The Lower Incisors In Theory And PracticeH. BERGER, DR. MED. DENT.Tel-Aviv, IsraelWe orthodontists customarily re-gard the denture as a whole; we do notconfine ourselves to the denture only,but also take into account the entirepatient and his constitution. Never-theless, there sometimes arises the needto concentrate one's attention upon acertain section of the denture. Such asection presents itself today in thelower incisor region. First, because ofthe fact that there has been a certaineclipse oE the six year molar as the"key to occlusion", lower incisorshave usurped a kind of "key position"in newer diagnostic procedures. Sec-ond, these teeth seem to have alsobecome a problem in therapeutic pro-cedures during the last few years.While in Bogue's and Angle's timeswidening was thc cure-all, today wehave quite a range of possibilities. Onemay be that of resignation, as ex-pressed by Howes: "The patients andtheir parents are prepared for someirregularities of the mandibular in-cisors." Another is the outright extrac-tion of two premolars. In between isthe extraction of an incisor, strippingand, of course, widening.Though all this has been treatedand discussed repeatedly so that therehardly appears to be any need to gointo it once more, it seems to the bestof the author's knowledge that theinterrelation between diagnostic andtherapeutic precedures has not yetbeen investigated, i.e., the double per-sonality of the mandibular incisors asdiagnostic subjects on the one handand as therapeutic objects on theother.The significance of the lower in-cisors as therapeutic objects is three-fold:(1) As the first to erupt they maybe the first sign of an incipient mal-occlusion.(2) They are difficult to treat asthey relapse easily.(3) Crowding of the mandibularincisors is the most frequent anom-aly.In this connection it might be op-portune to give a few statisticalfigures. STATISTICSIn Table I the results of some in-vestigations regarding crowding havebeen collected. They are rather uni-form and may be roughly summarizedas follows: (1) crowding of lower in-cisors occurs in about fifty per centand (2) crowding in the lower j.aw isbetween fifty and one hundred percent higher than in the upper. Table IFrequencies of maxillary and mandibular crowdingBarrow aiid Huber and Lundstrom* Moore Mmrrees* SeipelWhite Reynolds and ReedAnteriors Anteriors AnteriorsMaxilla . . . . . . . . 24% 32.2% 35% 26.4% 25%Mandible . . . . . 51% 52.6% 5 0% 48.3% 6% 51%* Estimate from histo or seattergram respectively133 134 BergerJuly, I959?.hcsc figures call for somc cxplana-tion. Generally, disrclations betweenthe widths of the upper antcrior tccthand of the lower oncs arc thoughtto bc the cause. Hut if this wcrc thconly cause, it should work both ways,i.c., lcading equally to crowding andspacing. Further, it docs not cx-plain thc twice as much occiirrcncc 01crowding in the mandible. Thus wcare led to the conclusion that thereexist still othcr factors responsiblc forthis peculiarity. It seems to the authorthat we might get a chic from studyingphylogenetic development.PHYLOGENETICS - ?`HE MANINISIXIt goes without saying that thoughthis paper is concerned with the lowerincisors, we cannot treat them in akind of "splendid isolation". Theremust be considered relations with thcother mandibular teeth, with their an-tagonists and with their very base,the iiiaiibib:e its~!f, and espc !!.?/with that part of it which has evokedso much interest among dentists, anat-omists, anthropologists and even lay-men: the chin.A few years ago the chin problemwas comprehensively treatcd by DU-Brul and Sicher under the ratherchallenging title The Adoptive Chin.After reviewing and rejecting the cur-rent theories, especially that of reduc-tion (Weidenreich), they ,proceed todevelop their own theory: the chin isa sort of buttress which has come intoexistence to reinforce this part of themandible against the masticatorystresses which converge here andwhich, after the mandible has under-gone phylogenetic changes, make thispoint a particularly vulnerable one.Thus, the chin has arisen in responseto changed conditions and they speaktherefore of the "adaptive chin".The author cannot but look at thcproblem differently. Siding with therepresentatives of the reductionFig. I Mntlcr~i skull (E.D.Cope) in ~`0111-Iwiuon with Na:iitlerthal (La CIi:~pe!lc-aux-Saints). After Boule.theory, hc thinks that not the chin,but the tccth and especially the lowerincisors arc "adaptive", since it isthey which had to adapt themselves toa reduced space. A few illustrationsshall elucidate this point. Fig. 1 showsthc changcs which have taken placein the build of the skull since paleo-lithic times. Of special interest for 11swill be the reduction which befell thcFzcc IEC! jaws. .As wr are nnw mainlyconccrnctl with the conditions in themandible, the next two pictures willgive us more details. Fig. 2 is a repro-duction of Schoetensack's comparisonof three mandibles of which only thcHcidclberg jaw and the modern onewill concern us here. Fig. 3 is Keith'scomparison of the mandible of a Ncan-tlerthal child with that of a recentjuvenile specimen. If we look at thesepictures, we are inimediately awarc ofthe fact that a substantial reductionhas taken placc, but we will not getany hint as to where it has taken placc;the superimpositions as thcy arc madehcrc might easily lead to the idcathat out of these changes the chin hasevolved as something new. If, however,wc superimpose the same mandibles asin Figs. 4 and 5, i.e., on a point cor-responding approximately to the gna-thion, wc shall get quite another im-press ion.From comparing different growthstages we know the difficulties we have Vol. 29. No. 3Lowerto contend with when we wish to fixgrowth ccntcrs in order to superimposetracings. 