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The keystone triadII. Growth, treatment, andclinical significanceROBERT MURRAY RICKETTS, D.D.S., M.S.Los Angeles, Calif.

INTRODUCTIONT H E keystone triad was described in an earlier publication1 as consisting ofthe chin, point B, and the lower incisor. This unit, located at the keystoneof the mandible, includes the symphgsis, the alveolus, and the lower anteriorteeth. Anatomic details, phylogenetic factors, and problems in cephalometricnomenclature were discussed in the earlie r article. The intent of the presentessay is to review growth findings together with treatment changes and at-tempt to transpose them inbo clinical understanding. This perspective is neededfor a knowledge of the ultimate prognosis of the orthodontic patient.

LABORATORY INVESTIGATIOSSIt had been generally assumed, from the early studies of Brash2 and of

Keith and Campion, that the mandible always grew forward in the face andthat the chin became more prominent. The first work of Humphrey, the conceptsof John Hunter,5 and other works all contributed to general acceptance of thebelief that the human mandibular body grew in length by means of resorp-tion of the anterior portion of the ramus and apposition on the posterior sur-face. The lack of increase in the size of the arc of the mandible led investi-gators to conclude that li ttle apposition occurred on the chin, and yet the factthat the chin seemed to become more prominent in many patients confoundedthe acceptance of that conclusion.It was this frame of knowledge that prevailed at the time that &hoar and

The second of two essays to be publish ed in the AMER ICAN JOURNAL OF ORTHO-DONTICS, based on materia l presented before the Midd le Atlantic Society of Ortho-dontists in Atlantic City, N. J., Oct. 28 and 29, 1963, an d before the JarabakFoundation in Chicago, Ill., Feb. 1 and 2, 1963.*Associate Professor of Dentistry, School of Dentistry, University of Californ ia.

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Massle? studied the growth of the monkey mandible by means of alizarin redS dye. Their findings showed that until the equivalent of human age 6. orduring the mixed dentition, there occurred a general apposition of bone aroundthr mandible on all surfaces. This early behavior gave evidence of a g~nc~ralgrowth on the external body of the bone in all dimensions. However, at aboutthe age of the mixed dentition, there was a change in growth characteristics.Ai eoncentrat.ion of the dye was seen at the alveolar border, the posterior b~rd(~l*of t,he ramus, the coronoid process, the sigmoid notch, and particularly thcLcaondylar head and process. All these areas were growing by apposition, butt.hr body of the mandible did not grow. This was confirmed by Jloores wor4~in 19-K).7

The early work of Engel and Brodie8 proved rather conclusively that, tllc,nlain impetus to growth occurred at the mandibular condple? which propcll~tlt,he mandible downward and forward, carrying the chin with it or pushing thecallin ahead. Sarnat and Engels study on monkeys that had undergone cotldyl-octomp also confirmed this observation.

CLINICAL STUDIESTn his cephalometric study of prognathism, Bjiirk pointed out &at OV~I

the long growth range the chin tended to become more prognathic in the fanthan the maxilla. Landell also observed this tendency. The average profile t,hustended to straighten out, and the ma.ndibular anterior teeth tended to mov11upward and backward in relation to the profile. This finding has been clrmort-strated repeatedly, but all patients do not behave in this manner.

The work of Schaefferl is of particular interest to our discussion of changesof the lower incisor. Schaeffer measured the eruption tendencies of the Loweincisors in cases taken from Broadbents files at the Bolton Foundation. Hcshowed that the lower incisor could erupt upward and forward but that typicall:-it was carried upward and backward. This could occur in some cases, evtnwithout a change in the tooths axial inclination. Schaeffer however, spoke oithe difficulty of superimposing the symphpsis in some of the C~SCS t,udicd due-ing the later phases of growth.

C~XDYLE GROWTH AS RELATED TO THE CHIS. The above work representrcl llrcstatus of our knowledge of the changes in tho keystone triad at, the timo thalI conducted growth studies with laminagraphy.13 The growing condple wascorrelated with the change in facial profile, and it was found that the growtht,endency of the average condyle was straight upward and backward, almos!up the long axis of the neck of the condyle as typically described. In this typtof patient the chin grew downward and forward in the face in the usual manncll..

In some patients, however, the condylar head seemed to incl ine mor*r$posteriorly when measured from the mandibular border than had been obscrvcflin the beginning (Fig. 1). The mandible became more obtuse in form. Thisled to speculation that the growth of the condple was following a more postcrjordirection and that there was a distal inclination in shape of the condylar ncc*kand ramus. These patients seemed to show more gro\Tth in anterior height 01the fact.


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730 Rich&s Am. J. OrthodonticsOctober1964

Fig. 1. A, Tracings of a male patient with predomin ant vertical growth between the agerof 12 and 21 years. It wil l be noted that the facial pattern is long and shallow and that theY axis opened even more in spite of efforts to close it with orthodontic treatment and theextraction of four first permanent molars.

B, Mandib ular comparison, superimposed in the manner suggested by BjSrk on thearea of pogonisn and the crypt of the develop ing third molar, shows an upward and back-ward growth of the mandib ular condyle and apparent resorption of the ante&r border ofthe ramus and an upward and backward change in the lower incisor. Notice also the heightincrease in the mandib ular incisor. Open-bite tends to develop more if the incisors do noterupt in patients with this type of growth pattern. It wil l further be noted that develop-ment of the apparent prominence of the chin from a bony standpoint is due to incisor re-traction with growth.

