Ricketts1964.pdf

The keystone triad I. Anatomy, phylogenetics, and clinical references ROBERT

MURRAY

RICKETTS,

D.D.S.,

M.S.*

Los Angeles, Calif.

THE chin, point B, and the lower incisor merit discussion as a triad. A f ~ knowledge of all the influential factors in this unit is often the key to clinic~ success or failure. Sophistication in orthodontics depends upon an understanding of the systematic totality of the behavior of this triad which is the keystone of the lower jaw. T o d a y concern over a receding chin is f r e q u e n t l y e n c o u n t e r e d b y the orthodontist. This concern also has been m a n i f e s t e d b y m a n y cultures of the past. ] n his writings K e i t h 1 mentions the conceit of certain groups as shown b y their description of the " c u t " of the chin. He describes the m e n t a l state which has impelled m a n to f a v o r his own k i n d and to be indifferent to all others. M a n y philosophers h a v e held t h a t the chin was essential to facial b e a u t y and thus was a significant f a c t o r in the desire f o r the m a t i n g of "superior races." D u B r u l and Sicher, ~ however, sober this ethnocentrism b y insisting t h a t " t h e chin is but a blob of bone subject to all the laws of bone a n d musculat u r e and is the result of the s h r i n k a g e of the d e n t i t i o n . " I n other words, the chin has developed as the teeth became less p r o m i n e n t in the face a n d less i m p o r t a n t to the life of the organism as a whole. Downs 3 first described " B " as a clinical point in 1948, although the point of deepest c o n c a v i t y at the chin alveolus on the a n t e r i o r portion was employed p r e v i o u s l y b y a n t h r o p o l o g i s t s a n d other investigators who called this point s u p r a m e n t a l e . 4 I stopped using this point, except for descriptive purposes, several years ago. P e r h a p s an explanation of m y reasons for not using it Tho first of two essays to be published in the AMERICANJOUI~NALOF ORTHODONTICS, based on material presented before the Middle Atlantic Society of Orothodontists in Atlantic City, N'. 5., Oct. 28 and 29, 1963, and before the Jarabak Foundation in Chicago, Ill., Feb. 1 and 2, 1963. *Associate Professor of Dentistry, School of Dentistry, University of California. 244

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clinically will help to strengthen other points that arc used instead, or perhaps my observations will provoke some interest and stimulate a controw,rsy, which is good for the clarification of any subject. The differences of opinion regarding the most proper measurement or position of the lower incisor are as old as dentistry. As neophyte dentists, we were taught to put teeth over the ridge in prosthetics. Yet we frequently observe clinically healthy mouths in which these teeth arc far forward ,v backward of the center of the chin. The resulting confusion has led many clinicians into a dilemma about who is right (ir which technique they shouhl follow. Some have come to use many techniques, i actually observed one clinical procedure in which an orthodontist used four different analyses in order to determine " w h i c h way to go." There has been little concrete found,tion because sophisticated research in large longitudinal samples, treated and untreated, is only now forthcoming. Agreement will lie difficult because ,kl the wide variation in nature and the differences in structure that any inveslL gator of human biology quickly observes. The prol/lem of relating the h)wer incisor should be defined and simplified, gemanties should be olarified, and known facts should be recognized. OSTEOLo,Y Although the orthodontist deals daily with the chin, a review of the fine anatomic points seems advisable in this discussion. The chin will be described as it is viewed in the mandible as a whole aud as seen in detail at the symphysis (Pig. ] ). When the dried mandil)le is held in the hand, one quickly realizes that its greatest bulk is seen in the area of the molars immediately anterior to the ramus. On elose inspection, it will be noted that a " c e n t r a l c o r e " of the mandible starts from the mandibular condyle and proceeds through the ramus and is reinforced by the coronoid process at the external oblique ridge. The mass of the bone at this point is extremely heavy, as any oral surgeon knows. Thb~ central core then terminates in the chin. Thus, the chin is directly connected to the mandibular condyles, and the mandible is one long bent bone. Looking on the medial side (Fig. 2), the outer heavy mass is eomple-mcnted by the inner mylohyoid ridge which joins it to proceed around the mandible toward the midline, forming a hard, compact internal and external cortical surface. On the outside, the external oblique ridge proceeds forward under the mental foramen and blends into a mass of bone which we call th~ chin. The two halves of the mandihle join at the mid]in(~ t . fm'm th~ symphysis. AnterioHy, on the inside in th~ area of the symphysis (Fig. '3), 1lie fob lowing landmarks should he noted: the inferior ridge of hone, slightly depressed at the midline and terminating in tlhe interdigastric spine, and on either side a depression called the digastric fossa. These points and surfaces form the posterior border of the symphysis as traced eephalometHeally. The inner surface of the chin is also of gre~lt interest for our pres,mt consideration (Fig. 4~. Immediately above, on either si,le ,,f the afot'emen-

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Fig. 1. Lateral view of the mandible. 0 shows area of greatest thickness of this bone at external oblique ridge. Stresses from condylar area (C) join with stresses from the coronoid process (CD), and these combined vectors terminate in the chin at mental tubercles (I'). Buccal occlusion is Class I I (Angle) malocclusion.

