Orthodontic Implications of Growth and.pdf

Orthodontic Implications of Growth andDifferently Enabled Mandibular Movementsfor the Temporomandibular JointRakesh Koul

Differently enabled functional movements of the mandible and different

types of maxillomandibular and occlusal relations may share a cause-and-

effect relationship with the disorders affecting the temporomandibular joint

(TMJ). The purpose of this article is to draw inferences with orthodontic

implications for the TMJ from an overview of adverse factors for growth and

biomechanics of the TMJ, dentofacial characteristics associated with tem-

poromandibular disorders, and mechanism of action of orthodontic inter-

ventions affecting the TMJ. Inferences drawn include the importance of

history taking, functional evaluation and the need for radiological evaluation

of TMJ condyle and disk, and position and function during procedures that

are expected to interfere with TMJ homeostasis, for example, surgical

craniofacial corrective procedures, functional therapy, and occlusal recon-

structive procedures. Extremes of form (eg, excessive overjet and overbite,

open bite and deep bite, skeletal hyperdivergence and hypodivergence) and

differently enabled mandibular functions resulting in overloading of TMJs

are all potential factors in the etiology of its disorders, thus enhancing the

need for its evaluation before, during, and after treatment; a reciprocal

relationship exists between growth and biomechanics of the TMJ, dentofa-

cial characteristics and articular afflictions, occlusion and TMJ, and mandib-

ular movements and TMJ. These interrelated, interdependent, and/or coex-

istent factors have a bearing on the diagnosis and treatment of the disorders

of the TMJ. Orthodontic therapy should be directed to achieve a structural

balance to facilitate physiologic adaptation and rehabilitation. Because the

movements of the mandible are not restricted by the joint structure per se,

other operative templates, for example, neuromuscular and psychological,

apart from the structural template, contribute significantly to its complex

functions and pathology. There is a need to find optimum values of structure

and function of the masticatory system and develop mechanisms that can

record and reproduce highly accurate geometric models of a subject’s TMJ

and teeth combined with recordings of chewing trajectories and 3-dimen-

sional TMJ movements to obtain subject-specific models of masticatory

system by either improving upon conventional mechanical articulators or by

application of virtual-reality techniques for the development of virtual artic-

ulators for diagnosis and treatment of the disorders of masticatory system.

(Semin Orthod 2012;18:73-91.) © 2012 Elsevier Inc. All rights reserved.

Professor, Department of Orthodontics and Dentofacial Orthopedics, Career Post Graduate Institute of Dental Sciences, Lucknow, India.Address correspondence to Rakesh Koul, MDS, Professor, Department of Orthodontics and Dentofacial Orthopedics, Career Post Graduate

Institute of Dental Sciences, Sitapur Bypass, Near IIM Lucknow, Lucknow, India. E-mail: [email protected]© 2012 Elsevier Inc. All rights reserved.1073-8746/12/1801-0$30.00/0
doi:10.1053/j.sodo.2011.10.004
73Seminars in Orthodontics, Vol 18, No 1 (March), 2012: pp 73-91

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74 Koul

T he degree, direction, and duration of func-tional mandibular movements depend on

he interactions in and between different typesf operative templates, that is, structural, phys-

ologic, neuromuscular, psychological, androwth. These movements are made possible byhe temporomandibular joint (TMJ), which isapable of both rotational and translationalovements. Rotational movement occurs be-

ween the condyle and the inferior surface of theisk during early opening (the inferior jointpace), and translation takes place in the spaceetween the superior surface of the disk and theossa (the superior joint space) during laterpening. The mandible has 3 primary functions:peech, mastication, and swallowing. Althoughhe mandibular movements are 3-dimensional inharacter, the direction of primary functionalovements is up and down and there are only

lightly lateral and protrusive excursions duringunction. Because of the viscoelastic propertiesf the disk, the articular load is dispersed over a

arger surface area, the size of the disk.1 Anothermportant feature is the presence of fibrocarti-age in the TMJ,2 whereas other synovial jointsre covered by hyaline cartilage. Advantages ofbrocartilage in the TMJ over hyaline cartilagere that it provides more strength against forcesn many directions than would be possible withyaline cartilage, particularly against shear

orces, and it has a better ability to repair.3,4 It isow generally accepted that mechanical loading

s essential for growth, development, and main-enance of living tissues.5 The envelop of masti-ation is a vertical tear drop with lateral move-ent of 5-6 mm during first phase of crushing;

s the teeth approach, the lateral displacementessens to 3-4 mm from the starting position.6

Normally, this involves the rotation of condylesaround a horizontal axis in combination with aforward and downward gliding movement incontact with the lower surface of articular disk,limited by its posterior attachment, until thecondyle articulates with the most anterior part ofthe disk and the mouth is fully open. Duringclosing, the sequence is reversed. The characterand nature of mandibular movement changes ifthe operating templates are not normal; func-tions are performed to the degree that the adap-tive plasticity of the operating templates permits.Gross and micro changes in the operative tem-
plates induce micro and/or macro changes in
the functional movements of the mandible, andvice versa. Biomechanical factors resulting fromfunctional activity of the mandibular joint arethought to influence the growth of the condylarcartilage. To elucidate the nature and mecha-nism of this influence, a number of investiga-tions have been undertaken in different animalmodels. Alteration of biomechanical forces andapplication of abnormal forces through altera-tions of normal function of the TMJ have been 2major experimental approaches. This research iscited as evidence that the condylar growth ismodulated in response to both protrusion andretraction of mandible.7-10 Because the condylarartilage is responsive to changes in the move-ents of the mandible, biomechanical altera-

ions by orthodontics are used to effect a cor-ective change in dentition, TMJ, and jaws.emporomandibular disorders (TMD) encom-ass a wide spectrum of changes in the operativeemplates and the functional movements of the

andible. Whether and to what degree thesehanges indicate potential disorders of the TMJs a matter of further investigation. This articleresents an overview of risk factors and dentofa-ial characteristics associated with the TMJ dis-rders. A rationale for orthodontic interventionor correction of disorders of dentofacial growth,tructure, and functions that may share a cause-nd-effect relationship with the TMJ disorders wille discussed, and inferences of importance to or-hodontics will be deduced. Elucidation of devel-pmental events leading to the formation of theMJ will provide an understanding of the contri-ution(s) of each of the TMJ components in theathogenesis associated with it and provide a ra-

ionale for therapeutics.

