Because of the severity of the pain it is sometimes impossible to eat or to talk. The pain from simply smiling can reduce me to tears. As you can imagine this severely cuts into my interactions with others. I cannot work. I cannot go to social events like Sunday morning church service. For those who know me you know how I love to laugh and gab and sing. I can no longer do this, the pain is often too much. I have become very depressed.
The masticatory system supports many functions vital to human health and well-being, including chewing, drinking, speech, and facial and emotional expression. Disorders of the masticatory system (generally termed temporomandibular disorders, or TMDs) involve the muscles of mastication (chewing), the temporomandibular joints (TMJs), and associated nerves and tissues (see Appendix D). Important outcomes of changes in the functioning of the masticatory system may include a reduction in the ability to use the jaw, ongoing pain, or many other subsequent impacts on an individual’s overall quality of life. These changes to the quality of life can include a decrease in the pleasure that one obtains from eating and an alteration in eating behaviors, not just in what is consumed but also in where, when, and with whom a person eats. Such changes affect the individual’s work and social life. Also, while much more needs to be learned about the impact of TMDs and orofacial pain on people’s lives, the evidence is clear that cultural, geographic, socioeconomic, and gender factors contribute to the impact of altered masticatory function on an individual’s
self-image, health, and well-being. This report focuses on the impact that TMDs can have on many aspects of a person’s health and well-being, which go well beyond the face and jaw. Many TMDs are multi-faceted and need interdisciplinary attention from clinicians in medicine, dentistry, and other health care fields. The goals of this chapter are to describe the complexities underlying TMDs and to establish the terminology and classification framework that will be used in the remainder of this report. The chapter discusses the scope, definitions, assessments, and classification of the disorders.
The musculoskeletal structures of the masticatory system and the neurological structures that control a wide diversity of functions are complex, and much about them remains to be investigated (see description of the anatomy in Appendix D and research directions discussion in Chapter 4). Additionally, the cervical system has a critical role in both the normal and abnormal functioning of the TMJ. As detailed in the overview of the history of the terminology (see Box 2-1), the labeling of the group of disorders affecting the masticatory system has varied across the years, reflecting the complexity of the disorders and adding to the confusion regarding what is a disorder, how the disorder should be treated, and by whom. These problems continue to the present.
This report follows the recommendations of the research community—as well as of the consensus report from the American Dental Association in 1983—and uses the term “temporomandibular disorders” (Laskin et al., 1983). TMDs are defined as a set of diseases and disorders that are related to alterations in the structure, function, or physiology of the masticatory system and that may be associated with other systemic and comorbid medical conditions. The committee emphasizes the multiple disorders that are encompassed by the umbrella term TMDs and the multiple causes of these disorders. Emphasizing the plurality of the conditions is important, as there are more than 30 individual TMDs (see discussion later in this chapter). The committee emphasizes that the single term “TMD” should only be used when referring to a specific TMD, such as myofascial pain of the masticatory muscles. It is important to note that neither “TMD” nor “TMDs” is a diagnostic term. Each condition, as based on the most current full taxonomy, has established diagnostic criteria, and the validity of the criteria range from untested, to tested and poor, to tested and excellent. “TMD” is not a single diagnosis but requires further diagnostic work to identify the specific disorder—or disorders—that an individual is experiencing, the potential involvement of multiple body systems and comorbid conditions, and the appropriate approaches to treatment or management. Patients often
present with more than one TMD as well as comorbid health conditions (see Chapter 3).
The use of the term “TMDs” should not be construed to reflect less attention by this committee on the importance of the TMJ and the mechanical problems involving the TMJ that lead to functional limitations, as compared with TMDs that are primarily characterized by pain and that
have received far more research attention and have, as a result, seen much more progress to date (Ohrbach and Dworkin, 2016).
Conclusion 2-1: Multiple disorders are encompassed by the terms “temporomandibular disorders” or “TMDs.” The committee defines TMDs as a set of diseases or disorders that are related to
alterations in structure, function, or physiology of the masticatory system and that may be associated with other systemic and comorbid medical conditions. TMDs can be usefully separated into two groups: the common TMDs with validated diagnostic criteria and the uncommon TMDs that do not yet have validated diagnostic criteria due to the challenges of conducting research on rare conditions. When possible, a patient’s diagnosis needs to be focused on the specific TMD or TMDs. “TMD” should not be used as a diagnostic term. An individual patient may have more than one TMD and may also have comorbid conditions.
Substantial efforts have been made to categorize the multiple types of TMDs and to develop validated diagnostic criteria. Additionally, attention has been given to exploring how the types of TMDs that are painful fit into broader categorizations of orofacial pain disorders. The following section provides a brief overview of the history of the categorization systems; more details are available (Ohrbach and Dworkin, 2016). Notably across this history the same dominant symptoms and signs of a small number of TMDs are seen in the general population and in those individuals seeking care. It is the manner in which those symptoms and signs are interpreted that changes across time and settings. These disorders are discussed in greater detail below. In addition to the categorizations developed by clinicians and researchers, patients often develop their own perspectives in their attempts to understand the disorder from the lived experience. Many of the patient quotes in this report highlight some of those disease perspectives.
Overview of the History of Categorization and Diagnostic Criteria for TMDs
1950s to 1980s
Diagnosis of TMDs between the 1950s and the 1980s followed several pathways. The TMJ syndrome approach (discussed in Box 2-1) tied the diagnosis specifically to various occlusal features. However, this was not found to be a workable approach for diagnosis because “abnormal” occlusal features can be found in most individuals (Proffit et al., 2013). A second pathway required appropriate history-taking coupled with an examination restricted to those parts of the masticatory system central to the disorder definition in order to determine if myofascial pain dysfunction syndrome was present; this pathway, resembling medical management, was less familiar to most dentists. In either of these first two approaches, there
was one diagnosis. A third pathway focused on the position of the TMJ disc, which led to two different diagnostic and treatment approaches. One involved a focus on TMJ surgery to repair or replace the disc. However, problems were identified with joint implants, and many patients experienced serious adverse effects from those implants (see Chapter 5). The second approach classified individuals based on the nature of the occlusion and its purported effects on the position of the clicking disc and the bony TMJ. This approach led to extensive (and often very expensive) orthodontic and prosthetic treatments in order to alter the occlusion (Bellavia and Missert, 1985; Laurell, 1985; Lundh et al., 1985; Kurita et al., 2001). A retrospective assessment of disc repositioning treatments clearly indicates that it was an unnecessary intervention (Greene and Obrez, 2015).
By the late 1980s, nine different published categorization systems had emerged for the diagnosis of TMDs. One evaluation of these systems used the following criteria: methodological considerations (sampling method, research suitability, and inter-rater reliability of clinical evaluation), diagnostic validity (specificity, inter-rater reliability of diagnosis), and clinical considerations (biological plausibility, exhaustive diagnostic framework, provision for multiple diagnoses, and clinical decision making) (Dworkin and LeResche, 1992). None of the evaluated systems met the criteria required of a diagnostic system.
Research Diagnostic Criteria for Temporomandibular Disorders and Initial Guidelines from the American Academy of Orofacial Pain
In 1992 the Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD) was released (Dworkin and LeResche, 1992). This effort, which was supported by funding from the National Institutes of Health, was based on the biopsychosocial model of health and disease and relied on epidemiological data. The RDC/TMD incorporated a dual axis system: Axis I focused on clinical history and physical examination findings in support of non-overlapping diagnoses, and Axis II focused on pain parameters, mandibular jaw function, psychological status, and the level of psychosocial function (Dworkin, 2010). This approach used strongly operationalized criteria in order to facilitate inter-examiner reliability, and it allowed for the provision of multiple diagnoses. The RDC/TMD was developed to provide an instrument to be used in epidemiological, observational, and clinical trials research (Dworkin, 2010). Use of the dual-axis system with TMDs was considered controversial by clinical dentists at the time of publication, but, in the intervening years, the dual-axis approach has become more accepted within dentistry and has become a model for other pain classification systems (Garofalo and Wesley, 1997; Deyo et al., 2014; Ohrbach and Dworkin, 2016).