Fig. 2. On the medial side, the mylohyoid ridge (M) proceeds forward from the mandibular foramen to blend into the plauum alveolar (P) at the midline at the symphysis (S).

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Fig. 3. I n f e r i o r aspect of symphysis showing interdigastric spine (is), digastric fossa ( d / ' , an4 genial tubercles ( g t ) . Note also bulk at X and prominence at P.

Fig. 4. Inside view of symphysis showing the inferior torus (it), the superior t ransw~rs~ torus (st), and the planum alveolar (pl). (Note the thickness of bone in "trea as sho~v~ i~ Fig. 6.)

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tioned digastric fossae, are p a i r e d spines os tubercles k n o w n as the genital tubercles. Between these rounded extensions, in the midline, are m a n y foranlina. Above the digastric fossa is a ridge of bone r e f e r r e d to as the inferior t r a n s v e r s e t o m s . A p p r o x i m a t e l y m i d w a y up the slope of the lingual surl'ace of the bone and in a direct line with the m y l o h y o i d ridge is a bridge of bone called the superior t r a n s v e r s e torus. A b o v e this ridge is a flattened area k n o w n as the p l a n u m alveolar, or the alveolar plane of the m a n d i b u l a r symphysis. Thus, the posterior surface has the s t r o n g e s t and most heavily s t r u t t e d site in the chin.

Fig. 5. Frontal view of chin. The area from the mental tubercles (T) blends upward and forward to meet the mental protuberance (Po) or pogonion to form the trigonum mentale (T-Po-T). There is a slight ridge for the attachment of the mentalis muscle (m). T u r n i n g the mandible and the chin to the f r o n t a l view and looking at the mandible in strict detail, we see several points t h a t are of interest (Fig. 5). Virchow s called the d o m i n a n t area of the chin the t r i g o n u m mentale. This t r i g o n u m is composed of the tubercle of the p l a t y s m a as it comes f o r w a r d to f o r m the mental tubercle. F r o m these lateral tubercles the bone bends u p w a r d and f o r w a r d to meet at the midline of the mental protuberance which terminates in a ridge of bone blending u p w a r d into the alveolus at the j u n c t u r e of the two halves of the mandible. These points are often confused in the eephalometric image. I n a lateral direction, at a p p r o x i m a t e l y a midpoint from the lower border to the m a r g i n of the alveolus, we see the ridge for the mentalis muscles. The area above this ridge at the roots of the incisor teeth is called the mental fossa.

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Details of the growth change of the ridge for the mentalis muscles are nol known.

In cross section, in the midsagittal area (Fig'. 6), the symphysis is c h a r a c ~erized by a thick outer cortical layer of ahnost horseshoe shape which is heavivr on its lingual surfaee than on its labial surface. The planum alveolar is con sistently thick from the genial tubercles to the alveolar border. The oute:' (;ompaet plate f r e q u e n t l y terminates in the r~gion of th~ m~ntal protuberanc~ and is quit~ thin at the curve leading up to the h~w~,r blcisor, esp~cially l~ll ~',ral to the midline.

Fig. 6. l , a t e r a l view. N o t e g e n i a l t u b e r c l e s (gt) a n d p l a n u m alveolar. Pogo~lion is lo,.at~d at, g, point B at X, a n d m e n t o n at Z. N o t e m y l o h y o i d ridKe (ml). T h e occlusion as l:lnss t | ,, ta tact( risti,,s

The true chin'-' is " s t r u e t u r e d in relief oil th(~ u p w a r d and /)ackw~Jr~i slanting outer surface of the jaw at the midline (protuberance) eantil~o: sharply out of the hollow just below the sockets of the incisor teeth. A rid~(~ divides below to enclose a t r i a n g u l a r bulge with a concave base forming lh~ lower b o r d e r of the jaw and ending on either side in a blunt knob, the m~nt~l tu, bercIe." A real subject of controversy is the area from the mental protuberanee to the lower incisor or the labial curve of the bone up to and includht~; the incisor root. MYOLOGY

