730 Mew American Journal of Orthodontics and Dentofacial Orthopedics December 2004ical or symptomatic. Any theory must be based on a during growth,12 adverse or otherwise, but newer workwide range of observations, and the more respected with Procrustes superimpositions might improve thisterm hypothesis should perhaps be reserved for theories situation in the future.13that accommodate all, or at least most, of the available Overall, the twin evidence is not as conclusive asevidence. Surprisingly few current theories can pass some suggest, with clear differences of opinion amongthis test; this article attempts to identify the single most accepted leaders in genetics. We are left with nolikely hypothesis. Some years ago, I put forward the explanation for the fact that some monozygote twinsfollowing deﬁnition. have substantially different skeletal relationships10,11 because it is clearly impossible for these to have been A sound hypothesis needs to ﬁt all the available evidence rather than rest on part of it. It needs to be carried in the genes. both logical and speciﬁc. It is additionally convincing The evidence of x-rays. The desire to measure if it is compatible with evidence other than that upon explains our heavy reliance on x-rays, which provide a which it was based, especially if no additional cor- convenient 2-dimensional image. Cephalometric arti- ollaries are required. The ﬁnal test is if new prognos- cles have dominated the orthodontic literature over the tications can be drawn from it, which when tested are last 60 years and have, in general, supported Brodie’s found to agree with both existing and future ﬁndings. conclusion in 1938: “The most startling ﬁnd was the In retrospect, the truth is usually simple. apparent inability to alter anything beyond the alveolar It might be helpful to consider how well past process.”14 This set the foundation for the subsequenttheories ﬁt this deﬁnition. Most are centered around the belief that environmental forces (including treatment)belief that the form and the size of the dental skeleton do not alter the inherited pattern of skeletal growth.is primarily inherited, but the position of the alveolar This view is currently maintained by random controlledbone and the teeth is open to environmental inﬂuences. trials15 that show minimal skeletal change after ortho-The reasoning is as follows. pedic treatment. We know that sutural growth causes the bones of the skull to move as whole units, and that these changesINHERITED FACTORS are followed by extensive remodelling. This combina- The evidence of twins. There are many studies of tion maintains the original contours while permitting anmonozygote twins, most of whom show striking simi- increase in size; this in turn makes superimposition onlarities between their malocclusions. This is true even if landmarks an unreliable means of evaluating craniofa-they have been separated soon after birth and brought cial form.13 Bjork and Skieller16 overcame this problem ¨up in different environments. Some studies show a high by using implants and noticed that the bodily move-rate of concordance in twins for speciﬁc malocclusions ment of bones tended to be disguised by the subsequentsuch as Angle Class II Division 2,7 but the issue is remodelling. Isaacson et al17 reassessed Bjork’s origi- ¨confused by nontwin studies that suggest that this type nal implant work and concluded that “[t]his rotationof malocclusion is not genetic.8,9 Horowitz et al,10 in a was not obvious in the past since it is masked by anclassic study of 35 pairs, concluded that “[h]ighly external surface remodelling that tends to restore thesigniﬁcant hereditary variations occur in anterior cra- relationship of the jaws to their original morphology.”nial base, mandibular body length, total face height and This can also be seen after sagittal split osteotomies,lower face height (P ϭ 0.001),” and many subsequent when the mandible sometimes appears to relapse to itsstudies have reported broadly similar ﬁndings. These original position despite rigid ﬁxation.18 In these cases,particular measurements appear to reﬂect variations in the metal ﬁxings act as bone markers and show that thethe direction of growth of the facial bones (either true relapse is less than it appears because there is ahorizontal or vertical) rather than their inherited forms. concomitant process of peripheral adaptation. As aThis possibility is supported by the work of Lobb,11 result, the lateral skull x-rays can give a misleadingwho, in a study of 30 pairs of identical twins, found that impression of the true jaw changes.