Myths of the TMJ
Hardly a week passes without an advertisement arriving for a course on TMJ diagnosis and therapy. Has there been a great increase in the incidence of TMJ dysfunction? If you measured disease volume by the quantity of remedial advertisements, you would think the United States was experiencing an epidemic.
Even more exasperating are the diagnoses based on anatomical and physiological myths that are accepted without question by so many orthodontists. Although the elimination of these myths will not guarantee accurate diagnosis and good therapy, it might help clinicians appreciate the truth about joint physiology and anatomy.
Myth No. 1
The first myth that needs to be destroyed is that in the normal joint there may be linear sliding movement between the disc and the condyle. The only movement that ordinarily occurs between disc and condyle is rotation. This rotation happens only in the lower joint cavity of the TMJ complex; when joint translation occurs, the condyle and disc move together in the upper joint cavity. The main function of the joint disc is to maintain sharp contact between the articulating surfaces and thus prevent joint dislocation.
Disc contour prevents the normal disc from being displaced forward--independent of the condyle. Before displacement can happen, the posterior margin of the disc generally must flatten by either severe acute trauma or chronic occlusal trauma (loss of posterior teeth, severe occlusal wear, partial denture wear, etc.).
If you were to believe most lecturers on the TMJ, you would think forward translation of the disc, independent of the condyle, is the most common TMJ aberration. In fact, it is rare.
Myth No. 2
A second myth is that joint noise normally results from the condyle sliding over different parts of the disc. Ordinarily, noise in any joint is due to obstruction to movement that muscles must overcome with greater effort.
Myth No. 3
Another myth is that hypercontractility of the lateral pterygoid muscle is responsible for the independent forward displacement of the disc. The lateral pterygoid is, of course, two muscles with reciprocal functions, and the superior head is responsible for forward movement of the disc and condyle--only in association with hard biting forces (power strokes, as opposed to empty-mouth movements). A companion myth is that hypercontractility of the lateral pterygoid muscles is common. The muscle fiber composition of these muscles makes them less likely to express hypercontractility than is generally believed.
Myth No. 4
There is also a myth regarding the palpability of the lateral pterygoid muscle. Johnstone and Templeton proved the impossibility of this technique in 1980, but cherished myths die hard--and this one shows unusual tenacity. If this muscle is palpable through the buccal vestibule, why can it not be anesthetized in this manner? The only reasonable way to probe and anesthetize the lateral pterygoid muscle is through the sigmoid notch of the mandible. This is not to be done indiscriminately, because the pterygoid plexus is quite near the muscle.
Myth No. 5
One myth with mysterious appeal is that TMJ problems can be diagnosed with radiographic techniques. No dental radiographic technique should ever be thought of as diagnostic. Radiographs can sometimes confirm a diagnosis, but they should never be considered as more than a confirmation.
The only useful imaging technique for soft tissues such as the TMJ disc is noninvasive magnetic resonance, which is, of course, a static evaluation like every other imaging technique except fluoroscopy. The expense and unavailability of magnetic resonance also limit its use.
Radiographic techniques show only hard tissue. When arthrographic contrast medium is used to delineate joint cavities, the joint space is distorted by the augmented joint pressure and the disc may be displaced forward. Arthrographic techniques are now passe in orthopedics, having given way to more accurate arthroscopic techniques.
The article in this issue by Drs. Nuelle, Alpern, and Ufema shows how arthroscopy can be used to diagnose and then treat TMJ soft tissue conditions that have been misdiagnosed as anteriorly displaced discs. Some of this misdiagnosis was due to diagnostic reliance on magnetic resonance, arthrography, and clinical symptoms.
Myth No. 6
Oral surgeons also participate in the promotion of TMJ myths when they operate to "recapture the disc". The relief from pain that results from such surgery may owe more to sensory denervation than to recapturing of the disc. Nor should surgical recapture be considered a permanent correction, unless the self-centering mechanism of the disc is restored. This is difficult because of lost disc contour, and there is little published literature to suggest that oral surgeons generally understand the procedure.
If permanent surgical repositioning of the disc is improbable, should we expect functional appliances to recapture an anteriorly displaced disc? Functional appliances and splints can indeed relieve pain symptoms, because they reduce electromyographic activity in the jaw-closing muscles and substantially decrease occlusal forces. However, if the discomfort is due to an anteriorly displaced disc that occurred because of irreversible damage to its posterior contour, then we shouldn't expect any permanent repair from the use of functional appliances.
The disc is not an adaptive, malleable piece of cartilage that can regenerate and reestablish its original contour. Any anatomical damage that permits it to slip forward is quite likely to stay unhealed.
Myth No. 7
It is commonly believed that pain in and around the TMJ can refer to areas caudal to the joint such as the shoulder, back, and arm. This is incorrect because referred pain is almost always cephalad and entirely a central nervous system phenomenon. That is, it is not reliant upon peripheral synapses or axons for its effect. When people suffer coronary infarcts, for instance, they often feel shoulder or jaw pain--not groin or leg pain. When we eat ice cream too fast and chill the glossopharyngeal nerve (ninth cranial nerve), we feel referred pain in the frontal area of our heads (supplied by the fifth cranial nerve)--not in our shoulders or arms (supplied by the brachial plexus). And when patients have pain in the masseter or sternomastoid muscles, it will refer to the TMJ area in front of the ear--not down and away from the origin.
A common complaint of TMJ patients is often obscure, deep pain that seems to focus in and around the ears. In fact, many patients with TMJ problems will consult physicians about their ears, only to be told there is no ear infection or inflammation. This pain is most likely referred pain occurring in the central nervous system.
Referred pain depends on the continuation of the primary initiating pain and will not stop until the primary pain is arrested or interrupted. When the therapist applies some nostrum and a natural cessation of primary source pain occurs, it is tempting to believe that the applied therapy was the cure. The truth is that patients often improve in spite of their doctors' therapies. That has obviously happened with many TMJ patients, or this myth would not continue to circulate.
Before you swallow whole some story about the TMJ, measure it against this myth list. Then see if you don't discover a more accurate diagnosis and a more reasonable treatment plan.