It is impossible to consider the chin from a biologic point of view without giving due consideration to the attached muscles and the total function p(rformed by the bone. Muscle pull wins out over bone and is a strong factor in a consideration of point B and the lower incisor. I like D u B r u l and Siehm"s ~ stat~'mcnt that bone can be r i g h t l y imagined as li~'i~tg calcified s~]st(:m,~

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of stress, and that these stresses may be elicited by forces acting directly upon1 the area or by the loading of a bone distant to the area. It is necessary, therefore, to remind ourselves of the muscles that are both directly and indirectly related to the function of the lower incisors and of the chin as a whole. In this study attention will be directed to only the pertinent muscle systerns. Let us first examine the muscles most directly concerned with the chin proper. The first of these is the mentalis muscle (Fig. 7), which arises from that aforementioned slight ridge of bone just below the incisal fossa and extends into the chin tissue where it is fused with the muscle of the opposite side. It elevates the tissue of the chin immediately over the trigonum. In orthodontics nearly all lip problems involving muscles have been rather loosely called "mentalis habits." Space will not permit a discussion of functional problems in this area; these vary extensively, and the reader is referred to my previous publications on classification of lip problems. 6 The second muscle of concern is the quadratus labii inferioris (Fig. 7), which is sometimes called the depressor of the lower lip. It arises from the slight ridge of bone posterior to the mental protuberance and extends obliquely upward to join the orbieularis otis around the mouth. These fibers cross at their insertion and blend with the same muscle of the opposite side. What is not well known is that they sometimes connect in the midline to form an almost horizontal band. We need to know much about the detail of variation in this area. Is this band, which is sometimes present, the cause of the sublabial furrow that we often see clinically ? The next muscle is the triangularis (Fig. 7) which covers the origin of the quadratus labii inferioris and is attached from a point just distal to the mental tubercle backward to a point on the external oblique ridge, ahnost at the level of the first molar. Its fibers converge toward the corner of the mouth and are continued as a part of the orbicularis otis into the upper lip. This muscle draws the mouth laterally and downward and is opposed by the caninus muscle from the upper lip. In many patients a thin slip from this muscle runs under the chin and joins a faseiculus from the opposite side to form a transverse mental muscle. Can this be a problem in the extremely narrow chin sometimes seen clinically? Anatomic research into the details of these chin muscles is still needed. We often forget the platysma muscle which originates in the lower part of the neck, extends obliquely upward to the chin and corner of the mouth, and inserts into the mental tubercle of the mandible and the inferior margin of the mandible. It often fuses with the quadratus labii inferioris and the orbicularis oris and can serve as a depressor of the integumental tissue and of the mandible. We usually do not appreciate its influence until we see a case of tortieollis in which the whole head is pulled to one sideJ The muscle system of great concern to us in the present discussion is the buccinator complex (Fig. 8). The buccinator muscle arises from the pterygomandibular raphe and from the sides of the maxilla and the mandible and runs forward to decussate at the commissure of the mouth, completely encircling it. Its function is to pull the lips against the teeth and to flatten the cheek.

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\ A

L/ @

f

f f

/f B

Fig. 7. A, Diagrammatic representation of pertinent muscles in the chin area as viewed from tracing of frontal head film. Note mentalis muscles (m); quadratus ]abii iuferioris (Li); triangularis (Tr); dotted line represents the plutysma. Quadratus muscles sometimes join at X. Note area of cortical bone viewed in frontal head fihn at arrows and O. B, Muscles se(m in lateral perspective.

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Fig. 8. Diagrammatic representation of buccinator complex as viewed from the lateral head film. The orbicularis otis blends with these muscles, bu, Buccinator; ml, mylohyoid. The digastric muscles are also represented. (da, Anterior belly; dp, posterior belly.)

This muscle literally wraps around the arch in the area of point B. It should be added that one school of thought labels the buccinator in common with the superior constrictor because they join posteriorly at the pterygomandibular raphe. This buccinator complex is of direct concern in the eruption of teeth and the development of occlusion. One might ask why the muscles of mastication should be related to the chin. The fan-shaped temporalis, the thick masseter, and the pterygoid muscles are of deepest concern to the biologist. In a discussion of the chin, the external pterygoid comes into consideration because of its possible importance to the bulk of the chin, although it is farthest removed from the chin at its insertion (Fig. 9). The arrangement of the closing muscles is well known, but the external pterygoid should be considered almost independently. This factor will be discussed later in connection with the vectors of pull of these heavy muscles and their influence on form of the mandible. Of direct consideration are the muscles which are attached to the inner aspect of the chin and the inside margins of the mandible : the muscles of the tongue and the hyoid muscle (Fig. 8). Of first concern, possibly, is the digastric muscle, particularly the anterior belly which originates in the digastric fossa at the lower border of the chin toward the midline and proceeds downward to a stirrup from the hyoid bone, only to go upward to the posterior belly and to end in an area beneath the mastoid process. Deep to this muscle is the mylohyoid muscle which, together with its twin on the opposite side, forms the