“the greatest variation in each group was in the spatial Situations such as these cloud the whole issue ofarrangement of the component parts of the craniofacial using x-rays to identify tooth or bone movement. Ifcomplex rather than within these components” and whole bones remodel toward their original positionsnoted that, even when identical twins had identical after movement, how can we assess changes in the baseocclusions, there was often a “considerable variation in of the skull where bone markers cannot be used? Forthe bony components.” One problem in this type of instance, we know that horizontal and vertical growth isresearch is that inherited variations of overall size associated with contrasting saddle angles.10,19 Occa-cannot easily be separated from variations in arch form sionally, patients show a change in the direction of
American Journal of Orthodontics and Dentofacial Orthopedics Mew 731Volume 126, Number 6facial growth accompanied by an altered saddle an- ate inheritance of the skulls, jaws, and teeth fromgle.20,21 In this situation, an analysis of x-rays super- parents of different racial backgrounds. Although sup-imposed along sella-nasion would give a misleading ported by some orthodontists, this theory is at odds withimpression of the change in growth direction. Batta- the geneticist’s teaching that inbreeding is more likelygel22 found that “vertical changes are not easily de- to cause genetic disproportion than outbreeding.32 Itected by conventional cephalometric investigations.” know of no evidence to support the idea that large parts It is unlikely that any part of a growing skull from 1 parent are mixed with small parts from theremains static,23 and Ricketts et al24 attempted to other. Consider, for example, a single species with aovercome this by constructing midpoints along the contrasting gene pool such as the dog. A Chihuahua cangeneral direction of growth to increase the predictabil- be mated with a Saint Bernard that is 100 times as largeity of forecasts. This has proved very accurate on and very different in appearance,33 but, even if both hadaverage, but individual forecasts have been less so.25 been purebred for many generations, malocclusion in Because Brodie et al14 could detect only minimal any offspring is extremely unlikely. Not only will themovement of the skeleton during treatment, he and tooth size be matched to the jaw size for each animal,many since assumed that the movement had no clinical but also both jaws themselves will also be matched.signiﬁcance. However, if remodelling masks such There will be some intermediate-sized offspring whosechanges, x-rays alone must be a less-than-certain means growth will also have been balanced even though theirof determining their true extent. No doubt the situation parents were purebred extremes. Where did their geneswill become clearer with the development of modern come from? Most evolutionary biologists would rejectscanning techniques.13 the concept of disproportionate inheritance on the dual Parent/child similarities. Rightly or wrongly, grounds that it is virtually unknown in animals and hardmost clinicians consider malocclusions to be inherited to relate to Darwinian theory.because of similarities between parents and their chil- Attrition. Begg and Kesling34 thought that lack ofdren. Although many children have the same malocclu- attrition in modern diets results in crowding. Althoughsions as their parents, many do not, and severe Class II this is an environmental concept to explain dentaland Class III malocclusions can appear and disappear in crowding, Begg still believed that skeletal form wasa single family over a few generations. Lavelle26 was inherited. They reported that the wear between the teethone of the ﬁrst to use a multivariate analysis to separate of primitive aborigines was equivalent to a premolar’sthe many factors he thought might be responsible. width in each quadrant and suggested that much of thisHowever, his ﬁndings, like those of many since, were was due to grit in their food. However, Corruccini35barely signiﬁcant, and he concluded that “a simple questioned this concept after ﬁnding that the mesiodis-Mendelian model is not compatible with most cranio- tal dimensions of the teeth “did not relate to crowding.”facial dimensions.” He found that few of Begg’s samples suffered from Much publicity has been given to a series of studies wear to this extent, even in old age. Primitive foodon the inheritance of mandibular prognathism in the certainly contained more grit, but modern bruxersHabsburg kings.27 These opinions, originally written in eating a diet of