Developmental Events Leading to theFormation of the TMJ

During human embryologic development, 2 sets ofarticulation form between the cranium and themandible. The first articulation forms from thecellular elements of Meckel’s cartilage and the firstbranchial arch serving as a hinge articulation until16 weeks of postnatal life, and this ultimately be-comes the joint between incus and malleus.11 Thesecond articulation develops from condensed mes-enchyme located lateral to Meckel’s cartilage be-ginning at 6 weeks of development. This structure
develops into complex articulation that is charac-

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75Orthodontics and Temporomandibular Joint

teristic of the human TMJ.11 The condylar carti-lage is derived from condylar blastema and thentransferred to the mandibular bone as a second-ary cartilage.12 The mandibular condyles repre-ent important growth sites within the facial skel-ton. Condylar growth is not a pacemaker ofandibular development, but it provides re-

ional adaptive growth, as the condyle’s upwardnd backward growth movement regulates thenteriorly and inferiorly directed displacementsf the mandible as a whole.13 Because of evolu-ionary origin, the endochondral growth of theondyle is appositional, and the adaptive growthapacity of the condyle is highly dependent onhe articular function of the joints.13,14 In the

orphogenesis of the condylar head, severalnique features must be considered. The embry-nic zone of condylar cartilage persists through-ut life, opening the possibilities to reactivatehe growth potential of this cartilage at any timend thereby increase mandibular growth.15

Growth and differentiation of condylar cartilageis regulated by local growth factors,16,17 such asvascular endothelial growth factor (VGEF) andinsulin-like growth factors I and II (IGF I and II),and changes in the cartilage’s local environmentupregulate or impair their endogenous expres-sion, leading to increased or decreased condylargrowth.18,19 The condylar cartilage is a second-ry fibrocartilage derived from the periosteumf the membranous mandibular bone; becausef the functional demands of evolutionary devel-pment, its histology reflects the functionaleeds of mandibular movement.20 Unlikepiphyseal growth cartilage of the long bones,he mandibular condylar growth cartilage has anique capacity for adaptive remodeling in re-ponse to external stimuli, both during and afteratural growth.19 Purcell et al21 concluded that

ormation of the TMJ requires 2 distinct hedge-og-dependent steps and that the TMJ is anique synovial joint not only in terms of itstructure but also in terms of the developmentalenetic pathways that govern its formation. Thisan be deduced from the fact that condylar car-ilage is not affected by gain-of-function muta-ions in fibroblast growth factor receptor 3FGFR-3) gene (a negative regulator of chon-rocyte differentiation in bones of primary car-

ilaginous skeleton) that cause achondroplasia
n humans.22,23 d
In the development of the TMJ, externalpterygoid muscle and its surrounding connec-tive tissues play an active role in outlining thefuture condylar process by forming the articulardisk and providing the source of fibrous coverfor condylar cartilage.11 The muscle attachmentto the condyle is also unique because it runs intothe bone of condylar head contrary to thosemuscles that attach solely to the fibrous layer ofperiosteum. This mechanism is of functional sig-nificance, as muscular tissue resists changes inlength to a greater degree than in any bonytissue. Therefore, muscle dysfunction of longstanding might not be apparent within the mus-cle itself, but rather result in adaptive bony re-modeling.24,15

Three phases of condylar growth discernedfrom the function of condylar cartilage duringgrowth and development have been describedby Copray et al as follows25: –phase I (embryonalnd early postnatal), phase II (childhood anddolescence), and phase III (post active growtheriod). In phase I, the condylar cartilage func-ions mainly as a growth cartilage, and the artic-lar function is subordinate. In phase II, therticular function becomes dominant over activerowth function. In phase III, the condylar car-ilage functions almost entirely as an articularartilage. Any imbalance between form andunction can result in adaptation and remodel-ng,26 during second phase, by stimulation ornhibition of proliferation and matrix synthesisy altered forces27; however, in third phase, this

imbalance can lead to TMD.28 The loss of carti-age on the articular surface, which must occuro allow a regression of contour and reductionf the articular tissue to its usual thickness, can-ot be explained by inadequate nutrition andetabolic factors; rather, such changes were at-

ributed by Blackwood29 to mechanical factorssuch as increased attrition and wear. Articularcartilage thickness varies in response to differenttypes of loading30-32 or even disappears at com-

lete immobilization.33 Loading in the TMJ maystimulate remodeling, resulting in increased syn-thesis of extracellular matrices.34 Remodeling of

ony skeleton is continual throughout the life ofn individual and occurs in response to altera-ion in the mechanical equilibrium of its skele-on and musculature and to changes in the met-bolic functions of the body. Such changes are
irected mainly toward the maintenance of con-

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gruity between opposing articular surfaces and ismediated through the proliferative activity ofarticular cartilage, a fact that has been recog-nized by Ogston35 as long ago as 1875. Adaptiveremodeling is slow morphologic change thatpermits alteration of joint components to main-tain adequate joint function.26 This processakes place as long as the functional demands doot exceed the adaptive capacity of the joint.nce the adaptive capacity of the joint is ex-

eeded, maladaptive tissue reactions and jointegradation develop.36 There is evidence to con-lude that the reason for impaired mandibularrowth, as a sequel to intra-articular afflictions, isegeneration of condylar cartilage and subse-uent erosive destruction of the condylarone.37-39

TMD and Facial Morphology

A basic tenet of craniofacial growth and devel-opment is that the individual growth of all com-ponents of the face constantly interact, workingtoward a functional and structural balance. Ifgrowth is disturbed in any part of the craniofa-cial complex, the physiological and structuralequilibrium changes as well.13 Features of facialskeleton that may share a cause-and-effect rela-tionship with the development of disturbancesof the TMJ through adverse changes in growthand function, and have implications for orth-odontic diagnosis and treatment, are discussedlater in the text.

