factor in this exam ple is unity, the new technic would utilize the same exposure time values as the old.
Time-voltage factor (Table 2) : 70 K .V .P ................................................ 3 /4 63 K .V .P ................................................ i
Example 3 .— Design a technic for increased con trast and definition (Fig. 7 ). In place of a technic employing 63 K .V .P ., 8 inch source-film distance, K odak Regular film and 10 M a., the operator would change to a technic employing 50 K .V .P ., 16 inch source-film distance, K odak Ultra-Speed film and 12.5 Ma.
Tim e-current factor (Table 3 ) : 7-5 M a................................................. 4 /3 10.o M a ................................................. i
Time-voltage factor (Table 2) : 50 K .V .P ........................................ .. 5/2 63 K .V .P ................................................. i Time-distance factor (Table 1 ) : 16 in c h e s ............................................... 8 in c h e s...............................................
4 1
Time-film speed factor (Table 4 ) : K odak Ultra-Speed film ..................... 1/8 K odak Regular film .............................. 1 Tim e-current factor (Table 3 ) : 12.5 M a...................................................4 /5 10.0 M a................................................... i 5 /2
4
1 /8
4 /5
■— X - X ---- X — 1
1
1
.
= 1, 1
the over-all time factor.
Tim e-film speed factor (Table 4 ) : K odak Radia-Tized film ................. .... 1/2 K odak Regular film .............................. 1 Time-developing factor (Table 5 ) : 5 minutes at 65 F ..................................3 /4 3 / ¡ minutes at 65 F ............................. 1 3 /4
4 /3
1 /2
3 /4
---- X ----- X — X — = 3/ 8 , 1 1 1 1 the over-all time factor. The new exposure time for a region which formerly required 4 seconds of exposure, would be 4 X 3 /8 or i / s seconds. Summary
T h e production and properties of shadow images are discussed with spe cial emphasis on how definition is im proved and enlargement is minimized. T h e role that source-film distance,
404
The Journal o f the A m erica n Dental A sso ciatio n
diaphragms, voltage, current, film speed and developing time play in the produc tion of dental roentgenograms is indi cated. Each of these items is carefully evaluated and assigned a conversion
factor which facilitates a change from one technic to another. Examples are given to show how a roentgenographic technic may be de vised to fulfill specific requirements.
FUNCTIONAL ANALYSIS OF OCCLUSION John R. Thom pson,* M.S., D.D.S., M.S.D., C hicago , and F. W inston C ra d d o ck,f Cert.Dent.(N.Z), M.S.D., Dunedin, N ew Zealand
term “ functional analysis” is used to describe a method of diagnosis of occlusal disharmony or frank mal occlusion which places proper emphasis on the biologic and functional aims of orthodontic, prosthetic, or restorative treatment rather than on merely ana tomic or cosmetic aims. Functional anal ysis aims to produce an occlusion which will not only possess the traditionally accepted characteristics of “normality” in the formal or anatomic sense, but which will be efficient in the functional or physiologic sense as well. To fulfil this condition an occlusion must be developed which is in functional harmony with the facial skeleton sup porting it, with the muscles of expres sion and mastication and with the tem poromandibular joints. The concept is thus a dynamic or kinematic one rather than a static one and looks beyond the mutual occlusal relations of the teeth to the masticating mechanism as a whole. It has important practical consequences for the description of the normal, for diagnosis and for treatment and method. he
T
to classify individuals of the same race and same general physical build; but they were even more apparent when an at tempt was made to apply them to differ ent races. It is now realized that occlusal harmony and biologic efficiency can exist over a wide range of anatomic variation; and conversely, an occlusion which has a superficial appearance of normality may be in fact abnormal or even patho logic when subjected to functional anal ysis. Diagnosis
It is also realized that merely by oc cluding upper and lower casts in the hand and looking at individual tooth relations a correct diagnosis is almost im possible except in the simplest cases. Even these may be deceptive because an apparently normal occlusion, judged in the static manner, may not function nor mally. The inclusion of functional analy sis as a supplement to traditional methods of diagnosis will go far toward substitu tion of rationalism for empiricism in treatment and toward the elimination of
Description of the Norm al
The inadequacies of the older norms established on a static consideration of occlusion became evident in any attempt
♦Professor of orthodontics, Northwestern University Dental School, Chicago. fSenior lecturer in prosthetic dentistry. University of Otago Dental School, Dunedin, New Zealand.