In other patients there was greater development in the body of the ramus,leading to upward and forward growth of the mandibular condyle. The man-dible and the face tended to develop more squareness or to be more consistentwith the brachyfacial type of facia l pattern and the chin tended to move moreforward than downward (Fig. 2).In attempting to explain these changes, I referred to the muscu lar sus-pension apparatus of the mandible. In the final analysis, it was concluded thatthe mandible was growing against the function of the muscles which held itin place. Thus, upward and backward growth of the condyle, with slow devel-opment of the angle of the ramus, tended to add length to the face or to causea downward dropping of the chin.

At the opposite extreme, an upward and forward direction of eondylcfgrowth with extensive depression of the angle of the ramus was consistent wi


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mandibular squareness and a more prominent chin. In other words, the devte-opment of a heavy angle seemed to characterize a lower face and chin that wer(*forward in the face. Bj6rk14 observed the same differences in growth pattern*but apparently did not quite agree with the consistency of these mandibula:,patterns with particular facial patterns (Figs. 1 and 2). We have observeliexceptions, but generally it has held true. Statistics are lacking for these factors.More recently Bjijrk15 published a follow-up of earlier implant studieh.Four tantalum pins were placed in the mandible at certain points. At, a laterdate, by orienting on these fixed pins, which did not change in their rtlatioll-ship, he demonstrated the mean and ext.remcs of directional behavior of tht~growing condyle. The average posterior plane tended to move slightly upwar~land forward about 6 degrees from the original mandibular plane. The variut.iotltoward gonial development was 20 degrees (in an upward and forward dirt.+tion) . The opposite d irection ranged to an dista l inclination of 16 degrees (momobtuse). Bjiirk did not correlate mandibular behavior with facial chi~~~pe.1)1tK.C. g-714-3

Fig. 2. A type o f growth opposite that shown in Fig. 1. a, Tracings of a femal e patient be-tween the ages of 9 and 14 years. The X-Y axis changed from 1-7 to +15 degrees as the chinmoved almost directly forward. It wil l further be noted that the ramus increased markedlyin height and that posterior height developed at a greater rate than anterior height. Kobicethat the c,hin carried the lower incisor forward. In contrast to Fig. 1, which demonstratedabout a 2.5 cm. dropping of the hyoid bone, very littl e dropping of the hyoid is evidckntin this patient.B shows pattern of growth, superunposed according to BjBrk, and indicates marked verticaldevelopm ent. However, instead of anterior ramus resorption, as shown in Fig. I, there WBSapparently resorption of the angle of the ramus and a vertical growth of the ramus. Thisproduces growth rotation to the chin in an almost forward direction as the mamli blc tendsto curl during growth. Notice that the eruption of the first molar is much greater thanthat of the incisor in this particular patient. These patients tend to develop closca-l)itcs :IR apart of their growth pattern. Note that thr symphynis tends t,o incrcnse in width with thl>additi on on the posterior margin.


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7 3 2 Ricketts Am. J. OrthodonticsOctober 3964since his findings so close ly followed my observations or showed even greatervariat ional behavior, I would assume that the same general observations wouldhold for facial change.

LOCAL CHIN MORPHOLOGY. Several observations support the contention thatthe lower incisor is suspended from t,he lingual plate of bone and from thedense compact of t.he planum alveolar on the lingua l aspect. Repeated sectionstaken by numerous invest igators over the past 40 years have substantiated thisfact. It can also be shown very effectively by midsagittal laminagrams of thisbone taken in living subjects.

An over-all thickening of the symphysis menti seems to occur occasiona lly,but in these cases one must make observations over a long period in order tomeasure the change. An example was seen in a nontreated patient who devel-oped a beautiful occlusion and a very strong mesognathic pattern, with a markedincrease in the width of the symphysis, in a period of 8 years (Fig. 2).It has been noted, however, that by the time the lower first molar haserupted the general width character istics of the symphysis will have been at-tained. The average symphysis is almost teardrop in shape. Extreme variationsshow either a long, narrow symphysis or one that is short and thick. Patientswith mandibular prognathism or with great alveolar height and retrognathiepatterns often have long, narrow symphyses. The brachyfacial or wide-eyed per-son often exhibit,s a symphysis with thick, more square outlines. Exceptionscan be found if a search is made, but these cha.racteristics can be identified inmany cases.It would seem that the presence of these facial types can be taken to in-dicate that certain functional and morphologic characteristics prevail. The merefact that these tendencies can be observed is worth while as a clue for use inestimating the future. This is a conjecture to be borne out or refuted by re-search. Clinicians have reflected this suspension in the often-heard expressionthem thats got-gets when it comes t,o a chin. Investigations of the cross-sectional area of the chin should be correlated with mandibular width. As shallbe seen later, this is only one part of a whole complex.

POINT B BEHAVIOR. It is of particular importance to visualize the differentgrowth patterns of the whole face in order to understand the behavior of theerupting t.eeth and of point B, which is directly related to the shifting of theseteeth. In 1953 Brodie16 completed the study which he orig inal ly started in 1940on the growth pattern of the face. He noted the tendency for the teeth to shifton the basal bone of the mandible after the age of 8 years. In h is own words:

The la te stages of growth have been shown to be accompanied by a continuation offorward and downward movement of the anterior nasal spine and of pogoni on, whil e the dentalarch and its supporting bone tends to move more slowly and thus drop behind. This de-creases the prominence of the denture. At the same time, however, such behavior is notnecessarily accompanied by a more upright position of the incisors. These teeth may becomeless procumbent, more procumbent, or many remain at their origin al axial inclin ation.

Brodie employed the occlusa l plane and dropped perpendicular lines fromthe occlusa l plane in order to reach his conclusions. He referred to t,he pre-vious ly mentioned work of Xchaeffer in support of those conclusions.