Three critical characteristics of the RDC/TMD were apparent subsequent to the 1992 publication. The first was the recognition that continued research was needed in order for this descriptive classification system to evolve. The second was that the RDC/TMD was not intended to be either inclusive of all TMDs or self-sufficient with regard to differential diagnosis requirements for distinguishing a potential diagnosis of a TMD from other diagnoses. And the third was that only the more common TMDs were included in the RDC/TMD because the prevalence of the uncommon TMDs was so low as to preclude effective research at a single clinic. The assessment and diagnostic reliability of the RDC/TMD was found to be acceptable (Lobbezoo et al., 2004; John et al., 2005; List et al., 2006; Look et al., 2010), but the diagnostic validity needed to be evaluated. The latter required clear decision rules for non-overlapping diagnoses.
Also during the early 1990s the American Academy of Orofacial Pain developed a clinical diagnostic system that aimed to be comprehensive of all the known TMDs. Disorders were defined in an inclusive manner of multiple features, and a diagnosis was based on presence of any of those features. However, the system was hampered by several problems that limited its validity.
Diagnostic Criteria for Temporomandibular Disorders (DC/TMD)
In 2001 a major multi-site assessment of the RDC/TMD was initiated (funded by the National Institute of Dental and Craniofacial Research), which led to published outcomes, a public symposium in 2008, and an international consensus workshop in 2009 (Ahmad et al., 2009; Anderson et al., 2010; Dworkin, 2010; Haythornthwaite, 2010; John, 2010; List and Greene, 2010; Lobbezoo et al., 2010; Look et al., 2010; Ohrbach et al., 2010a; Schiffman et al., 2010a,b; Stegenga, 2010; Truelove et al., 2010). The workshop was led by the International RDC/TMD Consortium Network of the International Association for Dental Research (now named INfORM [International Network for Orofacial Pain and Related Disorders Methodology]) and the Orofacial Pain Special Interest Group of the International Association for the Study of Pain (IASP), along with individuals representing a range of specialty areas. The working groups expanded and refined the TMD taxonomy, and their work led to the eventual publication in 2014 of the Diagnostic Criteria for Temporomandibular Disorders (DC/TMD) (Schiffman et al., 2014b), which maintained or improved all of the foundational principles underlying the RDC/TMD for the common TMDs. The Axis II assessment procedures were expanded and improved. The DC/TMD delineates 12 disorders, each based on clearly defined criteria; each criterion is well operationalized with regard to the required clinical procedures (Ohrbach et al., 2014), which permits reliable classification with
known validity for 10 of those disorders. DC/TMD-relevant interpretation of any indicated imaging is also available (Ahmad et al., 2009). To date, no other diagnostic approach for TMDs exhibits these characteristics of an integrated system for classification. Few other diagnostic systems for TMDs clearly identify and operationally define psychosocial constructs relevant to pain that need to also be assessed. While weaknesses of the DC/TMD are subsequently discussed, the importance of such a system cannot be overstated at this stage in the development of yet better diagnostic methods and more effective treatments. The literature and the Internet contain an abundance of diagnostic “classification systems” for TMDs (some of which are briefly summarized below), yet epidemiological data for incidence and persistence, clear operationalization of criteria, coherence between criteria and definition of the disorder, reliability, and validity are consistently missing from such systems.
Additional subsequent work has led to an updated and expanded DC/TMD taxonomy for the other TMDs which are far less common. Operationalizable criteria and clear decision rules, consistent with the definition, were created for each of these uncommon disorders.
In total—and depending on how one considers the disorders as organized in a hierarchical framework—more than 30 TMDs have been identified through the DC/TMD and expanded taxonomy. The criteria continue to be evaluated through ongoing research efforts. The resulting classification of the full set of TMDs is depicted in Figure 2-1 and encompasses the range of muscular, joint, headache, and other disorders that are considered TMDs. Other extensions of the DC/TMD include the:
- American Academy of Orofacial Pain’s Guidelines for Assessment, Diagnosis, and Management, now in its sixth edition, which includes the expanded DC/TMD (de Leeuw and Klasser, 2018);
- International Classification of Orofacial Pain, which includes the pain diagnoses from the DC/TMD into the broader pain taxonomy developed by the IASP and merges with the International Classification of Diseases, 11th Revision (ICD-11) (Benoliel et al., 2020); and
- ACTTION-APS1 pain taxonomy, which has a specific focus on the chronic painful TMDs (Ohrbach and Dworkin, 2019).
Since the initial review of TMD diagnostic systems in 1992, some of the systems have persisted, and new diagnostic or patient classification systems have emerged (reviewed in Klasser et al., 2018). Separate from the DC/TMD and its extensions, these other systems have varying levels of evidence for
1 Analgesic, Anesthetic, and Addiction Clinical Trial Translations, Innovations, Opportunities, and Networks (ACTTION) and American Pain Society (APS).
diagnostic reliability and validity (as distinguished from technical reliability and validity) (Wilkes, 1989; Pertes and Gross, 1995; Suvinen et al., 2005; Woda et al., 2005; Okeson, 2008; Simmons and AACP, 2009; Benoliel and Sharav, 2010; Stegenga, 2010; Cooper, 2011; de Silva Machado et al., 2012; Monaco et al., 2017; AES, 2019; Piper, 2019). Further contributions to the understanding of pain, clinical dysfunction associated with the masticatory system, diagnostic utility, and the biopsychosocial model applied to TMDs are promising for some of the systems (Suvinen et al., 2005; Woda et al., 2005; Benoliel and Sharav, 2010; Stegenga, 2010; de Silva Machado et al., 2012; Monaco et al., 2017), while the others are characterized by little to no evidence or have been surpassed by other developments. At present, only the DC/TMD (and its extensions) for the common TMDs meets the diagnostic system criteria pertaining to methodological considerations, diagnostic validity, and clinical considerations. Strong diagnostic systems call for further validation research through self-evaluation as part of planned future revisions.
Learning from Classification Approaches to Low Back Pain
An approach very similar to the DC/TMD has been taken for back pain. A consensus task force appointed by the National Institutes of Health Pain Consortium drafted standards for research on chronic low back pain with three sets of recommendations (Deyo et al., 2014). The first part of the recommendations of this task force focused on the classification of chronic low back pain and the classification of the person with this disorder. The recommendations included the following: operationalized criteria defining chronic low back pain; classification by its impact; and a minimum set of measures to characterize individuals with chronic low back pain, including medical history, physical examination, diagnostic testing, self-reported functional status, psychosocial factors, and mood disturbance. The second part of the recommendations focused on best practices for outcomes measures. The third part of the recommendations focused on the projected research by which the recommendations could be empirically evaluated.
Of particular relevance to this report, the task force on chronic low back pain also identified a number of key principles underlying the structure of the recommendations:
- Guidelines should be evidence-based and incorporate a biopsychosocial model of chronic pain.
- The absence of an identified pathology should not lead to the assumption that the pain is psychological or somatoform.
- The classification should incorporate the impact of pain on function.
- A minimal set of measures should be routinely used.
- The approach should be appropriate for population, observational, and interventional research.
- The evaluation should include both biomedical and psychosocial variables.
In addition, for further consideration, prognostic variables need to be defined, and research standards should evolve.
It is worth highlighting that, in parallel with information accompanying the DC/TMD framework, the report on chronic low back pain emphasized the role of a practical and evidence-based diagnostic system in moving forward to improve patient care and outcomes.
Next Steps for TMD Diagnostic Criteria
The diagnostic validity of the DC/TMD is excellent for painful TMDs, excellent for subluxation, good for disc displacement without reduction in the acute phase of limited jaw movement, and poor for other disc displacements and degenerative joint disease, with the stated recommendation within the DC/TMD to use appropriate imaging for the disorders that have poor diagnostic validity when a definitive diagnosis is clinically required. Further efforts to build on and improve the DC/TMD include:
- Better understanding of the orofacial pain disorders and how painful TMDs (a type of musculoskeletal pain) fit within that broader set of pain disorders (Benoliel et al., 2019);
- Better understanding of the interplay between pain mechanisms and mechanical problems, such as disc displacement and degenerative changes in the TMJ, and of where such pains fit into an orofacial pain disorder classification;
- Exploring how to categorize the painful TMDs within the IASP classification of similar pain disorders elsewhere in the body and ensuring that the linkage to ICD-11 (and beyond) facilitates better health care for TMDs (Benoliel et al., 2020);
- Highlighting the importance of chronic primary pain as a disorder and implementing that in health care settings for early and rational recognition of chronic TMDs (Nicholas et al., 2019); and
- Extending beyond the current two-axis (physical diagnosis, psychosocial and functional status) approach of the DC/TMD, and investigating additional axes such as genomic classification, mechanisms, and role of comorbid pain and general health problems, consistent with current approaches to all chronic pain problems; such expansion has implications for improved patient assessment, classification, and management (Fillingim et al., 2014; Ohrbach and Dworkin, 2019).
