The Role of Disk Dislocation

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 that act like chinstraps holding a hat down on a head.  In this location, each disk functions as a cushion between the condyle and the underside of the skull so these bones never rub directly against each other.  The disk in its normal position is shown below:alt

TMJ disorders nearly always start with a dislocation of the disk (below right).  The cause of the dislocation may be an injury, a retrusive facial growth pattern, or a strained bite that forces the lower jawbone backward into the front of the ear where it cannot fit with its disk on top, driving the disk off the top of the condyle in this highly lubricated environment like squeezing a watermelon seed out from between two fingers.  

Side view of the TMJ

The dislocation of the disk pulls the retrodiskal (behind the disk) tissues into the place where the disk used to be.  Trapped between the bones, these retrodiskal tissues are vulnerable to damage by even normal chewing forces, and they are especially vulnerable during the nocturnal grinding and clenching (bruxism) that affects everybody during sleep.  Initially the retrodiskal tissues acquire all the characteristics of a chronic bruise.   Eventually they undergo adaptation by losing their blood and nerve supply to become fibrotic pads of scar tissue called pseudodisks, which can usually withstand normal chewing and even nocturnal bruxism.

FORWARD DISK DISLOCATION
Half of all disks are dislocated in a forward direction where they form an obstacle to the opening pathway of the condyle.  The lower jawbone can open easily until it reaches the obstacle.  At that point, the ligaments still attached to the disk become taut and the jaw may feel stuck. Opening any further requires the condyle to jump over the back end of the dislocated disk and onto its thin center - causing a click or pop, as seen between boxes 2 and 3 in the illustration below.  The disk then stays in place until closing when it dislocates again between boxes 8 and 1.

                                                           CLOSING                                                  FULLY CLOSED                                                               OPENING

Forward dislocation of the disk in TMJ

SIDEWAYS DISK DISLOCATION
The disk may also become dislocated to either side (medially or laterally). Sideways disk dislocations often do not produce clicking, popping, or limited opening, because the disk is dislocated into a position that does not form an obstacle to the opening pathway of the condyle.  Sideways dislocated disks are more difficult to recapture (restore to normal position) than forward dislocated disks.

ROTATIONAL DISK DISLOCATION

The disk may also become dislocated by rotating to one side or the other, because one of the collateral ligaments (either the medial or lateral) holds while the other fails.  The affected TMJ behaves and responds to treatment like a forward dislocated disk, except that it is only forward on one side.

PATHWAYS

Whatever the direction of the dislocation, the natural course of the dislocated disk disorder takes one of three pathways - closed lock, deformed disk, or adhesed disk.

1. CLOSED LOCK

In many forward disk dislocations, at some point, the disk stops going back into place during mouth opening and thereby prevents the lower jawbone from opening beyond the point at which it used to click.  Jaw opening is limited.  When attempting to open wide, your chin deviates to the side of the disk dislocation, because the condyle of that side runs into the dislocated disk and stops moving, while the condyle of the other side keeps moving forward.   This condition is called a closed lock, even though the mouth is not actually locked in a closed position - it just cannot open as wide as before. The process of opening and closing with a folding locked disk is illustrated below.

 

                       CLOSING                        FULLY CLOSED                            OPENING

Forward dislocation of the disk in TMJ

 

INTERMITTENT LOCKING usually occurs for some time before the lock becomes permanent.  The transition from intermittent locking to a permanent closed lock is usually the most symptomatic stage of the TMJ disorder.

The PROGNOSIS for unlocking a TMJ depends on how long it has been continuously locked. A TMJ that has just locked can usually be popped back in easily, but unlocking a TMJ that has been locked for months or years is difficult or impossible. The unlocking process consists of a progression of more invasive and expensive techniques. These various unlocking processes are explained in TREATING THE LOCKED TMJ under TREATMENTS. 

2. DEFORMED DISK 
Some dislocated disks become misshapen after repeatedly getting run over by the condyle or balled up in front of the condyle until they no longer remain a significant obstacle to jaw movements. The click or pop may disappear, but the joint is still functioning without the benefit of an interposed disk.

3. ADHESED DISK

Occasionally the dislocated disk becomes permanently stuck (adhesed) onto the underside of the skull (the articular eminence of the temporal bone). If the disk is adhesed in the path of the condyle, it can prevent full opening like a locked disk can.  However, eventually it gets ironed out until it no longer forms an obstacle to normal opening and closing, much like a deformed unadhesed disk. 

