The Role of Disk Dislocation
SUMMARY - Most TMJ disorders begin with a dislocation of the articular disk from its proper position between the bones in the TMJ. This causes the clicking or popping that is often characteristic of the disorder. However, the dislocation is normally followed by a natural adaptation process that restores normal function and enables the dislocated TMJ to chew even tough foods. the process may require an hour or a decade, but it almost always occurs by middle age. It can also be facilitated by mandibular orthopedics, so it occurs in days rather than years.
ANATOMY - The temporomandibular joints (TMJs) connect the lower jawbone to the skull on each side of the head, just in front of the ears. Each TMJ holds a fibrous aticular disk down on the top of the condyle (the back end of the lower jawbone) with a pair of collateral ligaments, like chinstraps holding down a hat. When in place, this disk functions as a well lubricated cushion between the bones, and the ligaments allow it to roll forward and backward on the condyle to adapt to varied chewing conditions without separating from the bones. The disk in its normal position is shown below:
TMJ disorders nearly always start with a dislocation of the disk (below right).
The role of disk dislocation has been controversial, and opinions have swung like a pendulum, as they often do after new information is uncovered. The new information was Farrar's surprising discovery in the 1980's that most of the clicking and popping sounds in the TMJs are due to dislocated articular disks, unlike the clicking sounds commonly heard in other joints. For the next decade, many dentists tried to relocate (recapture) these dislocated disks without understanding how to establish the necessary therapeutic bite, and consequently most of their attempts failed. In addition, many of those dentists trying to recapture dislocated disks did not understand the self-limiting nature of TMJ disorders and were overtreating patients who actually had little pain and an excellent prognosis without treatment. About a decade later, the pendulum swung back the other way. Dental authorities warned that, because TMJ disk dislocation occurs in about 30% of the general population, it should be considered a variation of normal.
Now we know that dislocation of a TMJ disk can produce severe symptoms in some people and very few in others. The dislocated TMJ functions like a door off its hinges. Damage can occur to the door (the lower jawbone), the door frame (the upper jawbone), or its hinges (the TMJs); but all these components also heal naturally, even without treatment. Treatment can relocate the disk into its proper position (recapture) in some cases, especially when the dislocation is recent and the joint noises are clear, but such extensive treatment is rarely necessary to eliminate the pain, because TMJs adapt to disk dislocation by fibrosing of the retrodiskal (behind the disk) tissues to create a new elongated disk-like structure, which functions so much like the original disk that it is known as a pseudo-disk.
The dislocation of the disk is associated with a facial growth pattern that is displaced to the side of the dislocated disk, and the dislocation certainly removes about 1/2 mm in height from the condyle on the side of the dislocation. That loss of height produces a cant in the orientation of the mandible and the bite; but the cause of the disk dislocation is almost always the displaced facial growth (the mandible growing toward the side of the dislocated TMJ due to an asymmetrical bite), rather than the disk dislocation causing the aberrant facial growth pattern.
The cause of the disk dislocation is almost always a bite that forces the condyle into the back end of the joint, where it mashes the network of ligaments that intertwine to hold the disk down on the top of the condyle from behind and the sides, where they coalesce to form the collateral ligaments (the chinstraps), as seen in the illustration below of a condyle from behind. When the ligament network gets damaged, it releases the disk from behind. The disk then gets squeezed out forward or to either side from 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 the disk into the place where the disk used to be. These retrodiskal tissues are very vascular. The blood vessels are shown as seven dark blotches just beneath the ligament network in the illustration above. Trapped between the bones, they are easily damaged by chewing forces, especially during the nocturnal grinding and clenching (bruxism) that affects everybody during sleep. Initially these retrodiskal tissues undergo bruising, along with all its typical characteristics,- blood leaking out of vessels, swelling, heat, and pain. The bruise can last for minutes or years. Eventually they adapt anatomically, as described later.
The dislocation of the disk also alters the growth pattern of the face by slightly shifting the position of the mandible, which then shifts the positions of the teeth to fit the new mandibular bite position. Typically the mandible shifts toward the side of the dislocation, and the mandibular condyle on that side becomes shorter; which causes the head to tip toward that side, making the eye of the other side look higher in pictures.
FORWARD DISK DISLOCATION - About half of disk dislocations occur in a forward direction, where the disk becomes an obstacle in the opening pathway of the condyle and prevents wide opening. In early stages, the mandible can only open until the condyle of the dislocated disk side reaches the disk. Then the ligaments that once held the disk in place hold it in the opening path of the condyle, and the condyle is stuck behind the disk (frame 2). 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 frames 2 and 3 in the illustration below. The disk then stays in place until it dislocates again during closing, as seen between frames 8 and 1.
CLOSING FULLY CLOSED OPENING
ROTATIONAL DISK DISLOCATION - is a forward dislocation on only one side. It occurs when one collateral ligament holds, while the other fails. The disk then rotates around the side that held.
SIDEWAYS DISK DISLOCATION - occurs when a disk gets dislocated medially (inward) or laterally (outward). 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 the ear. Medial dislocations often cause a sudden sideways shift of the lower jawbone during opening. 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 onto its center; and the clicking or popping stops, because the disk prevents the lower jawbone from opening far enough to click. This condition is called a closed lock, even though the mouth is not actually locked closed - it just cannot open as wide as before. Most people experience intermittent locks that last for minutes or hours before the lock becomes permanent. The closed lock stage is usually the most symptomatic stage of the TMJ disorder.
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
PROGRESSION - Over time, the displaced disk may take one of three courses. It may flatten from being run over hundreds of times each day, it may ball up from being pushed forward like in the above illustration, or it may become permanently stuck (adhesed) onto the underside of the skull (the articular eminence of the temporal bone), where it can prevent full opening until it gets ironed out or degenerates. The retrodiskal tissues that initially became bruised get worn away, and jaw movements produce 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. The natural progression of the disk deterioration is illustrated below.
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 danger after a TMJ has healed by anatomical adaptation, as described in MANAGING THE ARTHRITIC TMJ. This natural adaptation may take anywhere from days to decades, but it almost always occurs naturally by middle age.
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 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. If you take 10 seconds to open, 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 no matter how slow or fast the 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 it 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 sounds depend on the speed of the condyle (moving very slowly changes the sounds), and the sounds can be made repeatedly without fully opening or closing.
IMAGING - of disk 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 you to years of misery. MRI studies show that thirty percent of the population have a dislocated disk in one or both TMJs, and the vast majority of them no longer suffer, because their TMJs have undergone natural adaptation. Although waiting for such adaptation is one option, simple orthopedic treatment can enable adaptation in days or weeks rather than years or decades.