Two distinct types of facial form have beencharacterized: the skeletal deep bite and theskeletal open bite. Schudy40 used the terms hy-podivergent and hyperdivergent, respectively, todescribe these facial types, the latter of whichhas also been referred to as long-face syndrome.The cause of these skeletal discrepancies is usu-ally related to positional and/or size variationsof the maxilla, the mandible, and/or the cranialbase.41 Hyperdivergent cases present a greaterondylar distraction and consequently present areater difficulty in achieving an optimumeated condylar position.42 These facial ex-remes commonly manifest clinically as dispro-ortionalities between certain facial dimensions,articularly the upper and lower face heights.43

Longitudinal studies by Nanda44 indicate thatthe fundamental difference between open bite
and deep bite faces is found in the anterior
segments of the face rather than in variations ofposterior facial dimensions. For example, sub-jects with an open bite have an increased lowerface height relative to upper face height. Incontrast, subjects with a deep bite generally havean increased upper lower face height relative tolower face height. The unfavorable anterior/posterior facial height ratio predisposes togreater condylar distraction (especially in thevertical dimension) to bring anterior teeth tofunctional contact. Some patients within the hy-perdivergent group presented with an anterioropen bite and minimal condylar distraction,42

and it was proposed that such patients wouldhave registered a greater condylar distractionhad the mandible been closed over the molarteeth for incisal contact. Anterior open bitelikely prevailed instead of significant condylarshift. Anterior open bite has been associatedwith compromised health and/or stability of thegnathic system.45,46 The nature of stress distribu-ions in the TMJ is substantially affected by ver-ical discrepancies of the craniofacial skeleton.n a study to investigate stresses in the TMJuring clenching in patients with skeletal dis-repancies in vertical direction, strain patternsithin the TMJ were shown to increase in sub-

ects with increased vertical facial height, partic-larly those with high mandibular plane an-les.47

The unfavorable anterior/posterior faceheight ratio of hyperdivergent facial pattern pre-disposes to greater condylar distraction (mostlyin the vertical dimension) to bring anteriorteeth to functional contact. It has been hypoth-esized that displacement of the condyle awayfrom the eminence may be detrimental to jointhealth and/or stability because there is subse-quent loss of juxtaposition between the condyle,disk, and eminence.48,49 The increased intra-articular space may predispose to internal de-rangement, through mechanical posterior dis-placement of the condyle50-52 and/or through

yperactivity of the superior head of the lateralterygoid muscle.53

In a cephalometric study on patients withinternal derangement, Stringert and Worms54

found a greater number of patients with internalderangement (ID) having “high plane” charac-teristics but found no association between occlu-sal characteristics and internal derangement.
Further, in subjects with low angle, Bacetti et al55

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found the position of glenoid fossa in relation tobasicranial structures to be more caudal than insubjects with high angle or normal vertical rela-tionships. Brand et al56 did not find a significantrelationship between ID of the TMJ and mor-phologic characteristics of the face, and theirresults indicated that patients with internal de-rangements had smaller mandibles and maxil-lae. Some studies57 indicate that ID in adoles-cent subjects may be associated with certaincraniofacial features: reduced ramal and poste-rior facial heights, dentoalveolar adaptation inposterior maxillary regions with reduced maxil-lary molar to palatal plane heights, and in-creased mandibular and palatal plane angles rel-ative to sella nasion. In a study on Class IIIsubjects, Muto et al58 found patients with disk

isplacement (DD) had higher gonial anglesnd/or sella-nasion–mandibular plane angles.n a study on preadolescent subjects with Class IIalocclusion and vertical or horizontal growth

eficiency–correlated condylar characteristicsnd facial morphology, Burke et al59 found con-

dylar head inclination and superior joint spaceto be significantly related to facial morphology.Patients with vertical facial morphology exhib-ited decreased superior joint spaces and poste-riorly angled condyles, whereas patients withhorizontal facial morphology demonstrated in-creased superior joint spaces and anteriorly an-gled condyles. Gidarakou et al60 compared skel-tal and dental characteristics in asymptomaticnd symptomatic subjects with bilateral DD witheduction. A decreased length of anterior andosterior cranial bases and reduced sella-nasion-oint A and sella-nasion-point B angles werebserved in the symptomatic group. An in-reased interincisal angle and more retroin-lined upper incisors were observed in theroup with bilateral DD with reduction. In an-ther study, Gidarakou et al61 evaluated the ef-ects of bilateral degenerative joint disease onhe skeletal and dental characteristics of symp-omatic and asymptomatic female subjects. It washown that the maxilla and the mandible wereetruded with a clockwise rotation of the man-ible. The mandibular plane angle, Y-axis, go-ial angle, and lower facial height were in-reased, whereas ramal height was decreased,uggesting a clockwise rotation of the mandible.n another study on female subjects with unilat-
ral DD without reduction (DDNR), Gidarakou
t al62 found a decreased ramal height, a steeperandibular plane angle, and relative infra-erup-

ion of the mandibular first molar. In femaleubjects with unilateral DD with reduction, Gi-arakou et al62 found decreased anterior andosterior cranial base lengths and increased cra-ial base angulation. Both upper and lower den-

ure bases were retroinclined. Posterior ramaleight was decreased in the symptomatic group.n a study by Byun et al63 on the relationshipetween ID of the TMJ and dentofacial mor-hology in women with anterior open bite, pos-eriorly rotated mandibular ramus, a smaller

andible, and a greater tendency for a skeletallass II pattern, ID of the TMJ was much morerevalent. These patterns were more severe be-ause the ID progressed to DDNR. They con-luded that some cephalometric characteristics,uch as a decrease in posterior facial height,ecrease in ramus height, and backward rota-ion and retruded position of the mandible, aressociated with TMJ ID in women with anteriorpen bite. ID of the TMJ can cause facial asym-etry. In a study to examine relationship be-

ween TMJ ID and facial asymmetry in women,n which the influence of condylar hyperplasian facial asymmetry was eliminated by selectingnly those subjects who had sella-nasion-point Bngles �78 degrees, Ahn et al64 found that sub-ects with TMJ ID of greater severity on thenilateral side had shorter ramal height com-ared with those with bilateral normal disk, bi-

ateral DD with reduction, or bilateral DDNR. Inddition, the mandibular midpoint deviated to-ard the side where the TMJ ID was more ad-anced. It was concluded that subjects with aore degenerated TMJ on the unilateral sideight have facial asymmetry that does not come

rom condylar or hemimandibular hyperplasia.n a study to discriminate ID of the TMJ byateral cephalometric analysis, Ahn et al65 re-

ported backward positioning of the mandible,clockwise rotation of the mandible, proclinationof the mandibular incisors, and an increase inoverjet, which intensified gradually with the pro-gression of TMJ ID; the subjects with bilateralDDNR showed the greatest changes in dentofa-cial morphology. Their results revealed thatsmaller mandibular incisor to Frankfort horizon-tal plane angles and larger overjets had highpossibilities of TMJ ID. They concluded that
some cephalometric variables can be used as an

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auxiliary diagnostic tool to help identify patientswith potential TMJ ID.

