Research related to malocclusion

American Journal of ORTHODONTICS VoZume 59, Number

ORIGINAL

1, January,

1971

ARTICLES

Research related to malocclusion A “State-of-the-Art” workshop conducted by the Oral-Facial Growth and Development Program, The National Institute of Dental Research Coenruad F. A. Moorrees, Charles J. Burstone, Ernest H. Hixon, and Samuel Weinstein l3oston, Mass.

Richard

1. Christiansen,

To assess the wide ramifications of research related to malooolusion, State-of-the-Art Worlcshops were conducted by the National Institute of Dental Research at the initiative of K. Kenneth Hisaoka, chief, and coordinated by Robert J. Tacy, program ofleer, of the Oral-Facial Growth and Development Program. In conjunction with the authors listed above, the format for a series of worlcshops was developed and participants were selected. The workshops were designed to provide in-depth review of the achievements, directions, and needs of research concerning malocclusion and dentofacial disfigurement (exclusive of cleft lip/pal&e). These worbhop conferences were held at the National Institutes of Health in Bethesda, Maryland, on Nov. lY and 18, 1969, and Feb. $5 and 86, 19YO. The ideas exchanged were integrated and summarized by the planning committee to produce this report.

M

an’s face and dentition serve as a mirror of expression and emotion, an instrument of speech and communication as well as in the vital functions of breathing, mastication, and swallowing.1 The psychologic and social implications of the dentition and its role in essential physiologic activities suggest that the treatment of malocclusion and facial disfigurement must be considered as a health service. The premise of orthodontic treatment has been that the change toward a normalized dentition enhances over-all functional adequacy and, as a consequence, the survival of the denture during the life span of the patient.2 This aspect of health care poses many problems because a precise and meaningful definition of malocclusion does not exist. Treatment need, or the threshold at which different malocclusions actually cause functional impairment and, conversely, treatment objectives, or the extent to which orthodontic 1

treatment must be carried to satisfy functional requiremelits, may be &fiicult to determine in individual instances. Moreover, the etiology of malocclusion remains largely unresolved, since information is not, available concerning the m&c of action of the genetic and environmental factors as determinants of both normal and abnormal development of face an(I dentition. Ecologic

and

psychologic

impact

of

malocclusion

and

dentofacial

disfigurement

The social (ecologic) and psychologic significance of the human fact, its role in human relations, coupled with cultural emphasis on external appearance, physical attractiveness, and conformity, points t,o the fact that the problem of the dentofacially handicapped is concerned directly wit,11 mental health.]~ 3, 4 Nonetheless, the severity of disfigurement does not invariably bear a direct proportional relationship to psychic distress, owing to the capacit,y for atljustment to malocclusion and facial disfigurement, or the lack of it, either in the presence of severe or minor discrepancies.” The study of the psychologic and ecologic sequelae of malocclusion requires identification and definition of a large number of variables,“, 7 which, together with their interrelationships, are of major importance in the design of such research. Attention may be given to obvious but complex factors as fundamental aspects of personality and self-image, attitudes, sex and age of the patient, family background, and social class, together with intellectual ‘endowment and educations Malocclusion is a combination of heterogenous traits or symptoms, each with a wide ra.nge of severity, and the implication of each trait as a handicapping condition remains to be clarified. In addition, the effect, of treatment may hc evaluated for determining to what extent correction of the malocclusion contributes to the patient’s well-being. This assessment must contend with the changing response of the patient during the lengthy treatment period of approximately 2 years and with the influence exerted on the patient by the orthodontist (his personality and behavior). In summary, a highly complex set of factors is involved in determining the impact of malocclusion and dentofacial disfigurement as well as the impact of treatment. Both cross-sectional and longitudinal, Peated and untreated samples, adequately representing the subgroups required, must be ascertained for testing with both psychologic and case history methods.’ Studies of the demand” or motivation lo, I1 for treatment among different population samples provide additional and invaluable clues with implications for public health services, manpower needs, and the logistics of orthodontic care. Fortunately, greater awareness of the significance of this kind of information has already become evident by the at,tempts to develop indices which relate anatomic aspects or traits of malocclusion to their presumed handicapping effect.l”-I4 Some of these efforts have been conceived for administrative screening to set treatment priority levels, ideally based on a rank order of the severity of malocclusion. However, differentiation among milder forms of malocclusion for treatment need by means of an index has not been achieved,l”, l” while the extent of facial disfigurement has been inferred only indirectly by the malpositioning of the teeth.

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A major advantage of these efforts has been to re-establish the need for epidemiologic studies, but approaches to such studies must be revised if they are to serve their intended purpose. Improved methods to select, assess, and measure pertinent morphologic traits of malocclusion are essential for statistical description of the incidence of each trait separately, their combinations, and interrelationships. This information is currently not available. Since the present anatomic model is inadequate for measuring the handicapping effect of malocclusion, new and meaningful methods are yet to be developed for objective and standardized determination of the impact of malocclusion and its treatment. This methodology should be independent from that used to evaluate the occlusal morphology itself, for the effect of malocclusion cannot be measured by measuring malocclusion.1F Physiologic

impact

of

malocclusion

Speech. The general awareness that the teeth are involved in the production of speech does not imply a causal relationship between malocclusion and speech problems.17 The vocal tract functions as an integrated unit during speech production. Since the tract is flexible and modifiable, a deviation in one portion can be minimized frequently by modification of ot,her portions of the tract. Sounds can therefore be produced in a variety of ways. Most speakers compensate automatically for all but the most severe structural deviations to maintain intelligibility of the acoustic signal because of the impelling need to communicate. The adaptability and compensatory action of the vocal tract to a wide variety of deviations do not negate the fact that in individual instances improvement in speech disability can be secured by orthodontic treatment in conjunction with speech therapy. Moreover, the greater the number of structural deviations, the greater the demands placed upon compensatory adjustments and, consequently, the greater the likelihood that defective speech and particularly consonant production can be remedied by correction of the malocclusion. The diagnosis of occlusion in its relation to defective speech is primarily concerned with the sagittal relation of the dental arches (overjet), the vertical relation of the anterior teeth (overbite), the continuity of the incisal edges of the anterior teeth (spacing), the configuration of the hard palate, tongue mobility, and the adequacy of the velar pharyngeal valve mechanism.ls Future research requires better identification and quantification of diagnostic procedures to determine the anatomic and physiologic adequacy of the oral structures for speech, tongue activities in swallowing to evaluate such assumed clinical entities as tongue-thrust,lg and conditioning techniques to modify neuromuscular behavior as well as the interaction of reflexive and voluntary behavior with oral sensation and perception.“O a~b These investigations do not pertain exclusively to orthodontics but transcend to the general area of oral physiology (speech, deglutition, and mastication) for which this fundamental knowledge is also desirable. Mastication and digestion. The mastication of food is a primary function of the dentition in the process of digestion. Masticatory efficiency is known to be

4

Moorrees et al.