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Fig. 9. Representation of geniohyoid (gh) and genioglossis (gg) traced from lateral head film which was exposed lightly. The external pterygoid directions i)~ lateral perspective are suggested ( P t ) .

floor of the mouth. It arises from the mylohyoid line in the mandible, which extends from the symphysis to the last molar and the mandibular foramen. The anterior portions are attached to a medial fibrous raphe, which often runs backward to the hyoid bone. Deep to this muscle is the narrow genio. hyoid muscle, which comes from the inferior genial tubercle and extends from the chin to the hyoid bone on either side of the midline. The tongue muscle with which we are mainly concerned in our study o~' the chin is the genioglossus which arises from the superior genial tubercle (Pie' 9). Some fibers of this muscle extend upward a)~d forward into tile tongue: others extend directly toward the dorsum of the tongue; and the lower fibers run downward and backward to the medial part of the superior border of th( ~ hyoid bone. Although this muscle serves to change tongue form, its main put' pose is to draw the tongue forward for protrusion from the mouth. In om

sense, the genio.qlossus muscle is, therefore, an antagonist of the buccinato) muscle. There arises the question of whether these two very important muscles have a reciprocal or synergistic relationship of value to the clinical orthodontist. This is at the very vital center of clinical practice, for the lower incisor lies in a state of equilibrium between the tongue and the lips. ANTHROPOLOGIC CONSIDERATIONS In their excellent work entitled The Adaptive Chin, DuBrul and Sieher:' list six structural changes which occur in the development of forms from th(

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•

Tupoio

~

Lemur

~Cercopithecu$ Cebus ~

Hyolbae ts Homo

Fig. 10. Six stages in alteration of the symphysis in the phylogenetic development of man. Note the horizontal relationship of the chin in the Tupaia to the vertical relation in the Homo. (From DuBrul and Sichcr: The Adaptive Chin, Springfield, Ill., 1954, Charles C Thomas, Publisher.)

lower primates to modern man. Time will not permit a thorough discussion of their outstanding work, but certain critical observations cannot be omitted. In the lower forms, such as the Tupaia and the Lemur, the mandible is long and V-shaped (Fig. 10). The lower teeth are seen to extend almost directly forward. In these animals the chin is flat, forming an almost directly horizontal shelf.

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Fig. 11. One theory of the function of the symphysis is to bridge the mandible from both of the external pterygoid muscles. A, Vectors of pull from superior view. B, Both are forcefully contracted in protrusive movement of chin. C, Unilateral contraction observed in lateral movement of chin. ( F r o m Dubrul and Sicher: The Adaptive Chin, Springfield, Ill., 1954, Charles C Thomas, Publisher.)

I n the Cercophithecus, the p l a n u m alveolar is t u r n e d slightly upward, and the inferior t r a n s v e r s e t o r u s has f o r m e d a slight shelf which is sometimes refl, r r e d to as the simian shelf. A f t e r this stage has passed, the chin starts to t u r n more u p w a r d and the teeth become more vertical in their relation to the