Kjellberg66 reported that in juvenile rheuma-oid arthritis showing destruction of the TMJ,he dentofacial morphology was characterizedy overall smaller dimensions of the mandible,andibular retrognathia, a steep mandibular

lane, Class II malocclusion, dental crowding,nd frontal open bite. In another study by Lar-eim and Haanaes67 on patients with juvenileheumatoid arthritis, severe radiographic changesere characteristic features in all the patients.andibular size (gn-ar distance) was significantly

maller in patients with complete destruction ofhe mandibular head than in those with partialestruction, indicating a causal relationship be-

ween mandibular underdevelopment and arthri-is of the TMJ. Flat fossa, anteroposition of rem-ants of the mandibular head, and reducedondylar movement at maximum opening of theouth were found. Stoustrup et al68 summed up

the skeletal and dentoalveolar characteristics injuvenile idiopathic arthritis growth disturbances:reduction of posterior facial height, retrog-nathism, increased mandibular inclination andjaw angle, antegonial notching, and an anterioropen bite with an increased horizontal overjet.The maxilla is affected by a decrease in verticaldevelopment.

Aberrant facial characteristics such as verticaldiscrepancies (excessive discrepancies betweenanterior and posterior facial heights and be-tween upper and lower anterior facial heights),displacement of glenoid fossa, anterior openbites and deep bites, smaller gnathic dimen-sions, steep mandibular plane angles, large over-jets, and horizontal discrepancies such as facialasymmetries can be probably predictive, but notdefinitively diagnostic, of the disorders of theTMJ.

More Adverse Factors in Growth andBiomechanics of the Joint

Associations between certain occlusal featuresand TMD have been mentioned in many re-ports. A decreased vertical overlap and an in-creased horizontal overlap have been associatedwith disorders of the TMJ.46,69-72 A significantassociation of TMD with unilateral crossbite andmidline displacement has also been reported.71

Abnormal overbite and overjet may be associ- m

ated with more extensive deviation in the tem-poral and condylar form; particularly, whencombined with age, it gives credence to the hy-pothesis that longer exposure to malocclusionmay be associated with more extensive TMJchanges.73 Several studies have reported more

MD in skeletal Class II than in other dentofa-ial deformities.46,72,74-77 However, some studiesave shown no association between condyle po-ition and untreated Class II deep bite malocclu-ions.78 O’Ryan and Epker79 have presented thatentofacial deformities and malocclusions may

ead to adaptive changes within the TMJ. John etl80 concluded that wide ranges of overbite andverjet are compatible with a normal function ofasticatory muscles and TMJ. There are other

tudies that have failed to confirm significantelationships between TMJ or muscle tendernessnd Angle’s classification or any occlusal contactelationships, or between functional occlusal re-ationships and TMD.81-83 Patients with “longace” tend to have occlusal contacts on the non-orking side during mandibular excursions andre at a risk of developing nonworking-side func-ional occlusal interferences.84 Functional inter-erences are considered to be important in theirssociation with TMD.85,86 In addition, contraryo ideas that ideal functional occlusions main-ain the TMJ,87 evidence exists that suggests oc-

clusal guidance patterns are not associated withTMD.88,89 Evidence for functional occlusion to

e important for TMJ homeostasis is not defini-ive.90,91

Effects of experimental overloading of jointby occlusal abnormalities on condylar cartilagehave been reported, including degradation inrat condylar cartilage accompanied by an in-crease in chondrocyte death.92 A decrease invertical dimension of occlusion may also be as-sociated with ID of the TMJ. Loss of verticaldimension of occlusion may be due to attritionof dentition or loss of posterior teeth. A positivecorrelation between tooth wear and TMD hasbeen demonstrated.93,94 However, in a study bySchierz et al,95 anterior tooth wear was not asso-ciated with self-reported TMD pain. Similarly,positive correlation between missing posteriorteeth and TMD has been reported, and thiscondition may act as a perpetuating and accel-erating factor.96-100

Schellas101 hypothesized that TMJ pathology
ay be the cause of malocclusion, rather than

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vice versa. Reynders102 and Seligman and Pull-inger89 concluded that there existed no scien-tific evidence for a causal relationship betweenocclusion and TMD. According to Pullinger andSeligman,103 combinations of occlusal variablesppear to be TMD specific. They found somextreme ranges of occlusion were the domain ofatients with TMD, but occlusions in most pa-

ients were within normal ranges. It was demon-trated by Takayama et al104 that the symptoms

of TMD correlated with age, sex, and dental andocclusal conditions. However, the prevalence ofbone change in the condyle correlated poorlywith these variables in patients with or withoutTMD.

Associations between orthodontic treatmentand TMD105 are based on assumptions that re-traction of upper anterior teeth and extractionsresult in enforced mandibular distalization anda posterior condylar displacement. Wadhwa etal106 studied 3 patient groups: one with normal

cclusions, one with untreated malocclusions,nd one with orthodontically treated malocclu-ions. They concluded that the role of orthodon-ic treatment in either the precipitation or therevention of TMD remains questionable. Aeta-analysis to examine the relationship of or-

hodontics with TMD concluded that traditionalrthodontic treatment did not increase the prev-lence of TMD.107 In the long term, there is noelationship between the prevalence of signs andymptoms of TMD and previous orthodonticreatment.108,109

Orthopedic terminology defines ID as inter-ference of the normal smooth action of a jointby intra-articular tissue. The most commoncause of TMJ ID is displacement of the TMJdisk.110,111 It has been suggested that overload-ing the joint can cause DD.112 The orthopedicproblem of a displaced TMJ disk without reduc-tion is known to affect the growth of the mandi-ble—unilateral62,113 and bilateral39,63, resultingn dentofacial asymmetry and retrognathia.