reﬁned foods might achieve equivalentGerman, were based on interpretations of royal por- wear; this makes it more likely that much of the wear istraits, most of which were three-quarter views. Mayor- due to chewing rather than grit. Although Begg’sal28 provided an English translation and observed that theory explains some aspects of crowding, his ownthroughout the Habsburg family “superior microgna- ﬁgures show the reverse of what might be expected,thism is a more constant feature than inferior progna- because both crowding and attrition increased with age.thism.” It is known that the whole family was heavily Functional matrix. The functional matrix hypoth-interbred. Thompson29 believed that they suffered from esis was put forward by Moss,36 who suggested that thecraniosynostosis, which would be an equally valid, and soft tissue units guide the hard to an extent that renderspossibly more rational, explanation for the inherited skeletal genes superﬂuous. Essentially a genetic theory,maxillary hypoplasia. Similar inherited pathologies are this explains how the soft tissues mold the hard.found in bulldogs. It would seem that geneticists However, it does not explain all aspects of bonycurrently stand on the middle ground that “[a]t least development and has proved difﬁcult to test. It has littlehalf of the phenotypic variation in this sample is due to favor with embryologists who point out that the longenvironmental differences,”30 but there appears to be bones from chick embryos will develop normally with-no strong evidence either way. out soft tissues. Moss’s theory also suggests that the Mixed inheritance. One hundred years ago, Case31 nasal cartilage inﬂuences the growth of the mid-face,suggested that malocclusion was due to disproportion- but this is unproven. Figure 1 shows the changes in the
732 Mew American Journal of Orthodontics and Dentofacial Orthopedics December 2004 Fig 1. Ten-year-old girl who developed tongue-between-teeth swallow. Note changes in shape of mandible, superimposed on inner symphyseal outline. (Used with permission of British Journal of Orthodontics 1981;8:203-11)mandible of a 10-year-old girl who developed a para- suggests that this has occurred; in any case, change infunctional swallow. As is often seen with vertically the genes themselves would require 100,000 years orgrowing faces (Fig 2), the vertical ramus has remod- more.39elled forward, in this case shortening the horizontal Soft tissue drag. In 1977, Solow and Kreiborg40ramus by a third, during maximum growth. There are suggested that some differences in craniofacial mor-few muscle attachments on the anterior or posterior phology could be explained by the drag of the softborders of the ascending ramus, and the trigger for this tissues on the facial skeleton caused when the mouth ismassive remodelling seems to have been nothing more dropped open. Solow suggested that there is a “den-than gonion sliding back passively between the invest- toalveolar compensating mechanism”41 that tends toing tissues of the pharynx. No other bone in the human restore the incisal relationship despite the skeletalbody changes its relative position (or form) to this disproportion. This theory is a good ﬁt in many situa-extent and, in view of the lack of muscle activity, the tions and has a more environmental slant, suggesting aembryologists’ concept that tissues grow and adapt in sequence of nasal obstruction, craniocervical extension,response to positional information37 from neighboring and increased tissue drag. However, it does not explaincells provides a more understandable explanation than how the alveolar compensation is effected or the originthe functional matrix. Although Moss’s theory ﬁts of some malocclusions such as deep bites or Class III,some situations well, it does not seem able to explain which the authors appear to accept as genetic.42all aspects of malocclusion. Most of these 8 theories accept that malocclusion Evolutionary change. It has been suggested that a has a genetic basis, but this appears to be at odds withgenetic shift is causing the jaws to become progres- other evidence.sively smaller.38 However, it is hard to understand how If malocclusion were inherited, one would expect athis could happen because genetic shifts can occur only sign of its progressive spread in the historical, geo-if the gene pool itself changes; there is no evidence for graphical, or epidemiological records, but this has notthis. A genetic change would have required selective occurred. Instead, we ﬁnd the classic malocclusionspressure that eliminated those with larger jaws. Nothing appearing in diverse populations wherever civilization