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With similar observations, Bji jrk placed implants as far down on the ~J%I-physis as he could from inside the mouth. In his preliminary studies OVV!short ranges, Bj iir k demonstrated that meta llic implants tended to remain illthe same position and that actual resorption of bone was occurring in the arc;1of point B. In a patient with condy lar arrest and alteration of pa,tt,crn, Hji itkdemonstrated that apposition did occur at the angle of the mandible in a CIUI-centric layer adaptation, possibly as a function for a masseter mu&c 01 ptcl: -goid internus bracing. In that particular case, superimposing on tnrtallic ir+plants revealed the probability of appositional changes on the bon?: chin. hrltin the majorit>- of normal persons resorption of the bonth occurrt~ l in the :t~c:tof point B.

BjS rks latest studies of the chin and point B have demonstrated no app+sition of bone in the region of pogonion. Some apposition was suggested in tijctarea below pogonion, but the area of the m.entnl protubwunce was not dwn,qr (1.ResorpCon was almost routine ly demonstrated in the area of point K 213 t iwinciso rs shifted vertica lly with growth. The increase in thickness of the s:v:~I-physis seemed to be appositional on the posterior and inferior borders.

By using t,he central core thus registered by implants, new light, Nil:>shed on the directional eruptive possib ilities of the teeth. The typical helmviol*was one of straight upward eruption. Extremes suggested, however, that th(bteeth could erupt either upwasd and forward or upward and backward 1r~~mthe body of t,he mandible. The lower incisor was shown to move hncA~~rr-tlnaturally in those eases of obtuse growth. of t,hc mandible. In brachyfacial tylj (lSthe incisor seemingly demonstra.ted for.~~~~l! eruption tendencies. Therc:For~~,it might be hypothesized that in the brachyfaciat typr the lower anterior tl)cthwill be stable in a more forward position when related to the symphysis.

Further discussion of the behavior of point, K is rc>lati\-c , to trc~atmrnt. JctintK tends to follow the lower incisor. It will bc obstirvcd t~lin ically that altrrforward movement of the lower incisor a ledge of bonct in the planum dvtYJlarwill a,ppear on the lingual side; this will 1~ retained for a time but, will grt~d-ually disappear. However, no such shelf is seen lahiall,v after lingual nlovrlnSJntof the teeth. This would imply that tho hclav>-. slow-rc~sorb ing bone is lo~nt cclon the lingual side.

BEHAVIO R OF THE LOWER INCISOR. (3ne of the first critical studies with I(-gard to positioning of the lower incisor during orthodontic treatment, was l)A -lishcd by IAtowitz17 in 1948. He referred to the earlier work of 8pGdcl ;tntLStoner,l who related the long ax is of the incisor to the mandible iu atlll1t.s.and also to the earlier work in cephalometric analysis conducted in 1936 byBrodie, Downs, Goldstein, and Myer. IQ Litowitn studied twent;v t.rc~at~t~cl SCSand reached the following conclusion:

The tracings of the mandib ular symphysis with the incisor tooth gave striking cvidenwthat teeth do not move through bone but rather that the alveolar process is remo&Ic~l ast,he teeth change their positions. When teeth are moved labiall y, the alveolar procw+ fo l lowsant1 when relapse occurs, the bone returns with rhe tooth.

Litowitz further stated tha.t a disturbance of the root apex or (rown wasfollowed by a return to the original position in almost every case, particnlady


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if SUCK movemeut had been in the labia l direction. In almost every case treated,however, there was much growth following orthodontic treatment. Litowitzpointed to the confusion of methods and the various interpretations that couldbe made by comparing individual cases in which different techniques had beenused.

I also became concerned about the multiplicity of factors to be known orunderstood in the behavior of the lower incisor as a key to treatment planning.20I studied 150 orthodontically treated pat,ients and compared them with 100nontreated cases. The behavior of the lower incisor was measured in fifty pa-tients during treatment with ordinary cervical extraoral anchorage, in fiftypatients treated by intermaxillary anchorage only, and in fifty patients inwhom a combination of intermaxillary elas tics and headgear had been employed.

That study was conducted in an effort to provide some data for the plan-ning of treatment and for the comparison of lower incisor movement duringtreatment. The observation period lasted almost 3 years, during the transitionfrom the mixed to the permanent dentition, or when the patients were at anage at which children are commonly treated with cervical t,raction and at whichthe 2.7 mm. shortening of arch length was supposed to be occurr ing. Thecephalometric technique employed called for dropping of the perpendicular linethrough pogonion to the mandibular plane and measurement of the lower in-cisor parallel to that line.

In nontreated Class I cases, the lower incisor was located at an average ofabout 8 mm. posterior to that perpendicular line and moved about 1 mm. back-ward during this 219, to 3 year observation period (Fig. 3, A). In the Class IIsample, this movement was only about one half that amount. Stretching thisbehavior over a long span would suggest that the lower incisor could possiblymove as much as 3 to 4 mm. backward from the mental protuberance from thetime it erupted until full growth was reached. However, a I to 2 mm. shiftshould be expect.ed often.A contrast in behavior of the lower inc isor under the various types oftreatment was noted. Although mandibular growth in the three treated sampleswas almost identical to that of the nontreat,ed samples, the behavior of thelower inc isor differed radically. It was shown that the effects of the headgearcontributed to an average lingual movement of almost 1.5 mm. during the timethat the neck strap was worn (Fig. 3, B). The lower incisor tended to movelingually even more than normal. In Class II patients treated with intermax il-lary elastics, the average lower incisor moved 3.0 mm. labially when measuredto the symphysis (Fig. 3, C). On the average, therefore, treatment of Class IIcases by headgear or by intermaxillary elastics alone eventually yielded, at re-tention of the incisor, an average of 4.5 mm. difference in forward or backwardrelation to the anterior border of the symphysis (Fig. 3, D).A position forward or backward can be related theoretically to arch lengthby adding the difference on both sides. Thus, a 1 mm. forffard or backwardposition of the lower incisor yields 2 mm. difference in arch length as measuredfrom the molar. Naturally, with changes in arch form, this is not always true.Given no change in the arch form, however, a difference of 9 mm. between the