111 the sanic way wc have, ofcoiirsc, no certainty wlicrc to placc-il one may say so-tlic Izdriction cento..Superinipositions like those in Figs. 2and 3 would imply reduction at tlicdistal part ol tlic corpus antl ramusand apposition in the lront. Iht, if wcsuperimpose tlie tracings as in Figs.4 and 5, we arc let1 to the conclusionthat reduction has taken placc rxtliercqiially in tlic corpis and thc rainiis,yet the tooth bearing part has changedtlrastically in extension as well ;is inposition. Sucli a process scciiis to tisvery probablc, as tlic extrcnicly iiias-sive corpus inantlibiilae should resistradical changes inorc cff'ectivcly thanthe cancel lous tooth bcari ng part.In addition to this wc also have toconsider what has beon called thcIncisors 135rhin of fhe irpper jnw-thc anteriorspina nasalis. 'l'liis proccss, too, docsnot exist in apes or early nicn, antlcoiiics into cxistcncc simii1tanco:islywitli the chin. Ibth tlicsc tlcvclop-incnts wc can cxplain by the same~)roccss, iianicly the rctluction of tlictooth hearing parts in tlic uppcr andlowcr ,jaw, a reduction which has notyct cnvclopctl tlie basal parts in eitlicr,jaw. For thc eincrgcnce ol tlie chin,tlic mantlilxilar prominence, pu-ticu-Iwitics ol niuscular bcliavior antl mas-ticatory stress liavc becn atltlucctl byI)iil rul ant1 Siclicr. It should I)c diffi-cult for tlicni to claini the sainc causesfor the tlcvclopnicnt of tlie antcriornas;iI spina.Ihlh-ul ;ind Siclicr try to discreditthc reduction theory further by statingthat reduction should have acted onImk corps antl alveolar process to-, MOOLRN LNGLISWFig. '2, upper left. Superinipositioii of mandibular profiles of Heidelberg m::~ (- ),rxcnt Europe:ni ( . . . . . ) and African negro according to Schoetensaek.Fig. 3, upper right. Superiniposition of ni:uidibul:ir profiles of Seanderthal child andrcrcnt one zcrording to Keith.Fig. 4, lower left. Superimposition of niandibular profiles of Heidelberg man and recentEuropean according to the author.Fig. 5, lower right. Superimposition of inandihular profiles of Neanderthal child and re-cent one according to the author. 136BergerJuly, 1959Fig. G Developmental stages: upper left, ape; upper right, early inan; lower left, recentman; lower right, child.gether. But why should it? Familiarwith quanta and mutations, we areused to the idea that nature does notproceed in a continuous way but byleaps. So the emergence of the chincan be explained as a temporary lackof synchronization in these leaps. Oneof the next leaps may easily concernthe chin which has so far escaped thegeneral reduction trend.BOLK AND THE FETALISATION THEORYThe above given explanation wouldalso be in conformity with Bolk's feta-lisation theory. Bolk has demonstratedthat the rise of Man is connected withthe fact that conditions existing inearlicr ontogenetic stages have becomefixed. Apes and men resemble eachother in their fetal stage much morethan one would expect, judging fromtheir adult state. Thus, while the mas-ticatory apparatus develops into thesnout in apes, this development inman comes to a standstill at a muchless progressed stage. Another ex-ample: in the course of ontogeneticdevelopment the relatively high fore-head of the child becomes flatter andmore receding. This process, too,comes to its end in recent man earlierthan in the Neanderthaler or in theape. Figure 6 illustrates both thesephenomena.To these representatives oE develop-mental types might very well be added Vol. 29. No. 3Lower Incisors137a constitutional type, to be insertedbetween the stages 3 antl 4 of Figure6. It is the Cerebral Type of theFrench school of constitutionalists.This type is characterized by a well-developed cerebral part of the skullwith its especially high forehead. Incontrast to this the facial part is lessdeveloped so the profile is nearlystraight, with a small nose, narrow lipsand a very flat chin which might betermed "undertlcveloped". Rut thismderdeneloped chin can be untler-stood as nrwsted in its development ata more juvenile stage. It is well knownthat the chin starts its tlcvclopmcntalcareer rather late, at antl after puberty.The high forehead too can be ex-plained in the same way, (Fig. 7).Comparing the developmental stagesin the light of the fetalisation theory,we easily get the impression that thepresent day cerebral type is the latestl'orni Nature has evolved on Alan'sway froni past to future. And it isjust here that the atavistic remnantcalled "chin" for the first time showssigns of a beginning reduction andthus falls into step with the generaltrend towards reductions of the wholemasticatory apparatus.PHYLOGENETICS, THE TEETH AND THEIR OCCI.IJSIONAfter having found that reductionto an essential degree has taken placein the very region of the lower incisors-the chin excepted-it seems necessaryto investigate how the reduction ofthese teeth themselves compares withit. This poses a certain difficulty, sinceearly man is generally found as onespecimen, rarely two, at a time. Therewere, however, two excavations wheregroups of Neanderthal people werefound and which lend themselves tostatistical treatment. The one place isKrapina (Yugoslavia), the other onhhmt Carmel (Israel). In Table I1we shall see that the mean value forFig. 