Efforts in the field of biomedical ontology—a field that develops frameworks and terminology—are expected to clarify and enhance the understanding of what the disorders are and their diagnostic criteria (Ohrbach and Dworkin, 2016). The differences in the diagnostic validity of the various subtypes of TMD as defined in the DC/TMD indicate that changes will likely be made to the current constructs within TMDs as more is learned about the underlying pathology and pathophysiology of TMDs, so that the defined disorders and their criteria can better represent the actual disease processes. More clinical and basic research will be required to identify all etiological processes that lead to TMDs and the pathological processes that develop from them. However, care must be taken that the terminology used to report such new insights is clear and at the correct level of detail. The basic formal ontology outlined in the ISO standard (ISO, 2019) in particular may play an important role. Not only does the basic formal ontology offer the vocabulary to represent the various biomedical entities involved in TMDs at the necessary level of granularity, but it does so in a way that the consequences are computable and predictable.
The joint disorders identified within the DC/TMD are disc displacements and other degenerative changes in the joint that are based on decades of research. The expanded DC/TMD adds another 14 joint disorders (Peck et al., 2014), but collectively the group represents conventional approaches to classification. The slow advances in understanding of disorders specifically of the TMJ suggest that the current approach to classification warrants further examination. For example, the attempted integration of tissue systems by Stegenga (2010) represents an obvious departure from the conventional classification based on simple changes in the anatomy. Findings from the RDC/TMD Validation Project point to possibly different constellations of signs and symptoms for defining soft-tissue disorders (currently, internal derangements) and hard-tissue disorders (currently, degenerative joint disease) (Schiffman et al., 2010a,b). Other approaches for the classification of pain disorders rely on a hierarchical modeling of clinical and imaging findings (Rudy et al., 1988, 1990).
Additionally, the central role of pain in persistent disorders of the TMJ may also be reconsidered in light of the primary findings from the Orofacial Pain Prospective Evaluation and Risk Assessment (OPPERA) study regarding the incidence of a painful TMD: TMDs seldom exist as isolated conditions and that general indicators of poor health, including comorbid conditions, increase the risk of developing a TMD (Slade et al., 2016). The implication is that the biopsychosocial model needs to be fully used with complex conditions such as TMDs, and one possibility is that the application of a full biopsychosocial model with early TMDs could prevent persistent disorders of the TMJ from occurring. Another possibility is that the medical and surgical treatments used to date to deal with the progressive disorders of the TMJ
have had limited success because the treatment has been provided in a biomedical context rather that the biopsychosocial context, the latter pointing to the simultaneous involvement of other forms of treatment.
For any advancements to occur, diagnostic test reliability, clear decision rules, and diagnostic validity are required before routine clinical use can be considered. The evidence-based DC/TMD is appropriate for use in both clinical and research settings for the common TMDs (Schiffman et al., 2014b). However, the DC/TMD, and in particular the use of Axis II, is presently under-utilized in most relevant clinical settings (Visscher et al., 2018; Sharma et al., 2019b). Looking ahead, a potentially more useful approach will integrate joint neurophysiology, the complex biomechanics exhibited by the TMJ, interactions of the peripheral and central nervous system, behavioral patterns, and longitudinal considerations such as life-course and psychosocial factors that increase risk for onset, transition to chronicity, and maintenance of chronicity—all of which bear on the status of the joint. Finally, a useful diagnostic system for TMDs will need to integrate behavioral, functional, biological, pharmacological, and surgical approaches to therapy, and those approaches must be linked to the proposed diagnoses if one is to fully understand the pathophysiology of these disorders and identify realistic expectations for which form of therapy is appropriate for which aspects of these disorders, and for which patient, given that psychosocial factors are critical for the expression and course of pain.
At present, the DC/TMD classification of TMDs falls short with regard to the additional considerations raised here, and more needs to be done to facilitate the use of reliable and valid patient classification by clinicians. The DC/TMD does fulfill the goal of classifying the most prevalent hard- and soft-tissue disorders, and with suitable training for the use of these tools (as with any new procedure that clinicians adopt) the DC/TMD is an excellent tool within the scope of its design. The intent is to assist clinicians in identifying the pertinent disorder and selecting appropriate treatments based on currently available information (where the clinical treatment studies use the DC/TMD almost exclusively), and to at least do no harm via unnecessary or inappropriately aggressive therapies. The widespread use of the DC/TMD for patient classification in clinical trials research further points to the benefits for clinicians to use the same tool, which would enhance transfer from research to clinical practice.
Another limitation of the DC/TMD is its conventional approach to anatomical separation into disorder groups. Further challenges center around the assumption that diagnostic systems will embed etiology and pain and disorder mechanisms, to the extent that such information is available. While the OPPERA study (described in more detail in Chapter 3) has provided etiological information on the painful TMDs (Slade et al., 2013a, 2016; Meloto et al., 2019; Sharma et al., 2019b; Ohrbach et al., in press),
an attempt to incorporate such information into a revised TMD diagnostic system might be premature at this time.
Conclusion 2-2: The Diagnostic Criteria for Temporomandibular Disorders (DC/TMD) provides the most thorough and accurate diagnostic criteria for the most common types of TMDs. Additional work is needed on the diagnostic criteria for other types of TMDs that are uncommon. Efforts to increase its utility to clinicians are priorities, including the brief DC/TMD assessment tool that is currently in development. Widespread dissemination of these tools, coupled with provision of training in order to maximize the information yield from these tools, to general practice dentists is needed; primary care clinicians need to be apprised of these developments so that targeted referrals and appropriate expectations of good clinical practice will occur.
Most masticatory system disorders fall into two groups: those associated with pain and those associated with functional or structural changes in the TMJ (and which may also include pain) (Scrivani et al., 2008; Schiffman et al., 2014a; Ohrbach and Dworkin, 2016). There has been substantial research and thus knowledge about the first group. In contrast, knowledge regarding the functional and structural changes in the TMJ lags (Ohrbach and Dworkin, 2016), for reasons that are addressed throughout this report. Each of these two major groups includes common disorders that account for the majority of the complexity in diagnosis and treatment of the patient, burden to the health care system, and controversy within the profession. There is also a larger number of uncommon masticatory system disorders about which less is known. The following descriptions highlight several types of TMDs as indicated above in Figure 2-1; these descriptions use readily agreed upon characteristics rather than specific diagnostic criteria.
As noted throughout this report, TMDs are often complex disorders that can have multi-system components (described further below) and multiple comorbid medical conditions. One patient often has multiple diagnoses (e.g., myalgia and disc displacement), with substantial overlap in history and impact. It is not unusual for the multiple diagnoses of myofascial pain, arthralgia, disc displacement with reduction, and headache attributed to a TMD to be present in the same individual. Such overlap can make it difficult to distinguish which specific diagnosis is primary or which is necessarily the best target for treatment. This overlap of diagnoses may be analogous to what has been observed in low back pain where “non-specific low back pain” is an established and useful term for early diagnostic and
treatment stages. See Chapter 5 for further information regarding the importance of diagnostic overlap among TMDs.