American Journal of Orthodontics and Dentofacial Orthopedics Mew 733Volume 126, Number 6 Fig 3. Skull of North American Indian whose head was bound as child. Kindly photographed and loaned by Smithsonian Institution, Washington, DC. Muscle tone and activity. Modern diets are rela- tively soft, and it is suggested that this has led to a reduction in muscle strength.47 Figure 2 shows aFig 2. Radiograph of 12-year-old girl with muscular 12-year-old girl who suffered from muscular dystro-dystrophy, with average outline for her age superim- phy, compared with the average proﬁle for her age. Herposed. (Used with permission of author and publisher. maxilla appears to have collapsed downwards, possiblyKreiborg et al. American Journal of Orthodontics 1978; under the force of gravity and drag from the “soft tissue74:121-41) mask,”41 to create a horrendous skeletal malocclusion. This has caused the mandible to hinge back and has been associated with a massive restructuring of thatprogresses above a certain level. Although skeletal bone, although clearly no force was involved. There isform certainly varies from 1 location to another, sig- now wide agreement that muscle weakness is linked toniﬁcant malocclusions in each area are restricted to the increased vertical growth.48 Figure 3 shows the skull oflast 20,000 years. a North American Indian whose head was bound as an Extragroup differences between humans are sur- infant. It is clear that these light forces caused aprisingly small even between white and black people, massive and permanent change that involved most ofsuggesting that our subspecies are genetically very the bones in the cranium. This applies to the sphenoidsimilar. However, intragroup variations of the facial and temporal bones that received no direct pressure butskeleton are often large, making it probable that epige- nevertheless underwent substantial remodelling. Thenetic factors are interceding. teeth and their supporting structures are known to be Malocclusion is also relatively less frequent in highly resistant to short-term force, but the 2 subjectsmodern man living in primitive conditions, and Cor- illustrated (Figs 2 and 3) show that light forces over anruccini et al43 found that deep bites were 9 times as extended period can have a dramatic effect on thecommon in a sample of privileged children in India as skeletal bones, especially in younger children. Thisin their close relatives who were poor. In the modern would suggest that muscle posture is more inﬂuentialindustrialized world, malocclusion has worsened44 so than function.that it is now endemic.45 There are many ways of assessing muscle force, Recent work on the human genome46 appears to and earlier research tended to rely on short-term clenchconﬁrm that our genes have been handed down over the tests that might or might not reﬂect long-term posture.last 30,000 years or more, with little alteration. Muscle thickness is probably a better guide than force measurements.48 Benington et al49 found that “a steeplyENVIRONMENTAL THEORIES inclined mandibular plane, a small posterior face height Other theories are primarily environmental. and an increased gonial angle were strongly related to
734 Mew American Journal of Orthodontics and Dentofacial Orthopedics December 2004short, thin masseter muscles of low volume.” The effect been supported by many others,12,57,58 with few to theof the muscles depends on the product of both time and contrary.force, together with the distribution of types 1 and 2 Many clinicians and especially occlusionists be-ﬁbers, which can vary within a muscle over time. All lieve that a resting freeway space of several millimetersthese variables can inﬂuence muscle thickness, and is normal. If “rest” were synonymous with “minimalfailure to allow for each factor separately can render muscular activity,” then the study of Rugh and Drago59research results questionable. It is sometimes suggested would suggest that the mouth should be open 8.6 mm;that poor occlusal contacts inhibit ﬁrm muscle contrac- this seems excessive. In this context, normal might betion, but Ingervall50 found that “strong muscles with misleading, because there is little evidence to suggestresultant hard biting and increased function cause better that large freeway spaces exist outside the industrial-occlusal stability,” suggesting the reverse. Electromyo- ized world.graphy is helpful in determining long-term muscle Few would doubt that letting the mouth hang opencontraction but tends to be qualitative rather than will encourage overeruption of