Amer.

J. Orthodont.

January

1971

impaired with the loss of teeth, but almost no difference has been reported between subjects with excellent occlusion and those with most types of malocclusion.‘l Although unmasticated food may leave undigested residues,“” the degree of mastication required for maximum absorption of foods is seemingly readily attained by subjects with inadequate dentitions. Actually, little research has been done on mastication, and no evidence exists that malocclusion (excluding conditions that cause severe functional impairment) affects the digestive process and general health. Nevertheless, the ease of chewing and swallowing, freedom from interdental food impaction, self-cleansing action, and the enjoyment of taste are factors which cannot be quantitated but which must be satisfied according to individual requirements. More refined nutritional studies arc needed to determine whether differences in the transport of food across the digestive membranes depend on the degree of preparation in the mouth and whether mastication has any bearing on resistance to disease or longevity of man. Periodontal health. Some forms of malocclusion have been linked with occlusal trauma and breakdown of tooth-supporting tissues.23* 24 Conversely, the assumption has been made that the change to a more optimal occlusion will safeguard periodontal health. Gross functional impairment and tissue damage, as observed in some patients, obviously requires correction. In the absence of destructive and degenerative symptoms, prediction of future periodontal breakdown as a result of occlusal trauma is hazardous,25 for measurement of magnitude and direction of occlusal forces and adaptability of tooth-supporting tissues to these forces in time remains beyond the present state of diagnostic art. In spite of repeated but undocumented statements, one may still question whether specific traits or characteristics of malocclusion initiate or accelerate periodontal pathosis. Experimental and therapeutic modification of occlusion should aim at clarification of the conditions under which adaptation of periodontal tissues is no longer possible and pathologic changes occur. This objective involves numerous investigations which include functional and nonfunctional force loading of the teeth during extensive time spans. The complexity of such a project should not be underestimated, because instrumentation has to be developed for measuring the three-dimensional distribution of forces and the tissue response to these forces. Occlusion has become a central core in dentistry, transcending the boundaries of various specialties. Considerable efforts have been made during the last two decades to study occlusal function and its sequelae in greater depth. Sophisticated instrumentation has been used to investigate patterns of closing, oeclusal contacts, tooth mobility, and muscular activity, as well as effects of overloading individual teeth. For instance, the use of telemetry to relay tooth contacts during closing26 has been helpful, although its application is obviously limited. Nonetheless, longstanding concepts are being challenged by new evidence which suggests that functional or habitual contacts are attained during chewing and swallowing rather than in the more posterior centric position.27

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Research related

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The use of electromyographic techniques in the recording of muscular activity in the presence of occlusal anomalies and cuspal interferences has attempted to define the timing, duration, and phasic relationship of muscle contraction.28* 2g From these action patterns, inferences have been made about muscle behavior which, in turn, has been linked to occlusal relationships. However, the inherent difficulties and limitations of the technique,30 as well as the inability to obtain simultaneous recordings from the large number of facial muscles, explain the limited contribution of electromyography to diagnosis and treatment of occlusal anomalies. Much work remains to be done in the area of occlusion, but expectations must be tempered until it becomes possible to record the many aspects of occlusal function, reliably and without interference, to the normal physiologic status of test subjects. Within individual variation in oral function requires special consideration in the experimental design of such research. Tooth

movement

The literature abounds with reports on tissue changes on which tooth movement for correction of malocclusion is dependent. The findings of these investigations had a profound effect, resulting eventually in modification of force dosage to prevent necrosis. During the last two decades analysis of force magnitude and force duration has been given continued emphasis for optimal biomechanical appliance design. 31,32 These attempts have been successful insofar as the development of a variety of methods for precise three-dimensional control of tooth movement is concerned. Nonetheless, variations in individual response33 to these perfected force systems demand further study of the biologic reactions to the carefully computed direction and strength of forces and couples, both for tooth movement and for preservation of anchorage, as needed. Research in tissue reaction to forces from orthodontic appliances involves ascertaining the mode of action of osteoblasts and osteoclasts in matrix formation and calcification as well as resorption of bone. It should be realized, however, that the mechanism by which pressure and tension are converted into two different cellular responses is unknown.34 Changes in tissue vascularity35 and altered oxygen-carbon dioxide levels3s have been implied, but more generally pressure has been linked with differentiation of osteoclasts and initiation of resorption and tension with osteoblastic deposition of new bone owing to distention of the collagenous periodontal fibers. Current thoughts suggest that the principles of bone remodeling characteristic of bone growth in the long bones37 and sku1138 are also involved in tooth movement following bending of alveolar bone,3g in addition to the compression and distention of the periodontal membrane for initiation of cellular changes. The mechanical energy expended in alveolar bone has been related to the piezoelectric effect, which is expressed in terms of transient electric fields that arise when bone is dynamically loaded and presumably induce profound metabolic changes in cells. The rate of bone deformation is proportional to amplitude and the polarity of the electric potential to the direction of deformation.40 Similarly, inferences may be made about root resorption, since piezoelectric potentials have also been demonstrated for the dentine and cementum of the

