Prevalence, Epidemiology and Etiology

Prevalence

TMD is the 2nd most common musculoskeletal pain, with low back pain being the first. About 33% of the population has at least one TMD symptom and 3.6 to 7.0% of the population has TMD with sufficient severity that they desire treatment.^1,3

As seen in Figure 2, TMD varies with age and there are significant gender differences. In a large study of adults (1,016) conducted in Seattle, WA,³  females had higher rates of TMD pain at all ages, including a peak prevalence of 18% in the 25-44 year old group, compared with 10% of males for the same ages. Since there is a decrease in prevalence after this age, this suggests TMD tends not to be a painful life-long disorder. For example, in the aforementioned study, only 2% of females and no males reported TMD pain at 65 years of age and greater.

Figure 2. Six Month Prevalence Rate (%) of TMD Pain.

Figure 2. Six Month Prevalence Rate (%) of TMD Pain.

TMD, like many other chronic pain conditions such as headache and back pain, typically has cycles in which the pain and discomfort are present and then diminish or go away completely. For the vast majority of people, a recurrence of TMD does not indicate a more serious problem. Only a small percentage of people with TMD pain develop significant, long-term problems.

Etiology

It is not always clear the reason a patient develops TMD symptoms, or what has caused an exacerbation of existing symptoms. As is true for all common chronic pain problems (e.g., common tension headache, low back pain) the scientific evidence overwhelmingly views these problems from a biopsychosocial perspective. The biopsychosocial model regards TMD as an integrated combination of biological, psychological, and social factors that interact together; TMD is not only a physical or biological condition. A thorough history, examination and patient observations that address these domains provide the information needed to render a diagnosis and most appropriate treatments.

Physical

Direct and indirect trauma to the face can cause TMD pain and symptoms. A direct blow to the face can cause pain and in some cases fracture of underlying bones. A motor vehicle accident, whether with direct or indirect trauma, can cause pain in the face. TMD symptoms can even be due to dental treatment, such as from a patient staying open for an extended period of time, forcing the patient's mouth beyond the patient's opening comfort zone with a bite block, or from an inferior alveolar nerve injection causing trismus of the medial pterygoid muscle.

There are a number of local and systemic conditions that can cause resorption of the condyle, which can result in TMD related problems. Local pathologies include: osteoarthritis, reactive arthritis, infection, and traumatic injuries. There are many systemic conditions that can also cause condylar resorption, the more common of these connective tissue and autoimmune diseases include rheumatoid and psoriatic arthritis, lupus erythematosus, and Sjögren’s syndrome. Additionally, scleroderma and ankylosing spondylitis can cause condylar resorption, as can infections as Lyme’s disease. Another condition that affects the TMJ is idiopathic condylar resorption (ICR). ICR is also called idiopathic condylysis, condylar atrophy and progressive condylar resorption.

Another cause for an abnormal condyle is congenital malformations of the condyle. Both hyperplasia (an overgrowth of the condyle) and hypoplasia (a shortened mandibular ramus and/or an underdeveloped condyle) can occur, but these generally do not predispose the patient to TMD.

Another potential contributing factor for TMD may occur from placing a restoration that is not in harmony with the rest of the patient’s occlusion.^3,4 While a patient’s natural occlusion is generally only a minor contributor to the etiology of TMD, changing a patient’s natural occlusion (e.g., through orthodontics) does not provide the long-term TMD symptom benefit that would justify this.²

Behavioral

Stress may cause or aggravate TMD. Most of us think of “stress” as an external situation (the “stressor”) that causes us to have negative physical and emotional reactions, which may overwhelm a person’s ability to cope effectively. If a stressful situation persists, or if a person has had a number of stressors over time, the body may develop a higher level of muscle tension, predisposing the person to TMD.

Some patients may clench or grind their teeth when they experience stress or hold excessive tension in their masticatory muscles and this can result in sore jaw muscles and/or TMJs, and can also cause headaches. Some patients will feel physically or emotionally fatigued or “stressed” as a result of their pain, and this can reduce their ability to cope with the pain. This may lead to even more pain, which for some patients, becomes a cycle of pain, stress, tension, and fatigue. Thus, while it is difficult to know whether stress has caused a patient’s TMD, stress generally makes TMD worse.

Nocturnal (nighttime) and diurnal (daytime) clenching and grinding can cause TMD pain and other symptoms. A patient may report clenching or grinding, or a dental examination may provide this indication through tooth attrition. For patients with significant tooth attrition, it is important to determine whether this attrition is associated with a current grinding habit, or if the attrition occurred in the past. Studies have not found significantly greater tooth attrition in patients with TMD and some patients exhibit very severe tooth attrition with no history of TMD pain or dysfunction.

Patients with cerebral palsy, dystonia, or Parkinson’s disease may grind their teeth due to an associated movement disorder. These sustained muscle contractions can cause jaw muscle fatigue and pain. Additionally, patients with a severe dystonia may grind excessively resulting in severe tooth attrition.

There are many other types of “parafunctional habits” (using the jaw in ways other than its intended purpose) people engage in that can cause or exacerbate TMD. These habits may be repetitive movements (e.g., gum chewing, biting lips or cheek) or other habits (e.g., chewing on pencils, pens, and fingernails; shifting the jaw and holding it to one side; and continually popping the TMJ). Some patients with a Class II malocclusion will posture the jaw forward in order to have more of a Class I appearance, which can also lead to muscle fatigue. Frequently patients are unaware of the contribution a parafunctional habit has on their jaw muscle tension and fatigue.