the teeth (Fig 2), and, ifquantitative. Profﬁt and Sellers’s work60 on animals is linked to their Bone is a rigid and inﬂexible structure, and changes work on humans,61 it is obvious that the teeth must besuch as these can occur only by organized apposition in contact for a sufﬁcient period each day to preventand resorption. We do not know how bones can adapt them from overerupting. Once overeruption has takenin this way, but embryologists believe that positional place, the increase in facial height is likely to maintaininformation is of crucial importance during organogen- a broken lip seal (Figs 1 and 2) and so perpetuate aesis,37 again suggesting that posture is more important vertical pattern of growth. On the other hand, if thethan function. teeth are in contact for long enough with enough force, they will not overerupt, and the occlusion will be Open-mouth postures. The term mouth breather excellent.62,63 There seems strong evidence to showcan be misleading because Vig et al51 showed that that open-mouth postures are associated with bothmany people who keep their mouths open still breath increased vertical growth and malocclusion.nasally. Fratto et al52 suggested that “mouth breathing Tongue between teeth swallowing and parafunc-may be due to acquired habits following previous tion. It was accepted for many years that the tonguerhino-pharyngeal obstruction.“ It would seem that these should suck against the palate when swallowing, butchanges might be habitual rather than obligate. For this Rix64 demonstrated that many people swallow withreason, I have always preferred “open-mouth posture” their tongue between their teeth. These people willrather than “mouth breather” or “incompetent lips.” usually recruit the lips and buccinator muscles to sealResearch that ignores these distinctions is liable to the margins of the tongue, and this is associated withprovide ﬂawed results. visible parafunction and hypertrophy of the muscles The natural lip seal of most healthy infants in concerned. Levine65 found that nonnutritive suckingindustrialized populations has been lost by the age of (eg, thumb sucking) might be as high as 95% in many4,53 and, for them, open-mouth postures must be western countries but is almost unknown in primitiveconsidered normal. As they reach puberty, some, for cultures, where breastfeeding often continues for 3 or 4social reasons, will develop a competent lip seal, but years.66this will tend to be achieved with voluntary rather than There is universal agreement that the teeth andinvoluntary muscle ﬁbers, and many will still leave alveolus lie in a position of balance between the cheeks,their lips apart for long periods, especially at night. lips, and tongue; however, research in general has By its nature, posture is long term, and thus found that the tongue is the more powerful. To someaccurate measurements have been difﬁcult to record or extent, this is at odds with the observed fact that themonitor, resulting in many negative ﬁndings. However, teeth in many malocclusions are inclined lingually.within the last few years, several articles have sup- Lundeen and Gibbs62 and Melsen et al63 have all notedported the concept that closed-mouth postures are that those who rest and swallow with their teeth inbeneﬁcial.54 Linder-Aronson et al55 studied postade- contact have less malocclusion, suggesting that tongue-noidectomy patients and found that those who devel- between-teeth postures could also cause malocclusion.oped a natural lip seal showed substantial forward The effect of suction during swallowing was de-movement of the incisors and increases in arch length. bated at length in the 1930s but subsequently ignoredGross et al54 found that patients with “high levels of because of inconclusive ﬁndings. Harvold et al67 ex-open-mouth posture manifested signiﬁcantly smaller perimented with lumps of plastic placed against thegrowth of the maxillary arch.” These ﬁndings have palates of monkeys. This appeared to have little imme-