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lar retrusion with iIlt,~~~ltlas ilIiI~y dastics i~lom! , all I10ugl~ in sucli cas(ls t.li~~r~~was a calcdatrd ris k of forward movement of the incisors. The IOW~I incaisorswere observed to bc moved forward c\rasticaIIy in thcst: patic>nts, thr nlor~mwntof some cvcn approaching 1 full ccntimetcr. After a time, the alveolar ?KNK!seemed to re-form on the labial aspect of the tooth. Most cases treated !n thismanner appeared to be holding well several years after orthodontic treatment,which would be contrary to the findings of Litowitz. Even though these teethwere upright or moved lingually during retention, it should be pointed out thata slight lingual drift M-as consistent with normal development and was there-fore to be expected.

Clinicians often fear a loss of bone or a clefting of tissue labial to the in-cisor, called stripping. Often stripping of the lower incisor has occurredeven before the start of treatment. I have claimed that this is due to extremetightness of the mentalis muscle and to a pressure ischemia. In other subjectsit was consistent with a history of traumatic conditions.I should state immediately that the lower incisor should never be movedpromiscuously, as it has been with many techniques. In many cases, however,particularly those in which patients have short brachyfacial patterns, the lowerincisor is actually intruded as it is brought forward into a wider portion ofthe symphysis and therefore results in greater covering on the labial surfaceinstead being shoved out of the bone.

In the treatment of some patients with stripping, various c linicians recom-mended extraction to prevent forward movement of the lower incisors for fearof further bone loss . Other c linicians, however, have indicated that a forwardmovement is desirable in spite of stripping and prescribe long retention periods.In st ill other patients, it has been recommended that the lower incisors be re-tracted lest they move backward under tension of lip musc les if the arch wereexpanded.These arguments show t,hat a factor entire ly independent from estheticconsiderations has prevailed. In discussing behavior of the incisor clinically,two other observations should be mentioned. It wi ll be noted that, without ex-ception, a change in point B is directly related to the behavior of the lowerincisor-so much so that a measure of the lower incisor is a measure of thechange in point B. Furthermore, the thickness of the soft tissue labia l to pointB remains the same to a remarkable degree. A comparison shows that the thick-ness of tissue of the lower lip labial to point B is almost identical in all but avery few cases. A change in position of the lower incisor with its accompanyingalveolus very definitely affects the soft-tissue contour of the sublabial area ineither direction (forward or backward). 23 In this manner, an esthetically cf-fective chin can be either produced or reduced.

FACTORS IN U)NG-RANGE PROGNOSIS. In a longitudinal study in which agegroups were used in order to dovetail for the full age span, Bencll3 correlatedchanges of the hyoid bone relationship with the growth of the cervical vertebraeand the behavior of the denture profile. He showed that the hyoid bone con-tinued to drop downward from the chin at ages later than those at which activestages of change are usually thought to be taking place. In addition, Bench


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measured the position of the lower incisor and showed that these changes Jve rtconsistent w ith the alterations of the hyoid posture. He further showed a strongcorrelation with changes in the hyoid bone and growth of the cervical vert(bra(.In patients in whom growth of the cervical vertebrae was no t occurring and inwhom bra&ycephalic tendencies were seen, he was able t,o demonstrate markedforward growth of the chin. In other patients with severe dropping ol tl,c chin a,nd vertical cervical growth, there was an upward and backward alt(r;l-tion in the outline of the symphysis as the lower inc isor drifted posteriorl?According to all the foregoing basic and clinical studies, t.he natural formof the chin and the natural position of the teeth seem to be correlat.ed wit 11deep underlying nebulous factors and tend to behave in a biologic spiral. Thxheight of the indiv idual, growth of the cervical vertebrae, the form of TII~~mandible, the width of the face, the changes in the tongue and hyoid I)oII(~.and the individual characteristics of the lip and mouth were seen to play :trole in determining the relationship of t,he chin to point, B and the lower in-cisors . It was hypothesized that most crowdin, v of lower inciso rs was due I*)growth changes rather than to the force of third-molar eruption, as commnnlhclaimed. In other words, the forces producin g the rollapse were coming froiilthe anterior area to crowd the anterior teeth backward. The third molars l), -came a buttress in this sense.

In describing various characteristics of t,he mandible as a background t(11the estimation of growth during orthodontic treatment, I included a cons id-eration of all these factors. It will be recalled that I further described t(.rrdifferent characteris tics of the mandible to be considered in an estimation (s-ercise, O~C of which was t,he width of the symphysis. The natural f()rc(bs Irlgrowth and change have been shown to he of vital (aoncern to a full ~oml,~-hension of orthodontic planning and secure? ultimate results. The secret ,itstability of point B or the lower alveolar bone is, t.hcrefort, a prime consider;i-tion. Thr key to this understanding is a knoxledge of the function of the JI,IIS-culat ure of the tongue and lips and the natural growth of the (ahin wit11 itsalveolar boric. My clinica l criterion for proper managt~mc~nt, of the lolv~ r jr,-&or thus follows.