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symphysis. In Homo sapiens, the main body of the synlphysis is usually inclined posteriorly (Fig. 11). DuBrul and Sicher offered four principal theories in explanation of these phylogenetic changes in the chin: (1) the shifting theory, (2) the development of articulate speech, (3) the mechanical effects of reduction and retrusion of the teeth, and (4) the influence of muscle activity. After a rather complete critique, DuBrul and Sicher concluded that probably the best explanation that could be offered involved the effects of muscle activity; thus, they agreed with the theories of Weidenreich s and Grunewald2 By means of a stress coat painted on the mandible, it was shown that the compressive strength exerted by the external pterygoid muscle in the normal lateral and protrusive movements of the lower jaw was enough to produce a strain in the symphysis (Fig. 11). In the protrusive movement of the mandible, DuBrul and Sicher were able to demonstrate a 0.5 ram. contraction in the width of the intermolar distance. They concluded that this force was probably important in the formation of the chin. Thus it is strongly hinted that the development of the chin is linked with good lateral function of the mandible. DuBrul and Sichcr favored this theory rather than that of Klaatseh, 1° who claimed that the form of the chin was due to the inward or outward rotational effect of the temporalis or masseter muscles. In patients with large mental tubercles, Klaatsch had reasoned a temporalis dominance, while patients with large mental protuberances were thought to reflect masseter dominance. DuBrul and Sicher agreed with Weidenreich s that the human chin was the result of a long evolutionary process and that the triangular form of the chin was due to a buckling of the alveolus which was filled in with bone as a result of the total transformation principle. They pointed out that the chin was not developed as an isolated factor and showed that it was consistent with the uprighting of posture and the change in locomotion of the animal (Fig. 12). Thus, the development of the chin is consistent with a long line of structural changes in the whole body and is not a single entity unto itself. From a clinical point of view, I would readily accept the thesis of DuBrul and Sicher. However, I tend also to inspect in a very critical nlanner the shifting theory developed by Bolk 11 in 1924, especially in light of more recent cephalometric growth findings and certain conclusions of Brodie. 12 Bolk described various forms of the chin. A chin with an angle greater than 90 degrees, or no chin, was termed ageniotic. This would be represented in our current thinking by chins in which point B lies anterior to the body of the symphysis. Chins with a 90 degree relation to the body of the mandible were described as neutral, or nlesogeniotic. Definitive chins, or those with angles less than 90 degrees, were described as eugeniotic. These types delight those following the Tweed school of thought and demonstrate what is called a " b u t t o n . " Bolk pointed out that in fetal life most chins are mesogeniotic and that in the ape, since the second molars erupt prior to the shedding of the deciduous teeth, the chin tends to become ageniotic. In man, however, the basilar part grows faster than the tooth-bearing parts, and the second molar does not erupt until after the deciduous teeth are shed; hence, man becomes more

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,,,~~'t ~ ' ~ Tupaia Fig. 12. Changes in the chin are associated with deep underlying structural and funetion~l phenomena, such as uprighting of posture and vertical locomotion of the organism. Diagram ~hows direction of opening of foramen magnum, change in the angle of the ramus, and change in cant or inclination of symphysis. (From ])uBrul and Sicher: The Adaptive Chin. Springfield, Ill., 1954, Charles C Thomas, Publisher.) e,ageniotie. Thus, although it has not been stated directly, one eouhl infer t h a t f o r w a r d positions of point B were eharaeteristie of more primitiw~ races. Both H u x l e y '~ and Cuvier 14 described a flattening of the faee during th,: individual life cycle, and this idea has been accepted by m a n y anatomists and

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anthropologists. ]'he inference is that the face becomes less convex and the denture less prominent. ~ ut it has also been observed that some of the lower creatures, born with a shortened maxilla, grew alveolar processes on the lower jaw which leaned backward so that the teeth could work against the upper opponents. Thus, in an ahnost pt~ssive manner the lower border of the mandible has come to lie in an advance position in the lower jaw. These structural stages of development are a prologue to the beautiful act of Nature in the ontogeny of the human face. Knowledge of these observations from studies in basic science helps the clinician to become better qualified for making decisions at the chair. CEPHALOMETRIC LANDMARKS AND DESCRIPTIONS

Studies of mandibular growth are usually started postnatally or after infancy, and various tools for measurement are needed. With the advent of cephalometrics for growth studies, the description of various landmarks in the lateral x-ray film has been controversial. Certain points of reference have come to be accepted, although frequently they are given different names. One subject of our present study is point B, which Downs described as a point at the deepest curvature of the outline of the symphysis in the chin. This point allegedly was to separate or to be located at a junction between the basal bone and the alveolar bone (Fig. 6). Downs was concerned with the relationship of the alveolar bone, since at that time he feared the alteration of tooth position on the mandible because of experience with so-called " s t r i p p i n g " or recession of the investing gingival tissues when the lower incisor' was moved forward. The use of point B was simply a convenient device for description. A vertical deepening of the body of the symphysis occurs with eruption of the teeth. The bony tissue that is alveolar in the young becomes incorporated in the body and becomes basilar in the adult, so that point B moves both vertically and horizontally. The need for description, however, makes necessary the identification of one area to separate it from another, at least for purposes of measurement. Point B was referred to earlier by BjSrk as supramentale, which was the anthropologie nomenclature described by Martin and Salles. 15 Therefore, either SM or B will suffice as a symbol for our use. There seems to be little disagreement that the most anterior point on the curve of the outline of the symphysis can be termed pogonion. This is a point on the n|ental protuberance at the midline and should not be confused with the mental tubercles, which are lateral structures. Different symbols have been used to designate pogonion, including PG, POG, pg, Po, Pg, P, Pgo, and pgo. I choose to label it Po, following the original work of Downs. The simple P has been employed for porion, so I hope to implement the understanding of this point with the continued use of Po. The selection of gnathion (Gn) has become confused by various statements of different authors. Brodie 16 selected gnathion as the lower anterior point on the chin by bisecting the distance between the most anterior and inferior points on the bony chin. Its use became confused as Brodie selected the most inferior curve on the body of the mandible at the angle and at the