hether the adverse craniofacial growth predis-oses for TMJ DD, or vice versa, remains unclearntil the cause-and-effect relationship is estab-

ished in longitudinal experimental studies. Dis-lacement of the disk can occur in differentirections, but the most common are anteriornd anterolateral.114 Nonreducing DD is associ-

ated with impaired mouth opening ability and in-
flammatory and degenerative reactions in the
joints, and it is frequently associated with pain anddysfunction of the masticatory apparatus.115-117 Atudy by Ribeiro et al found the prevalence ofD in asymptomatic children and young adults

o be 34%, whereas 86% symptomatic TMD pa-ients had DD. Their study reported that 13.8%ad bilateral symptomatic but normal joints,8% had unilateral DD, and 58% had bilateralD.118 Bilateral affliction was reported in about0% of both symptomatic and asymptomaticroups.118,119 Because bilateral nonreducing

TMJ DD induces retrognathia, the prevalence ofnonreducing DD ought to be higher in subjectswith mandibular retrognathia, and this is sub-stantiated in part by a study by Link and Nicker-son,120 which reportedly found 88% of 33 pa-ients who had undergone orthognathic surgeryor Class II malocclusion had bilateral DD. In atudy of children with Class II malocclusion, pre-reatment DD frequencies of approximately 5%,2.5%, and 7.5% for medial, lateral, and ante-ior displacements, respectively, were revealedn asymptomatic children.121 Schellas et al,122 in

a study on pediatric population, reported that93% of 60 patients with mandibular deficiencyhad a displaced disk, usually the nonreducingdisk. Studies investigating the prevalence of DDin patients with painful joints reported a preva-lence of 77%-94%.118,123,124 However, an ante-rior or posterior disk position also can be pres-ent in asymptomatic subjects.114

It has been shown that the altered biome-chanics of the TMJ, following a faulty function-ing TMJ disk, causes histologic reactions of themandibular condylar cartilage.125-127 Age, apartrom systemic and local factors, appears to play aignificant role in the severity and progression ofartilage changes.127-131 Bryndahl39 pointed to a

reparative compensation for an extensive re-sorption of subchondral bone due to displace-ment of the TMJ disk. This reparative compen-sation was not sufficient for the maintenance ofnormal growth. Nebbe et al132 showed that func-ional alterations due to temporomandibularisk dynamics are an important factor whenorecasting craniofacial growth in orthodonticreatment planning.

An association between a posterior condyle po-ition and anterior DD has been reported.133-135 A

study on asymptomatic volunteers with normaldisk position evaluated condyle position and
compared it with different stages of ID.133 It

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concluded that the condyle positions of TMJswith normal disk position are distributed ran-domly, although posterior condylar position wasmore prevalent in anterior DD. However, a pos-terior condyle position cannot be interpreted asa diagnostic sign for internal derangement, asanterior or centered condyle positions also areoften seen in patients with ID.133 Nonetheless,according to Bonilla-Aragon et al,136 a posteriorposition of the condyle has a higher prevalencein symptomatic patients than in asymptomaticvolunteers.

Factors for dysfunctional articular remodel-ing leading to TMJ disorder can be classifiedunder broad headings of macrotrauma due toexternal source of injury (eg, blow to the man-dible, whiplash, mandibular hyperextension)and microtrauma due to forces that overload thejoint complex or disturbed normal relationshipof the condyle disk and eminence (eg, parafunc-tional overloading, unstable occlusion, and in-creased joint friction); these factors may occuralone or may be interrelated, interdependent,and/or coexistent.131,137,138 Both macro- and

icrotrauma can be acute and/or chronic.ther factors include developmental and ac-uired defects that can alter the structural integ-ity of TMJ components, such as hypoplastic andyperplastic condyles, and disorders of condylarovement along with various skeletal facial

symmetries. Various systemic illnesses, hor-onal and nutritional disorders, and tumors,hich affect the host adaptive capacity, can in-olve the TMJ, leading to TMJ dysfunction. Be-ause the functional condition of the joint isssential for the differentiation and mainte-ance of the condylar cartilage, trauma to the

ace and jaw during growing years has been de-cribed as a disturbance to growth and develop-ent of the mandible.139-142

Mechanical fatigue is the result of tractionalforces and compressive stresses applied repeat-edly to the cartilage surface.143 Magnitudes oftractional forces imposed on articulating sur-faces are affected by the compressive stress dis-tribution over the cartilage surface because apart of total compressive force is supported bytangential forces on the cartilage surface duringloading.144 Factors that increase compressivetresses on the TMJ disk during loading includeecreased cartilage thickness and decreased
ongruency between articulating surfaces.145,146
Compressive mechanical stress was observed toenhance osteoclast formation through inflam-matory cascade reactions, and continuous com-pressive force may induce osteoclastic bone re-sorption in the TMJ.147 Intra-articular disorders,such as inflammatory disease of the TMJ, areknown to result in adverse mandibulargrowth.37,148-150

Factors that result in altered biomechanics ofthe masticatory system and consequently elicithistologic changes in the condylar cartilage canrange from altered dentofacial relationships, in-cluding occlusal relations, to abnormal relation-ships of TMJ components. These abnormal rela-tionships, in turn, can have their etiology ingenetic, epigenetic, and/or environmental fac-tors. Altered biomechanics will result in non-physiologic stress that can result in dysfunctionalremodeling of the TMJ and cause adverse histo-logic alterations. Dibbets and Carlson151 ob-served in 1995, and their observation is still rel-evant, that the influence of TMJ pathology,myofacial disorders, and disk interferences onfacial growth are areas in need of further inves-tigation; further, because condyle plays a prom-inent role in mandibular growth and facial de-velopment, categories of TMD that involvedysplasia of condylar cartilage could be associ-ated with aberrant facial growth and form.

Mechanism of Action of OrthodonticInterventions Influencing the TMJ

Two types of intraoral appliances are indicatedfor the treatment of disorders affecting the TMJ:one type allows the mandible to function withnociceptive input from the nonspecific occlusalcontacts, for example, bite appliances/planes/splints, and the other type provides specific andguided occlusal schemes that encourage man-dibular repositioning, for example, mandibularocclusal repositioning appliances and myofunc-tional appliances.