CLISIChT, ISTERPRETATIONTOOTH AXGIJLATION VER SUS SPATIA L RELATIOR. ~?%tiCCd~y a i l i?W.?stigat(~r,&front 19.i.5 to 1955, were measuring the angulation of the lower incisor rrrthrlthan it s spatial relationship to the symphysis or profile. For the most part,

the orth,odontist was trying to avoid the wovenlent of the ape3: of the incisorfor fPar of root resorption or destruction of the alvcbolar bone or gingival I is -SW. TtJ appeared to mc, however, that the angulation of the tooth was less iln-portant, than its t,rur anterior post,erior position . Tl l~ question was : "T o w11 ;1 isho~ltl it 1~ rc~latrd? We accepted, ant1 stil l clt~frnd, pogonion and l)oint ,\as 111~. atilc~r ior limits 01 bnsal lwttt: in 111~ tn;~tldilml;~r ant1 niasilla ry IWIIC~.rctspwt ivthlv. Thcrcforc>, I started measuring tl iv I)ositiou of 111~ tip of tlrc. St,-cisor to the lint A-pogonion for various B( i lSOIlS (Fir. 1, -1 2nd Ii 1. l+'or* tit,-scriptive purposrs I called this thcl f l (~n l l l tY~ 1, l i lT l i .


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738 Ricketts Am. J. OrthodonticsOctober 1964

A BFig. 4. The different inclinations of the A-pogonion plane as revealed in concave faces (A)and convex faces (B). This is the line thought to be mo st useful and critical in the clinicalevaluation of the lower incisor.

We learned from the study of comparative anatomy that certain animalswith diminished development of the maxilla have a lower incisor that is linguallyinclined in such a position that it functions as a reciprocal member for bothjaws. The tooth serves both jaws, not just the mandible or the maxilla alone.With this functional consideration in mind, the A-pogonion plane was laterreferred to as the reciprocal denture plane. In order to describe the labio-lingual location of the lower incisor, I have employed the APO plane as themost useful c linical reference; today, after 15 years, I stand more firmly onthat conviction than ever (Fig. 5). Why have I become more insistent con-cerning the value of the A-pogonion plane as a sensible reference for the lowerincisor? What is the controversy?

The chief argument advanced by the opponents of this approach is thatpogonion is unstable because of secondary growth characteristics or the devel-opment of a button on the chin, which makes it unusable because of the additionof bone on the chin. Furthermore, many are concerned over the acceptance ofnarrow standards or a single ideal incisor relationship for the population as awhole. I also have sensed that clinicians fear any method which would eversuggest that the lower incisor should be moved forward. There are also thosewho believe that planning should be initiated not with the lower incisor but,with the molars. Finally, and hardest to comprehend, there is the need forrealizing changes in point A or the chin with growth and treatment-a con-


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sideration of dynamics rather than statics in interpretation of the lower incisolfrom the APO plane.For t,he purpose of constructive argument, I would pose three questions 511defense of the use of the reciprocal denture plane reference for the lowN* in-cisor : (3 ) If point A changes with orthodontic treatment, why is it maintainedas a basal point? (2) What true basal point at the anterior border of the marl-dible is most usable? (3) When a so-called button develops, what is it. liondoes it form, and how frequently?In answer to the first question, point A, selected at the deepest point ORthe contour of the bone between the anterior nasal spine and the alveolus, doeschange when the upper anterior teeth are moved. However, the nasal spine isdefinitely a process and the alveolar process is a process, and between thtJst&two lies the most basic part of the denture base at the anterior limit. It shoul(\be mentioned that al l bone in the anterior pnrt of the maxilla is thin and, zmni-nated. NO true heavy compact areas are available for reference in the same:sense as the symphysis. Since point A was originally defined, various clinicia,nsand investigators have sought a better reference point., but clinicians somehowhave continued to go back to the basic point first described. Point A must 1~:selected carefully, but it has proved to be a vastly useful point to most of tll()scawho have understood its problems and variations.

Fig. 5. Range of variation in the two standard deviations from the mean in l,OOU ortlro-rlontic eases. The darkened area in the center shows average or typical position relativet.o the A-pogon ion plane, that is, 0.5 mm. forward and at 31.5 degrees to the A-pogon ionplane. The striped tooth to the lingu al, for practical interpretation, is -6 mm., and the dottetltooth is +8 mm. to the A-pogonion plane. Phnnin g is usually conlirrtrd to one standrtrd tlwin-tion, or -3 to +3.


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740 Ricketts Am. J . Orthodonticsoctotw 1964

DFig. 6. Patient S. L. Tracings of a girl from age 9 to age 15 depict a patient with severebrachyfacial type of growth similar to that seen in Fig, 2. Note the retraction of the lowerincisor, which explains tbc severe button increase. Very littl e increase in facial height wasseen in this brachyfacial patient, as the facial angle increased from 92 to 95 degrees andthe Y axis closed 3 degrees during this treatment period. This behavior occurred despitethe fact that the patient was treated only with intermaxillary elastics and every attemptwas made to bring the lower arch forward.


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L.R.. 9-l

A

18 3 \

/-v-f// * f7/-?/

. .Y. .* IC \

II\IIIIIII

\

DFig. 8. 8, Tracings of boy between the ages of 9 and 18 with marked vertical developm entduring growth and treatment, similar to the type shown in Fig. 1. Very little change wasseen in the facial angle, wh ich usually increases in a normally growing face. Note that tbY axis changed from 0 to -5 degrees during this interval.B, Comparison with B reveals that the great amount of growth appeared in the area ofthe outlin e of the chin; however, careful vieTYing suggested that this was lateral to the mid-line in the area of the menta l tubercles. A severe clefting of the bony chin was found bypalpati on. Notice, finally, that both the lower incisor and the lower molar erupted extensivt>lyin this type of growth pattern (C and D).