7 Maturation cli:iiigrx of the face.Above, after Gerhardt; below, after Martin.upper and lower incisors in ortho-dontic patients is only almut fifteenper cent smaller than in the extremelymacrodont Krapina people and evenca. five per cent only in the MountCarmel people. These reductions areof a rather moderate extent in com-parison with the reductions whichhave taken place in the mandible.(Hased on figures published byHrdlicka, the author once computedthe over-all length of a modern man-dible to be thirty per cent smaller thanthat of the Heidelberg mandible andthe thickness in the median line,measured midway from above, is re-duced as much as even fifty per cent.)Though this difference in reduc-tion would explain the occurrence ofcrowding generally, it would not ex- 138BergerJuly, 1959Table I1Incisor Widths in Palaeolithic people and orthodontic patientsKrapina Mount Carmel Heidelberg Ortho. patientsSum of the four upper iiicisor widthsMinimum ..................... 34.8Mean ......................... 37.6Maximum .................... 39.8Minimum ....................Mean ........................ 27.4Maximum ....................plain the higher percentage in thelower jaw and for this we shall haveto consider the changes in position andocclusion which have taken place dur-ing the same period.Psalidodonty (scissor bite) is todayregarded as the "normal" bite; wemust, however, not forget that this is arather recent acquisition and thateven at present labidodonty (edge toedge bite) is still the bite of Australianaborigin~ and Eskiii~s. It Swiiis iothe author that it is this change-overfrom labidodonty to psalidodontywhich is responsible for the high fre-quency of mandibular incisor crowd-ing. Looking at modern den talarches, we see well r ou n d e dincisal segments in the upperjaw; the lower incisal sector, however,is nearly straight, for the lower teethhave to arrange themselves behind theupper ones. If we compare these condi-tions with those of earlier times, wefind the lower incisal segment equally.well arched. Not confined behind theupper teeth but arranged oppositethem, the incisors can fan out andadapt themselves to their antagonists.It should be mentioned that even inthese early times labidodonty was notoriginally existent but was a productof ontogenetic development connectedwith the attrition of the teeth. Butthen, at a very early age, cusps andedges were worn down by attrition andabrasion due to primitive food habits29.433.839.625.031.838.0Sum of the four lower incisor widths21.0 17.524.0 23.6 23 127.4 27.5and the admixture of sand and gritto the food. The lower incisors couldleave their restricted position behindthe crowns of the upper ones andoccupy a segment, which was not onlywider, but curved like the upperfrontal arch. In Fig. 8 an attempt ismade to arrive at an estimate of thereduction which has been caused bythis change. The curved line A-Bmeasures 26 mm., while the nearlybiidigiic iirie A' - E' is oniy 2i mm.long, i.e., ca. 20 per cent less. We have,however, just seen (Table 11) that theteeth of recent men are on the averageonly between five to fifteen per centsmaller. We must not overlook thefact that, apart from averages, therecrop up in recent man maximumvalues which almost reach those ofFig. 8 Loss of space throuqh transitionfrom labidodonty to psalidondonty. Vol. 29, No. 3 Lower Incisors 139early man, while we shall never find amandible today which could comparebut slightly with that of Heidelbergman, for instance. It seeins that herewe find the underlying reason for to-day's mandibular incisor crowding.Now, it might be argued that, whenthe lower incisors descend, phylogenet-ically speaking, from their edge toedge position and glide back to ar-range themselves behind the upperincisors, they bring the whole lowerarch into a more posterior position.Rut, as a matter of fact, skulls withlabidodonty show the same occlusalpattern as exists in modern neutroc-clusion interdigitation. Thus, the pos-terior segments remaining stable, thewhole loss of space makes itself feltin the anterior sector which reacts bycrowding.Though we cannot follow DuRruland Sicher when they speak oE ancidapti-iw chin, it would be possible tocall the inantlibular incisors adnpti-oeteeth. Crowding is their response tothe phylogenetic changes which havetaken place. That this kind of adap-tion is not an ideal one need hardlybe said. Reduction in size or numberwould bring a real solution, of course,but this process has scarcely started.Table I1 has shown the modestamount of reduction in size which hastaken place up to