Myalgia and Myofascial Pain
Both myalgia and myofascial pain refer to pain originating from muscle. While differences in their respective proposed mechanisms are suspected, the terms are often used interchangeably, and the distinctions in clinical characteristics and proposed mechanisms may have little clinical significance. Myalgia refers to pain in the muscle without a specific mechanism causing the pain. Generally, myalgia is identified by complaint of pain localized to a particular area and the presence of enhanced pain upon provocation. Provocation can be tested either by the application of pressure to the skin overlying the muscle or by testing the individual’s range of motion. Myofascial pain is denoted by spreading pain (extension of pain beyond the initial focal point) or pain referral (pain located in another body structure, remote to the source). Because of the lack of evidence for specific mechanisms for myofascial pain, the diagnosis of myofascial pain is controversial (Cohen and Quintner, 2008; Quintner et al., 2015). No substantial evidence is available demonstrating that different treatments must be used for myalgia versus myofascial pain. Consequently, myalgia is often the preferred diagnostic term. While myalgia and myofascial pain can persist for years or even decades, there is no evidence that there is a progressive underlying disease; rather, the pain is more accurately considered to be chronic primary pain (Nicholas et al., 2019). Chronic primary pain includes the subtype of high-impact chronic pain, which is associated with higher usage of health care for pain, lower quality of life, more pain-related interference with activities, and more frequently reported pain at multiple anatomic locations (Von Korff et al., 2016). Jaw injury has been strongly associated with incident TMD (Sharma et al., 2019a), and stress can impact behavior (e.g., in the form of oral parafunction), which increases TMD pain (Ohrbach and Michelotti, 2018). However, the cumulative impact of multiple risk factors has greater evidence (Vlaeyen and Linton, 2000; Slade et al., 2016). A distinction between initiating factors for initial onset and perpetuating factors for the continuation of the condition is a dominant theme for this type of pain (Fricton, 1985; Simons, 1985). The current model of care for myalgia emphasizes the importance of self-management for symptom control and of addressing the behavioral factors that contribute to persistence.
Arthralgia refers to pain in the joint. The characteristics are parallel to those for myalgia. While there might be value in differentiating the source
of the pain within the TMJ to include the specific structure where pain is present, the small size of the joint renders such distinctions neither reliable nor clinically useful, as based on the available evidence. The same caveats regarding treatments for myalgia apply to arthralgia. However, arthralgia may also accompany TMJ disc disorders or degenerative joint disease (see sections below).
Headache Secondary to TMD Pain
Whether headache secondary to a painful TMD is a headache disorder or a TMD, headache and TMD pain overlap and share underlying pathophysiological mechanisms, clinical characteristics, and neurovascular anatomy as noted by Benoliel and Sharav (2010). It is one of the few identified comorbidities of TMDs that has a specific name and criteria for diagnosis. The headache may be of any type (e.g., migraine, tension type), a painful TMD diagnosis must be present, and the headache pain must be replicated by clinical examination procedures normally used to provoke pain identified as myalgia or arthralgia. One primary goal in making this type of diagnosis is to better integrate the list of disorders affecting a specific individual with the goal of clarifying what kind of pain is present and what treatments are needed. For example, if headache is secondary to a painful TMD, it may not require any specific treatment beyond that indicated for the painful TMD. The reverse can also occur.
Internal derangement of the disc refers to the displacement of the articular disc from its normal functional relationship with the mandibular condyle. Disc displacements of at least one of the joints are common and are estimated to occur in about one-third of the adult population. For the majority of those with disc displacement of the TMJ, there is little to no functional impact and no pain. Other TMD problems may co-exist with a symptom-free disc; this type of joint condition generally requires no treatment.
For a small number of individuals, the disc displacement is associated with substantial pain, limitation, and disability. The cause of disc displacements is largely unknown; growth discrepancies between the condyle and the developing occlusion have been suspected, but the available evidence does not support this (Farella et al., 2007). Trauma—particularly that associated with whiplash-associated disorders—has also been proposed as a cause of disc displacements, but the evidence is limited by the predominantly cross-sectional study designs whereas prospective designs are essential (Lee et al., 2018).
In the mild form of the disorder, the disc typically returns to its normal position during movement of the jaw as the condyle moves forward; a popping or clicking noise may accompany the reduction (i.e., the return of the disc to the normal position) during opening or closing or may accompany the displacement (the return of the disc to the abnormal position) during closing. This disorder is diagnosed using magnetic resonance imaging (MRI). In a more severe form of the disorder, the disc remains displaced throughout the maximum opening cycle. In the more acute phase of the severe form, the displaced disc results in a mechanical obstruction to opening and is symptomatic, while in the more chronic phase the posterior attachment to the disc stretches and normal mobility is usually restored, though normal function may be limited and symptoms may remain. Histologic evidence indicates that the posterior attachment of the disc may undergo change or remodeling from elastic connective tissue to dense connective tissue (Scapino, 1983). Based on clinical data, it is likely that individuals with such remodeling of the disc tissues into a pseudo-disc regain full masticatory function as well as normal mobility of the mandible (de Leeuw et al., 1994).
None of the disc displacements can be diagnosed based on a clinical assessment of clicking sounds or jaw deviation during opening; while diagnosis has been attempted with various instruments, MRI remains the standard method for diagnosis (Li et al., 2012; Sharma et al., 2013; Schiffman et al., 2014b) and should be used for significant mechanical joint problems, suspicion of significant disease, or when treatment has been unsuccessful (Schiffman and Ohrbach, 2016) and only if the prognosis or selection of treatment will depend on an exact diagnosis (Schiffman et al., 2014b).
The human TMJ is a unique structure, and clinical disorders associated with disc displacement are poorly understood. For most individuals disc displacements are relatively minor and self-limiting, but for some individuals disc displacements represent substantial problems. It is not yet known which types of disc displacement in the early stages are indicative of later problems.
Degenerative Joint Disease
Breakdown of the cortical bone of the TMJ condyle has been termed osteoarthritis, osteoarthrosis, and degenerative joint disease. All three terms refer to the same underlying bony changes. The term osteoarthritis is used when pain is present, while the diagnostic term osteoarthrosis is used when pain is absent; these terms are used in this distinctive manner within the research literature pertaining to the TMJ, perhaps because asymptomatic adaptive bony changes are common to the TMJ. By contrast, osteoarthritis and osteoarthrosis are used interchangeably within the medical literature.
Degenerative joint disease is the term used by the DC/TMD, based on extensive considerations regarding terminology.
Degenerative joint disease in the TMJ, as in other joints, is the consequence of chronic abnormal mechanical loading to the joint. This may occur as a result of a long-term advanced internal derangement in the TMJ, but only about 15 percent of persons with internal derangements are so affected. When degeneration does occur, it may lead to pain and further mechanical joint dysfunction. Such degeneration may require surgical treatment to improve pain and function, such as arthrocentesis, arthroscopy, open joint arthroplasty, or total joint replacement. Due to its chronicity, however, osteoarthritis is frequently associated with pain comorbidities, and evidence from other load-bearing joints such as the knee clearly indicates that pain and disability associated with degenerative joint disease are not predicted by the extent of bony destruction but rather by the same full range of biopsychosocial factors applicable to TMDs as a whole (Summers et al., 1988; Salaffi et al., 1991; Dekker et al., 1993; McAlindon et al., 1993).
TMJ Subluxation and Luxation
As noted in Appendix D, the condyle has an expected extent of motion; however, in some cases the condyle exceeds that range and problems can ensue. In some individuals the additional movement can result in the condyle being momentarily stuck in that position, which is termed subluxation, or the condyle can be stuck in a more extreme manner and may require manual reduction (external assistance) to relocate the condyle back to the fossa, which is termed luxation or dislocation. The literature is not consistent in how these three terms are defined. This condition is typically highly distressing and often painful. In other individuals, such extreme movement of the condyle is not associated with dislocation. There are a few suspected causes for this condition. Angulation of the eminence that bounds the anterior extent of the joint space is believed to contribute to dislocation, especially to recurrent dislocations, which occur in a small number of individuals, but this observation is largely anecdotal; nevertheless, surgical correction of the bony shape, bony augmentation, or injection therapies appear to reduce if not eliminate the frequency of recurrent dislocations (Fernandez-Sanroman, 1997; Moore and Wood, 1997; Undt et al., 1997; Caminiti and Weinberg, 1998). Recurrent yawning or external injury to the jaw can contribute to this condition; however, for most individuals the onset is without an identified contributing factor. While this type of problem has a set of reasonably applicable treatment procedures, recurrent dislocations are nevertheless believed to be associated with a stretching of the capsule and TMJ ligaments and thereby represent a form of joint instability.
Consequently, exercises for joint stability as well as injections for facilitating connective tissue growth in the joint capsule and consequent decrease in joint mobility have been proposed (Bell, 1979; Zhou et al., 2014), but they are not well understood, and this lack of understanding highlights the gap between the apparent physiology of the TMJ and its surprising complexity.