tt?eth.4’ StrcsS tlistribution in a mat,h~l~lutical modrl 01: tlic pcrio([ontal ligi]m(‘[)t indicates that the largest stresses oc(+ur at the ~mentnltl snrfa~ rat,hcr t ban it, the bone surface of’ the alreolns. The highest ratio of thcsstresses WI tlltls(a s~f’a~s for Cllly locat,ion in this 11~od~l is follnil tht: ;ll)ir*al ill'Vil. \\hfxW ro(jt rc5orption also occursL.,I’! Yet the origin of the piczoelcctric efYWt, and its function a tr;LnsduccI mechanism in living systems for cellular rcsponsc to force applic*ation require further investigation. The electric potentials at thcl tension alIt1 compression sites of the periodontal membrane may bc studied to cletrrmine \vhether they result from the elastomer tissues of the ligaments or Cram an interaction at the bone-elastomer interface. Likewise, tlic polarit>- of electric potential differc~ncrs remains to be determined in the metlullary spact~s on the innrr surface of t,he cortical plate at areas of undermining resorption. The time lag in the response and tlifferentiation of osteoblaxts and ost,eoclasts after force application and the registration of an c1lectric potential difference would clarify the significance of the piezoclectric effect as a transducing mechanism or as an associated phenomclnon to a primary rcll-mediated biochemical process. Conversely, the piezoelectric effect may be used as an experimental tcchniquc to study changes in bone architecture during tooth movement to reveal the site and extent of alveolar bone bending. As is often the case, attempts to csplain the mediating mechanism of tooth movement demand fundamental biologic, research, particularly since electric potential differences are known t,o (K’PW already at the intracellular level and across cell membranes. The unrarcling of a causeand-effect relation is therefore implied between ionic exchange anal an electrical phenomenon. ilt

ilS

Stability

of

tooth

position

Teeth are known to drift from the time that the deciduous teeth emerge, throughout the mixed dentition,4” to t,he early thirties in the unmutilated permanent dentition,4” and probably throughout the entire life span of the individuaL4” Thus, tooth position and occlusion are dynamic entities probably in response to functional forces and growth changes of the face, however minute these may be. Tooth positioning for the correction of malocclusion is guided by a wellestablished anatomic norm modified 1)~ individual criteria, such as maintaining the original dental arch shape and breadth as well as the position of the mandibular incisors. Dental arch continuity through contact relations of the proximal surfaces of the teeth, proper root positioning of anterior and posterior teeth, and overcorrection of overbite and rotations arc additional objectives of treatment. Furthermore, assessment of occlusal relations during functional activities in the last phase of treatment is undertaken to eliminate excessive forcaes and grm occlusal interferences. In spite of adherence to these objectives, varying degrees of relapse occur after orthodontic treatment and even after prolonged retention. These relapses cannot be readily explained other than by the fact that alveolar bone responds

Rescad

related to malocclusion

7

to mechanical stresses on the teeth to maintain homeostasis by differential growth, as in any stimulus-response or closed-loop negative feedback system.46 The obvious conclusion must be that the teeth are not in coronal equilibrium when subjected to the system of forces produced during the patient’s physiologic activities.47 These force vectors cannot be determined at the present time, as stated above, and it may be concluded that empirical concepts of crown and root positioning do not necessarily correspond to the physiologic requirements of each patient. Since teeth are in equilibrium at all times (that is, the sum of all coronal forces and all moments equaling zero), any deviation in coronal equilibrium must be compensated by forces developed through stresses in the periodontal ligament. When such stresses exist for periods of time sufficient to permit differentiation of osteoblasts and osteoclasts, the teeth will migrate toward a position in which coronal equilibrium is re-established. This concept may also explain the seemingly continuous migration in the untreated dentition. The tolerance limits of this force equilibrium remain obscure. Moreover, it is not known to what extent cusp-fossa relations of opposing teeth, slippage in approximal contacts of neighboring teeth, root positioning, as well as gingival and transseptal fibers, singly or jointly, contribute to a train of events that disturbs a seemingly excellent treatment result. Furthermore, the value of prolonged retention to prevent relapse has not been demonstrated in terms of its buttressing effect on the dentition to withstand facial growth changes after treatment, particularly in the case of differential growth of the two jaws, and to allow adjustment of muscle behavior. The last, factor may not be pertinent, for adaptation of muscular activity to the changing tooth environment., if it occurs, should already have t,aken place during treatment. Persistent tongue-thrust in the presence of open-bite tendencies poses special problems for correction and stability of results as a neuromuscular disorder uncorrectable by conditioned learning experience. Its incidence is, however, low and recognizable. The balance between tongue, lip, and cheek forces has been considered especially decisive for maintenance of arch form, and myometric studies have been conducted to verify this hypothesis. Results so far have not been conclusive ; in fact, buccolingual forces seem to be rarely in complete balance at any moment in time. A number of factors can be cited which point to the limitations of these studies. Transducer design, transducer placement against the buccal or lingual tooth surfaces, presence of lead wires, and inability of transducers to withstand prolonged use in the mouth all point to the need to improve instrumentation. Some progress has been made, for transducers have been incorporated in artificial teeth,48 and it has been possible to design transducers with strain gauges that accurately represent the force exerted by muscles without artifacts resulting from the extent of muscle contact on the transducer platform area.49 The marked variation during repeated testing of the same person requires that force measurements are obtained during long time spans. So far, muscle balance has been estimated only from 24-hour activity and swallowing records,

8

Moorrees et al.

Amtw.