American Journal of Orthodontics and Dentofacial Orthopedics Mew 735Volume 126, Number 6diate effect other than to displace the tongue from its with a familial tendency to heart disease, where thenormal position. However, in the longer term, there actual cause is more probably obesity, smoking, or lackwere widespread occlusal and skeletal consequences, of exercise. These hidden genetic differences mightand “in every juvenile animal the maxillary arch was surface only when the person is put under environmen-considerably reduced in width.”67 This suggests that tal stress. Philosophically, this is a more rationalchanges in muscle posture are more inﬂuential than explanation, which appears in keeping with the evi-muscle function. At times, as we have seen, this will dence, and would explain why skeletal malocclusionresult in massive restructuring of the dental skeleton, does not obey Mendelian rules.26 This could be stated:compared with which the genetic contribution to skel- “environmental factors disrupt resting oral posture,etal variation must be accepted as small. increasing vertical skeletal growth and creating a dental malocclusion, the occlusal characteristics of which areDISCUSSION determined by inherited muscle patterns, primarily of A sound hypothesis must be able to accommodate the tongue.” This theory could offer a simple explana-all the evidence. We have just considered 8 hypotheses tion for the mixed genetic and environmental featureslargely based on inheritance and 3 environmental the- of many, if not most, malocclusions.ories, none of which can stand alone. This leaves us in Figures 1 and 2 demonstrate that muscle functionthe same position as many previous assessments, with and posture can at times produce a large but unpredict-some partial explanations but no agreed hierarchy of able disruption of facial growth. Current scientiﬁcprobabilities. The objective of this article was to ﬁnd protocol precludes single-case evidence; however, inthe single most likely explanation. many studies, variables of this size might increase the The lack of positive evidence conﬁrming genetic standard deviation so that there is little chance thatinﬂuence is surprising. There must be some inherited existing statistical techniques can identify the muchfeatures somewhere, and, if so, why can they not be smaller genetic differences. This might be why thisclearly demonstrated? In other research, Harvold et al68 debate has remained unresolved for so long.took groups of young monkeys with perfect occlusionsand changed their oral posture by blocking their noses. Available hypothesesEvery monkey developed a severe malocclusion, but I am aware of 4 theories that offer viable explana-the pattern of irregularity varied between groups. Here tions for the etiology of malocclusion, those of Beggwe have an identical environmental assault precipitat- and Kesling,34 Moss,36 and Solow and Kreiborg,40ing different malocclusions. This is highly relevant to already quoted, and my own “tropic premise.”72 Thisour discussion, and we should consider how this infor- states that “Because the genetic control of skeletalmation ﬁts the human model. growth is not precise, the articulation of the teeth and We have just discussed how young children from jaws depends upon additional guidance from oral pos-industrialized countries leave their mouths open most ture.” It is further suggested that “If the tongue at restof the time.53 This presumably creates a similar situa- is against the palate with the lips lightly sealed and thetion to the open-mouth postures in Harvold’s monkeys teeth in or near contact, there will be ideal facial andthat produced a malocclusion every time. But why dental development.” This posture is rare in industrial-should a single environmental factor, such as open- ized societies, but the same could be said of the numbermouth posture, produce a range of different malocclu- of adults with ideal occlusions, possibly for the samesions, the characteristics of which appear to run in reason. It seems to be restricted to those who have goodfamilies? muscle tone and swallow with their tongues sucking We know that muscle patterns are inherited,69 against the palate. There is a simple logic to the tropicespecially of the tongue,70 and that the teeth, the premise, because if the tongue rests against the palatealveolus, and even the skeletal bones are very sensitive with the lips sealed, the erupting teeth have little optionto muscle posture. Is it possible that bizarre tongue but to slide down the space between them, until thepatterns are expressed only if the environment is cusps contact and guide them into occlusion. If, on theprejudiced in some way— eg, if the child keeps his other hand, the tongue rests between the teeth, they willmouth open or develops a dislike for hard food? have no guidance and are likely to erupt haphazardly asSharma and Corruccini,71 in their study of identical is seen in aglossia. According to this hypothesis, everytwins, came to a similar conclusion, that “the muscular type of malocclusion, including Class III73 and tem-balance achieved by the lips, cheeks and tongue may be poromandibular disorders,74,75 is closely linked to spe-disturbed by abnormal or habitual behaviour patterns, ciﬁc oral postures.perhaps shared in families.” A similar situation exists Begg and Kesling’s attrition theory,34 Moss’s func-
