The temporomandibular joints (TMJs) connect the jawbone to the skull much like two hinges, one located on each side of the head just in front of the ear.  Each TMJ consists of a fibrous disk held down on the top of the condyle (the back end of the jawbone) by 2 collateral ligaments which act like chinstraps holding a hat down on a head.  The only disk movements they permit are rotations forward and backward.

TMJ disorders nearly always start with a dislocation of the disk (below right).  In the normal TMJ (below left) the disk functions as a cushion between the condyles and the underside of the skull so these bones never rub directly against each other.   When the jawbone gets driven backward into the front of the ear, the disk gets squeezed out forward from its highly lubricated joint compartment, much like squeezing a watermelon seed out from between two fingers.  Subsequently, the retrodiskal tissues are pulled into the place of the disk between the opposing bones where they get mashed by normal chewing forces.

The disk may get dislocated forward (as above), to either side (medially or laterally), partly forward and partly to one side, or rotationally so that only one side of the disk gets dislocated forward.  Whatever the direction of the disk dislocation, it pulls the fragile vascular retrodiskal tissues from behind or beside the disk into the space between the two bones where the disk had been.  Trapped in this location, the fragile vascular tissues can get easily injured by normal functional forces.  In early stages of disk dislocation, the retrodiskal tissues acquire all the characteristics of a chronic bruise - with blood outside of the vessels, swelling, heat, and pain.  In later stages of disk dislocation, they toughen up by losing their blood and nerve supply to become fibrotic pads of scar tissue which may be able to withstand chewing pressures.

FORWARD DISK DISLOCATION
At least half of disk dislocations occur in a forward direction where the dislocated disk forms an obstacle in the path of the condyle during jaw movements.  In these cases the lower jawbone can open part way without clicking, because the condyle is able to slide forward and push the dislocated disk in front of it until the ligaments become taut.  Further jaw opening requires the condyle jump over the back end of the dislocated disk and onto its thin center.  It is the condyle landing on the thin center of the disk which makes the clicking or popping sounds so frequently heard in early stages of TMJ disk dislocation.  Usually the opening click occurs near wide opening, because it doesn't occur until the ligaments attached to the back of the disk can stretch no further.   The closing click is generally softer and occurs when the jaw is more nearly closed, because the disk doesn't re-dislocate until the condyle reaches the depth of the socket where the disk can no longer fit with the condyle.

In later stages of forward disk dislocation, the disk no longer goes back into place during mouth opening, and the clicking stops.  In some cases, the transition to this later stage happens gradually due to the disk getting ironed out.  In other cases the transition to this later stage may be accompanied by a phase of intermittent locking followed by a permanent lock which is characterized by limited jaw opening and significant worsening of symptoms as the lower jawbone can open only to the place where it used to click.  In most cases, the only way to prevent intermittent locking from progressing to a permanent lock is a treatment called disk recapturing (see the paper entiled RECAPTURING THE DISLOCATED DISK in the drop down menu under SPECIFIC TREATMENTS).  Once your TMJ goes into permanent lock, if you try to open wider, your condyle on the affected side will run into a mechanical stop.  As a result,  your chin will deviate off to the affected side, because that side stops moving while the other condyle keeps moving; and you'll experience pain just in front of the ear on the side of the locked TMJ where the ligaments are being stretched..

SIDEWAYS DISK DISLOCATION
While forward dislocations are the most common, the disk may also get dislocated by shifting or rotating to either side.  Rotational dislocations occur when one collateral ligament fails while the other one holds.  Sideways and rotational disk dislocations frequently cause no clicking, popping, or opening limitations, because the disk is dislocated into a position which does not form an obstacle in the opening pathway of the condyle.  The lower jawbone may shift more easily to one side thatn to the other side.

ADAPTATION
In some people, dislocation of the TMJ disk produces no significant symptoms.  When MRI began showing  that the vast majority of the noises in peoples' TMJs were due to dislocated disks, researchers could not understand why the same condition could cause severe symptoms in some people and no symptoms in others.    Some patients reported, “It’s been clicking like that for decades and it’s never bothered me.”

