What is TMJ?
The human jaw is a U-shaped bony structure which constitutes the lower part of the face. The lower jaw does not have a bony attachment to the skull, but it attaches to the skull via muscles, ligaments and soft tissues.
The temporo-mandibular joints are located in front of the ears. All the movements of the lower jaw are based on movements occurring at the connection point of the skull and the lower jaw, the TMJ.
The TMJ is both a hinge and a sliding joint. When the mouth is opened, the movement begins as a circular motion typical of a hinge joint (eg. knee). When the mouth is opened further, the movement continues as a sliding motion.
THE TWO-PART MOTION OF OPENING THE MOUTH
- Circular motion: the temporo-mandibular joint rotates against the joint surface, as the tip of the chin moves downward
- Sliding motion: the lower jaw slides forward and down from the glenoid fossa (shallow depression in the bone)
Thanks to this sliding motion, the mouth opens approximately 1.6–2.4 inches (40–60 mm) as measured between the tips of the front teeth, for some people even further than 2.8 inches (70 mm).
Between the bony structures of the temporo-mandibular joint is an articular disc formed of connective tissue, whose task is to act as a protective barrier between the bony structures of the temporo-mandibular joint – the joint head of the lower jaw, and the skull. The job of the articular disc is to soften the movements of the bony surfaces, as well as to reduce the abrasive forces in the temporo-mandibular joint.
Jaw popping or jaw clicking
Popping or clicking sounds in the temporo-mandibular joint are quite a common problem, and can occur when the mouth is opened or closed. The most common cause is reversible displacement of the articular disc of the TMJ (reciprocal disc displacement).
Due to fatigue or structural reasons, the articular disc between the head of the jaw joint and the skull can stretch and weaken. The articular disc of the temporo-mandibular joint can be displaced, and generally the disc slips in front of the joint.
The articular disc can nevertheless return to its place between the joint and the skull when the mouth is opened. This is called reversible displacement of the disc; the articular disc is displaced when the mouth is closed, but returns to its place when the mouth is opened.
When the mouth is opened and the articular disc returns into place, sometimes you may sense a rattle in the joint, and sometimes a click. Joint cracking may not always occur when opening the mouth, and it may even vary on a daily basis. The temporo-mandibular joint can also crack when the mouth is being closed, when the articular disc slips to the front of the joint.
Less common causes of temporo-mandibular joint cracking are temporo-mandibular joint attachments’, ligaments’, strains. Rough surfaces of the joint sliding against each other (TMJ wear, Osteoarthrosis) can also cause a click.
The temporo-mandibular joint is a fluid-filled synovial joint which may click as a result of the so-called cavitation effect, the same as cracking knuckles. Fluid cavitation is caused by a joint stretching and achieving low pressure, such as when the mouth is opened, which causes a gas bubble in the synovial fluid. When the bubble collapses, a crack is heard.
The TMJ rasps
The most common cause for a rasping noise in the temporo-mandibular joint is temporo-mandibular joint osteoarthritis, osteoarthrosis. Osteoarthrosis often preceded by displacement of the temporo-mandibular joint articular disc.
If the temporo-mandibular joint articular disc is displaced, the cartilaginous joint heads of the lower jaw and the skull rub against each other. In particular, Bruxism ie teeth grinding can cause stress in the temporo-mandibular joint, and with time the cartilaginous surfaces can wear down.
Eventually, the jaw joint bony structures can rub against each other, which can cause rasping when opening and closing the mouth.
In addition to osteoarthrosis, a lack of temporo-mandibular joint synovial fluid can cause a rasping noise.
Mouth does not open, or opens at an angle
The opening of the mouth may be limited if the temporo-mandibular joint articular disc is displaced. In most cases, the articular disc slips in front of the joint. If the articular disc does not return to its correct place when the mouth is opened (irreversible disc dislocation), the articular disc remains on the front surface of the joint, limiting the movement of the joint. In this case, the mouth does not open normally.
Temporo-mandibular joint articular discs are rarely displaced at the same time. For example, if the right temporo-mandibular joint articular disc is out of place, the right joint will not open normally, and the lower jaw rotates to the right side when opening the mouth.
The TMJ is sore
Temporo-mandibular joint surfaces have a lot of pain-sensing nerves that are sensitive to joint overload and damage.
The most common cause of TMJ pain is temporo-mandibular joint osteoarthritis. Osteoarthritis refers to the temporo-mandibular joint inflammatory condition that can cause erosion or wearing of temporo-mandibular joint cartilage and the bone under the cartilage.
Osteoarthritis, or temporo-mandibular joint inflammation can be caused by trauma, such as a strike against the lower jaw, or joint overstraining such as bruxism.