now, and congenitalabsenrc: of lower incisors is so seldomand irregular as to be quite a negli-gible factor.DENTAL DISHARMONIESThe phylogenetic approach shows usthere exists an inherent tendencytoward disharmony between the man-dibular incisors and the space at theirdisposal. If this fact is regarded hereas the primary reason for their so fre-quent crowding, there should, how-ever, not be excluded the possibilitythat other causes might be involvedtoo.Various authors have mentioned thedisrelation in the size of the upperand lower front teeth. Neff in 1949proposed an anterior coefficient: intwo hundred cases he found this co-efficient ranging from 1.17 to 1.41. Inhis opinion the "ideal" ratio would be1.20. Varying the degree of overbiteaccording to the value of this coeffi-cient, he believes in the possibility ofa "tailored occlusion". In 1957 Neffhas once more taken up the subject.He now introduces an anterior per-centage relation (APR) and states thatin three hundred malocclusions themaxillary anteriors are between 18 and36 per cent larger than the lowers. Hestill recommends compensation fordisharmonious segments by varyingthe overbite and even gives a tableshowing the "indicated overbite" fordifferent values of APR. However, healso thinks that in some cases the ex-traction of a lower incisor might benecessary, and that in other casesstripping might be s~ifficient.Lundstroin has treated the problemof anterior disharmonies on differentoccasions. In 1955 he tested an anteriorinclex in 195 boys and 124 girls andfound a range of 73 to 85, with a meanof about 79. He states that the degreeof crowding is higher in individualswith big teeth, while those with smallteeth tend toward spacing. In contrastto Neff he declares: "An adjustmentof the overbite or overjet does notseem to be the method used by Naturefor accommodation of disharmonies inthe tooth width ratio between upperand lower jaws."Bolton (1952), according to Neff,investigated fifty-five excellent occlu-sions and found a range of 74.5 to80.4 with a mean of 77.2. He, too,could not find a relation between theratio and the degree of overbite.Rallard (1956) believes, in additionto disharmonies of upper and lowersegments, left-right discrepancies, too, 140Berger July, 1959Table I11Ifeans, stand:~rd dcriations, coefficients of vari:lbility sildcorrelation coefficients of incisors and of molar circ.uniferences3Iem St. Del-. Coe. Tar. Coe. Cor. 0.70Sum of upper incisor widths ............. 31.81 2.24 7.07%Suui of lower incisor widtlis ............ 23.10 1.8.) 7.885Upper molar circumferences ............ 36.90 1.46 3.9.57;Lower molar circumferences ............. 36.16 1.72 4.i.iY;0.78should be taken into account. In fin!hundred cases he found that in 90 percent one or more pairs of teeth showedsuch discrepancies. Ballard regards 75per cent as the "normal or ideal totalof the mesiotlistal widths of the lowwincisors." In four hundred orthodonticpatients he found that this total waslarger in 90.7 per cent; in 50.3 percent it was larger by 2 mm or mort.,and in 31.5 per cent it was larger by3 mm or more. On the basis of thesefindings he recommends stripping or,width of a central incisor, the extrac-tion of such a tooth.All these publications were con-cerned with the front teeth, i.e., in-cisors plus cuspids. As this paper isdevoted to the lower incisors, theauthor thought it opportune to makea special investigation limited to therelationship between upper and lowerincisors. It was found that the meanfor the Incisor Index was 73 per cent,with a range of 63 to 86 per cent, andtherefore not much different from themeans found for the anterior indices.It deviates from them by the muchgreater range which is nearly twice aslarge. The material for this investiga-tion consisted of three hundred ortho-dontic patients.There was also computed the cor-relation coefficient for the sum of theupper and lower incisor widths whichwas SO.70 -+ 0.029. To give some in-terpretation of the meaning of thisfigure, the author has also calculatedwhen the diFer~~c~ ZinGints to th~the correlation coefficient for the cir-cumferences ot the upper and lowerfirst molars: -1-0.78 2 0.023. As weknow how different upper and lowermolars can sometimes be in the samepatient and as we see that their cor-relation, nevertheless, is higher thanthat between the incisors, we get someidea what disharmonies we may haveto contend with, Table I11 and Fig. 9.To this might be added that the in-cisors especially have a rather highgenetic variability. This was demon-tion Tooth Size and Occlusion inTwins (1948). Ballard's (1956) obser-strated by L::n:!strom ir? his investi- e-3 7.213 0.52 3.8AAFig. 