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Fig. 0. Before- and after-treatment roentgenograms of patient shown in Fig. 8.

at a later time, even after premolars had been extracted. It would be hard toimagine anything other than a shift of the denture on the base of the mandible,since the width of the symphysis did not thicken at all. An esthetically Lstrongchin was developed, with no bone increase in the symphysis.

CLINICAL USE OF THE APO PLANE. Many clinicians have had trouble in usingthe APO plane for reference because of the dynamics of growth change. Beforeplanning treatment, the clin ician should have a knowledge of the variat ionsin lower-incisor position that exist in nature. It was for this information thatI measured the first 1,000 clinical orthodontic cases seen in my private prap-tice (Fig. 5). The findings revealed that the average lower incisor was 0.4 mm.ahead of this plane, or about 0.5 mm. for practical purposes. Its variation wast2.7 mm. in standard deviation, but again for prac tical use I have called itt2.5 mm. In general, convex faces displayed lower inciso rs that were forwaladon the chin, and in concave faces teeth were more retrusive from the symphysisbecause of the seeming adaptation of the inciso r to the contour o-f the fact>.

I related the lower incisor to different age groups in that same cross-sectionalst,udy. It was seen that the convexity of the face diminished in the age groul.is,but the lower incisor remained within 1 nlnz. of the same relationship to theA-pogonion plane in all groups. I concluded t,hat the uprighting o-f t,he toothnnit the rctlmaction of t,hc lower incisor were commensurate with a change incaonvexity and were part of the normal development, partic*nlar ly when Itr~-opinion was reinforced b.y other longitudinal studies.Therefore, a critical interpretation was gained by relating only the lowc~rincisor and forgetting about point B. The lower incisor studied in rehtion tnthe, A-pogonion plcrne automatically consitlers the fncial pnttern of th.e ikldi-


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16-I

Fig. 10. Tracings of patient P. I?., a girl, from age 5 to age 16. Clinical ly, this patient had acomplete bilateral cleft lip and palate, and the facial angle changed from 85 to 91 degreesduring the course of treatment and growth. An attempt was made to bring the maxillarysegments forward during treatment and the mandib ular teeth were employed for anchorage,but the teeth moved posteriorly (H). f:, A comparison shows the openin g of the Y axis,possibly produced by the prolonged use of Class TrI intermaxillary elastics. D, Tracingssuggest a movement of the lower incisor lingua lly to the planu m alveolar of the symphysis.Note tho upward and backward movement of the lower first molar in spite of premolar ex-traction in the lower arch. It is difficult to imag ine anything but alveolar and tooth rc-traction producing the prominence of this chin, since no anteroposterior increase whatsoeverwas present in the thickness of the symphysis.


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vidual and also is useful at his state of growth ctnd de~~elopmen~t. Downs hasreferred to the arc of the face in a quite similar manner of measuring rclatiotl -ship to the lower incisor.

The final question for this essay is : HOW do we consider the needed changesin the lower incisor, alteration of point B, and the contour of the c~h in in ortho-dontic treatment planning?

Lets fare it. In spite of recognized biologic pitfalls, most of us are treatingtoward but not necessarily to some sort of average or standard. We a ll sock :Iccrta.in amount of security in plannin g because, as clinicians, we must obscrvc,study, and make decisions and cannot continually procrastinate or take uncai-culated chances. Therefore, we are concerned with some conceived goal, whcthcrwe admit, it or not. We can ill afford to take chances repeatedly when we -Facc~the respons ibilities of finished stable results. As one student said to mc: I km)\\I can move teeth but what I need is a target.The correct use of the Downs-Ricketts APO plane, Steiners NA lint, the>Steiner-Holdaway NB line, or the Tweed triangle rests upon the predicte!tl orestimated alteration of facia l relationship resul t)ing from growth and trcat-ment changes. We have concentrated first on the behavior of the chin, thealveolus, and the lower incisor as a keystone unit, but in addition we have men-tioned the reciprocit,y of this tooth to both jaws. Therefore, we must consitl(~ ralso t,he behavior of the maxilla, or point A, in order to plan effectively.

I have stated that., in my opinion, the orthodontist should s trive to playthe lower inciso r within one standard deviation of the APO plane as point A cxntlpogonion will be located at the end of treatment and possibly later. In other*words, the lower inc isor should be related to the jaws that will exist at thtl ~1of treatment.


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0I/

C DFig. 12. 8, Tracing of patient with severe Class II, Division 1 malocclusion, slight retrognathicpattern, severe convexity of the face, and a lower incisor located lingua lly in relation tothe A-pogon ion plane. Correction of relationsh ip of the lower incisor to the A-pogo nion planecan be accomplished as shown in B, C, an d D .B, Correction by forward movement of the lower incisor typically seen with the use of inter-maxillary elastics or the monobloc. Notice that the upper incisor is uprighted and slightlyretracted, with very little movement of point A.C, Orthopedic retraction of the maxi lla and point A very frequently seen with forceful head-gear and vigorous intermaxillary elastic traction. In this situation point A is brought backover the lower incisor, with no movement of the lower incisor relative to the chin, in orderto promote a more ideal relationship of the lower incisor to the A-pogonion plane.D , The change in position of the lower incisor relative to A-pogon ion plane as manifestedby forward movement of the chin by growth. The upper incisor is held in place while thechin in the brachyfacial patient moves forward in a manner similar to that seen in Figs.2 and 7.