Relationship of TMDs to Orofacial Pain Disorders
The common painful TMDs (myalgia, myofascial pain, and arthralgia) are similar to pain disorders found elsewhere in the body, and the same diagnostic principles and treatment principles are applicable. The translation of the current state of the scientific evidence among medical and dental researchers and clinicians is key to furthering the understanding of these pain conditions. The major classes of orofacial pains are musculoskeletal, neuropathic, and visceral. However, several types of TMDs overlap these areas, particularly the musculoskeletal pain conditions. Currently the painful conditions within TMDs are now simultaneously also a subgroup of a broader set of orofacial pain conditions within the International Classification of Orofacial Pain (Benoliel et al., 2020). Such pains, whether primarily identified as muscle (or fascial) in origin or as stemming from the TMJ (such as during function) should be fully assessed within the biopsychosocial framework.
The mechanical disorders may also exhibit pain, but pain is typically not a required diagnostic criterion. Consequently, attempts to organize TMDs as part of orofacial pains more broadly have only been partially successful. The mechanical TMJ problems do not align well with the orofacial pains and rather should be considered as primarily orthopedic joint disorders. In summary, TMDs comprise two large classes of disorders: painful disorders and mechanical joint disorders. Evidence from other joints indicates that mechanical TMJ problems should also be fully assessed within the biopsychosocial framework.
As noted above regarding the evolution of terminology for this set of disorders, varying approaches have been explored over the years as to what causes TMDs and, as a result, where the areas of emphasis should be concerning TMD management and treatment. Ongoing controversies and divisions within the practice of dentistry continue regarding these issues; Box 2-2 provides an overview of some of the more frequently encountered approaches that are not evidence based with respect to the required criteria stated in this chapter which a diagnostic system must meet. In addition the issue of occlusion is discussed separately in the next section because it has
been such a large part of the TMD discussion in the published clinical and scientific literature as well as in beliefs commonly held by many clinicians. The committee aims to provide an overview of where the evidence is and is not regarding the role of occlusion and occlusal treatments (see also Chapter 5).
This report focuses on the evidence-based biopsychosocial approach. This approach maintains that conditions such as TMDs should be managed with an understanding of the multiple physical, psychological, and social factors that play a role in the onset and progression of the condition (Fricton, 2014). The biopsychosocial model focuses on the whole person,
including the mind, body, emotions, spirituality, lifestyle, social relationships, and physical environment. At present, only the biospsychosocial approach has both strong evidence and strong theory relating clinical findings to symptoms and approaches to treatment that are consistent with what is known about chronic pain elsewhere in the body.
Because this has been an area of ongoing discussion, particularly within the field of dentistry, the committee focuses this section on occlusion before discussing its adoption of the biopsychosocial model for etiology and treatment.
The field of dentistry has historically focused to a great extent on dental occlusion—how teeth fit together—as an assumed cause of TMDs and a basis for diagnosis of a TMD, and consequently emphasized treatments for TMDs designed to alter the occlusion. Dental occlusion also includes the anterior-posterior position of the lower jaw relative to the upper jaw and, more broadly, skeletal alignment between the upper and lower jaw. Two publications foundational for the clinical practice of dentistry described the purported role of a specific antero-posterior reference jaw position as causal to TMDs (Ramfjord, 1961a,b), but both publications used inadequate research designs and thereby did not provide any causal evidence. Those assertions and others based on equally poorly-designed studies continue to be published (e.g., Dawson, 1996; Racich, 2018).
In contrast, in a critical review of 68 years of research regarding TMDs and occlusal interferences—probably the most common characteristic of occlusion that dentists focus on and investigate—Clark and colleagues (1999) evaluated 18 human and 10 animal studies that examined experimental occlusal interferences and did not find evidence in this narrative review that these interferences resulted in TMDs. Structured systematic reviews as well as major textbooks and other narrative reviews have consistently come to the same conclusions: there is a notable absence of sufficient evidence that deviations in the dental occlusion are an important contributor toward TMDs (Clarke, 1982; Mohl et al., 1988; Zarb et al., 1994; Tallents et al., 2000; Fricton, 2006; Klasser and Greene, 2009; Manfredini et al., 2012; Türp and Schindler, 2012). A few specific studies, taken from different geographic regions and investigator teams, may be illustrative regarding the relationship of occlusal characteristics to TMDs.
In perhaps one of the most comprehensive studies of occlusal attributes, five characteristics were identified in a U.S. study that had a relationship to various types of TMDs; these characteristics included an asymmetric discrepancy (the so-called “slide”) of at least 2 mm from a ligament-determined posterior position of the mandible to where the teeth maximally
came together, excessive horizontal space between the lower anterior teeth and the upper anterior teeth, an anterior open bite (i.e., inability to incise food with the anterior teeth), five or more missing posterior teeth, and chewing on one side of the mouth (Pullinger et al., 1988; Seligman and Pullinger, 1991). The latter three abnormalities may lead to altered compensatory function. Collectively, these abnormal characteristics only accounted for 5 percent of the variability in the clinical signs and symptoms of TMDs, which meant that 95 percent of the variability was due to other non-occlusal factors, highlighting that occlusion alone had a weak relationship to TMDs. Similar findings were noted elsewhere in a study conducted in Italy; while the same “slide” had an odds ratio of 2.6 in favor of a TMD, the diagnostic value was equivocal, with 72 percent sensitivity and 58 percent specificity, making such findings poor to useless for diagnosis (Landi et al., 2004). In a study in Finland, abnormal occlusal characteristics were, again, not associated with pain or TMDs but were associated with quality of life in males but not females (Rusanen et al., 2012). Finally, studies of masticatory function indicate that it is pain, not the nature of the occlusion, that can affect the ability to chew when some types of TMDs are present. As Chapter 5 will address, treatment of the occlusion for TMDs also has no supporting evidence.
The use of devices (to measure muscle activity, to track jaw movement magnetically or optically, to measure vibrations from the TMJ) continues to be considered an important and valid approach to diagnosing TMDs by different parts of the dental profession. These tests are often used as proxies for demonstrating the need for treating the occlusion as a purported cause of TMD. However, the evidence demonstrates that such measurements have little or no diagnostic utility for TMDs beyond established methods defined by, for example, the DC/TMD (Mohl et al., 1990a,b; Manfredini et al., 2011; Sharma et al., 2013, 2017).
Efforts to move away from the focus on occlusion—as either a cause of TMD or a treatment objective—are needed in clinical practice and in dental training and education. The structure of dental education relies heavily on clinical training, and efforts are needed to ensure that the transfer of formal evidence to current disease models is conveyed to students. The evidence base also needs to be emphasized in continuing education curricula. One example of knowledge that has been discovered but not applied to clinical practice concerns the interactions between the cervical and masticatory systems; of relevance here, the status of the cervical system affects the dental occlusion (Mohl, 1984), yet clinical management of the occlusion typically ignores head posture and health of the cervical structures.
Recent experimental studies of occlusal deviations in maximal closure lead to some important insights of where occlusion might matter and indicate that acute alterations of the occlusion in individuals without current
TMD symptoms result in a decreased activation of the masticatory muscles (i.e., avoidance behavior) during sleep (Michelotti et al., 2005), which is the opposite of what would be expected based on theories about the occlusion and TMDs. Current experimental evidence, reviews, and weak occlusal theory indicate that occlusion should not be considered a contributing cause for the common TMDs. To the degree that dental structure may matter for TMDs, fresh research is needed, starting with a better conceptual analysis of the problem, followed by developing rigorous operational definitions and establishing reliable clinical measurements. Whereas occlusion research to date has focused on characteristics that have not led to any real understanding (Clark et al., 1999), it might be more productive to start with the concept of occlusal stability. As Skármeta (2017) noted, occlusion lies in front of each clinician, but a critical characteristic, simple stability, seems to be poorly operationalized, poorly understood, and, in the end, ignored. In contrast to the focus on occlusion and modification of the occlusion (adjustment, orthodontics) in some parts of dental care, the larger pain field has clearly organized treatment recommendations within the biopsychosocial model of pain management, emphasizing the necessity for integrative treatment across multiple levels. As discussed further in Chapter 5, a range of treatments is available and needs further research to allow clinicians to most effectively target specific types of TMDs.