Eventually researchers learned that TMJs with asymptomatic disk dislocations had undergone an adaptation response in which the normally fragile retrodiskal tissues were converted into a tough fibrous pad that looked and functioned very much like true disk tissue.  Surgeons removing dislocated disks had been frequently diagnosing them as elongated, but researchers pointed out that TMJ disks were made of collagen with little elastic tissue and therefore could not really elongate.  Then researchers examining these apparently elongated disks under a microscope found that their rear portions contained remnants of blood vessels – proof that they were really comprised of normal vascular retrodiskal tissue which had been converted by adaptation into avascular scar tissue.  These pads of avascular scar tissue are so similar to true disks that they are called “pseudodisks”.

Because of pseudodisk formation, disk dislocation is a condition in which the symptoms eventually “burn out”.  The adaptation process which eliminates the symptoms may take anywhere from days to decades, however it occurs eventually with age in everybody.  As a result, although TMJ disorder symptoms are common among people in their twenties, thirties, and fourties; they are rare in the sixties and even rarer in the seventies.  Thus there are millions of older people with dislocated TMJ disks that are asymptomatic due to adaptation.   Whatever the rate of this adaptation process, the course of the disorder usually takes one of three pathways.

CLOSED LOCK
In the first pathway, the disk maintains its shape, but at some point the condyle is unable to get over its back end.  Thus the disk no longer goes back into place during mouth opening, and the lower jawbone cannot open beyond the point at which it used to click. This situation usually occurs on an intermittent basis before it occurs on a permanent basis.  It is called a closed lock, even though the mouth is not actually locked in a closed position - it just can't open as wide as it used to. This is the stage at which TMJ disorders are usually the most symptomatic.

Sometimes a locked TMJ can be easily unlocked, but the unlocking needs to be immediately followed up with orthopedic treatment or the involved TMJ will simply lock up again.  Disk recapturing requires permanently changing the bite, so this treatment is only indicated when the symptoms are severe enough and simple treatment has not worked or when most of the teeth need restoration anyway.  

The prognosis for unlocking depends on the length of time the involved TMJ has been continuously locked and the shape of the disk.  A TMJ which has only been locked for days or weeks can usually be unlocked, but a TMJ which has been loced for months or years is unlikely to unlock. .

The process of trying to unlock a locked TMJ consists of a progression of brief unlocking attempts, using simple inexpensive measures first and more involved measures only if simpler ones have failed.  The various unlocking processes are explained in a separate paper called  TREATING THE LOCKED TMJ.  If your TMJ can be unlocked and you want it to stay unlocked, it will need to undergo disk recapturing (a non-surgical procedure) as explained in the  paper entitled, RECAPTURING THE DISLOCATED DISK.

DISK  RESHAPING
In the second pathway, the disk gets misshapen so it no longer remains an obstacle in the path of the condyle of the lower jawbone during opening.  The disk's posterior band may become flattened after the condyle has run on and off it hundreds of times each day, progressively softening the noise made by the condyle jumping over it during opening.  Alternatively, the disk may degenerate or get balled up.  In these cases, the click or pop may disappear completely, but the joint is still functioning without the benefit of an interposed disk, and the arthritic process continues.

STUCK DISK
In the third pathway, the dislocated disk gets permanently stuck (adhesed) onto the underside of the skull.  Without the mobility of the disk, the joint loses some of its lubrication and circulation as well as its ability to cushion the opposing bones.  Initially the dislocated and adhesed disk may prevent full opening, but eventually it gets reshaped by  flattening, balling up, or degenerating so it no longer forms an obstacle to normal opening and closing.

DIAGNOSIS
Diagnosing a dislocated TMJ disk is not always easy.  Disks which have been dislocated sideways, ironed flat, balled up, or degraded by cellular activity may produce no noise.  Also the TMJ can make clicking sounds that do not indicate any serious pathology, much like the harmless clicking that frequently occurs in other joints.

Fortunately, the noises made by TMJ disks in early stages of disk dislocation usually have unique characteristics that easily distinguish them from the more common types of harmless joint noises.   Because of these characteristics, diagnosis can sometimes be made with a high degree of certainty based on nothing more than history and clinical signs.  In other cases, MRI is required to determine the stage and direction of disk dislocation as well as the shape of the dislocated disk.