J. Orthodost. January 1971

transmitted with telemetry, Subsequent test measurements, in the laboratory, of various oral functions, such as chewing, speech, swallowing, and rest, with different head postures were used to compute the estimated contributions of these activities during the 24-hour period.4g Although myometric research may well produce clinically significant data,501 51 additional variables must be brought to bear on the balance concept. That is, a lack of balance between labial and lingual muscle forces may be compatible with other forces which, together, determine the total force system. Moreover, the effect of muscle forces, per se, may differ in various segments of the dental arches. Growth

and

development

Malocclusion is only rarely conditioned by gross anomalies in facial development, for the face with malocclusion is generally well within the range of normal variation, although characterized by disharmony of parts5” Family research has indicated a strong hereditary component of the development of the face and dentition. 53 Polygenic control is assumed because simple Mendelian models have failed to yield an explanation of genetic mechanisms. Because of the known phenotypic plasticity, this multifactorial system is also susceptible to environmental modification. In this context, postnatal factors have been given greatest importance in the etiology of malocclusion, perhaps because the embryo and fetus have been assumed to develop in a protected environment and the prenatal period has been described more in terms of a static than a dynamic process. Since etiology, diagnosis, and treatment of malocclusion are closely linked to growth of the face and dentition, a variety of approaches for research in this area is needed. Twin studies can define the contribution of genetic and environmental factors to facial morphology. 54y55 Heritability can be determined at various immature levels of facial development in twins and, when longitudinal data are available, the twin method can be applied to the study of heritability of growth increments. Through cross-twin analysis,5G genetic and environmental components can be determined for the interrelationships between different growth parameters, both absolute and incremental. Families are less efficient than twins for defining the contribution of genetic and environment factors, because of the lack of genetic unity that exists in monozygotic twins, but family data arc needed for the study of genetic mechanisms. However, differences in the ages of subjects and changes in environment restrict the usefulness of two-generation family data. Moreover, single-generation studies of full siblings, half-siblings, and cousins require extremely large numbers of subjects as well as age controls. Thus, only limited results c;w1 be anticipated from present analytic techniques, particularly since the mode of inheritance of most attributes is complex.57 Comparison of siblings to their parents has been used to make inferences about sex chromosome effects upon certain growth processes.58 When propositi with chromosomal aberrations can be ascertained, family comparisons can identify the relationship of a particular abnormality with specific chromosomes. Inbred-animal studies can serve to test theoretical models for the analysis

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of craniofacial growth in man. For example, breeding studies may be carried out to determine the efficiency of a mathematical model derived for the purpose of partitioning quantitative differences in a growth parameter by the use of marker genes, such as red cell antigens. Through embryologic research in animals, it would be possible to associate the effects of gene action on different tissues, systems, or traits. 5s This research is just in a beginning state in genetic laboratories. Nongenetic

factors

The contribution of nongenetic factors to phenotypic variation brings into focus the fact that, invariably, hard tissues have been used for human and animal studies, which leads to oversimplification of the true complexity of the problem of growth and development. Bone exists in a milieu and, irrespective of its form at a given developmental horizon, it is subject to modification by adjacent tissues. This interrelationship may be so prominent that primary genetic control cannot be readily discerned, as, for instance, in the mandible in the Pierre Robin syndrome. The proportionate effect of genetic (biochemical) and environmental factors will be particularly difficult to evaluate at earliest levels of morphogenesis. At present, models of bone growth are concerned with the effect of functionform interrelationships, an adaptive capacity that concerns size, shape, structure, and position of bony elements, recognizing a degree of individuality of bones or areas thereof within the totality of the organism and discarding a bone as an isolated anatomic specimen.@‘~w 85 This concept does not negate the role of genes, as is often assumed, but, rather, broadens genetic study to include morphogenesis of all tissues (soft and hard) and their subsequent interaction. It involves the central nervous system, which guides voluntary and sensory-receptor-induced muscle activity. Thereby, architectural aspects become subjugated to the underlying factors that create. The definition of function as a property and vitalistic force and of a functional component as a part of a structure which performs a function, as well as the definitions of functional matrices and functional units, has so far been made in operational terms. 61 In view of the true nature of this concept, the functionform intera,ction works both ways, since adaptation of form yields adaptation of function as a form-function relation. In fact, it is not known whether the latter is the proper term for the earliest developmental horizons or whether at that moment in time both soft and hard tissues are differentiating within a genetically coded pattern. The experimental model for pursuing the functional approach to morphology should consider the interdependence of functional factors on each other, expressed either at the site of the functional component studied or beyond this area. Neuromuscular

patterns

and

bone

development

This research also involves the field of neuromuscular physiology to investigate the role of muscle activity as controlled by the motor neurons and

10

Moorref~s

et ul.

the experimental induction of disturbances in the ~~~ntral n~r~-ous systf:m.‘:z Muscle removal itself is a relativ-cly cructc techniques and more cstensivc use Of SUCh methods as sectioning of the fac.ial n(‘rvc 01’ tri#kmina] J))[)tw nmt ;it specific stages of postnatal development and unilateral sretionillg 0f the h~p0glossal nerve to produce asymmetric tong;u(~ action is indicated. Unilateral sectioning of the al~ola 13IWJY’ soon aft,chr birth should rc~veal the effect of the trophic influence on tooth formation, t oath eruption, and bone gro\vth. Bilateral sectioning prevents sc~nsory input from the mandibular dentition to the central nervous system. It also interferes with the inhibitory brake mechanism on the jaw-closing mus(~Ics, at least by c4iminating prriodontal receptors in the mandible, and thereby affects mus~lc action ancl muscle force. Activation of n~nsclcs through elect rodcs implanted in the mcsrncephalic or motor nucleus of the trigcminus may 1)~ another approach to overactivation of muscles. Similarly, removal of the affercbnt feedback from the tongue pan reveal effects on bone growth owing to 1ac.k of control of the tongue or its orrractivit,g.“” At birth, an intricate pattern of‘ the organization of the central nervous system in response to input from oral sensory receptors has been achieved to assure the vital oral-pharyngcal function of the nowborn. The additive effect of the teeth, onc~~they emerge, and their related sens(~ organs as modifiers of these patterns in the developing ner~us system will bc of inter&. Furthermore, the changes in tooth position during Orthodontics offer additional experimental conditions owing to concomitant modification of il(:l1roIr1usc~~lar activit,y and thereby mandibular posture. Current research on oral-pharyngeal rcflexeP, (i5may provide tests to characterize reflex patterns and their relation to bone development. At present no firm experimental evidence is available t,o ascertain the role of the nervous system, and methods for its study are yet to bc perfected. Nevertheless, this a.rca of research requires attention because it is directly relat(ad to the investigations that have occlusion of the dentition for their objcctivcl. Patients with neurologic disorders, developmental anomalies, endocrinopathies, and chromosomal defects provide another source for the study of growth and devclopment.6F-G” Careful documentation and longitudinal follom-up by a team of investigators representing various disciplines remain to be attained as a prerequisite for interpretation of the data which these experiments by nature can reveal. For instance, subjects with significantly altered growth and development associated with specific hormonal deficiencies or excesses, for which appropriate therapy is available, may be used to define the role of the endMarine system in growth. This information is especially valuable with respect to hormones responsible for normal adolescent development, since specific techniques are now available for measuring separately gonadal, adrenal, and growth hormone concentrations under different physiologic states. The role of sex hormones on the rate of development of the face during adolescence would be of particular interest, possibly for growth prediction. Further study in man or experimental animals of the effect of hormones on tooth formation, tooth eruption, and tooth emergence is still indicated for a more definitive understanding Of this aspect of oral