The effectiveness of intraoral appliances can beattributed to any of the following mechanisms:maxillomandibular realignment, neuromuscularadaptation, occlusal disengagement, TMJ reposi-tioning, restored occlusal vertical dimension, cog-nitive awareness and placebo,152,153 or a combina-tion of any of these mechanisms.

Although the occlusal splints are of limited
value and only adjunctive treatment modalities

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with unestablished efficacy,152 they can be usefulas habit management aids and protect the den-tal/periodontal structures against some of theadverse effects of prolonged overloading. Eluci-dation of the multifactorial etiology and progres-sion of the TMD; improvements in experimentalresearch designs, which simulate the clinicalconditions; technical improvements of the in-struments (for electromyography, intra-articularpressure (IAP) measurement, radiography) thatevaluate therapeutic effectiveness; standardiza-tion of treatment outcome measures; and sup-plementation by long-term follow-up data arerequired to establish the clinical efficacy, or oth-erwise, of occlusal splints. Kriener et al154 con-luded that the literature evidence is sufficientor the use of occlusal appliances in managingocalized masticatory myalgia and arthralgia oroth. However, if the behavioral modification oflenching is not corrected, even the best splintill not be effective. A need for randomizedontrolled trials that pay attention to method ofllocation, blind outcome assessment, sampleize, and duration of follow-up for evaluation ofhe effectiveness of stabilization splints was sug-ested by Al-Ani Z et al,155 although their studyoncluded that the stabilization splint therapyay be beneficial for reducing pain severity at

est and on palpation and for improvement inhe level of depression, when compared with noreatment.

The precise mode of action of functional ap-liances for growth guidance is obscure. Know-

ng the mechanism of modifying mandibularrowth by functional appliances has importantlinical implications. Removal of restraininguscular forces by myofunctional appliances

onditions the patients’ musculature to sustainn altered, favorable mandibular position andubsequent shift to a structure-controlled posi-ion by growth and remodeling of the TMJ.156

That increased neuromuscular activity promotescondylar growth is questioned by electromyo-graphic studies, which show no change or de-creased transient masticatory muscle activity, in-cluding that of lateral pterygoid muscles, withfunctional appliances.157-162 Clinical results ofunctional jaw orthopedics can be attributed notnly to the repositioning of teeth by creatingew reflexes in the perioral musculature but also

o stimulation of condylar growth and remodel-
ng. After applying a constant retracting force on
he mandible of young (age, 14-23 months) ma-aques for 140 days, Janzen and Bluher7 re-orted resorption at the posterior surface ofondyle and posterior wall of the glenoid fossand apposition at the anterior surface of theondyle. The experiments of Melanson and Vanijken163 have shown that concomitant with the

regaining of growth activity after glenoid fossaextirpation, the absence of functional loadingresulted in a remodeling of the condyle frommature functionally mediolaterally skewed ap-pearance to the round embryonal shape. In anin vitro system, Copray et al10 could initiate op-posite shape in growing condylar cartilage bysidelong application of biomechanical stimuli.Petrovic et al8 reported an increase in the thick-ness of the articular disk and prechondroblastic(resting) and chondroblastic (proliferative andhypertrophic) zones after anterior displacementof the mandible in young rats. The increasedactivity of the lateral pterygoid muscle was re-flected in its decreased length and hypertro-phied fibers. Displacing the mandible backwardby means of a chin cup is followed by a decreasein the thickness of prechondroblastic and chon-droblastic zones and an increase in the length oflateral pterygoid. In these experiments, forcewas applied 8-12 hours a day for 1, 2, and 4weeks, simulating the force application of orth-odontic treatment. Kantomaa164 has demon-strated that an altered position of the condyle inthe fossa and subsequent altered loading canchange the shape of the condyle. Using artificialcraniosynostosis, Kantomaa provoked an in-creased posterior displacement of the glenoidfossa in young rabbits. The resorptive process atthe anterior aspect of the condyle appearedmore rapid than the apposition at the posterioraspect. Despite the increased condylar growth,the condyles were located more anteriorly andinferiorly in relation to the fossa from the fifthpostoperative day onward. A subsequent shiftingand thickening of the condylar cartilage to thenew side of compression was observed.

In general, growth redirection in response tocontinuous condylar distraction remodels theglenoid fossa anteriorly, and the condylargrowth is redirected in a more posterior andsuperior direction; furthermore, in response tocontinuous retraction, resorption at the poste-rior surface of condyle and posterior wall of
glenoid fossa and apposition at the anterior sur-

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face of the condyle occur.8,10,164-168 Increase inandibular length is thought to be due to in-

reased condylar growth.169 Dentoalveolarchanges associated with functional appliances in-clude the labiolingual tipping of incisors and ver-tical manipulation of the occlusal plane.170-175 Thebone remodeling in the TMJ after functional ap-pliance treatment is also not consistent.121

Alterations in gene expression following pro-trusive function have been reported, and integ-rins have been found to be important mediatorsin this response.176-179 It has been shown, in a rat

odel, that transcription factor Sox-9 and itsarget gene (type II collagen) and X collagen arepregulated in the glenoid fossa, and cell-signal-

ng molecule Indian hedgehog expression is in-reased in the cells of proliferative zone anddjacent chondroblasts, following mandibularorward positioning,178,180 coinciding with an in-

crease in cell proliferation within the prolifera-tive zone. Immunohistochemical analyses byMarques et al179 demonstrated that the use ofthe propulsor appliance for different periodsmodulated the growth of the rat condylar carti-lage and that arginine–glycine–aspartic acid-binding integrins participate in mechanotrans-duction.

Petrovic et al8 reported that growth at theondylar cartilage results via a mechanism thatepends on “messages of local origin” and that

he “co-ordination of masticatory apparatus” de-ends on a regional, structural homeostasis.athews181 questioned the stimulation of the

ondylar cartilage growth by Class II mechanicseyond its genetic potential. He said that al-hough the orthodontist may not be able to stoprowth of the mandibular condyle, this does notean he is unable to modify and perhaps coor-

inate growths of the dentoalveolar processes ofoth dental arches during orthodontic treat-ent. He also concluded that the changes inMJ articulation occurring during the growtheriod of the child undergoing orthodontic

reatment occur not because of but rather de-pite any particular mechanics being used. Inonclusion, the crucial element in most orth-dontic appliances is their capacity to produce

ong-term change in condylar position and, witht, the ongoing pattern of functional loading,ith the condylar cartilage providing an adjust-ent mechanism essential to the health and

ntegrity of the TMJ.