One unique characteristic of TMJ sounds which are caused by disks going in and out of place is that the opening sound occurs when the mouth is opened wider than when the closing sound occurs.  During opening, the dislocated disk gets pushed in front of the condyle until the ligaments holding it back get stretched tight and won’t let it advance any further.  Then, to allow further opening, the condyle must jump over the back of the disk and restore proper disk position.  The landing of the condyle back on the disk is often accompanied by a snapping, popping, or clicking sound which usually occurs when the mouth is already at least half way open and the ligaments have stretched until they can stretch no more.  Further opening is uneventful, because the disk is in place.  During closing, the disk gets carried all the way back with the condyle until it can’t fit together with the condyle into the back of the joint cavity.  At this point, the condyle gets eased back off the disk, often accompanied by a softer snapping, popping, or clicking sound.  This softer closing click usually occurs just before the mouth is fully closed.  Thus the opening noise and the closing noise occur at very different locations during the process of opening and closing the mouth, and the mouth has to close far enough to make the closing noise if it is to make an opening noise at the next mouth opening.  In contrast to this pattern, when TMJ noises are due to the condyle running over a fixed obstacle (like a fibrous adhesion or a bony irregularity) in its path, the opening and closing sounds occur at the same location, often when the mouth is opened quite wide, and the mouth doesn’t need to close very far at all in order to create a click or pop on the next jaw opening.

Another unique characteristic of TMJ sounds made by disk dislocation is that they are independent of the speed of opening.   The sound made on opening is due to the disk snapping rapidly into place after its attaching ligaments have been stretched as far as possible.  Thus the sound doesn't change – even if the opening movement of the condyle is very slow.   In contrast, TMJ noises due to the condyle running over a fixed obstacle in its path change depending on the speed of jaw movement.  For example if you take 10 seconds to open,  the joint noises will be softer because the condyle is striking the obstacle more slowly.

More advanced stages of TMJ disk dislocation are usually accompanied by a sound called crepitus.  Crepitus is caused by bones rubbing together.  It usually begins as a rubbing sound (fine crepitus), and then it becomes more coarse over time.  Eventually it may sound like footsteps in gravel.

The only reliable way to diagnose the condition and location of the dislocated disk is with MRI.  X-rays show bones very clearly, and they can detect even small arthritic changes in the bones of the TMJ.  However X-rays can’t produce an image of the disk unless dye is injected above and below the disk so the presence of a disk can be inferred from the presence of a space where no dye goes.  This dye injection assisted X-ray procedure (called arthrography) can be very painful, because it requires injecting fluid into a joint capsule that is already swollen with excess fluid.  In addition, squirting dye into the joint can change its dynamics, diminishing the accuracy of the test.  In  contrast, MRI is painless, much safer (because it uses no ionizing radiation), and more clinically useful because it shows the condition of all the soft tissues including the disk.

THE PROBLEM OF SURGICAL FAILURE
The only TMJ disorders which do not eventually resolve due to natural adaptation are surgical failures.  TMJ surgery often works, but surgical failures in the TMJ can be disasters.  The scar tissue left from the surgery seems to be able to prevent natural pseudodisk formation.  Because TMJ surgical failures have such a high morbidity rate, statistics show that the primary indication for TMJ surgery is previous TMJ surgery.  Even the oral surgery journals agree that TMJ surgery should only be performed as a last resort - after all other types of treatment have failed.

SYMPTOMS
Displaced disks only occasionally cause pain directly in the TMJs.  Joint surfaces are designed to withstand large compressive forces, so they don’t have pain transmitting nerves in the area between the bones.  Instead, their pain sensitive nerves are confined to the capsule at the outer edges of the joint.  They signal pain only when the joint capsule is stretched by swelling inside the joint.  In such acute TMJ conditions, the swelling which produces TMJ pain also pushes the condyle down and away from the skull a little, thereby slightly changing the cant of the whole lower jawbone.  Therefore severe TMJ pain is often associated with inability to make biting contact with the back teeth on the affected side.

The most common effect of tissue damage in the TMJ is jaw muscle tension.  Damage to tissue in any joint produces a reflex tightening of the muscles which cross that joint.  You can see this effect in the way walking on a hurt knee automatically produces leg muscle tightening which results in limping.  The muscles which cross the TMJ are the jaw muscles.  When they must function on a dislocated TMJ, they hold themselves braced in readiness to protect the damaged joint, so they can never fully relax.  As a result, their circulation is impaired and they don’t adequately flush the waste products out of their tissues.