736 Mew American Journal of Orthodontics and Dentofacial Orthopedics December 2004tional matrix,36 Solow and Kreiborg’s soft tissue Helman77 suggested in 1921, we should pay moredrag,40 and the tropic premise all offer an explanation attention to the “biologic laws underlying the develop-for the etiology of malocclusion. At this point, we can ment of malocclusions.” In 1968, Harvold78 said, “Fortest them using Popper’s best-ﬁt method2 on the basis years clinicians have realized that the tongue and facialof the broadly accepted evidence we have just dis- muscles are the factors which determine the size of thecussed dental arches and the crowding and spacing of teeth and that the skeleton has a subordinate role.” However,1. Our direct ancestors of 30,000 years ago had little Harvold’s powerful research had little inﬂuence on malocclusion. clinical practice, and, as Machiavelli (1469-1527) said,2. There is no evidence that genes have changed “The reformer has enemies in all those who proﬁt from signiﬁcantly since then.46 the old order, and only lukewarm defenders in all those3. Fewer than 5% of humans suffer from inherited who might proﬁt from the new.” dentofacial deformities.4. Malocclusion is endemic in industrialized countries. CONCLUSIONS5. There has been no genetic pattern in its geographical or historical spread. Our current environment and diet are dramatically6. Skeletal and dental structures are highly resistant to different from those of our ancestors of 30,000 years short-term heavy forces. ago when malocclusion was rare; no evidence suggests7. The skeletal and dental structures are very pliant to that there has been a signiﬁcant change in our genes long-term light forces. since then. This questions all hypotheses that are based8. On occasion, changes in oral soft tissue function and on the long-held belief that most malocclusions are especially posture are followed by major alterations inherited. There is strong scientiﬁc, logical, and clinical in the growth of the facial skeleton. evidence to suggest that the weak muscles and open- mouth postures that are now endemic in our society can Begg’s theory explains crowding but not skeletal cause increased vertical growth, whereas parafunctiondisproportion. Moss’s theory suggests that soft tissues of the tongue, lips, and cheeks is known to displacecarry the genes causing the malocclusion; this is hard to both the teeth and their supporting bone. Such arelate to factors 2 and 5 above. Solow and Kreiborg’s malocclusion would be a postural deformity.theory40 ﬁts all the above but does not explain the Perverse oral postures have proved difﬁcult todifferent types of malocclusion. The tropic premise measure, diagnose, and treat, and, in these circum-appears to be the only theory that is compatible with all stances, we should perhaps be guided by basic research8, and it also offers an explanation for the inherited in preference to clinical evidence. The tropic premisevariations in malocclusions. This is not conﬁrmation seems the best ﬁt of the hypotheses that have beenthat it is correct, but it does seem to ﬁt the available considered, and no evidence appears to disprove it. Itevidence and appears to be a sound basis for further suggests that environmental factors disrupt resting oralresearch. It replaces the many partial explanations that posture, increasing vertical skeletal growth and creatingnow exist with a single hypothesis covering 95% of a dental malocclusion, the occlusal characteristics ofmalocclusions. Having used it for many years,75 I think which are determined by inherited muscle patterns,clinicians will ﬁnd it a helpful guide in most circum- primarily of the tongue. The last entry in Victor Hugo’sstances. diary states “There is one thing stronger than all the The ﬁnal test of the tropic premise will be if new armies in the world; and that is an idea whose time hasprognostications can be drawn from it, which, when come.”tested, are found to agree with both existing andsubsequent research results— eg, whether speciﬁctongue and lip postures are found to be associated with REFERENCEScertain malocclusions (Fujiki et al76). If malocclusion is 1. Angle E. Treatment of malocclusion of the teeth. 7th ed.inherited, then our armamentarium is limited to me- Philadelphia: S. S. White; 1907.chanics and surgery, but, if the tropic premise is valid, 2. Popper KA. Conjectures and refutations. 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