9 Comparison of minimum, mean andmaximuni values of iiiaxillary and mundi-bulur incisors. Vol. 29, No. 3 Lower Incisors 141vations have been reported above, andonly recently Horowitr, Osborn andDeGeorge (1958) in their publirxtioiiHel-edital-y Factors in Tooth Dimen-sions came to the following conclu-sion: "When mesiodistal tooth dimen-sions are used to establish orthodonticdiagnosis ratios, the findings of thisstudy suggest that the anterior teethbe considered as two separate groups,the `variable' incisors and the relative-ly `stable' canines".TEETH - JAW DISHARMONIESAmong the possible disharmonies re-mains that between the siLe of theincisors and the size and form of themandible to be investigated. As theauthor has repeatedly treated the sub-ject of disrelations on a constitutionalbasis, it might suffice here to mentionthat crowding can result from the ap-pearance of large teeth in relativelysmall or narrow niandiblcs. The im-portance of such disrelations was firststressed by Axel Lundstrom and thisproblem has since been dealt with un-der the term of "apical base". Therewas, however, always a certain vague-ness about it, and the author has triedto put the teeth-jaw relationship on amore concrete basis by his ZygomaticMethod. This method has been suffi-ciently described in earlier publica-tions, so that there seems to be no needto go into it once more. This might,however, be a good opportunity tomention some recent investigationsconcerning the validity of this method.Markowitsch of Bask University(1957) has confirmed the zygion-molarrelationship in an investigation oftwelve hundred children and adults.He thinks, however, to get still betterresults by replacing the bizygomaticmeasurement with the bitragialbreadth.Another and especially interestingtest of the author's Zygomatic Methodhas been made by Moorrees in his in-vestigation of the Aleut Dentition(1 957). In this Eskimoid people, hefound the index: bizygomatic breadthover biinolar width to be 33.2 asagainst the required 33.3. Commentingon this result and comparing it withinvestigations by Meredith and Hig-ley, Moorrees writes: "These authorsfrom their own data and those ofothers reported a wide range of vari-ation (0.18 to 0.88) for this correla-tion coefficient. It should be noted,however, that the different samples re-viewed varied in age, in methods ofmeasurenients and in the manner inwhich growth increments for bizygo-matic breadth in children were takeninto account, a fact which explains thehighly variable findings reported bydifferent investigators. For thirty-fourAleuts the coefficient of correlation be-tween arch breadth and bizygomaticbreadth is +0.74 2 0.076 which indi-cates a rather high degree of associ-ation between these two measure-men ts".It semis that these rather critical re-marks by Moorrees also hold good forthe latest publication by Hixon andMeredith (1957). Regarding this pa-per the author cannot but point outone rather astonishing inaccuracy. Indescribing the Disrelation Table whichis an off-shoot of the Zygomatic Meth-od, Hixon and Meredith write: "Thechart is diagnostic in that it is parti-tioned into zones designated `lack ofspace', `harmony' and `excess of space'.The major interpretation made is thatfor those patients falling in the `lackof space zone', the only solution forsuccessful treatment is extraction . . .".Now, as a matter of fact, the originalchart has not three zones as reportedabove but five, it., besides the zonesmentioned above there is a zone of "ex-treme excess of space" and one of"extreme lack of space". And theauthor's remark about extraction asthe only solution as quoted by Hixon 142 BergerJuly 1959and h4eredith did refer to this lattcrzone of extreme lack of space. Itmight well be stated here that theauthor never thought of the Disrela-tion Table as the one and only crite-rion of extraction therapy. It was pro-posed as an addition to existing diag-nostic procedures in order to facilitatea sometimes difficult decision.Returning to our problem, it willnot seem astonishing after the pre-ceding discussion that the author triedto demonstrate the occurrence of teeth-jaw disharmonies with the help of thebizygomatic measurement, though theoutlook was not promising. For evenin the upper jaw the relation betweenthe bizygomatic breadth and the an-terior parts of the dental arch hadproved to be rather weak in contrastto that existing in the molar region.The correlation coefficient of +0.23 -t0.77 for the relation between crowdingand the bizygomatic breadth did notcome as a surprise; it is iow and justat the level of significance. As a kindof surprise, however, there appeareda coefficient of practically nil for thecorrelation between crowding and thebigonial breadth, which was com-puted at the same time and with thesame material (one hundred fifty or-thodontic patients). On the otherhand, a correlation coefficient betweencrowding and the sum of the lowerincisor diameters proved to be as highas -0.56 -t 0.56, which means that thebigger the teeth, the greater the lack ofspace as expressed by amount ofcrowding, Figs. 10 and 11.Though we cannot directly proveteeth-jaw disharmonies, the very factthat crowding is so strongly correlatedwith incisor size makes them very prob-able. As a matter of fact, the dishar-monies produced by the greater reduc-tion of the niandible and the lesser oneof the teeth during phylogeny-des-cribed above- would already fall intothis category of teeth-jaw disharmo--E -5 -? t? 45 InNCRO'"D1 KG SPCCING-8 -5 -7 +: +5 mr.CROW1 NG SPP CI NGr=-0.56~0.056 n=ljOFig. 