Furthering the Evidence Base
The biopsychosocial approach was adopted by the committee because it is a broad model that can encompass the range of TMDs and apply the best science from medicine, dentistry, physical therapy, integrative health, and multiple other fields to the care of individuals with a TMD. This approach acknowledges that TMDs are not a single entity and consequently most often have varying causes (e.g., trauma, genetics, environmental etiologies) that affect differing parts of the masticatory system and potentially other body systems and require varied, and sometimes multiple, treatment modalities (see Chapter 5). As knowledge is gained across a number of scientific disciplines (see Chapter 4), the understanding of the etiologies of types of TMDs will continue to evolve and will provide more of the insights that are necessary to improve treatments.
Conclusion 2-3: The biopsychosocial model is most closely aligned with and has the best evidence for addressing the range of temporomandibular disorders (TMDs) with the goal of improving quality of life—including physical, psychological, and social function—for individuals with a TMD. It will be necessary to incorporate evidence-based medicine principles into all theoretical views in
order to address the long-held divisions within the dental profession regarding the causes of TMDs. Clinical experience, while important, must be augmented by epidemiological data and controlled evidence regarding disease mechanisms, appropriate and necessary diagnostic methods, and strongly theory-based interventions.
Individuals with a TMD vary in the nature of their symptoms, the severity of the disorder’s impact on their daily lives and health, the duration of the disorder, and the extent of the pain—all of which are part of the disease course. This section focuses on four issues that are the focus of ongoing research (see Chapter 4). These issues—acute versus chronic disorders, high-impact chronic pain, multi-system disorders, and pain as a disease—are not independent but instead overlap with differing emphases. A disease course can be formally defined as “the totality of all processes through which a given disease instance is realized” (Scheuermann et al., 2009, p. 118), and for a pain disorder the disease course typically refers to time-dependent changes in symptoms.
Acute Versus Chronic Disorders
The terms “acute” and “chronic” are often used in reference to pain, and they point to a temporal spectrum from symptom onset to chronic disorder. Acute pain refers to pain of recent onset. A substantial proportion of the population experiences symptoms of TMDs but does not meet the criteria for a diagnosis of a TMD; that is, they have subclinical TMD symptoms and such individuals would not be considered to have a condition. Using data from the OPPERA study, Slade and colleagues (2013b) found that one-third of individuals with no history of a painful TMD reported at least one episode of TMD pain symptoms during the follow-up period (median follow-up time was 2.3 years), with nearly 15 percent of people reporting two or more episodes. The vast majority of these episodes remained subclinical, as only 18 percent of episodes culminated in classification of an acute TMD according to the RDC/TMD. There were no differences by sex, but older and African American individuals had higher episode rates. More than two-thirds of the TMD pain symptom episodes were accompanied by other bodily pain. The results also showed that the subclinical episodes of TMD symptoms were associated with a greater use of analgesics and with health care attendance. These findings demonstrate that subclinical TMD symptoms occur frequently, are accompanied by other bodily pains, and are associated with increased health care use. Thus, even among people without a diagnosed TMD, symptoms of TMDs can be an important health concern.
When acute pain is accompanied by overt tissue damage as well as other characteristics associated with tissue damage (heat, redness, or swelling of the tissues localized to the pain complaint), then an acute condition is present. When the pain and the signs of heat, redness, and swelling have resolved and when active tissue damage is no longer apparent, then the acute condition is considered to be coming to an end. If pain is still present but the signs of tissue damage have resolved, then the condition becomes an acute pain disorder, which may then transition to a chronic pain disorder (Wall, 1979). However, the onset of acute TMD pain most commonly occurs without any evidence of tissue damage. Without markers for underlying biological correlates of the pain experience, there is no general agreement regarding when the acute phase of a painful TMD ends and when chronic begins (Kent et al., 2017). The consequence is that the end of acute TMD pain is most often indeterminate, which has a major impact on research into the disease course of painful TMDs and their transition to chronicity.
Chronic pain, which may arise from acute injury, is often defined (particularly in the absence of identified initiating factors) simply based on time since onset, such as pain persisting longer than 3 months (IASP, 2017). A period of 3 months for “acute” clinical pain allows time for the patient to engage in treatment and offers the potential for recovery before the “chronic” label is assigned. Because of inadequacies with a purely time-since-onset definition for chronic pain, the National Pain Strategy defined chronic pain as “pain that occurs on at least half of the days for 6 months or more” (Deyo et al., 2014; HHS, 2016; Von Korff et al., 2016).
In looking at the time course of a TMD, it may be helpful to characterize the disorder as acute or chronic. Disc displacements of the TMJ clearly have an acute phase with a sudden onset of clicking or locking (with a diagnosis confirmed by imaging), and they may later go into a chronic phase. A set of criteria (Wilkes Criteria) has been developed to assess the stages of disc displacement (Wilkes, 1989). While the distinctions of acute versus chronic may apply to degenerative joint disease, the period of development of degenerative changes (as compared to when they can be detected on imaging for a confirmed diagnosis) may preclude actually making the distinction and the diagnosis of degenerative joint disease is most likely to refer to a chronic stage. In contrast, more is known about the time course of painful TMDs. Following initial lifetime onset, approximately 50 percent will continue to have sufficient pain at about 8 months to still be classified as having a disorder; the other 50 percent probably remitted across the 8-month period between observations (Meloto et al., 2019). Once TMD pain becomes chronic both pain and disorder fluctuate (see Chapter 3 for additional discussion on course of the disorders). “Recurrent” and “persistent” are also terms used to characterize pain disorders.
Recurrent refers to a disorder that has clear onset and offset for major bouts that recur across time; if that time extends to years, such a disorder could be considered chronic based on the period of years, yet each episode may be like an acute episode. Importantly, a long-term recurrent disorder, if it occurs on less than half of the days, would not be considered chronic pain from the perspective of the National Pain Strategy. Menstrual migraine is an example of what may be a non-progressive and isolated recurrent pain disorder. Some patients with a TMD also can exhibit this recurrent pattern extending over years to decades. Persistent pain refers to pain that never goes away. How a patient copes with pain can heavily influence the reporting of episodes and whether a chronic pain is persistent or is not persistent but chronic (i.e., with clear pain-free periods). Effective coping skills, such as distraction or behavioral activation, can make persistent pain appear to be episodic because of the sufficient blunting of low-intensity periods, often referred to as background pain.
High-Impact Chronic Pain
High-impact chronic pain has been defined as persistent pain with “substantial restriction of participation in work, social, and self-care activities for 6 months or more” (HHS, 2016, p. 11). Previously known as pain-related disability, whose severity was often measured with the Graded Chronic Pain Scale (Von Korff et al., 1992), the construct of high-impact chronic pain focuses on the impact of pain on day-to-day activities of the patient. High-impact chronic pain can be assessed based on a patient’s responses to questions such as how often, over the prior 6 months, pain limited his or her life or work activities, including household chores. When the response is “usually” (compared to never, rarely, or sometimes) or the patient indicates “severe” interference with these activities (compared with none, mild, or moderate), the pain is considered to be high-impact chronic pain (Von Korff et al., 2016).
As succinctly reviewed for chronic low back pain by Deyo and colleagues (2014), high-impact chronic pain is the major complication associated with pain localized to the region of the back, and such pain has poor correspondence with physical findings based on either examination or imaging. Similarly, the evidence suggests that high-impact chronic pain associated with the TMJ also has poor correspondence with physical findings of disease within the joint. However, as indicated by the description and available evidence regarding complex anatomy and bilateral functioning of the TMJs (see Appendix D), the mechanical function within the TMJ is complex, and mechanical TMJ problems often trigger a cascade of events that lead to worse functioning within the joint.
Any major pain condition, including TMDs, can manifest as either a localized or a multi-system disorder. Chronicity and high-impact pain are part of this constellation of multi-system disorders that are characterized not only by overlapping diagnoses but also by multiple dimensions of vulnerability for broad domains of symptoms and other disorders (Naliboff, 2007). Another related consideration is the primary versus secondary nature of the pain (see Box 2-3).
A localized TMD is more likely to be acute, is clearly isolated to a specific part of the masticatory system, and often can be attributed to a specific event (e.g., a recent injury, such as a ball hitting the face, or a dental event such as a broken tooth that leads to an immediate alteration in chewing pattern). In general, localized TMDs are not associated with comorbidities such as other pain conditions, depressive symptoms, anxiety symptoms, general stress reactivity, multiple unexplained physical symptoms, and sleep
disorders. The presence of such comorbid conditions increases the probability that a TMD, regardless of local causes, should not be considered as a localized condition. Moreover, the presence of a comorbid condition increases the probability that a localized condition at onset is more likely to act like a chronic pain disorder.
