Evolution Epidemiology and Etiology of Temporomandibular Joint Disorders

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10.5005/jp-journals-10011-1061 JIAOMR

REVIEW ARTICLE

Evolution, Epidemiology and Etiology of

Temporomandibular Joint DisordersSujata M Byahatti

Reader, Department of Oral Medicine and Radiology, Maratha Mandals NG Halgekar College of Dental Sciences and Research

Center, Belgaum, Karnataka, India

Correspondence: Sujata M Byahatti, Reader, Department of Oral Medicine and Radiology, Maratha Mandals NG Halgekar

College of Dental Sciences and Research Center, Plot No. 49, Sector No. 9, Malamaruthi Extension, Belgaum-590016, Karnataka

India, e-mail: [email protected]

ABSTRACT

The temporomandibular joint (TMJ) is unique to mammals, but among different mammalian groups, its morphology and function vary enormously. It

is likely that various species show less loading of jaw joints during chewing than humans do. It took approximately 130 million years for the

evolutionary process to stabilize this composite, basic vertebrate head skeleton to the jaws. Very little documentations are noted on anthropology of

TMJ. When it is concerned with epidemiology, temporomandibular disorders (TMDs) have a number of consistent findings and various causative

factors that can contribute to TMDs. The aim of the following article is to provide detailed information regarding its evolution, epidemiology and

various causative factors leading to TMDs.

Key Message:The masticatory system is extremely complex.

Keywords: Temporomandibular joint, TMDs, Epidemiology, Biomechanics, Animal, Mastication.

INTRODUCTION

TMJ is a unique joint in which translatory as well as rotational

movements are possible, and where both the ends of bone articulate

in the same plane with that of other bone. It is also called as

ginglymodiarthroidal type ofj oint where in it has sl iding mo vem ent

between bony surfaces, in add ition to hinge movement, com mon

to diarthroidal joint.1

Jaws and jaw joints h old a particular importance in the history

of vertebrates. Comparative studies of many different species have

also documented to understand the idea of perpetual change over

time. Complicating features of jaws and their joints leading to

those o f mammals have derived by selective playing with adaptive

modifications of several original elements.2 The following article

provides detai led informa tion r egarding its evo lut ion along with

the epidemiology and etiology o f TMDs.

DISCUSSION

The masticatory sy stem is extremely complex. It is primarily made

of bones, muscles, ligaments and teeth. Movem ent of mandible

needs coordination between them to maximize function and

minimize the damage to surrounding structures. This movement

occurs as a complex series of interrelated three-dimensional

rotational and translational activities, which is determined by

combined activities of both TMJs. This complex biomechanics of

TMJ m akes it difficult to understand the insight of temporomandi

bu lar joint .3

Temporomandibular joint (TMJ) is a cardinal feature that

separates mammals from other vertebrates. As befits its late

evolutionary origin, the TMJ also makes a tardy developmental4

appearance.

The TMJ is interesting because of its constituent bones,

mandible and squamous temporal, which are intramembranous in

origin. Thus, the tissue that covers each articulating surface is a

secondary cartilage with a fibrous skin derived from the periosteum.

Another nearly constant feature is the intra-articular disk. The disk,

even when incomplete, is associated with the lateral pterygoid

muscle ,5 which has led some authors to speculate tha t it arose as a

tendon, which later formed the new joint.6

Origin of the Vertebrate Head

A complete vertebrate is a species not only with backbone but

also with biting jaws. The first vertebrates had no biting apparatus

bu t the proc es s o f ce ph al izat ion als o prod uc ed the sin gu lar

structures called the jaws. Furthermore, complete mammals have

long been distinguished from all other vertebrates and from

animals2 very nearly m ammalian largely on the basis o f their jaw

ear structure. However, there is an even greater significance to

jaw jo ints in v ertebrate evo lution. The true diarthrod ial jo in t was

first formed in fishes, and its basic structure remained essentially

unchanged when it was appropriated by the limbs o f land animals.

Diarthrognathus7and P robainognathus8have contact between

two bones, squamosal and dentary, which form the jaw joint in

mammals.