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Keystone triad i-47Let us, therefore, enumerate the possib ilities of corrcct,ion OC the lowcr :ill-

cisor in relation to the A-pogonion plane. Let us suppose that, we have a patient,whose lower incisor is slightly lingual to the APO planca, which is more or lesstypical of many Class II malocclusion cases (Fig. I?, d ). We know that thelower incisor normally lies slightly ahead of this reference plane and that wcmust treat the patient. We must realize that we can either move the tooth oteffect the end points of the reference line, that is, point A or pogonion.

Again, what are the possibilities? First, we can move the lower incisorforward (Fig. 12, B). Sometimes th is is a correct choice; at other times it isdangerous to stab ility. Therefore, we must be careful in recommending forwardmovement of the lower teeth. However, in profiles that are already straightwe may have no choice but to do so.

Second, we can effect an alteration of point A in two ways. We can, 1)~toryuc control, retract the upper incisor and move point 9 backward by localalveolar alteration (Fig. 9, B). In addition, research has suggested that extra-oral force reduces the entire hard palate or nasal floor by downward and back-ward tipping. Therefore, it is suggested that, the maxillary growth bchav ic~ris altered (Fig. 8, C).

Another means by which the APO plane may bc uprightcd is through for-ward growth of the chin at a faster rate than that at which point A movesforward (Fig. 8, D) . As has been shown in many cases of natural normal growth,such processes do occur, but they occur slowly. Forward growth of t,he chin,carrying with it the lower teeth, thereby corrects thtso teeth to the jaw rcla-tionship.

The final possib ility, and probably the most common, is the combinationof any two, three, or four of the foregoing. Given a case in u-hich the lowclincisor is in a retroposed or retruded position, one could move it forwardrapidly in the denture profile by a combination of backward movement of point,-4- loca l forward movement of the lower incisor, and forward growth of thechin, carrying with it the lower incisor. Such cases show marked improvompnlin a matter of a few weeks!

However, in given cases of severe convexity and mandibu1a.r retrognathisnl.in which lower incisors need to be moved posteriorly, the problem is more (as-treme a.nd it, takes longer to achieve the desired results. In such cases point A,or the maxilla, must be retracted over greater distances, which takes longer toachieve. Also, forward mandibular growth is needed. Longer periods are ad-voretcd t,o take advantage of growth in thpsc patients.

CLINICAL PROBLEMSIn the final analysis, the orthodontist is dealing mainly with occlusion (don-

tal or orthodontic) and profile relationships (orthopedic, skeletal, or structural).He works with the profile of the face but, perhaps more important, also withthe profile of the denture. The profile of the face can be measured from thefacia l plane (N-PO), and the profile of the denture can be measured to thedenture plane (A-PO). These lines or planes serve as references. Findings andstandards serve as guides for the orthodontists discretion in correction. He


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has his choice. .If he wants to correct convexity or concavity, hc attempts toline UP the anterior part of the maxilla toward the facial plant. Tf 11~ lv;ltitsto correct the dental convexity, hc attempts to lint 111) he teeth to t,hc dcnturcplane.Al l faces arc not alike, however, and in many casts the wisdom of correct-ing skeletal convexity can be questioned. Likewise, the need for full dentalcorrection to flat relationships is doubted when the lips are loose and flaccid,when the tongue is large and forward, when the nose is long, or when a fullnessof the denture wi ll be stable, where the mouth can be closed with no strain andis esthetically acceptable, and where the teeth are long-lasting in service tothe patient. It is within these parameters that wc differ in value judgments.

In actual practice, and in the prognosis of a trca,tment, I try t,o hold tothe princ iple of accepting a range of variation within the confines of one stan-dard deviation from the mean of the antcroposterior position of the lower in -cisor to the APO plane. This usually yields a range of approximately 5 mm.(-2 mm. to +3 mm.). In indiv idual cases with aberrant muscle problems, theselimits are extended sometimes to beyond one standard deviation (-2 to +5 mm.)without harmful effects when the teeth are stable, the lips are smooth in con-tour, and the mouth can be closed with relaxation.

S U M M A R YThis is the second of two articles discussing the chin, point B, and the

lower incisor. I have called this unit the keystone triad. Bas ic science andlaboratory investigation have suggested t,hat deep biologic phenomena areresponsible for chin form and denture behavior. Clinica l studies of growthhave been numerous and point to an average tendency toward natural retractionof the lower incisor with growth and maturation.

The development of the chin may be considered in two aspects: (1) theforward thrust of the chin as it is carried forward by growth of the condyleand ramus of the mandible and (2) the loca l alteration in shape and contourof the symphysis by remodeling resorption in the area of point B and ap-position on the lingual and inferior borders of the symphysis. Little or noapposition has been noted in the mental protuberance or pogonion area. Thesymphysis seems to be unaffected by orthodontic treatment, except for theadaptation of the planum alveo lar and alveo lar bone as the teeth are movedforward or backward and point B follows.The prevailing concept is t,hat masses of enveloping muscles move the den-ture as growth proceeds. Long-range prognosis depends upon the clin icianssophistication in understanding these biologic forces. The difference in be-havior of the incisor with various treatment procedures was reviewed. A 4.5mm. mean difference in position of the lower inciso rs in relation to the symphy-sis was noted between extraoral anchorage and intermaxillary anchorage ClassII correction in nonextraction cases.The position of the lower incisor in the jaws seems to bc more adequatelydefined by an anteroposterior measurement than by its axia l inclination. Ourcriterion for clinical analysis is the incisors relationship to the A-pogonion


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plane, measured in millimeters from the in&al tip and the angle of its longaxis to the APO plane. Even when so-called buttons are present., the pointA-pogonion relationship for the lower incisor st ill seems to be useful.