In contrast, a TMD with multi-system components is characterized by an identifiable TMD (meeting the diagnostic criteria for a localized disorder) associated with, in particular, comorbid pain conditions as well as with other systemic or behavioral disorders. Chronic overlapping pain conditions appear to be more common than any single chronic pain condition (Mayer and Bushnell, 2009; see also Chapter 3), and there is substantial overlap in the condition-specific pain intensity and pain interference measures (Ohrbach et al., in press), which highlights the mutual contributions that multiple pain disorders make to each other. Other comorbidities, such as depression or anxiety, appear to exhibit the same type of impact on a given pain condition, such as a painful TMD (Fillingim et al., 2011, 2013).
Available information suggests that a TMD with multi-system components occurs most often as a result of a single underlying process. Rather than two disorders existing with one as primary and the other as secondary, both disorders might have a common underlying cause or mechanism. For example, in the case of a painful TMD co-existing with low back pain, it could be that the TMD is secondary to the low back pain, but it could also be that the low back pain occurred after the TMD onset and that the TMD flareups aggravate the low back pain, with low back pain secondary to the TMD. Or both disorders could represent a process, perhaps triggered by regional trauma, in which manifestations of common mechanisms have been facilitated by each of the disorders. Such mechanisms include central nervous system dysregulation, which could result in the perpetuation of both a TMD and low back pain in an individual. In such instances, the process that initiates the pain and that which maintains the pain may be different. The peripheral nervous system may play a role in both, and it is suspected that in some cases, including fibromyalgia among the chronic overlapping pain conditions, the central nervous system may be an autonomous site of pain amplification or generation.
Conclusion 2-4: In many individuals, temporomandibular disorders (TMDs) resolve without medical or dental treatment interventions. In other individuals, TMDs progress to becoming chronic conditions; in addition, TMDs may be components of a multi-system disorder across biopsychosocial domains. Research is needed to identify why symptoms resolve in some cases and progress in others and how to better target different types, intensities, and timings of interventions.
Pain as a Disease
While pain has classically been considered a cardinal symptom of an acute condition, chronic pain was proposed in 2001 as a disease (Niv and Devor, 2004), the result of a process independent of the original cause, such as direct injury, but leading to disability, sleep disturbance, depression, and other consequences. The persistence of pain across time and the complexity of a disorder as multi-system provide the basis for pain to become a disease that fully represents the biopsychosocial model of health and disease (see Figure 2-2). As noted in the preface to the 2011 IOM report, “While pain can serve as a warning to protect us from further harm, it can also
contribute to severe and even relentless suffering, surpassing its underlying cause to become a disease in its own domains and dimensions” (IOM, 2011, p. ix). Chronic pain as a disease has become an increasingly accepted perspective, although not without critical discussion as well as criticism (IOM, 2011; Cohen et al., 2013; Taylor et al., 2015; Treede et al., 2019).
The well-documented functional and structural brain changes that take place in response to chronic pain may represent the mechanisms underlying pain as a disease, or they may represent the role of adaptive responses to pain (Tracey and Bushnell, 2009). The latter, however, may also justify disease status in the sense that the feedback loops between response patterns and disease perpetuation may be among the many components constituting a complex disease (e.g., TMDs). Finally, evidence for similar responses to the same treatment across different pain disorders suggests common mechanisms, which may underlie chronic pain as a disease (Turk and Rudy, 1990; Rudy et al., 1995). The mechanisms of nociceptive and nociplastic pain are discussed in detail in Chapter 4.
In a clinical setting, determining whether a specific patient’s pain is, at one extreme, a localized condition or, at the other extreme, an indicator for pain as a disease can be challenging. One proposed solution to this challenge is the new ICD-11 diagnosis of chronic primary pain, which is defined as pain in one or more anatomical regions that has been present (continuous or intermittent) for at least 3 months, associated with emotional distress and/or functional disability, and not better accounted for by another diagnosis (Nicholas et al., 2019). The intention of the use of the term chronic primary pain is to replace other established but vague terms such as “somatoform,” “nonspecific,” or “functional,” which are often used to classify a bodily pain when the clinician (or researcher) is not certain about a better diagnosis. It should be noted, however, that the diagnostic criteria for the most common painful TMDs are substantially better in terms of rigorous validation procedures than most of the other conditions subsumed within the chronic primary pain rubric. Nevertheless, the application of the term “chronic primary pain” for settings where clinical expertise, such as those needed to use the DC/TMD, is absent represents a substantial improvement over current practices, in that chronic primary pain explicitly incorporates the biopsychosocial model and thereby sets the direction for treatment. If acute pain from jaw injury, for example, was regarded as solely related to injury-related tissue damage and was to persist, the new terminology would classify such pain as chronic secondary pain (Treede et al., 2019).
To look at this topic from another perspective, consider the question, If chronic pain is not a disease but rather only a symptom, then what is it a symptom of? The poor correspondence between physical examination findings and reported pain highlight the problems inherent in diagnosis when
pain becomes chronic. The criteria developed for the painful TMDs in the DC/TMD use measurements that exhibit high reliability, yet the determinant of the measurements is the report of pain as influenced by pain processing. Considering chronic pain to be a disease helps the clinician move out of the biomedical model and instead link management of the disorder to the biopsychosocial contributors to ongoing pain experience (Taylor et al., 2015), that is, to treat the pain disorder as such rather than continue further diagnostic testing to identify underlying presumed ongoing tissue damage as the cause (IOM, 2011).
Yet, the determination of whether chronic pain in a given individual is a symptom of an as-yet undiagnosed primary disorder or it is a disease remains complex, and the pain may be both at the same time as well, particularly in the early stage when an anatomical derangement is clearly identifiable. An illustrative example is diabetes. The disease of diabetes does not typically start as a disease; rather, a pre-diabetes state of insulin resistance or impaired glucose tolerance may exist for years, which if left untreated can become the identified disease of diabetes that affects multiple organ systems beyond the endocrine system. In the same way, chronic pain can start as a symptom of another condition, but if the pain is untreated then over time it can become the basis of pain as a disease.
There is considerable variation in how any of the TMDs may manifest, be experienced by an individual, appear clinically to the observer, and be associated with measurable physical changes. For example, the four issues regarding disease course discussed in the prior section indicate that a recent onset TMD in the presence of other already established pain disorders might be considered chronic pain from the beginning simply because multiple risk factors for chronicity are already active. When pain persists there are many opportunities for it to influence and be influenced by psychological and social factors (Fordyce, 1976). Numerous studies have examined psychosocial factors in other chronic pain conditions, and emergent themes can provide insights into how individuals adjust to chronic TMDs. Among these factors, there are three that are particularly consistent and salient for TMDs.
- Adjustment to living with persistent pain: Individuals can differ markedly in how they adjust to living with persistent pain (Karayannis et al., 2019; Mun et al., 2019). These variations in adjustment are evident in persons with TMDs and, as in other chronically painful diseases (e.g., arthritis or cancer), are not well explained on the basis of medical or background factors such as the severity of the injury,
disease activity, or the duration of pain. The manner of adjustment or coping and its emergence across the life-span vary enormously across individuals, to the point that it is not possible to describe an overall natural development or even categories of natural history trajectories.
- Role of psychological factors in adjustment: Historically it was believed that longstanding trait-like personality factors play a key role in shaping how one adjusts to persistent pain. Over the past two decades, however, it has become increasingly evident that more dynamic psychological factors are important in explaining individual differences in the impact of pain. These factors include a person’s choice of pain coping strategies, the person’s beliefs in his or her own abilities to control pain (i.e., self-efficacy), and the person’s tendency to ruminate about and feel helpless (or not) about pain, and emotional distress (e.g., anxiety) (Turner et al., 2007; Fillingim et al., 2011).
- Role of social context in adjustment: Epidemiological studies have found that the prevalence of chronic pain conditions is much higher among those who have a low socioeconomic status, limited education, and limited access to satisfying work (Bergman et al., 2001; Brekke et al., 2002). Direct observational studies of patients with chronic pain and their partners (or caregivers) have highlighted the impact of the partner’s responses to pain (e.g., invalidating the pain, being supportive, or offering empathy) on the patient’s pain experience (e.g., Verhofstadt et al., 2016). Although partners and caregivers clearly can be affected by the individual with a chronic TMD, there is a dearth of research in TMDs using novel observational methods to better understand patient–partner interactions relevant to TMDs. Such research is important because it could be used to tailor new interventions designed to help patients and their families learn how to work together to optimize adjustment to a TMD.