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physiology.6s~ 69 In view of the recent advances in hormone assay, endocrinologic research may prove to be valuable in clarifying mechanisms involved in skeletal growth. Topographic study of the normal child with and without malocclusion has been a traditional approach in orthodontics. Cephalometric tracings and dental casts have been used, but sometimes they have been augmented with anthropometric data, as well as hand-wrist and dental radiogra.phs, to assess skeletal maturation and dental age, respectively. In spite of the battery of reports from cross-sectional or longitudinal studies, a precise description of normal facial and dental development is not available. Relationships between somatic growth and facial growthi and the value of using a physiologic age scale remain to be clearly defined. As a result, attempts to pursue prediction of facial growth hare been premature and unfruitful. limitations

of

growth

prediction

In fact, the unrealistic premise has been entertained that growth in absolute terms may be predicted for the postadolescent from quite immature levels at childhood (say, 5 years of age), Such efforts have generally involved correlations of univariate parameters.” They have been useful not so much for their findings but, rather, for pointing to the need for identifying the sources and their contribution to individual differences in the growth and developmental process. Recognition of the multivariate nature of the problem is a first requirement in any experimental design aimed at prediction. Yet it implies recognition of the limitation to which the precision of growth prediction can be pushed in absolute terms. Instea.d, the objective may well be limited to ascertaining growth trends, and that at relatively short time intervals, to gain better insight into the nature of growth changes. The shortcomings of the cephalometric technique as a two-dimensional projection and the inability to locate all desirable landmarks for defining growt,h in various areas of the head do not condemn this method. In the first place, the radiographic record is indispensable for the study of the living, and a great deal of information currently not available can be obtained from these films. Metallic implants have given an improved means for superposition of serial records to define the effect of growth more accurately.72 Their use demands accurate duplication of head positioning during serial study with image screening prior to exposure. Moreover, the combined study of frontal and sagittal head projections for a partial three-dimensional analysis has already been developed,731 74 and metallic implants can further enhance their yield. Finally, the caudad-cephalad projection can be obtained for complete t,hree-dimensional study.= Longitudinal studies generally evoke criticism based on limitations in sample composition and sample size. Consequently, the representativeness of the sample to the general population is questioned. In spite of the difficulty and cost of conducting longitudinal research, it should be remembered that individual variations in growth increments can be defined only by the repeated observation of the same subjects. Any attempt

12 Moorreeset

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J. OrthocZmat. January 1971

to predict growth must rely on the longitudinal method, and these data furnish the basic reference for the study of children with abnormal developnlerlt,. It is particularly unfortunate with reference to malocclusion that little longitudinal research has been conducted on growth from the early postnatal period to the establishment of the deciduous dentition.76 The dentition counts as a separate tissue system in the growth process, and the timing of its growth changes is dependent on the formation of the teeth.77 Facial growth, on the other hand, may be studied in relation to somatic growth and bone age, particularly with respect to its acceleration during adolescencc.78 Comprehensive documentation of the timing, duration, and magnitude of the adolescent growth spurt is of particular interest because of its pronounced but. variable effect on the individual’s growth cycle.7s Precise assessment of the relationships between dental, facial, and somatic growth demands careful selection and precision of observations. For instance, an over-all rating of epiphyseal and carpal development in the hand-wrist region will not reveal to best advantage the detail required for scaling growth data on physiologic age. 8o Likewise, for sibling or twin studies, meaningful information may be lost unless differences can be accurately determined. The statistical analysis of growth data is now generally described in terms of multivariate analysis. Yet specific statistical approaches and computer programming must be developed according to the inferences which the investigator wants to make from his growth data. Comput,er technology provides no more than a means of realizing the biostatistical objectives of a study. Research in growth and development cannot be lirnited in any case to one specific approach; that is, neither the experimental study7’jWi8nor the clinic for human studyTg alone can give the answers needed to clarify the etiology of malocclusion and to determine the probable effect of growth during orthodontic treatment and thereafter on the result obtained. Manpower

needs

for

research

related

to

malocclusion

The wide ramifications of research related to malocclusion obviously require a highly diversified group of investigators with competence in a variety of disciplines of the basic and related sciences. For this reason, it is necessary that orthodontists, particularly those in a university setting, seek active collaboration for the joint conduct of research. The complexity of the problems involves sophisticated approaches to experimental design and methods of study. Consequently, orthodontic departments cannot be expected to accumulate a staff to deal with these problems efficiently. The establishment of research institutes and clinical research centers pro vides a means to realize two objectives, namely, to interest faculty of the university at large in the challenging research projects relating to malocclusion and to provide the orthodontic department with expert assistance. The orthodontist has a primary role in research undertakings, for he must pose the problems, define their significance, and interpret the findings. Moreover, he will be responsible for clinical observations and clinical application of test procedures which will be part of most research. His motivation and his skills are indispensable for the programming of teamwork. Moreover, he must

