The Temporomandibular Joint (TMJ) is located just below the ears, is the joint attaching the lower jaw, called the mandible, to the upper jaw, called the temporal bone. It is a unique joint in that it is bi-lateral. The Mandible being all one bone and is connected to the temporal bone in two places, one a mirror image of the other. The ends of the mandible are rounded and called the condyles.
The condyles are in turn separated from the bony surface of the temporal bone by soft disks. These disks not only act to separate the hard bones but also to absorb and cushion vibrations and impacts transmitted through the joint.
As in the other body joints, the TMJ is controlled by muscles. The muscles controlling the TMJ are predominantly the masticatory muscles, these being located around the jaw and the TMJ. These would include the internal pterygoid, external pterygoid, mylomyoid, geniohyoid and digastric muscles. However, other muscles may have an effect on the functioning of the TMJ, such as the neck, shoulder and back muscles.
Ligaments and tendons, are present filling their usual roles of attaching the bones and muscles to each other and to other bones. Damage to any of these components can alter the working of the TMJ, affecting the opening and/or closing motion of the mouth.
Figure 1, below is a lateral view of the TMJ showing its position and its main components, including the mandible, condyle, temporal bone and the disk separating them.
Figure 1
The TMJ is the most frequently used of all the body joints being active during talking, chewing, biting and any action that involves the jaw moving. Even when the jaw is closed the muscles around the TMJ are being tensed to hold the jaw closed.
Only when that body is at rest, e.g. asleep, do the muscles time to relax. Although, for some people the joint is either being made to work or it can suffer stresses. For example, people grinding teeth while asleep or sleeping in a manner that the jaw is being pushed into the bed cause the muscles and ligaments to be stresssed. Temporomandibular Joint Dysfunction (TMD).
All these basic everyday uses of the jaw that are taken for granted can and often do lead to a dysfunction of the TMJ in some way. The TMJ is susceptible to a variety of damage due to both external events, such as trauma, or to internal deterioration, such as tumours or arthitis. A wide range of dysfunctions of the TMJ can occur. While some dysfunctions cause problems with the use of the TMJ, most do not. Severe dysfunctions, such as a fused joint, can be very disruptive to the use of the jaw, being painful to the person involved and even requiring extensive surgery.
Trauma is considered to be the leading cause of TMD. According to the Journal of American Dental Association in 1990, 40% to 99% of all TMD cases were results of trauma. Here, trauma can refer to something a simple as a blow to the jaw or even something as subtle as a whiplash injury to the head or jaw, cervical acceleration/deceleration, (CAD). Recent studies are also aiming to show that airbagdeplyment in vehicles can also lead to TMD.
Every joint in the body has limited movement. If the jaw is opened too wide for a long time or is forced open, ligaments may tear. Even when the jaw is opened normally, there is a partial dislocation of the TMJ. However, as the jaw is opened beyond the norm, a dislocation can occur or the separating disk may even become dislodged. Continuous bruxism, (the abnormal grinding of the teeth), can lead to TMD.
Bruxism usually occurs during sleep and often the sufferer does not realise that it is happening. Although in some cases the grinding can wake the person. Usually the person will awake with sore jaw muscles in the morning. This is often the only indication of its presence. Bruxism produces muscle pain as well as sensitive and worn teeth. Constant grinding of the teeth puts abnormal pressures on the TMJ which may lead to ligament damage, in turn causing the dislocation of the joint’s disk.
Malocclusion is the dental term describing when the teeth do not make contact in the normal way, often simply refered to as bad bite. This may be produced in several ways including the removal of teeth, lack of posterior prop, advent of pre-molars, (wisdom teeth), poor fitting dentures.
Another cause of TMD is stress. Stress for whatever reason, can cause the tightening og muscles, particulary about the shoulder, nech and especially the TMJ. This tightening or tensing of muscles puts abnormal pressure on the TMJ wihch in turn can lead to TMD, if only temporarily, (finishing after the stress disappears). However, prolonged stress can also produce permanent muscle and ligament damage about the TMJ and so create permanent TMD.
Various diseases can cause or aggravate TMJ problems. Immune disorders such as rheumatiod arthritis or oesteoarthritis can inflame the TMJ. Also, some viral infections like mumps and measles can cause damage to joint surfaces, which can then lead to internal derangement of the TMJ.