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lateral area of the chin as a mandibular plane. To confuse the issue further, Downs selected the mandibular plane from the lower border at its angle to the lower border of the symphysis and, at the intersection of this mandibular plane with the facial plane (nasion--pogonion), erected a point in space som(~times referred to as gnathion. ]n an effort to straighten out this protflem of description, some writer's have chosen to refer to the lower border of the symphysis as mentott (M), the anterior-inferior point at the crossing of the Y axis as gnathion, and the anterior border as pogonion. BjSrk, Jensen, and Palling '7 suggested the term prognathio~ for that point selected at the crossing of the Y axis and reserved the use of gnathion for the inferior border. Menton was to be employed for lateral structures usually seen at the mental tubercles rather than at the men~al protuberance and thus would represent a lateral structure. I suggest that H~ese points be labeled Mt for mental tubercles. Swartz used a point at the anterior curve of the symphysis and called it the SM or spina-mentalis (Fig. 13). He drew a distinction between crista mentalis (CR) and tubercle mentalis (T) in the outlines seen cephalometrieally. Studies using the posterior border have been conducted by Bench, 's who termed this point genial (G). In actual practice most authors describe their" reference points at the start of their studies, which has helped to reduce ~]~e confusion, but many have not done this. Another reference point has been suggested by Steiner2 ~' The eenter of the' symphysis wets picked by inspection, in nmeh the same manner that the center of sella-tureiea was selected. Recognizing the inadequacies of using point 1~ for changes in the chin, Steiner employed it as a reference point which h~ called point D for the purpose of evaluating serial behavior of the chin 1o SN. Several arguments have been offered against the use of pogonion or

A

B

C

D

E

Fig. 13. Concepts of A. M. Sehwarz on the bone of the chin and lower incisor. A, tlorizontal lines represent retroposed alveolar bone. Vertical hatched portion is area of spongiosa. B , Vertical hatch shows sylnphysis spongiosa; horizontal small hatched area lateral to midline shows prominent tubercles and yields double line for anterior border of the symphysis. True midline is posterior contour. C, Lower incisor can be 90 degrees vertical and be either forward or backward. The angle does not describe its anteroposterior position. D, Lingually inclined lower incisor at 70 degrees can still be either forward or backward. E, Labially inclined lower incisor (110 degrees) also can be either forward or backward. Note that point B is determined by the lower incisor position.

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gnathion for the growth reference, since it was believed that both of these structures reflected secondary growth characteristics, particularly in males; however, these objections have not been based on direct study but are simply hypothetical ones based on a few cases thought to grow buttons. Thus, the so-called " b u t t o n " on the chin has sometimes been referred to in the litera.. ture. In actual practice, some refer to the button (or spina mentalis) as any bone anterior to point B. Since little concrete work has been done in this regard, it will be necessary to review some of the studies of growth in a longitudinal manner to reflect some bona fide information on the subject. CLINICAL REFERENCES

Cephalometric references vary among different authors. Several planes of reference have been constructed in order to study the chin, point B, and the lower incisor in morphologic analysis and for growth and change. The mandibular plane has been the one most commonly employed for growth reference (Fig. 14). Brodie used a line tangent to the inferior border in discussing the growth of the mandible, but for analysis he used a plane between Go and Gn. This method was modified by Steiner. With the advent of Downs' analysis, a line from the inferior border of the angle of the mandible to the lower border of the symphysis or point M has become most popular. For routine measurement, I have selected this plane by superimposing on the " h o r s e s h o e " outline of the chin with the mandibular plane parallel, hereinafter called the mental osseum. In 1960 I started using still another plane of reference for growth of the mandible and behavior of the chin after viewing BjSrk's '~° studies on implants, discussing it with him personally, and reviewing my own series of mandibular growth problems. 2~ With laminagraphy, the mandibular canal and the mandibular foramen could be located. Therefore, I registered on the symphysis at the anterior border of the mental osseum and superimposed with the mandibular foramen parallel. My impression was that this technique closely followed the pattern expressed by BjSrk's implants. More recently BjSrk 22 has advised superimposing over the inferior border of the crypt for the third molar in line with the mental osseum or area of pogonion. In assessing development of the chbb the use of the ~'acial plane (N-Po) has also been popular because nasi(m represents the terminus of the anterior limits of anterior cranial base as pogonion represents this point on the ma~ldible (Fig. 14). The lower incisor and point B have been related to this line. The line N-B has also been employed by Reidel and was adapted for measurement of the lower incisor by Steiner. ~ When point B is in line with the facial plane, the two would be synonymous and recordings would be identical for either NB or NPo. As a reference for the spatial position of the lower incisor, I have continued to use the point A--pogonion plane (Figs. 6 and 14). In a perfectly straight face, with point B forward on the facial plane, all three planes (NB, NPo, and APo) would be identical. Downs employed a line from point A to point B in an effort to relate the denture bases to the facial plane. So-called " b a d " A-B relationships were