Diarthroses are the most complex joints with a wide single

articular cavity surrounded by a specialized sleeve lined with a

synovial membrane.2 Schizarthrosis are joints made more mobile

by severa l small sep ara ted splits or jo in t spaces interru pting the irbonding tissues. The simplest join ts are syn arthro ses in w hich the

pa rts are mad e sl ig ht ly mov ab le by a bi nd in g o f ca rt ila ge ,

fibrocartilage or connective tissue between them.2

Evolutionary Stages

The primary jaws were mo dified gill arches that were connected

at their caudal ends by a simple hinge joint. T here was an obvious

mechan ical deficiency in this sort of arrangemen t because the joint

was left hanging-free, protruding slightly on each side like

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Sujata M Byahatti

bowlegged elbows of a primitive reptil e with in the body wall. The

jaws were further streng thened , stif fen ed and protecte d by the

armor of numerous dermal bony plates plastered to their outer

surface. Some of their bones at the edge of the mouth also

developed teeth. These outer tooth bearing plates braced by

adjacent dermal bones are known as secondary jaws. However,

the primary jaws still formed the single joint on each side for the

entire jaw complex.2

Despite its status as a mammalian identifier, the TMJ shows

remarkable morphological and functional variation in different

species, reflecting not only the great mamm alian adaptive radiation

in feeding mechanisms, but also freedom from constraints, such

as bearing body weight. The most extreme evolutionary variants

include: (1) loss of the synovial cavity in some baleen whales;

(2) loss (or possibly primitive absence) o f the disk in monotremes,

some marsupials, and some edentates (anteaters and sloths);5,9

(3) variations in the orientation of the joint cavity from parasagittal

(many rodents) to transverse (many carnivorans); and (4) reversal

of the usual convex/concave relationship so that the mandibular

condyle becomes the female element.10 In addition, the relative

size of the joint is exceedingly variable. Soft tissue details, suchas capsular lig aments11and collagen orientation in the disk12 are

also highly species specific. The striking anatomical differences

in TMJs are clearly tied to biomechanics. The features mentioned

above are either correlates of loading (e.g., size of articular

surfaces) or movement (e.g., orientation of the joint) or both.

Loading of the TMJ is a reaction force arising from the

contraction of jaw muscles; its magnitude depends strongly on

the position of the bite point relative to the muscle action line.

The evolution of the TMJ is thought to have coincided with a

perio d of lo w r eact ion loads with hig her loading h av ing evolved

repeatedly in different lineages,13 including ou r own. Rodents fall

in the category o f minimal TM J loading, especially during chewing.

In contrast, carnivorans such as dogs probably sustain TMJ loadsthat are higher than those in primates.14 Complex movement in

three planes of space is also a primitive characteristic of TMJ, at

least if embryology is any indication 6 but there is no uniformity in

how movements are accomplished. Opening of the jaw usually

involves a combination o f forward sliding and rotation around a

transverse axis, but some carnivorans have lost the ability to slide

and some specialized anteaters instead use a rotation around the

long axis o f the curved mandible.9 Similarly, transverse movemen t

is usually ac complished by moving one condyle forward and the

other one backward, b ut carnivorans use a combination o f lateral

sliding and rotation around the long axis o f the mandible .15

Amphibia

The single joint for the double jaws (Figs 1A to C). The jaws o f

these creatures were used only for protection. The skull became

kinetic, new movable joints appeared within the cranium. This

ingenious device was a shuttling pterygopalatine component

coordinated with joints between the frontal and nasal bones.

Proliferation of loose, movable parts imparts the architectural

stability of the skull. This kinesis has disappeared in the crania of

reptiles, such as turtles and crocodiles those feed in water.2

In the carnivorous Pelycosauria, with which we are more

directly concerned since they include the ancestors of all the later

Figs 1A to C: Reptilian jaw (A) Agnathia (B) Gnathostome

(C) Osteichthyes

mammal-like reptiles and so o f the mammals, the teeth are all o f a

simple blade-like form. They hav e distal and mesial cutting edges,

often serrated like a bread saw. This demonstrates that the

sharpening of the teeth by direct tooth-to-tooth contact,16 which

is so important in there in mammals, did not take place in these

mamm al-like reptiles .17

The skull of a primitive amphibia tends to be long, wide and

remarkably flat. The first notable change from the amphibian

structure was the disappearance of the otic notch that supported

the tympanum of the primitive ear. The teeth began to lose their

continuity o f repetition that is different segm ents along the dental

line developed teeth o f different sizes and incipient functions.2

Reptilia

Reptiles cann ot chew.2 In general, they use the ir teeth only to seize

their prey, and if this cannot be sw allowed whole, it is torn apart

by a num ber of animals or by a single anima l worry ing it. The

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group o f reptiles which later gave rise to mammals - the mamm al