What factors predispose the TMJ to diseases?
The temporo-mandibular joint endures normal activities, such as eating and talking, very well. Excessive strain may be caused in particular by bruxism, or the grinding of teeth, harmful habits such as the long-term chewing of gum, as well as different bite errors.
Overwork of the temporo-mandibular joint can cause damage to the temporo-mandibular joint articular disc, the support fibers attached to the articular disc, and the cartilaginous joint surfaces.
Traumas of the head and jaws may also damage the temporo-mandibular joint. In many sports (eg. fighting and team sports), strikes to the lower jaw are common.
Impacts to the lower jaw or directly to the TMJ can fracture the lower jaw joint head as well as damage to the articular disc, which may require restrictions on use of the lower jaw and even TMJ surgery.
If TMJ damage is suspected, you should always seek examination by a dentist. If TMJ trauma is suspected, medical investigations are often complemented with X-ray and magnetic resonance images.
General disorders and hormones
Rheumatoid arthritis is a common disease of the joints, which usually begins in small joints such as fingers or small joints in the foot. Rheumatoid arthritis may progress to the TMJ, causing jaw pain and limiting opening of the mouth, especially in the mornings. In particular, smoking has been found to increase the risk of rheumatoid arthritis.
The female sex hormone estrogen may increase the risk of TMJ disorders and painful conditions. The hormone estrogen has been found to cause temporo-mandibular joint hypermobility, as well as changes in the composition of articular connective tissue.
How are TMJs examined?
The functioning of the temporo-mandibular joints and movements of the lower jaw can be clinically examined.
TMJ FUNCTIONING MADE CLEAR WITH EXAMINATION
- the extent of mouth opening and side-to-side motions
- is jaw pain, a cracking noise, jaw locking, or TMJ noise associated with the opening the mouth
- do you feel soreness in the masticatory (chewing) muscles
The examination of temporo-mandibular joints generally also examines the bite and investigates general diseases and medications.
In addition to the examination of the temporo-mandibular joints by a dentist, X-ray and magnetic resonance images give additional information about possible changes in the temporo-mandibular joint.
When temporo-mandibular joint wear and tear such as osteoarthrosis is suspected, generally either a panoramic radiograph or a cone beam computed tomography image is taken. A cone beam computed tomography image is especially helpful in examining temporo-mandibular bone structure changes.
Problems with the temporo-mandibular joint articular disc can be determined most accurately by means of MRI. However, imaging costs limit the use of magnetic images.
Arthroscopy (joint endoscopy) can be used when other examination techniques have not resulted in a sufficiently accurate diagnosis. For temporo-mandibular joint endoscopy, the temporo-mandibular joint is anesthetized, a small incision is made in the joint area, and a small camera is inserted in the joint. The camera shows joint structure and the potential causes of problems.
When necessary, joint maintenance procedures can be done in connection with joint arthroscopy.
Sonography and motion sensors
The function of the temporo-mandibular joints and the motions of the lower jaw can also be examined by sonography, as well as with devices to measure the motion of the lower jaw.
Sonography measures potential temporo-mandibular joint noises and presents them graphically. Sonography can be used to examine, for example, rasping and cracking noises in the temporo-mandibular joints.
The use of sonography is limited by many factors. For example, temporo-mandibular joint cracking sound often reveals reversible displacement of the articular disc, but a cracking sound may also be related to abnormalities in the temporo-mandibular joint bony structures, stretching of ligaments or fluid cavitation. Also, various distractors such as background noise can interfere with sonography.
Temporo-mandibular joint noises do not necessarily reflect temporo-mandibular joint disease, but they also do not exclude it.
Lower jaw movements can be measured with a variety of commercial motion sensors and thus obtain information on the functioning of temporo-mandibular joints and masticatory muscles. An assumption for the use of motion sensors is that for a person suffering with temporo-mandibular joint or or masculatory muscle symptoms, jaw movements are slower or narrower than normal, or trajectories are otherwise disturbed.
However, this has not been demonstrated. Differences between the lower jaw movements of people suffering from temporo-mandibular joint or masculatory muscle problems and those of healthy individuals are smaller than the general differences of the lower jaw movements between individuals.
Treatment of TMJ symptoms
The treatment of TMJ disease aims to reduce TMJ pain as well as (where possible) restore the normal functioning of joints. Treatments may be different physiotherapeutic treatments such as lower jaw exercises, heat and cold treatments, acupuncture, and jaw pain medication.
Find further information about physiotherapy and other treatment methods in our Stomatognathic treatment article.
The appliance is also commonly used in the treatment of TMJ symptoms. The video below shows various stages of appliance manufacture.