The reflex tightening of the jaw muscles caused by TMJ disk dislocation explains why earlier muscle oriented TMJ treatments usually provided only limited success.  Treating the jaw muscles provided short term relief, because it helped restore circulation in muscles which were clogged with waste products, but it ignored the cause of the muscle tightness.  Treating the jaw muscles without also improving the conditions in the joint was like massaging the leg muscles of someone who has an untreated broken ankle.  The massage helps relieve the pain, but the muscles tighten up again as soon as the patient gets off the table and starts walking again.

One effect of chronic jaw muscle tightness is jaw muscle pain.  Muscles are full of sensory nerves, and pockets of waste products which build up in muscles that can’t fully replenish their tissues can cause pain signals from those nerves.  However, the body’s muscles are all connected, so the pain and other symptoms resulting from chronic jaw muscle tightness frequently cause symptoms at locations far from the jaw area.

The jaw muscles attach all over the sides of the head, so jaw muscle tension can apply significant pressure to the head.  In monkeys, biting has been shown to bend the whole head by slightly separating its right and left sides at the midline (parietal) suture.  In addition, the jaw muscles are powerful controllers of blood flow to and from the head.  As a result, headaches are a primary symptom of TMJ disorders.  A surprising number of different headache types (including common migraine) have been shown to respond to TMJ disorder treatment.

The tightening of the jaw muscles affects head posture.  The jaw muscles form an integral part of the head posture mechanism.  Their pull downward at the front of the head balances the pull down at the back of the head from the strong muscles of the shoulder and back.  When the jaw muscles become chronically tight, they disrupt the balance between tensions at the front and back of the head.  This interdependent relationship is explained in detail in the paper entitled JAW AND BODY POSTURE.

A dislocated TMJ disk can also cause a number of ear symptoms.  Anatomical studies of TMJs with dislocated disks have shown that most of the tissue bruising occurs at the extreme back end of the TMJ, which is only 1.5 millimeters from the front of the middle ear.  Bruising causes swelling, and swelling can exert fluid pressure across the thin membrane bones separating the TMJ and the middle ear.

The balance mechanism, which is located in the inner ear, may be affected. Severe injury to the balance mechanism results in dizziness and nausea, but mild injury to the same system produces feelings of disorientation, inability to concentrate, a tendency to bump into things, and “spaciness”.  These symptoms are frequently produced in TMJ disorders by swelling from the bruised retrodiskal tissues just in front of the ear.

The eustachian tube also may be affected, causing a blocked or stuffy feeling in the ears and difficulty clearing them after changes in altitude. The eustachian tube has the job of equalizing pressure between the middle ear and the outside air so that no differential pressure is created across the eardrum.  However, the eustachian tube passes very close to the back of the TMJ.  A swollen TMJ can push on the outside of the tube until it is completely blocked or just so narrow that blockage can be caused by a cold, allergy, or anything else that further narrows the inner diameter of the tube by causing swelling of its mucous membrane lining.

Tinnitus (ringing, roaring, or buzzing sounds in the ears) is a third ear symptom which is frequently found in TMJ disorder patients. Tinnitus has been shown to respond to TMJ treatment in only about half of the patients studied.  Normally a 50% success rate is not impressive, but in the case of tinnitus it is considered highly significant because there are almost no other treatments that have a better success rate.

Ear pain is a fourth ear symptom that may be caused by a TMJ disk displacement.  The ear pain caused by a TMJ disorder is often misdiagnosed as ear infection and treated with antibiotics.  However the actual cause of the pain is sterile swelling from bruising of the TMJ tissues in front of the ear.

Subjective hearing loss is the fifth ear symptom frequently found in TMJ disorder patients.  In subjective hearing loss, hearing tests are normal even though you frequently miss things people say.  The problem is due to disruption of the background tonus maintained by the ear muscles.  These little muscles control hearing by selectively applying tension to the eardrum, and they are supplied by the same motor nerve as the jaw muscles.  Therefore increased tension in that motor nerve may disturb both ear and jaw muscles together.

SUMMARY
Having a dislocated TMJ disk does not condemn you to years of pain or other symptoms.   Studies have shown that about 20% -25% of the population have a dislocated disk in one or both of their TMJs.   Most of these people with a dislocated TMJ disk don’t have significant symptoms.  Of the people who do suffer severe symptoms, even without any treatment, adaptation would take place and eliminate most of the symptoms eventually anyway.   Early treatment can sometimes restore normal disk position. Later treatment can enable adaptation and consequent symptom relief to occur much more rapidly than would occur naturally.