The Role of Disk Dislocation

ANATOMY - The temporomandibular joints (TMJs) connect the lower 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 lower jawbone) by 2 collateral ligaments that act like chinstraps holding a hat down on a head, as seen below.  They allow some rolling of the disk forward and backward to adapt to varied chewing conditions, but they keep it on the top of the condyle so that the bony surfaces of the condyle and the underside of the skull never rub directly against each other.  The disk in its normal position is shown below:

TMJ disorders nearly always start with a dislocation of the disk (below right).  

The cause of the dislocation is almost always a bite that forces the condyle backward into the posterior border of the joint, where it damages the network of ligaments that hold the disk down on the top of the condyle.  That ligament network extends around the sides and behind the disk and condyle, holding them together.  The network of ligaments and accompanying blood vessels can be seen in a view of the condyle from behind, as illustrated below.  When it gets damaged, it releases the disk from behind.  The disk then gets squeezed out forward or sideways from its proper position between the bones, like squeezing out a watermelon seed out from between two fingers.       

                                                                

 

The dislocation of the disk pulls the tissues that were behind it (the retrodiskal tissues) into the place where the disk used to be. Trapped between the bones, these retrodiskal tissues are vulnerable to damage by chewing forces, especially during the nocturnal grinding and clenching (bruxism) that affects everybody during sleep. Initially they undergo bruising with all its typical characteristics,- blood leaking out of vessels, swelling, heat, and pain.  The inflammation caused by the bruising triggers reflex muscle tension. However, eventually these joints heal. They adapt by converting the previously vascular tissue  into a tough fibrotic pad of scar tissue called a pseudodisk, which can withstand functional forces. The conversion to a pseudodisk can take days or decades, but it almost always occurs naturally by middle age.

The dislocation of the disk disturbs the growth pattern of the face by causing a shift in the central bite position, the bracing position for the mandible. The mandible on the side of the dislocation shifts toward that side and upward, which then causes the whole head to tip toward that side. The head cannot sit straight on the top of the spine if the mandible cannot rest beneath the center of the skull. That relationship is summarized in THE ROLE OF POSTURE under the tab TMJ DISORDERS, and it is fully explained in BITES AND BODY POSTURE under the FOR DOCTORS tab.  

FORWARD DISK DISLOCATION - About half of disk dislocations occur in a forward direction, where the disk becomes an obstacle to the opening pathway of the condyle. In early stages of forward disk dislocation, the lower jawbone can only open until the condyle of the dislocated disk side reaches the  disk. Then the ligaments that once held the disk in place now hold it in the opening path of the condyle. The condyle is thus stuck behind the disk. To open further, the condyle has 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 it dislocates again during closing, as seen between boxes 8 and 1.

                                                           CLOSING                                                  FULLY CLOSED                                                               OPENING

ROTATIONAL DISK DISLOCATION - occurs when only one side of the ligament network gets disrupted, and the other side stays in place. The disk then rotates around the side that stayed in place, and it becomes dislocated on only the other side, where the ligament is disrupted. 

SIDEWAYS DISK DISLOCATION -  occurs when a disk gets dislocated medially (inward toward the brain) or laterally (outward toward the ear). Sideways disk dislocations rarely limit opening, because the disk is dislocated into a position that does not form an obstacle to the opening pathway of the condyle. Lateral dislocations can often be felt with a finger in front of the ear.  Medial dislocations often cause a sudden sideways shift of the lower jawbone during opening. These sideways dislocated disks are more difficult to recapture than forward dislocated disks.

CLOSED LOCK - In many forward disk dislocations, at some point, the condyle becomes unable to jump over the back end of the dislocated disk, and it stops clicking or popping back into place. At this point of partial opening, the disk becomes an obstacle that prevents the lower jawbone from opening beyond the point where it used to click. 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. In many cases, the transition from clicking to locking occurs multiple times on an intermittent basis before the lock becomes permanent. 

When a dislocated disk is in the closed lock stage, attempting to open wide causes your chin to deviate toward the side of the disk dislocation, because that condyle on that dislocated side stops moving forward (stopped by the ligament that once held it in place) while the other condyle keeps moving forward. The process of opening and closing with a disk that is in closed lock (non-reducing) is illustrated below.

                       CLOSING                        FULLY CLOSED                            OPENING

The closed lock stage is usually the most symptomatic stage of the TMJ disorder. Some locked TMJs can be unlocked, as explained in TREATING THE LOCKED TMJ. 

DEFORMED DISK  -  Many dislocated disks deform rather than undergo locking. Either they flatten as a result of repeatedly getting run over by the condyle or they ball up in front of the condyle.  In both cases, the click or pop disappears, but the TMJ has still lost its cushion.

ADHESED DISK - Occurs when 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 until it gets ironed out or degenerates.  

IN ADVANCED STAGES of TMJ disk dislocation, the jaw movements 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. Fine crepitus becomes more coarse over time, and eventually may sound like footsteps in gravel. These joint noises are scary, but they do not indicate any particular danger; because they eventually heal by anatomical adaptation, as described in MANAGING THE ARTHRITIC TMJ. This natural adaptation may take anywhere from days to decades.  

DIAGNOSIS -  is often but not always easy. Disks that are flattened, anatomically deformed, or dislocated sideways may produce no sounds, and some TMJ sounds do not indicate disk dislocation or any serious pathology, much like the harmless clicking sounds that occur in other joints.  

The noises made by TMJs in early stages of disk dislocation normally have two unique characteristics which make them easy to identify - the opening sound occurs when the mouth is opened wider than when the closing sound occurs, and those sounds are not affected by the speed of jaw movement. During very slow opening, the dislocated disk is pushed in front of the condyle until the ligaments holding it back are stretched tightly and snap it back into place, producing the same click or pop that occurs during fast opening. 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 gets forced off the top of the condyle near the end of the closing pathway, often accompanied by a softer sound just before the mouth is fully closed. 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, the sound it makes depends on how fast the condyle runs over it, and the mouth need not close all the way to create a click or pop on the next opening.   

IMAGING -  of disk displacement requires MRI. Although X-rays can image bones with more detail than MRI, they cannot produce an image of the disk unless dye is injected above and below it so that its presence can be inferred from the lack of dye. This arthrography procedure can be very painful, because it requires injecting contrast fluid into a joint capsule that is already swollen. It is generally considered outdated. 

CONCLUSION

A dislocated TMJ disk does not condemn a patient to years of misery. Studies of a random population using MRI have shown that thirty percent of them have a dislocated disk in one or both of their TMJs. At least 29 of those 30 people no longer suffer from significant symptoms. They may have had symptoms at some point, but eventually their TMJs underwent natural adaptation and their symptoms resolved. Although waiting for such resolution is one option, simple orthopedic treatment can enable adaptation that eliminates symptoms in days or weeks rather than years or decades.