10, above. Correlation of bizygomaticbreadth with cro\rding and sparing of thelower incisors.Fig. 11, below. Correlation of the sum ofthe lower incisor \ridths with cro\dl~~g andspacing.nies. To these would now have to beadded those where big teeth appear inconstitutionally small and/or narrowmandibles as connected with lepto-somic or cerebral types.DYNAMICSHaving up to now and at somelcngth treated what might be termedstatic conditions concerning the lowerincisors, we shall now have to con- Vol. 29, No. 3Lower Incisors143sider some aspects which might con-stitute their dynamics.First among them is growth andtlevclopment. The spacing ol the de-clduous incisors is a good indicator ofthe growth taking placc in this regionbetween the agcs of thrcc to six. Ifthcre is not enough room at that time,the lateral incisors erupt lingually. Butthey may still find their normal places-under the influence of function andmuscle pressure-if more space willaccrue by local growth at this "late"age. To understand the meaning of"late" in this context, it should beremembered that, as far as addition ofspace is concerned, growth in this par-ticular region stops at that time. Inan investigation of twenty-eight indi-viduals from 3 to 15 years of age,Woods found that the canine width,measured between the tips of theircusps, increased on the average by 0.9mm. in boys, by 0.6 mm. in girls, Le.,remained practically stable. There-fore, hopes for the relief of crowdingby later growth are doomed to disap-pointment. Space gained by the re-placement of the deciduous molarsthrough their smaller successors isgenerally taken by the mesial drift ofthe six-year molars.There have been interesting investi-gations with respect to changes inplace and inclination of the lowerincisors. Schaeffer as well as Bjork andPalling found that such changes do notoccur in one direction only. Some in-cisors increased their labial inclin-ation, others their lingual onc, andsome did not alter their inclinationat all.Occasionally one reads that besideschanges of inclination, the over-allposition of the incisors is changed intoa more lingual one. Such statementsare accompanied by pictures where thebackward movement is measured frompogonion as the point of departure.But pogonion is in an especially vehe-ment developmental stage during andafter puberty. Gerhardt in his mono-graph about Maturation Changes ofthe Htirnan Physiognomy has foundthat not only the inclination of thechin alters, but also changes of its con-figuration. He distinguishes three basicshapes. Meredith reports changes of upto 2.6 mm in the depth of the anteriorconcavity, reflecting upon the develop-ment of the pogonion, cven during thcprepubertal period (4 to 14 years). Itwould, therefore, seem that we haveto observe extreme reserve when meet-ing with conclusions regarding the in-cisor position in relation to pogonion.(See also Fig. 7.)The influence of the muscles mustbe mentioned among the dynamics ofthe lower incisors, too. One is inclinedto think that an equilibrium betweenthe pressure of the tongue from with-in and the buccal and labial muscula-ture from without is conducive to astable position of the teeth generallyand the incisors in particular. These,however, are general considerationsand the recent investigation of a spec-ial instance of muscle behavior niightbe of interest in connection with thisproblem. Sims measured perioral andlingual muscle pressure exerted uponthe maxillary and mandibular centralincisors and came to the conclusionthat there exists no relation betweenthe inclination of the incisors and thcamount of pressure. We should, there-fore, for the time being still use acertain caution when speaking aboutthe influence of the inuscles on thelower incisors.Another question to be consideredin this context is that of bite raising.This was regarded rather optimisti-cally until a few years ago. ButThompson's investigations broughtabout a more realistic approach andthe limiting influence of the musclesis now recognized. It should be withextreme caution that we try to get 144 Berger July, 1959more space for the arrangement of thelower incisors by bite raising, as, forinstance, envisaged by Neff in histailored occlusion or indicated ouey-bite proposals.Related to the influence of musclesis that of habits, part of which is buta perverted inuscle action. It seemsrather superfluous to state that thelower together with the upper incisorsare the teeth most of all exposed todisplacement by habits. Differences inthe resulting anomalies are due to thevarying combinations of duration, fre-quency and intensity, as has recentlybeen emphasized by Graber. It might,however, be added that these habitswill superimpose their influence on theexisting pattern only and will notchange an originally existing tendencytowards crowding or spacing.DIAGNOSISOn the basis of the above theoreticalconclusions we can draw for the prac-tical application in diagnosis andtreatment.Newer diagnostic procedures follow-ing the lead of Tweed are centeredaround the lower incisors. At the startcame the postulate of an angle of 90between the axis of the lower incisorand the mandibular plane. This waslater complemented by the introduc-tion of the Frankfort mandibularplane angle; and finally the Frank-fort-lower incisor angle was arrivedat. Thus, all the angles of the Frank-fort-mandibular plane