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be expected to have a broad scientific background if he is to utilize the resources of the biologic sciences and the health sciences of the university. Massive expenditure of funds into areas of research without assurance of collaborative efforts by those trained and knowledgeable in both the clinical and the relevant scientific disciplines is no longer warranted. The major key to research manpower is the team effort, whenever required, and a realistic appraisal of the actual collaboration that can be obtained. The training programs for orthodontists aiming at full-time academic careers must, of necessity, include the perspective that research related to malocclusion demands. Although much information is needed to better define indication and need of orthodontic treatment, it may be safely assumed that present manpower is insticient to deliver this care to larger segments of the population, regardless of the outcome of such research. Apart from extending facilities for specialty education, greater efficiency can be obtained by allowing a multitrack approach in the latter part of the undergraduate curriculum, whereby students can pursue their specific areas of interest in either the clinical or the basic science areas. In the future, these elective studies may well become acceptable as part of the educational requirements for specialty practice. Furthermore, a large percentage of routine tasks in orthodontic practice can be performed by auxiliary personnel. Their training should not raise serious problems, for auxiliary personnel in orthodontics, as in medicine, serve as a link in a chain rather than providing complete services. Orthodontics, as an integral part of dentistry, should also receive greater emphasis in undergraduate education in accordance with the guidelines of the Council on Orthodontic Education of the American Association of Orthodontists and the Council on Dental Education of the American Dental Association. Moreover, the wide implications of research related to malocclusion may serve in postgraduate programs to broaden horizons and educate future generations of orthodontists in true academic tradition and, hopefully, in preparation for their participation in new and imaginative research. Conclusions

This report has raised a number of fundamental questions regarding malocclusion and facial disfigurement which are ultimately related to improved patient care. Answers to these questions demand recognition of the complex nature of the problems to be solved. Much of this research involves active collaboration of investigators from various disciplines. Likewise, it requires a variety of approaches to experimental and clinical studies as well as advanced methodology. Future investigations may include focus on the following areas: 1. Dentofacial disfigurement and its relation to mental health, with specific attention to self-image, personality, social acceptance, and behavior. 2. Functional impairment as a result of malocclusion, particularly in terms of tooth-supporting tissues and dental health, speech, and possibly mastication. 3. The contribution of orthodontic treatment to functional improve-

14

Moorrccs

et

07.

ll~cllt and psychosocial well-being in I)oth the yo~u~g ;tncl adults t’o18<‘;lvIl of the different, traits that constitute? malocclusion. 4. Tissue response to forces from orthodontic appliances, bone forma tion and remodeling, I~~L~~OI~USCLI~~~ adaptation to tooth rnovemc~nt~ ill~tl effect of tooth-supporting ligaments on tooth movement. 5. Mechanisms through which I’orcc application res,nlt,s in cellula.1 activity. G. Oral physiology in terms of occlusal function at various developmental levels. 7. Embryology of dentofacial development, tissue differentiation, and their interrelationships during prenatal growth. 8. Growth of face and dentition, its relation to orthodontic treatment, and long-term stability of treatment results. Manpower provides a key to the advancement of knowledge and improvcment of clinical orthodontics, while education provides a key to the production of the qualit,y, quantity, and diversity of the manpower required. Acknowledgment I<. Kenneth Hisaoka, Chief, Oral-Facial Growth and Development Program (NITlIt), Robert J. Tacy, Program Officer, and the authors are indebted to the following consultants for their valuable contributions to this State-of-the-Art Workshop which made these conferences a most stimulating experience: C. Andrew I,. Bassett, Professor of Orthopedic Surgery, Columbia University College of Physicians and Surgeons; David Bixler, Associate Professor . -James P. Carlos, Chief, Biometry of Dental Sciences and Medical Genetics, Indiana 7Jniversity, Section, National institute of Dental Research; Donald B. CherJk, Professor of Pediatrics, The Johns Hopkins University; Lois R. Cohen, Chief, Applied T
1. Macgregor, F. C.: Social and psychological Angle Orthodont. 40: 231-233, 1970. 2. Moorrees, C. F. A.: Occlusion, J. Periodont.

38:

implications

of

‘i:il-$60,

7967.

dentofacial

disfigurement,

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3. Linn, E. L.: Social meanings of dental appearance, J. Health Hum. Behav. 7: 289-295, 1966. 4. Secord, P. F., and Backman, C. W.: Malocclusion and psychological factors, .T. Amer. Dent. Ass. 59: 931-938, 1959. 5. Macgregor, F. C., Abel, T. M., Bryt, A., Lauer, E., and Weissmann, S.: Facial deformities and plastic surgery: A psychosocial study, Springfield, Ill., 1953, Charles C Thomas, Publisher. 6. Stricker, G.: Psychological issues pertaining to malocclusion, AMER. J. ORTHODONT. 58: 276-283, 1970. 7. Cohen, L. K.: Social psychological factors associated with malocclusion, Int. Dent. J. In press, 1970. 5. Maj, G., Squarzoni Grilli, A. T., and Belletti, M. F.: Psychological appraisal of children facing orthodontic treatment, AMES J. ORTHODONT. 53: 849-857, 1967. 9. Baldwin, D. C., Barnes, M. L., Baldwin, M. -4., and Papajohn, J. T.: Social and cultural variables in the decision for orthodontic treatment, Int. Ass. Dent. Res., 45th General Meeting, 1967 (Abstract, p. 114). 10. Barry, J. R.: Patient motivation for rehabilitation, Cleft Palate J. 2: 62-68, 1965. 11. Baldwin, D. C., and Barnes, M. L.: Patterns of motivation in families seeking orthodontic treatment, Int. Ass. Dent. Res., 44th General Meeting, 1966 (Abstract, p. 142). 12. Howitt, J. W., Stricker, G., and Henderson, R.: Eastman esthetic index, New York Dent. J. 33: 215-220, 1967. 13. Salzmann, J. A.: Handicapping malocclusion assessment to establish treatment priority,

AMER. J. ORTHODONT.54: 749-765, 1968. 14. 15. 16. 17. 18. 19. 26a. 20b. 21. 22. 23. 24.