Inferences and Observations WithOrthodontic Implications

Mandibular repositioning is indicated not onlyin some primary intracapsular cases but also insome primary extracapsular cases. The purpose ofrepositioning appliances in many cases is to restorepotentially lost vertical dimension and/or removefunctional interferences and also to provide struc-tural stability and thereby promote neuromuscularefficiency and normative growth guidance. Cau-tious interpretation of clinical results is required ifthese are not drawn from stringently designedstudies. Loading in the TMJ may stimulate re-modeling, which is an essential biological re-sponse to normal functional demands, ensuringhomeostasis of joint form, and functional andocclusal relationships. However, excessive or sus-tained physical stress to the TMJ, exceeding thenormal adaptive capacity, can lead to degrada-tion and deterioration of the TMJ articular struc-ture. IAPs, when high and prolonged, have apotentially harmful effect on TMJ homeostasis.Nitzan182 reported that females generated signif-icantly higher IAP than males and that IAP inthe TMJ was significantly reduced after place-ment of an interocclusal appliance to uniformlyelevate the occlusal plane. Even if the stress tothe TMJ is within the normal range, degenera-tive changes can occur when a decreased adap-tive capacity of the articulating structures of thejoint is present. The latter can be associated withgeneral conditions, such as advancing age, aswell as with systemic illness and hormonal fac-tors. Convincing evidence exists that DD often,but not always, is responsible for the mechanicalsymptoms seen in patients with TMJ pain anddysfunction, but DD also may be seen in asymp-tomatic persons. Improvement of symptoms, forexample, joint sounds, as an outcome variable tomeasure the success of splint therapy for DD isquestionable, as displaced disks visualized witharthrography in the absence of joint sounds havealso been reported.183 Positive clinical outcomein the presence of an unchanged position of thedisplaced disk has also been reported.184 Treat-ments are necessitated by the fact that DDs, ifleft untreated, would lead to increased risk ofdeveloping pain, impaired mobility, degenera-tive joint diseases, avascular necrosis, and conse-quently, condylar degeneration and dentofacial
deformity.122,150,185-187 Bite jumping therapy in

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TMJ DD has positive therapeutic effects on his-tology and positioning of the condyle, whoseforward positioning is likely to recapture thedisk, and limit the retrusive translation to pre-vent the disk from redisplacing.188,189 Kinzingeret al,190 in their study, concluded that in joints

ith initial physiological disk–condyle relation,he orthopedic treatment of Class II malocclu-ion did not have any adverse effects, and inoints with partial or total anterior DD, an im-roved condyle–disk position was achieved. Mal-daptive tissue reactions and joint degenerationan result with the forward positioning of theondyle by repositioning appliances if nonre-ucing displacement of the disk is a pretreat-ent condition due to an increased biome-

hanical stress on the condyle that may exceedoint adaptive capacity.132,188

The incidence of painful DDs has a peakduring the puberty for both boys and girls and atendency to peak during the third and fourthdecades for women.191 In asymptomatic sub-ects, the prevalence of DD is about 11.8% (Hanst al192) in children, about 34% (Riberio et

al118) in children and young adults, and 31%-34% (Kirkos et al; Katzberg et al124,193) in adults.

linical implications of this include the impor-ance of history, functional examination, andadiographic evaluation of patients presentingor orthodontic treatment. DD is a frequentnding in preorthodontic patients,194 which

may necessitate early orthodontic interventionin patients for its correction. Bryndahl,39 in histhesis on TMJ DD and subsequent adverse man-dibular growth, proposes that failure of growthstimulation with resultant retrognathia andtreatment response in cases of DD due to degen-erative condylar erosions in growing childrenwith intact articular layers (due to increasedadaptive plasticity of growing condyle) may im-plicate a clinical risk of false-positive radio-graphic finding of degenerative changes in theTMJ of children and adolescents. These obser-vations further enhance the need for a full eval-uation of the TMJ condyle and disk, position,and function, during procedures that are ex-pected to interfere with TMJ homeostasis, forexample, surgical craniofacial corrective proce-dures, dentofacial functional therapy, and occlu-sal reconstructive procedures. Functional evalu-ation of articular forces must be undertaken,
especially if the joints are loaded asymmetrically. s
In patients with TMJ DD, orthodontic treatmentshould be undertaken carefully to avoid adversechanges in TMJ components and facial architec-ture. A functional disk position is essential forprevention of damage to the articulating sur-faces in areas susceptible to load concentrations.

Some cephalometric variables can be used asan auxiliary diagnostic tool to help identify pa-tients with potential TMJ ID. Lateral cephalo-metric analysis, according to Bósio et al,195 inases of TMJ DD, only improves predictability,ut it is neither diagnostic nor does the assess-ent explain any cause-and-effect relationship.ephalometric studies as an aid to assess theffects of disturbed condyle–disk relationshipn facial growth during orthodontic treatmentave been proposed.193

Chate196 is of the opinion that hominid inter-cuspation that persists is anthropologically ab-normal, and yet incorrect deductions aboutfunction, such as incisal guidance, continue tobe made on the assumption that overbites arenormal or that there is a relationship betweenthem and the slope of the glenoid fossa. Hefurther states that contacts made during func-tion are unphysiological, whether they are de-scribed as “gnathologically ideal” or as an “inter-ference,” and both could have an equalpropensity to influence the joint. On the con-trary, theoretically, both gnathologically ideal orinterference contacts made during function and“overbite” or “edge to edge bite” can be physio-logical, but differently enabled, and their pro-pensity to influence the joint is a time-depen-dent factor of interactions between variousoperative substrates and the biomechanical in-fluence of the type of functional movementsthey generate. Investigations should be directedtoward finding which of the above stated condi-tions has a lower threshold for transforming intoa diseased state, when all other factors are keptsimilar.