like reptiles diverged at an early stage from the main reptilian stem,

and by the Permian, some 250 million years ago, were widespread

and might reasonably be regard ed as the Lords of Creation. Later

their place has been taken by the dinosaurs, which remained the

dominant group until the latest Cretaceous. The mammal like

reptiles did not, however, quite disappear. Small members of the

group survived, perhaps in areas where there was insufficient food

for the large dinosaurs, or by eating items of food too trivial for

their notice. In this group of tiny mammal-like reptiles, a number

of changes took place, all probably associated with the acquisition

of the ability to maintain a body temperature above that of their

surroundings. T his increased their activity and their ability to avoid

their enem ies, but it carried the penalty that it vastly increased the

food requ irement.17

Rep tilia2 have loss o f bony parts (Figs 2 A to D ). In the

Pelycosauria, the lower jaw is of an essentially reptilian form. The

Figs 2A to D:Amphibian Jaw (A) Amphibian (B) Reptile

(C) Mammals like reptile (D) Mammal

tooth bearing bone or dentary has a strictly limited extension distal

to the tooth row; the other bones of the jaw are well-developed

and comprise the posterior third of the jaw. The quadratethe

skull bone which forms with the articular in the lower jaw, the

ja w hing e, is a large bo ne with a co ns iderab le do rsov en tra l17extension.

Mammalian Architectural Theme

The viscerocranium provides roofs and walls to the upper food

way, the neurocranium houses the brain w hereas the mandible is

a long, straight, level bar with its fulcrum, the jaw joint at the

backend. The m andib le walls the u pper food way, w hich is roofed

above by the palate and floored below by a m uscular hammock

hung between the bony bodies of both sides. This is the basic

characteristic of mammalian skull, from it, two adaptive feeding

modifications labeled carnivores as opposed to herbivores exist.2

The early American forms are grouped as the Pelycosauria,

comprising carnivorous, piscivorous and herbivorous forms. The

herbivores are o f interest as they were the first group o f vertebrates

to exploit plant life directly as food. The specialization of the teeth

and masticatory apparatus is such in herbivorous forms that theircapacity for further evo lution is limited, so that major evolutionary

changes are always initiated by carnivorous or insectivorous

forms.17

Austrolopithecus africanus skull (Fig. 3) is the least differen

tiated. The jaw s protrude sharply housing a sho rt cheek teeth. The

upper jaw has outstanding canine buttresses whereas the lower

ja w is fairly shallow. The zyg omatic a rches are quite slender. The

teeth are broad, spatulate, bulky and long rooted as they project

from their sturdy b uttresses.2 Austrolopithecu s boisei skull

(Fig. 4) is considerab ly differentiated. Th e skull dorsum is deeply

vaulted in a short asymmetrical arch, the supraorbital ridges are

bulky, jaw s are severe ly retruded, no canine buttresses, and massive

zygomatic buttresses give the face a dished in frontal plane.The teeth are smaller and occlusal surfaces are more than three

times greater than in modern man.2

Masticatory Action in Primitive Man

Animals, such as pig, bear, and humans can masticate a variety of

foods to a satisfactory degree. It is readily accepted that muscle

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attachments in the humans depend upon the functional activity of

the individual for their development and that muscle development

can reflect the exercise and activity that each muscle experiences.2

Modifications of the muscular elements (Figs 5A to C) o f the

apparatus were evident in the specialization of lateral pterygoid

muscle and a decrease in the bulk o f temporalis muscle. Bilateral

contract ion of the lateral pterygoid muscles permit ted a

modification of condylar movement within the glenoid fossa to

include a protrusive excursion. It has been suggested that primitive

ma ns protrusive movement has m erely been a bilateral sequel to

the individual forward movemen t of each blanching condyle during

lateral excursions of the mandible.3

It must, of course, be remembered that nature has evolved

satisfactory modifications over countless periods of time and that

adequate adaptation may not be possible in a single lifetime.

Indeed, with observation o f a wide range o f functional activities

that have been satisfactorily accommodated by modification ofthis joint in the animal kingdom, one is drawn to the b elief that

the lack of time to respond is responsible for masticatory

dysfunction in the most cases.2

Changes in the masticatory apparatus of primitive man were

not only the result o f his omnivorous diet, but also o f the adaptation

of the upright posture that necessitated a change in the position of

the head relative to the spine. The inclusion of a protrusive m andi-

bu lar mo vement and modifica tion of the ass ociated muscu lature

are characteristics of the hominoid masticatory apparatus.2

Epidemiology of TMDs

It is evident from the numerous epidemiologic studies on the

occurrence of TMDs in the general population where there are anumber o f consistent findings. F irstly, signs of T MD appear in

about 60 to 70% of the general population and yet only about one

in four people with signs are actually aware of or report any

symptom s.18 Furtherm ore, only about 5% o f the popula tion will

have symptoms severe enough for them to seek treatment.