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Fig. 14. Cephalometrie scheme employed by Rieketts. Mandibular plane for cephalometri(~ work is constructed from lower border of angle of mandible to lower border of symphysis at, midline, labeled M. Go is located at the arrow at the angle. In clinical usage, Gn is located in space at point of intersection of facial and mandibular planes and sometimes labeled point M. Technically, it is downward and forward in position on chin at crossing of Y axis (S-Gn). N-Po, Facial plane; A-Po, denture plane. Tip of lower incisor related at distance from A P, plane (4). those in which either A or B, or both, was distant to the ~acial plane or :from a s t r a i g h t line. I n an a t t e m p t to simplify this description, Reidel measure~l the angle A-N-B. Thus, " b a d " A-B relationships have come to mean disharmony of maxilla and mandible. W e have expressed skeletal c o n v e x i t y with ~1 straight linear measurement of point A to the facial plane, with no a t t e m p t to consider point B per se. One might ask why we use point A as a reference in the maxilla and object to point B in the mandible. The answer is t h a t we simply cannot find a better anterior terminus of basal bone in the u p p e r jaw. The anterior nasal spine is definitely a process and so, of course, is the alveolar process. A junction between these two, therefore, represents basal bone and allegedly compact bone. W e k n o w t h a t " A " is influenced by movement of the upper' incisor, but still we must accept it as the best reference point. Bone p r o p e r in the maxilla and the mandible is vastly different in c h a r a c t e r and functiom and this is a f u r t h e r consideration. I have r a t h e r serious objections to the common interpretations of point B, p a r t i c u l a r l y to the use of NB as a reference for the lower incisor. This is due to the fact that point B is determined by the position of the incisor in the fir.st place. Consequently, relating the incisor to point B is relating it to itself. Point B will not relate a t o o t h ' s incorrectness in the same light ~ls

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basal landmarks, because point B is an alveolar point after the mixeddentition period. Holdaway '-'~ recognized the weakness of B point and has suggested that a certain relationship of B to the bony chin be established its treatment. In the final analysis, he was referring back to pogouion, ltoldaway went further to extend the NB line through the chin. He strove to establish a relationship of the lower incisor which was consistent with the distance from pogonion; hence, a one-to-one relationship. It should be added that Holdaway did not rigidly prescribe these measurements but was flexible with respect to individual variation. Lower incisor measurements have also been subject to great controversy. Downs and BjSrk measured the incisal tip of the lower incisor to established planes, from a point on the incisal tip. Steiner chose a point on the most labial surface of the crown of the tooth to measure it in space. Probably the biggest error ever to appear on the orthodontic scene, and one which affected the course of orthodontics for many years, was the mistaken notion that the angle or axis of the lower incisor was a measure of its forward or backward position in the mandible or to the face.

The occlusal plane has been employed to measure the lower incisor and points on the chin. The relationship of the A-B plane to the occlusal plane has been taken as a measure of denture base problems. Changes with growth have also been measured for this plane. Time will not permit a discussion of the occlusal plane; however, this plane is vastly important to clinical orthodontic understanding, especially its change with certain types of treatment. Finally, the axis of the lower incisor has been related to the upper incisor by the so-called interincisal angle. While this describes their mutual relationship, it cannot be taken as a value for lower incisor behavior, since the upper incisor can change radically with growth and treatment. One final means of relating the lower incisor is its angle to the F r a n k f o r t horizontal. While this value reflects the lower incisor's position to a horizontal plane and serves as a guide for orientation to the earth's surface, it is a poor criterion for measuring its spatial relation forward or backward. SUMMARY

For purposes of convenience, we have used the term keystone triad in referring to the chin or symphysis, the lower alveolar process or point B, and the lower incisor. In order to understand the chin, the clinician should know the biologic forces that produce the growth and make the chin of man what it basically is--functioning bone serving nlany purposes. ]t is important to conceive of the t rigonum mentale in order to understand the difference between mental tubercles and the mental protuberance for the purpose of identification and tracing for serial comparison. In addition, the function of the heavy bone on the lingual side must be recognized as supporting bone for the lower incisor. Muscles are important in the production of an esthetically effective chin.