For cont,emporary orthodontics, a knowledge of this unit (the keystoncltriad) is a key to treatment planning. Proper execution of this method dependsupon conceived a.lteration of point A, because the lower incisor is referred tcrpoint A through this plane in a reciprocal relationship to both jaws that itmust serve. The growth of the chin looms as a matter of equal concern. In or&t,to use point A and PO references, their behavior must be estimated duringtreatment.

I have attempted to treat toward the mean (to.5 mm.), or to within on(standard deviation of the mean of the anteroposterior position of the lowet*incisor to the APO plane. In the broad sense of variation, this is -2 to -6 mm.If I have not achieved stability with th is arrangement, then I am will ing toaccept the risks of relapse, because this range seems to be a sensible guide it,the light of contemporary esthetic and functional objec tives.Mouth esthetics wi ll almost always be complimentary to the face when t,hr*teeth are arranged in the manner suggested in this art icle. I believe that whenesthetic harmony is achieved, the chances of functional equilibrium are vastl!,improved. The goals described here have been developed from studies of thenormal and from more than 1,000 successfully treated malocclusion cases. Thisis the manner in which normal faces are arranged, and I simply try to ximulati>normal conditions by using the criterion proposed here.

REFERENCES1. Ricketts, R. M.: The Keystone Triad, AM. J. ORTHODONTICS 50: 244-264, 1964.2. Brash, J. C.: The Growth of the Jaws and Palate , London, 1924, Dental Board of i&r

United Kingdom.3. Keith, Sir Arthur, and Campi on, George G.: A Contributio n to the Mechanism of Gromii l

of the Human Face, D. Record 42: 61-88, 1926.4. Humphrey, G. M. : On the Growth of the Jaws, Tr. Cambrid ge Phil . S ot. 11: 1866.5. Hunter, John: The Natural History of Human Teeth, ed. 3, 1803.6. Schour, I., and Massler, M.: Post-natal Cranio-facial and Skeletal Development in the

illb ino Rat and the Macacus Rhesus Monkey as Demonstrated hy Vita l Injections ofAlizarin e Red S, Anat. Rec. 76: 94, 1940.

7. Moore, A. W.: He ad Growth of the Macaque Monkey as Revealed by Vita l Stain ingEmbedding and Undecalcified Sectioning, Asl-. J. ORTHODONTICS 35: 654-671, 1949.8. Engel, M. B., and Brodie, A. G.: Condylar Growth and Mandi bular Deformities, SurgStlr>

22: 976-992, 1947.9. Rarnat, B. G., and Enge l, M. B.: A Serial Study of Xandib ular Growth After Removal

of the Condylc in the Macaca Rhesus Monkey, Plaat. & Reconstruct. Surg. 7: 364-380,1951.

10. BjSrk, Arne: The Significan ce of Growth Changes in Facial Pattern ant1 Their lbrlatiorr-ship to Changes in Occlusion, D. Record 71: 197, 1951.

11. I,ande, M. J.: Growth Behavior of the Human Bony Profile as Revealed by Seria l (~phalo-metric Roentgenography, Angl e Orthodontist 22: 78, 1952.

17. Schaeffer, Aaron: Behavior of the Isis of IIurnan TIl&~r Iwtl ~ I Wing Gro\\ I II, ,111~l~~Orthodontist 19: 241-254, 1959.

13. Ricketts, R. M. : A Study of Changes in Tcrr~l~oroma~ldihular Relations Associated \i-i I IIthe lrc~atmrnt of Class II Malocclusion, JIM. .J. OIY~~~~D~~ST~CS 38: 91X-933, 1952.


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750 Ricketts Am. J. OrthodmticeOctober 196414. Bjijrk, Ame: Cranial Base Developm ent, AX. J. ORTIU.ILIONTICS 41: 198.225, 1955.15. BjSrk, Ame: Variations in the Growth Pattern of the Human Mandible: Longitudinal

Radiographic Study by Implant Method, J. D. Res. 42: 400-411, 1963.16. Brodie, A. G.: Late Growth in the Human Face, Angle Orthodontist 23: 146, 1953.17. Litowitz, Robert: A Study of the Movements of Certain Teeth During and Follo wingOrthodontic Treatment, Angl e Orthodontist 28: 3-4, 1948.18. Speid el, T. D., and Stoner, M. M.: Variatio n of Mandib ular Incisor Axis in Adul t Nor-

mal Occlusion, AM. J. ORTHODONTICS 6; ORAL SURG. 30: 536, 1944.19. Brodie, A. G., Downs, W. B., Goldstein, A., and Myer, E.: A Cephalometr ic Appraisal

of Orthodontic Results: a Prelimina ry Report, An gle Orthodontist 8: 261, 1938.20. Ricketts, R. M.: The Influence of Orthodontic Treatment on Facial Growth and Dcvelop-

ment, Angl e Orthodontist 30: 103, 1960.21. Ricketts, R. M.: Cephalometric Synthesis, AK J. ORTHODONTICS 46: 647-673, 19 60.22. Ricketts, R. M.: Cephalome tric Analysis and Synthesis, Angl e Orthodontist 31: 141-156,

1960.23. Bench, Rue1 W.: Growth of the Cervical Vertebrae as Related to Tongue, Face, and Den

ture Behavior, AM. J. ORTHODONTICS 49: 183-214, 1963.

ricketts triad

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