Another consistent finding is that of those who seek treatment

for TMD, by far the greatest majority is females outnumbering

males by a tleast four to one. It is suspected that TMD affects both

males and females in almost equal numbers in general population

although females are possibly more likely to seek treatment.

Figs 5A to C: Different groups of muscle attachments in

mandible

Although, TMD may occur at any age, the most common time of

pre senta tion is early adulthood.19

The prevalence o f TMD signs and symptoms20 in a large elderly

sample was composed of medicare recipients, where the samplesize was 429 subjects derived from an eligible sample of 866

subjects. The mean age of the sample was 74.4 years and 42%

were males ov erall, 12% of the subjects reported a history of TMD

and 6.5% reported pain with jaw function. Joint noise was

documented in 35.2% of the sample, joint tenderness in 8.4%,

muscle tenderness in 12.8% and limitation ofjaw mo tion in 22.4%.

In pediatric pop ulations,20 it has been repo rted that on a sam ple of

11 and 15 years subjects, who had been examined for signs and

symptoms, the sample was composed of 119 subjects where the

point prevalen ce of subjects rep ort ing one or more symptoms of

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TMD increased with age (35% at age 7, 61% at age 11, and 66%

at age 15), 60% of the sample reported one or more TMD

symptoms, 66% reported one or more symptoms at age 15.

Etiology

The causes of TMD range from traumatic injury to immune-

mediated systemic disease to neoplastic growths to incompletely

understood neurobiologic mechanisms.21,22 Less common but

be tter recognized causes of TMD are (1) a wide range of direct

injuries, such as fractures of the mandibular condyle (2) systemic

diseases, such as immune-mediated arthri t is (3) growth

disturbances and (4) tumors. Some nonfunctional movements of

the mandible (bruxing) and tooth clenching habits are clinically

associated with a variety of jaw muscle symptoms and associated

less clearly with internal joint disk derangements.23 Chronic

parafun ctional clenching is suspected o f assoc iat ion with chronic

TMD and has been shown experimentally to cause acute TMD in

human beings.24 However, these behaviors are not established as

causes of TMD and may be pro pag at ing factors only.

Malocclusion is not established as an important factor in

TMD.25-27 There is no compelling evidence that orthodontictreatment increases or decreases the chances of developing TMD

nor is there any evidence of increased risk for TMD related to

any particular type of orthodontic mechanics or to orthodontic

treatments wi th tooth extractions.28,29 TMD is also cau sed by hea lth

care manipulations, e.g. holding mou th open wide, prolonging w ide

opening of mouth, stretching or forcing the mouth open, forcing

the jaws shut, relate to routine dental examination, oral

endotracheal intubation for general anesthesia, and the entire range

of dental services from restorative and orthodontic treatments to

tooth extraction to orthognathic surgery. There is no scientific

evidence that common or routine dental or medical procedures

cause TMD. Orthognathic surgery, orthodontic treatment,

pro sthodont ic rehabil itation and mandib ula r fracture repairs havebeen ass ociated wi th morph ologic TM J changes and worsening

of pre-existin g T MD .30-33

Diagnosis of TMDs

The gold standard of diagnosis in TMDs consists of (1) patient

history (2) physical evaluation, and in the most chronic cases,

(3) behavioral or psychologic assessment.23,34-39 This evaluation

should include a detailed pain and jaw function history as well as

objective measurements of such jaw functions as interincisal

opening, opening pattern, and a range of eccentric jaw motions.

TMJ sounds should be described and related to symptoms.

CONCLUSION

The different evolutionary stages have made this TMJ as complex

structure.

Clinical manifestation may not be completely contributory in

the diagnosis of TMDs. They need to correlate with standard

diagnostic modalities for the confirmation of origin o f particular

problem . Various etio log ica l factors have made the management

of TMDs challenging. Treatments of TMDs are behavioral,

ps ychologic an d psycho social factors ra ther th an ph ys ical or

structural factors. Poor outcome could be related to depression,

somatization, anxiety, and low self esteem. Combination therapy

(education, pharmacotherapy, trigge r point therapy, physiotherapy

and intraoral appliances and relaxation therapy) may be helpful in

pro vid ing a bett er outcome.

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Evolution Epidemiology and Etiology of Temporomandibular Joint Disorders