Epidemiologic data do not relate a specificdentofacial malocclusion to an individual’s riskfor developing TMD.197 There is no evidencefrom randomized controlled trials that occlusalinterferences cause an exacerbation of TMD andmandibular dysfunction or that occlusal adjust-ment treats or prevents TMD. Many clinicianscontinue to use occlusal adjustment for thetreatment of trauma from occlusion.198 The po-
ition of the mandible during the function of

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full intercuspation, while clenching, is impor-tant. Because forces generated by clenching canexceed 300 lb/in2,199 it becomes important thathe condyles are in their seated position to re-eive these strong forces. Correction of lateralorces is desirable for health and stability oftomatognathic system because under normalircumstances, people eat vertically and grindaterally.199

A distal condylar position has been associatedwith symptomatic TMD.134,136 The large range

f condylar positions in joints without DD andhe fact that a retroplaced condyle moves backo a normal position with advancing DD explain,n part, the reasons for low predictability of pres-nce or absence of DD by a posteriorly displacedondyle.134 Some studies indicate that the supe-ior head of the lateral pterygoid muscle insertsnto the articular disk.200-202 Posterior distrac-ion in conjunction with the activity of this mus-le could hypothetically put the joint at risk foromplications such as internal derangement. Be-ause of this well-accepted causality, childrennd adolescents are at a risk of developing de-eneration of the mandibular condyle when aetrognathic mandible is found contemporane-us to TMJ affliction. Conversely, TMJ afflictionan be a possible cause of growth impairment toe found in intracondylar bone reduction and aailure of condylar growth layers to respond toncreased growth velocity during the growthpurt.

Dawson203 points out that condylar access toentric relation is not dependent on verticalimension, and increasing the vertical dimen-ion does not unload the joints, if the startingosition is centric relation position. The posi-

ion of condyles in centric relation is indepen-ent of vertical dimension and occlusion, but if

he condyles are in a deranged reference posi-ion within the fossa, to begin with, alteration ofcclusion, is a logical consequence. For the or-hodontist, a diagnosis of a larger discrepancyetween seated condylar position and intercus-al position is indicative of condylar distractionway from the eminence, which may be detri-ental to joint health and/or stability because

here is loss of juxtaposition between the con-yle, disk, and eminence in hyperdivergent fa-ial patterns, and such knowledge prepares himetter for treating such cases to his desired ob-

ectives.42 e

The most pertinent questions that arise fromthe foregoing discussion are as follows:

How much of occlusal form is dictated by thestructure and function of the TMJ?

What is the effect of condylar displacementon the vertical dimension of occlusion?

To what degree is the anterior guidance afunction of condylar guidance, or is itunique to each individual and depends onlip pressure, lip closure path, phonetics,and esthetics?

The fact that cartilage plays the most promi-nent role in orthopedics204 can be extended to

rthodontics as well. The biological fact thatondylar growth may be directed and stimulat-d/remodeled by orthodontic therapy is of par-mount interest to orthodontists. The entirechool of functional jaw orthopedics is based onhis philosophy. The exact prediction of tissueesponse and the growth behavior in an individ-al case remains highly speculative. To increaselinical efficacy and effectiveness of the myo-unctional appliances, the areas that need to benvestigated further are as follows: dentoskeletalharacteristics that can help in selection of ap-ropriate patients in whom functional therapyan be initiated for favorable results, cellularnd molecular mechanisms involved in TMJ re-odeling and adaptation, biomechanical evalu-

tion of the type of forces they generate andheir effect on the component hard and softissues in different areas of the joint, methodsor estimation of growth potential and bone de-elopmental stage of the patient. Informationbout the genetic make up of a patient will helpiscern remodeling- and adaptation-compliantatients from noncompliant ones. The goal ofrthodontics should be to achieve a structuralalance to facilitate physiologic adaptation andehabilitation.

A reciprocal relationship exists betweenrowth and biomechanics of the TMJ, dentofa-ial characteristics and articular afflictions, oc-lusion and the TMJ, vertical dimension and theMJ, and mandibular movements and the TMJ.xtremes of form (eg, excessive overjet and over-ite, open bite and deep bite, skeletal hyperdi-ergence and hypodivergence) and differentlynabled mandibular functions resulting in over-oading of TMJs are all potential factors in the
tiology of afflictions of these interdependent


entities, thus enhancing the need for temporo-mandibular evaluation before, during, and aftertreatment. The quantity and quality of such af-flictions are dependent on the adaptive plasticityof that individual and the time of exposure tothe adverse factors. Because the movements ofthe mandible are not restricted by joint structureper se, other operative templates, for example,neuromuscular and psychological, apart fromthe structural template, contribute significantlyto its complex functions. Therefore, the com-plex set of operative templates that are requiredfor masticatory functions are all afflicted in thedisorders of this system. The positive influenceof these operative templates is that they makethe masticatory system an efficient machine forprecise, accurate, and controlled movements forits required functions, and the negative aspectincludes the introduction of complexity in diag-nosis, prognosis, and the treatment of the disor-ders of this system. The solution, perhaps, lies intreating the disorders of the TMJ by reducingthe complexity of these operative templates by aprocess of elimination and/or by simplifying(modifying) the end purpose of these tem-plates—the mandibular functional movements—during treatment. Treating to the optimum is anecessity because malarticulation and malocclu-sion can be mutually exclusive to some extent, ashas been observed in the foregoing discussion.In a given set of operative templates for differ-ently enabled functions of the TMJ and the oc-clusion, it is desirable that treatment objectivesmust be directed toward creating vertically ori-ented, active, and reactive forces at the inter-faces of the masticatory system (periodontal lig-ament, occlusion, TMJ), with lateral forces keptto a basic minimum required. Therapeuticsshould consider the functional envelop, which isunique to each patient. To this end, we need todevelop mechanisms that can record and repro-duce highly accurate geometric models of a sub-ject’s TMJ and teeth, combined with recordingsof chewing trajectories and 3-dimensional tem-poromandibular movements, to obtain subject-specific models of masticatory system by eitherimproving on conventional mechanical articula-tors or by application of virtual-reality tech-niques for the development of virtual articula-tors for diagnosis and treatment of the disorders
of masticatory system.
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