Grainger, R. M.: Orthodontic treatment priority index, Public Health Service Publication No. 1000Series 2, No. 25, 1967. Carlos, J. P., and Ast, D. B.: An evaluation of the HLD index as a decision-making tool, Public Health Rep. 81: 621-626, 1966. Carlos, J. P.: Evaluation of indices of malocclusion, Int. Dent. J. In press, 1970. Spriestersbach, D. C.: Dentition and speech. In Horowitz, 5. L., and H&on, E. H.: The nature of orthodontic diagnosis, St. Louis, 1966, The C. V. Mosby Company, pp. 177-191. Weinberg, B.: A cephalometric study of normal and defective -s- articulation and variations in incisor dentition, J. Speech Hearing Res. 11: 288300, 1968. Subtelny, J. D., and Sakuda, M.: Muscle function, oral malformation and growth changes, AMER. J.~RTHODONT. 52: 495-517,1966. Grossman, R. C., Hattis, B. F., and Ringel, R. L. : Oral tactile experience, Arch. Oral Biol. 10: 691-705, 1965. Bosma, J. F. (editor) : Symposium on oral sensation and perception. I and II, Springfield, Ill., 1967 and 1970, Charles C Thomas, Publisher. Hixon, E., Maschka, P., and Fleming, P.: Occlusal status, caries, and mastication, J. Dent. Res. 41: 514, 1962. Farrell, J. H.: The effect of mastication on the digestion of food, Brit. Dent. J. 100: 149-155, 1956. Stallard, R. E.: Occlusion: A factor in periodontal disease, Int. Dent. J. 18: 121-132, 1968. Geiger, A. M.: Occlusal studies in 188 consecutive cases of periodontal disease, AMER. J.

ORTHOD~NT. 18: 330-360,1962. 25.

Stahl, S. S.: The role of occlusion in the etiology and therapy of periodontal disease, Angle Orthodont. 40: 347-352, 1970. 26. Schaerer, P., and Stallard, R. E.: The effect of an occlusal interference on the tooth contact occurrence during mastication, Helv. Odont. Acta 10: 49-56, 1966. 27. Glickman, I., Pameijer, J. H. N., Roeber, F. W., and Brion, M. -4. M.: Functional oeclusion as revealed by miniaturized radio transmitters, Dent. Clin. N. Amer. 13: 667-679, 1969. 28. Ahlgren, J.: The silent period in the EMG of the jaw muscle during mastication and its relationship to tooth contact, Acta Odont. &and. 27: 219-227, 1969. 29. Meller, E.: The chewing apparatus; an electromyographic study of the action of the

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37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50.

51. 52. 53. 54.

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J. Orthodont. Ja.nuarg 19 7 1

muscles of mastication and its correlation to facial morphology, Arta Physiol. &and. 69: Supp. 280, pp. l-229, 1966. Ralston, H. J.: Uses and limitations of electroxnyograpby in the quantitative study of skeletal muscle function, AMER. J. ORTIIOI)OXT. 47: 581529, 1961. Weinstein, H.: Minimal forces in tooth movement, A~~E:K. J. ORTHO~XXT, 53: 881-903, 1967. Burstone, C. J.: The mechanics of the segmented arch techniques, Angle Orthodont. 36: 99-120, 1966. Hixon, E. H., Aassen, T. O., Arango, J., Cllark, R. A., Klosterman, R., Miller, 8. S., and Odom, W. M.: On force and tooth movement, AMER. J. ORTHODONT. 57: 476-489, 1970. DeAngelis, V.: Observations on the response of alveolar bone to orthodontic force, AMER. J. ORTHODONT. 58: 284-294, 1970. Bien, S.: Fluid dynamic mechanisms which regulate tooth movement, Advances Oral Biol. 2: 173-201, 1966. Goldhaber, P.: Oxygen dependent bone resorption in tissue culture. In Grecp, R. O., and Talmage, R. V. (editors) : Parathyroids, Springfield, Ill., 1961, Charles C Thomas, Publisher, pp. 243-354. The human face, New York, 1968, Hoeber Medical Division, Harper S; Enlow, D. H.: Row, Publishers. DeAngelis, V.: Autoradiographic investigation of cnlvarial growth in the rat, Amer. J. Annt. 123: 359-368, 1968. Baumrind, 5.: A reconsideration of the propriety of the “pressure-tension” hypothesis, AMER. J. ORTHODONT. 55: 12-21, 1969. Bassett, C. A. L.: Biologic significance of piezoelectricity, Calcif. Tissue Res. 1: 252s 272, 1968. Cochran, G. V. B., Pawluk, R. J., and Bassett, C. A. L.: Stress generated electric potentials in the mandible and teeth, Arch. Oral Biol. 12: 917-920, 1967. Haack, D. C.: Unpublished data, 1970. Lebret, L.: Physiologic tooth migration, J. Dent. Res. 43: 610-618, 1964. Moorrees, C. F. A.: Unpublished data, 1970. Beyron, H. L.: Occlusal changes in adult dentition, J. A.mer. Dent. Ass. 48: 674-686, 1954. Christiansen, R. L.: Some biological considerations in orthodontic research, AMER. J. ORTHODONT. In press. Weinstein, S., Haack, D. C., Morris, L. Y., Snyder, B. B., and Attaway, H. E.: On an equilibrium theory of tooth position, Angle Orthodont. 33: l-26, 1963. McNulty, E. C., Lear, C. S. C., and Moorrees, C. F. A.: Variability in lip adaptation to changes in incisor position, J. Dent. Res. 47: 537-547, 1968. Lear, C. S. C., and Moorrees, C. F. A.: Buccolingual muscle force and dental arch form, AMER. J. ORTHODONT. 56: 379-393, 1969. Gould, M. S. E., and Picton, D. C. A.: A study of pressures exerted by the lips and cheeks on the teeth of subjects with Angle’s Class II Division 1, Class II Division 2 and Class III malocclusions compared with those of subjects with normal occlusions, Arch. Oral Biol. 13: 527-541, 1968. Proffit, W. R., Chastain, B. B., and Norton, L. A.: Linguopalatal pressure in children, A~XER. J. ORTHODONT. 55: X4-166, 1969. Moorrees, C. F. -4.: Normal variation and its bearing on the use of cephalometric radiographs in orthodontic diagnosis, AMER. J. ORTHODONT. 39: 942-950, 1953. Hunter, W. S., Balbach, D. R., and Lamphiear, D. E.: The heritability of attained growth in the human face, AMER. J. ORTHODONT. 58: 128-134, 1970. Hunter, W. S.: A study of the inheritance of craniofacial characteristics as seen in lateral cephalograms of 72 like-sexed twins, Europ. Ortbodont. Sot. Rep. Congr. 41: 59-70, 1965. Kempthorne, I., and Osborne, R. H.: The interpretation of twin data, Amer. J. Hum. Genet. 13: 320-339, 1961.