triangle havesuccessively come into play and, ifone is inclined to say so, one couldstate that this triangle has now runfull circle. As the angles of a tri-angle add up to 180 and two angleswere already fixed at 90 and at ca.25O, this new relationship does notmean anything essentially new, butsimply follows as a mathematical con-sequence.consideratinns we ha11 nnw zcc whatThe question now arises whetherthe angulation of these teeth is real-ly as important as these continual di-agnostic endeavors would have it ap-pear. To a certain degree this ques-tion has already been answered. Wylie(1 955) , testing cases treated by John-son and by Tweed himself, found nocorrelation between the uprighting ofthe lower incisors and changes in theangle ot convexity. The greatestchange (16O) actually occurred in apatient where the lower incisors weretipped even lo forward while thegreatest amount of tipping (24O)produced a change of 9O only in theconvexity a n g 1 e. Wylie therefore,comes to the conclusion that "allthese years orthodontists have beenattaching exaggerated importance tothe angulation of the lower incisors,so far as it is concerned in orthodonticdiagnosis and treatment planning."Essentially the 90 angle was a prosthe-dontics, and its usefulness there hasbeen doubted. Wylie's investigationproved that these doubts were ab-solutely justilied.Another point should also be con-sidered: in the introduction it was re-marked that apparently the six yearmolar had lost its position as key toocclusion and has been replaced, atleast to a certain extent, by the lowerincisors. Massler and Frankel, in aninvestigation of 2758 children, cameto the conclusion that the lower in-cisors were the most frequently displaced teeth, the upper first molarsthe least ones. There does not seciiito be much advantage in replacingthe relatively stable teeth by such un-stable ones in diagnostic procedures.How unstable these teeth really are,the author hopes to have shown in theearlier part of this paper. There theteeth have been characterized as"adaptive", because they adapt theni-selves to the reduced space at their!ic prlT1cip!e : e!? ever iEtG Grtho- Vol. 29. No. 3Lower Incisors145disposal and according to their six.Their position is basically resultantof these two factors, and the correla-tion coefficient of -0.56 show thatsize is the more important one.Now, following up the idea ofadaption, we should have to ask:to what special condition have theincisors to adapt themselves? Theanswer would be: to the position oftheir neighbors, the cuspids. Earlier inthis paper it was reported that thebicanine width remains practicallystable. So it seems that it is thiswidth which determines the fate ofthe incisors. If the incisors are small,they will arrange themselves well with-in this space; but they will have tocrowd into it, if they are big. Whenthey are very small and there is muchroom at their disposal, there will bespacing.We are led to believe that ratherthan to center our interest upon theincisors and their angulation, weshould consider the cuspid position asthe point of departure. There is, how-ever, no action without reaction, andwe have learned from our anchorageproblems that a tooth cannot be mov-ed without to some degree influencingthe anchor tooth or teeth. Conse-quently, the retention of the incisorsproduces some reaction in the cuspids.This reaction in the cuspids expressesitself by tipping. And, if just now theposition of the cuspids was proposedas a point of departure, we will haveimmediately to correct ourselves andto add that in the last instance itis the cuspid apex position which isdecisive. The same action - reactionmechanism is, of course, at work inthe cuspid - bicuspid relation, too, andthus the tipping of the cuspids mayalso come to some degree under thesecondary influence of the bicuspids.Though this need not concern us here,a careful analysis will have to be madewhen planning treatment.TREATMENTIf one accepts the foregoing argu-mentations, one of the first practicalconclusions would be: not to base diag-nosis and treatment planning uponthe four lower incisors and their an-gulation mainly. Taking into accountthe reduction of the anterior part otthe mandible and the possible dis-harmonious size of the teeth, ourmain concern would be to find asolution which would disturb the posi-tion of the lower cuspids, i.e., theirapices, as little as possible. Thismeans that we would first of allhave to imagine them in a uprightposition and to calculate how muchspace would then remain available forthe arrangement of the incisors. Ifthere would be lack of space of morethan about 3 mm, the best solutionwould be to extract an incisor. Sucha procedure has occasionally been ad-vocated by Neff, Ballard and others.It is, however, apparently not acceptedas a routine measurc for Salzmannrecommends: "Incisor teeth shouldnot be extracted unless damaged bc-yond satisfactory repair".The author thinks that the recenttendency to use these teeth as a kind of"keys" to diagnosis and treatmentplanning has elevated them to such astate of importance as to make theweakening of the "key ring" as some-thing not to be thought of. The ap-proach to the problem, however, aspursued