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The immediate facial muscles are important, but of greatest importance are those of the tongue and of mastication. The external pterygoid has been considered a very important single muscle. Authors differ with respect to nomenclature and planes of reference for the study of morphology and growth of the chin. Recent implant studies hay( ~ shed much light on the true nature of proper references for study, 1)m investigations are still under way. Anthropologic, osteologic, myologie, and eephalometric principles haw, been related to the "keystone triad" located at the keystone of the dental arch and the arc of the mandible. [ have found the A-pogonion plane most useful as a plane of reference for the lower incisor. However, growth and treatment behavior of this wilt be discussed later. Clinical interpretation and use will be described in Part [I of this study.

REFERENCES

1. Keith, A.: A New Theory of H u m a n Evolution, 1949. 2. DuBrul, E. L., a n d Sicher, H.: The A d a p t i v e Chin, Springfield, Ill.. 1954, Charles (: Thomas, Publisher. 3. Downs, W. B.: V a r i a t i o n s in Facial Relationships: T h e i r Significance in T r e a t m e n t a n d Prognosis, AM. J. ORTHODONTICS 34: 812, 1948. 4. BjSrk, A.: The Face in Profile, Lund, B e r l i n g s k a B o k t r y c k e r i e t . 5. Virchow, It.: M u s k e l m a r k e n am Schadel, Itsder. Ethnol. 42: 638, 1910. 6. Ricketts, R. M.: A F o u n d a t i o n for Cephalometric Communication, AM. J. ORTHODONTICS 46: 330, 1960. 7. P r u z a n s k y , S.: The Application of E l e c t r o m y o g r a p h y to Dental Research, J. Am. Dent. A. 44: 49-68. 1952. 8. Weidenreich, F.: Das M e n s c h e n k i n n und seine E n t s t e h u n g . Eine Studie und Kritik. Ergetm. d. Anat. u. Entwcklngsgesch. 31: 1-124, 1934. 9. Grunewald, J.: i2ber die B e a n s p r u c h n u n g und den A u f b a u des menschlichen Unterkiefers and die mechanlsche B e d e u t u n g des Kinnes, Arch. f. Anthropol. 46: 100, N.F. 18, 1921. 10. Klaatsch, H.: Kraniomorphologie and K r a n i o t r i g o n o m e t r i e , Arch. f. Anthropol. 36: 101, N.F. 8, 1909. 11. Bolk, L.: t?ber Lagerung, Verschiebung und Neigung des Foramen Magnum ant Schadel der P r i m a t e n , Ztschr. f. Morphol. u. Anthropol. 17: 611, 1915. 12. Brodie, Allan: Muscular Factors in the Diagnosis and T r e a t m e n t of Malocclusiolls, Angle Orthodontist 9.3: 71-77, 1953. 13. Huxley, T. H.: Evidence as to M a n ' s Place in Nature, London, 1862, Williams & Nor ~ gate, Ltd. 14. Cuvier, The Baron: The Animal K i n g d o m - - T h e ('.lass Mammalia, E d w a r d Grittitb et al., London, 1827, Geo. B W h i t t a k e r . 15. Martin, R.y and Salles, K.: L e h r b u c h der Anthropologi% ed. 3. S t u t t g a r t , 1956, Gustav Fischer Verlag. 16. Brodle, A. G.: On the Growth and P a t t e r n of the H u m a n Head From the Third Month to the E i g h t h Year of Life, Am. J. Anat. 68: 209, 1941. 17. BjSrk, A., Jensen, E., and Palling, M.: M a n d i b u l a r Growth and T h i r d Molar ]mpac. tion, Acta odont, scandinav. 14: 231, 1956. 18. Bench, R. W.: Growth of the Cervical Vertebrae as Related to Tongue, Face, ami Denture Behavior, AM. J. ORTHODONTICS 49: 183, 1963. 19. Steiner, C. C.: Cephalometrics in Clinical Praeti~e, Angle Orthodontist 29: ~, 1959.

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20. Bjbrk, A.: Facial Growth in Man, Studied With the Aid of Metallic Implants, Acta odont, scandinav. 13: 9, 1955. 21. Ricketts, R. M.: Facial and Denture Changes During Orthodontic Treatment as Analyzed From the T e m p o r o m a n d i b u l a r J o i n t , A~. J. ORTHODONTICS 41: 163, 1955. 22. BjSrk, A.: Variations in the Growth P a t t e r n of the Human Mandible: Longitudinal Radiographic by the I m p l a n t Method, J. Dent. Res. 42: 400, 1955. 23. Steiner, C. C.: Cephalometrics for You and Me, AM. J. ORTHODONTICS 39: 729, 1953. 24. Holdaway, R.: Personal Communication, 1963.