ADAPTATION

Eventually the natural course of the dislocated disk disorder leads to adaptation that eliminates the symptoms - no matter which pathway your disk dislocation takes.  This natural adaptation may take anywhere from days to decades, however it almost always occurs by middle age. Although TMJ disorder symptoms are common in the twenties, thirties, and forties; they become less common in the fifties, and they are rare in the sixties and beyond.  In these age groups, there is little correlation between TMJ noises and symptoms.  Many older people have loud clicking, popping, or grinding sounds when moving the mouth, but few retain any symptoms.

DIAGNOSIS

Diagnosing a dislocated TMJ disk is not always easy, because the sounds of the dislocation and relocation may be difficult to detect.  Disks that are dislocated sideways or have deformed may produce no audible signals. Conversely, TMJs can make sounds that do not indicate disk dislocation or 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 often be made with a high degree of certainty based on nothing more than history and clinical signs. In other cases, MRI is required.

One unique characteristic of TMJ sounds caused by dislocated disks is that the opening sound occurs when the mouth is opened wider than when the closing sound occurs. During opening, the dislocated disk is pushed in front of the condyle until the ligaments holding it back are stretched tightly and will not let it advance any further. As a result, the click or pop caused by the condyle landing back onto the middle of the disk usually occurs when the mouth is already at least half way open.  Then, during closing, the disk gets carried all the way back with the condyle until it cannot fit together with the condyle into the back of the TMJ, and it slips back out of place, often accompanied by a softer sound just before the mouth is fully closed. Thus the opening noise and the closing noise occur at very different degrees of mouth opening, and the mouth has to close far enough to cause redislocation of the disk if it is to make a noise the next time the mouth is opened.  In contrast, TMJ noises that are due to the condyle running over a fixed obstacle (like a fibrous adhesion or a bony irregularity) in its path occur at the same location during opening and closing, often when the mouth is opened widely, and the mouth need not close very far from that location in order to create a click or pop on the next opening.  

Another unique characteristic of TMJ sounds made by disk dislocation is that they are not affected by the speed of opening. The opening click is due to the disk snapping back into place after its attaching ligaments have been stretched as far as possible, therefore its sound does not change – even if the opening movement of the condyle is very slow - like taking ten seconds to open. In contrast, TMJ noises that are due to the condyle running over a fixed obstacle like a bone spur or fibrosis in its path depend on the speed of jaw movement, and slow opening will produce a softer sound, because the condyle is striking the obstacle more slowly.

ADVANCED STAGES of TMJ disk dislocation are usually accompanied by a sound called crepitus, which is caused by bones rubbing directly together without the cushioning and lubrication provided by an interposed disk.  Crepitus usually begins as a rubbing sound (fine crepitus), then becomes more coarse over time. Eventually it may sound like footsteps in gravel.  

There are companies claiming that the equipment they sell (including portable doppler machines used to detect fetal heartbeats and a similar technology called electronic vibration analysis) can enhance the accuracy of auditory diagnostics.  Some of these companies have devised a model for "staging" TMJ disorders based on the diagnostic markers that can be obtained using their equipment; however there is no evidence that these techniques provides better diagnostic ability than a stethascope or even just an ear, and there is no evidence that such "staging" of TMJ disorders provides information that is clinically useful.  

DIAGNOSTIC  IMAGING -  of disk displacement requires MRI. Although X-rays can detect even small changes in bones, they cannot produce an image of the disk unless dye is injected above and below the disk so that the presence of the disk can be inferred from the presence of a space lacking dye. This dye injection assisted X-ray procedure (called arthrography) can be very painful, because it requires injecting contrast fluid into a joint capsule that is already swollen. In addition, injecting fluid into the joint can change its dynamics, diminishing the accuracy of the test. Therefore arthrography is an outdated procedure. 

SURGERY has not yet been able to reliably put dislocated disks back in place and keep them there.  80% become dislocated again within two years.  In the near future, mini-implants may be able to attach the disk more securely to the condyle.  However, at this time, all authorities warn that TMJ surgery must be considered a last resort, because failures can be disasters.  Apparently the scar tissue left from the surgery can prevent natural adaptation. Fortunately, as a result of recent improvements in oral orthopedic appliances, TMJ surgery is rarely necessary.  Unfortunately, there are still surgeons who do not know much about using oral orthopedic appliances, and they perform surgery when it may not be necessary.

SUMMARY

A dislocated TMJ disk does not condemn a patient to years of misery. Studies have shown that thirty percent of modern populations have a dislocated disk in one or both of the TMJs, and most of those people do not suffer from significant symptoms. Many report that they have had symptoms at some point for a period of time varying from weeks to decades, but the symptoms eventually resolved. Although waiting for such resolution is one option, simple orthopedic treatment of the TMJ can enable adaptation and consequently relief of symptoms in days or weeks rather than years or decades.