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56. Osborne, R. H., Horowitz, 8. L., and DeGeorge, F. V.: Genetic variation in tooth dimensions: A twin study of the permanent anterior teeth, Amer. J. Hum. Genet. 10: 350-356, 1958. 57. Osborne, R. H.: Personal communication, 1970. 58. Garn, S. M., Lewis, A. B., and Polacheck, D. L.: Sibling similarities in dental development, J. Dent. Res. 39: 170-175, 1960. 59. Alexandersen, V.: The odontometrical variation of the deciduous and permanent teeth in Downs’ syndrome, M. S. Thesis, University of Wisconsin, 1969. 60. Dullemeijer, P.: Some methodology problems in a holistic approach to functional morphology, Acta Biotheor. 18: 203-214, 1968. 61. Moss, M. L.: The primacy of functional matrices in orofacial growth, Dent. Pratt. 19: 65-73, 1968. 62. Goldberg, L. J., and Nakamura, Y.: Lingually induced inhibition of masseterie motor neurons, Experientia 24: 371-373, 1968. 63. Goldberg, L. J.: Personal communication, 1970. in the 64. Goodwill, C. J.: The normal jaw reflex, measuring of the action potential masseter muscles, Ann. Phys. Med. 9: 183-188, 1968. 65. Bosma, J. F.: Maturation of function of the oral and pharyngeal region, AMER. J. ORTHODONT.49: 94-104, 1963. 66. BjGrk, A., and Kudora, T.: Congenital bilateral hypoplasia of the mandibular condyles, AMER. J. ORTHOD~NT. 54: 584, 1968. 67. Cohen, M. I., Crigler, J. F., and Wittenborg, M. H.: Systemic disturbances in relation to general and dentofacial growth and development in children, AMER. J. ORTHODONT. 48: l-20, 1962. 68. Car-n, S. M., Lewis, A. B., and Blizzard, R. M.: Endocrine factors in dental development, J. Dent. Res. 44: 243-258, 1965. 69. Keller, E. E., Bather, A. H., and Hay&, A. B.: Dental and skeletal development in various endocrine and metabolic diseases, J. Amer. Dent. Ass. 81: 415-419, 1970. 70. Bambha, J. K.: Longitudinal cephalometric roentgenographie study of face and cranium and relation to body height, J. Amer. Dent. Ass. 63: 776-799, 1961. 71. Hixon, E. H.: Prediction of facial growth, Europ. Orthodont. Sot. Rep. Congr. 44: 127. 139, 1968. 72. BjGrk, A.: Sutural growth of the upper face studied by the implant method, Acta Odont. &and. 24: 109-127, 1966. 73. Schwartz, H.: A method of measuring points in space as recorded by the BroadbentBolton cephalometric technique, M. S. Thesis, Northwestern University, 1943. 74. Savara, B. 5.: A method for measuring facial bone growth in three dimensions, Hum. Biol. 37: 245-255, 1965. 75. Berger, H.: A neglected view in cephalometrics, Europ. Orthodont. Sot. Rep. Congr. 57: 335342, 1961. 76. Sillman, J. H.: Some aspects of individual dental development, Anna. J. ORTHODONT.51: l-25, 1965. 77. Moorrees, C. F. A., and Reed, R. B.: Changes in dental arch dimensions expressed on the basis of tooth eruption as a measure of biologic age, J. Dent. Res. 44: 129-139, 1965. 78. Krogman, W. M.: Growth of head, face, trunk and limbs in Philadelphia white and Negro children of elementary and high school age, Monogr. Sot. Res. Child Develop. Serial No. 136, Vol. 35, No. 3, pp. l-80, 1970. 79. Barton, W. H., and Hunt, E. E., Jr.: Somatotype and adolescence in boys: A longitudinal study, Hum. Biol. 34: 254-270, 1962. 80. Tanner, J. M., Whitehouse, R. H., and Healy, M. J. R.: A new system for estimating the maturity of the hand and wrist, with standards derived from 2,600 healthy British children. Part II. The scoring system, Paris, 1962, International Children’s Centre. 81. Hoyte, D. A. N.: Experimental investigations of skull morphology and growth, Int. Rev. Gen. Exp. Zool. 2: 345-407, 1966.

82. Stockard, 0. It.: The genetic and entlocrinic basis for tlifl’erences in form and 1~ havior, Philadelphia, 1941, Wistar Institute of Anatomy and Biology. 83. Baer, M. J., and Gavan, J. A. (editors) : Symposium on hone growtli as revealed by in vivo markers, Amer. J. Phys. Anthrop. 29: 155310, 1968. 84. Gruneberg, H.: The pathology of development, New Work, 1963, Johu Wiley & Sons, Lnc. 85. Dullemeijer, P.: Craniofacial biology: h zoologist’s ricxw, AWER. .T. ORTHOWJNT. 59: 19-23, 1971. Sennior author: Forsyth Dental Center 140 The Fenway I3oston,

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It should be obvious to the student that orthodontia is not a mechancial art as popular rumor would have it, but is largely a biological problem. However it does not necessarily follow that mechanics has no part in it, for such an inference would be as far from the truth as to say that it was wholly mechanical. Just as in orthopedics and in other branches of medicine and surgery, mechanics and mechanical means must be used, so in orthodontia we must utilize mechanical means to influence and control certain physiological processes which in turn make possible the normal development and functional perfection of the teeth and their correlated parts. (McCoy, James David: Applied Orthodontia, ed. 3, Philadelphia, 1931, Lea 8, Febiger, p. 58.)