TMDs include several features common to other chronic pain conditions, such as back pain, widespread pain, and headache, among others. All of the major chronic pain conditions are quite heterogeneous with respect to the severity of the symptoms, the quality of life, and the psychosocial impacts, and a substantial proportion of individuals with these conditions experience marked impairment in physical or psychosocial function, as is the case with other conditions (Dworkin and Massoth, 1994; Manfredini et al., 2010). The development and persistence of these conditions are driven by complex interactions among multiple biological (e.g., genetics, nociception), psychological (emotional distress, coping), and social (socioeconomic status, social support) factors (Dworkin, 1994; Furquim et al.,
2015; Maixner et al., 2016). Specific common risk factors across these conditions include female sex, enhanced pain sensitivity, a family history of chronic pain and mood disturbance, adverse childhood experiences, and multiple somatic symptoms (Clauw, 2015; Harper et al., 2016).
However, in many instances, TMDs represent a distinct group of conditions that differ from other pain conditions in important ways. First, the symptoms of TMDs occur in the masticatory system, which includes arguably the most complex joint in the body, combined with an intricate neuromuscular apparatus that must be effectively coordinated for healthy functioning. Hence, understanding the factors contributing to TMDs and their associated symptoms requires a consideration of this complex musculoskeletal system. Second, TMDs are associated with fewer disability days than back pain and have a lower pain impact than either headache or low back pain (Dworkin and Massoth, 1994), suggesting that despite comparable pain and psychological distress, individuals with TMDs may continue to function more than their counterparts with some other pain conditions. Third, for many individuals with a TMD, as with many other chronic pain conditions, there are no observable physical changes. This may contribute to the stigma and the need for those with a TMD to feel that they must convince others of the symptoms and impact (see Chapter 7 for discussion on stigma). These various factors affect the self-image of individuals with chronic pain. Whether the higher level of function among individuals with painful TMDs represents, for example, healthy adaptation, symptom repression, or the impact of stigma is not known. Fourth, a significant proportion of TMDs appear to be self-limiting, such that the prevalence of TMDs declines later in life (see Chapter 3), which stands in contrast to the pattern observed with some other conditions, such as low back pain, chronic widespread pain, and osteoarthritis. This emphasizes the importance of avoiding harm when providing treatment for TMDs. Finally, among chronic pain conditions, TMDs are unique in their management being carried out largely within the dental rather than medical setting. This can create considerable challenges with access to care, and the dental–medical divide can impose substantial negative impacts on the effective management of people with TMDs (see Chapter 6).
As noted throughout this chapter and in Appendix D, the complexity of the masticatory system permits a wide range of functions that include far more than only mastication. In terms of the complex muscle vectors required for joint stability, the scapular system (supporting the shoulder) has some similarities to the functional requirements of the TMJ. Despite
the importance of the masticatory system, the TMJ, as a joint, has not been well studied compared with other synovial orthopedic joint systems such as the knee, shoulder, and hip joints, all of which are far better understood. Knowledge about these other joint systems cannot, however, be directly translated to the TMJ because of the following considerations:
- Striking differences, based on genetic analyses, indicate that the TMJ develops in response to distinct molecular biological mechanisms.
- The TMJ has a uniquely complex anatomy involving bilateral joints that function as one, resulting in a transmission of load to one side whereas traction may be the force in the joint on the opposite side.
- The contacting components in the movable joint are often not well matched; incongruent surfaces within the components of the jaw are therefore not able to truly minimize stresses and strains during condyle movement, resulting in shear forces.
- TMJ movements involve both rotation and translation combined with six degrees of freedom (that is, movement in three planes), requiring complex coordination of an extensive muscle system, as described in Appendix D.
- The masticatory system has multiple muscles containing muscle fiber groups that can be activated in a highly variable manner across individuals, yet can accomplish similar behaviors and oral functions. This variation among people makes it challenging to identify universal principles that may underlie TMJ function and its disorders.
These anatomical and functional distinctions of the TMJ, in contrast to other joints, underlie part of the complexity of TMD when it is a local condition, and these distinctions are further compounded when TMD becomes a multi-system disorder because of overlapping comorbidity. Consequently, there is great potential for unique interactions between the complex subsystems making up the masticatory system and comorbid disorders, and knowledge obtained from other pain conditions may well not help in the development of a better understanding of the masticatory system and of the TMJ in particular. A discussion of relevant biomechanics research on the TMJ can be found in Chapter 4. Since the review of the masticatory system published in 2008 (Scrivani et al., 2008), little has been added to our understanding of the TMJ at a complex system level. And while degenerative joint disease of the knee has been studied extensively (e.g., Mora et al., 2018), considerable controversy continues regarding its pathophysiology and natural history. Knee osteoarthritis is influenced by local, systemic, and external factors, and both its progression and its response to treatment vary across individuals. Current management is primarily oriented toward symptom reduction. Non-pharmacological treatments include avoiding excessive
joint loading and routine exercise. Pharmacological treatments include limited use of non-steroidal anti-inflammatory medications and corticoid injections. Most findings from soft-tissue imaging of the knee joint are unrelated to the symptoms (Kornaat et al., 2006). Overall, knee osteoarthritis and TMJ osteoarthritis appear to share the same major characteristics. Self-management approaches to osteoarthritis, through group approaches and Internet-based education, have been successful, with improvements in both multiple health-status measures and in self-efficacy (Lorig et al., 2008), with both outcomes consistent with more than just symptom relief. Taken together, the evidence supports the further development of self-management approaches for TMDs as a viable treatment direction that can have appropriate impact at multiple levels (Nicolakis et al., 2002; Mulet et al., 2007; Riley et al., 2007; Lindfors et al., 2019).
As noted throughout this chapter, TMDs are a set of disorders that are often complex and overlapping. The committee’s conclusions for this chapter are restated here, followed by thoughts on research priorities (see Box 2-4) relevant to this chapter.
Conclusion 2-1: Multiple disorders are encompassed by the terms “temporomandibular disorders” or “TMDs.” The committee defines temporomandibular disorders (TMDs) as a set of diseases or disorders that are related to alterations in structure, function, or physiology of the masticatory system and that may be associated with other systemic and comorbid medical conditions. TMDs can be usefully separated into two groups: the common TMDs with validated diagnostic criteria and the uncommon TMDs that do not yet have validated diagnostic criteria due to the challenges of conducting research on rare conditions. When possible, a patient’s diagnosis needs to be focused on the specific TMD or TMDs. “TMD” should not be used as a diagnostic term. An individual patient may have more than one TMD and may also have comorbid conditions.
Conclusion 2-2: The Diagnostic Criteria for Temporomandibular Disorders (DC/TMD) provides the most thorough and accurate diagnostic criteria for the most common types of temporomandibular disorders (TMDs). Additional work is needed on the diagnostic criteria for other types of TMDs that are uncommon. Efforts to increase its utility to clinicians are priorities, including the brief DC/TMD assessment tool that is currently in development.
Widespread dissemination of these tools, coupled with provision of training in order to maximize the information yield from these tools, to general practice dentists is needed; primary care clinicians need to be apprised of these developments so that targeted referrals and appropriate expectations of good clinical practice will occur.
Conclusion 2-3: The biopsychosocial model is most closely aligned with and has the best evidence for addressing the range of temporomandibular disorders (TMDs) with the goal of improving quality of life—including physical, psychological, and social function—for individuals with a TMD. It will be necessary to incorporate evidence-based medicine principles into all theoretical views in order to address the long-held divisions within the dental profession regarding the causes of TMDs. Clinical experience, while important, must be augmented by epidemiological data and controlled evidence regarding disease mechanisms, appropriate and necessary diagnostic methods, and strongly theory-based interventions.
Conclusion 2-4: In many individuals, temporomandibular disorders (TMDs) resolve without medical or dental treatment interventions. In other individuals, TMDs progress to becoming chronic conditions; in addition, TMDs may be components of a multi-system disorder across biopsychosocial domains. Research is needed to identify why symptoms resolve in some cases and progress in others and how to better target different types, intensities, and timings of interventions.
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