here, would make the re-duction of tooth material between thecuspids the logical solution. And totranslate this into practice is rathereasy, if, as has just been stated, thereis a lack of space of 3 or more mm.The author prefers the extraction ofa lateral incisor as he finds that thedistal side of a central incisor fitsrather well the mesial part of the cus-pid, whereas the extraction of a cen-tral brings the mesial side of a central 146BergerJuly, 1959Fig. 12, left; Fig. 13, right.and the mesial of a lateral togetherand, even when the teeth are perfectlyuprighted and parallel, there some-times persists an awkward empty tri-teeth and the gingiva.Extraction of this kind is easilydecided upon when there exists a cer-tain asyinmetry, one lateral in a lin-gual position and the adjacent cuspidtipped outward or backward. If thereis more lack ob space than that of anincisor width, the extraction of twopremolars is the best solution. Theremay, however, be cases where bothcuspids are tipped backward to aconsiderable degree. To bring themback into the premolar space wouldnot only entail uprighting but alsoa consecutive extensive bodily move-ment. In these rare cases cxtractionof both laterals seems, if not ideal,still the relatively best solution. (Fig.The direction of tipping may alsobecome decisive when the incisor posi-tion is symmetrical, both laterals be-ing lingually displaced, of which,however, only one need be extracted.If in such a case one cuspid shouldafig!P betT.+Teefl thc !C!ATer !?a!f of12).be tipped forward and one backward,the lateral beside the latter should beextracted for uprighting this cuspid isalready half the treatment, while ontipped cuspid into the extractionspace of the lateral would requiremoving it farther in the direction oftipping and would necessitate exten-sive bodily movement (Fig. 13) .If the lack of space is less than 3mm, the decision might becomerather difficult. One just can resignoneself to the situation and take acertain amount of crowding into ac-count, as advocated by Howes; orone could resort to stripping. Up tonow the author has never practiced it,but he remembers certain cases wherethe peculiar form of the incisors,wide at the edges and strongly taper-ing towards the gingiva, posed areal problem and stripping mighthave been the solution. Progress inimpregnation techniques, like fluori-dation, might overcome one's stillexisting reluctance. Finally, in thesecases of not-so-pronounced lack ofspace, there still remains the con-servative treatment by widening, es-the other side bringing tlie forward- Vol. 29, No. 3Lowerpecially if this would be in conformitywith the need of some expansion inthe upper jaw.Needless to say that these are ele-mentary points only, to be consideredin lower incisor treatment. Therecertainly is room for further improve-ment in differential treatment plan-ning. Besides there is a possibility fora wide range of combinations as ex-traction of a lateral on one side andof a bicuspid on the other, or extrac-tion plus widening, when just a littlemore space than one incisor width islacking, etc.SUMMARYConsidering the lower incisor prob-lem from different points of view, weare led to the conclusion that thecrowding of these teeth is mainly aresult of an evolutionary process. Itcan also be caused or may be ag-gravated by disrelations either withthe upper incisors or with the bonybase. Consequently, crowding has tobe understood as an adaptive re-sponse to changed conditions, as theincisor's contribution to an equili-brium of static and dynamic forceswhich it would seem unwise to dis-turb. Therefore, for practical pur-poses, the position of the lower cus-pids, or still more exact, the positionof the lower cuspid apices should beregarded as stable and the treatmentof the incisors should be planned insuch a way as to arrange them withinthe given limits, if need be, by re-duction of tooth material. It goeswithout saying that though thetreatment of the lower incisorsshould be planned in each case in-dividually and on its own merits, itshould fit into the over-all treatment,but it should be regarded there asof secondary importance only.Of recent years it has become ac-cepted to speak of the science andart of orthodontics. So we mightIncisors 147do worse than to go to the world ofart in order to find a simile for lowerincisor treatment. The author likesto compare the lower incisor treat-ment to the cadenza in a concerto.The cadenza has its place at theend of the first movement, after theprincipal themes have been playedand developed; it is here that theartist has the possibility to improvise,keeping himself only loosely to thetheme or themes. In the same waytreatment of the lower incisors shouldbe initiated when the case has pro-gressed for some time along the prin-cipal lines, and the play of the ap-pliances and the counterplay of thepatient and his tissues has suffi-ciently developed to reveal the char-acter and peculiarities of this specialcase. Then the treatment of the lowerincisors may become a rather short,though important, episode within theentire treatment as also is the cadenzawithin a concerto. 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