This exploration of clinical characteristics found that 59 of 71 assessed variables demonstrated a significant association with pain-related TMD. Some of these findings provide important confirmation and validation of previous studies, while others are presented for the first time (eg, selected health conditions and anthropometric findings). However, we believe this to be the first study to address all of these findings in 1 cohort. An important consideration when interpreting these findings is that TMD cases were volunteers recruited from communities in and around 4 US academic health centers. As reported in an accompanying paper,82
clinical pain reports, symptom profiles, and history of treatment among TMD cases were consistent with TMD as it occurs in the community at large, rather than in treatment-seeking TMD cases. The accompanying paper also shows evidence that controls were selected from a similar community-based population of volunteers, and that the methods of selection produced good internal validity in estimates of associations.
All 3 forms of trauma were strongly associated with TMD. While the finding is consistent with previous studies, these case-control results do not resolve the controversy as to whether such trauma plays a direct causal role in TMD.9,15,44
Equally strong was the association between TMD and yawning or prolonged opening, yet these everyday sources of plausible repeated strain have received no systematic research. Past orthodontic treatment was only weakly associated with TMD, a consistent finding with other studies showing that orthodontic treatment, considered alone, does not appear to either cause or improve TMD.61,78,84
Parafunctional activity such as teeth clenching has historically been considered a potential source of so-called microtrauma, but with limited evidence.63,77
In this study, where parafunctional behaviors were assessed on a continuum using a comprehensive questionnaire, odds of TMD was elevated 17-fold among people in the upper tertile of the distribution relative to people in the lowest tertile, a strong relationship consistent with other findings.11
Other studies that have reported weaker associations have asked only a few questions about parafunction.33,92
Comprehensive item sets, such as the OBC used here, might be more valid because they have greater potential to elicit state-specific memory.43
While findings from recent experimental and observational studies support a causal relationship between parafunction and pain,11,31,32,41,56
parafunction may be both cause and consequence of the pain experienced in TMD, and the very magnitude of the odds ratio observed here suggests the presence of a bidirectional relationship, which casues doubt as to whether parafunction is a sufficient cause for TMD onset.57
Pain or other functional disorders were very strongly associated with TMD, most notably for headache. The OR of 10 for each additional type of headache represents a pragmatic though crude indicator of headache symptoms. While the ICHD-2 based headache symptoms will be further assessed in a future publication, we consider the present findings as an intriguing perspective regarding the underlying complexity of headache. Potential causal effects of headache on TMD should be interpreted cautiously; headache that co-occurs with TMD might represent the same disorder but called by another name, or it might be secondary.6,88
Furthermore, enrollment criteria excluded controls, although not cases, who reported 5 or more headaches per month, thereby amplifying odds ratios. Our examination protocol sought to disentangle this possible confounding of headache with TMD, especially among the controls. Specifically, individuals who only had headache (less than 5 per month) but not jaw pain were accepted as “controls.” Given that headache and TMD may coexist due to overlap in their classification, we refer to headache as an overlapping condition and will rely on future analysis of longitudinal data to examine causal direction.
Those caveats did not apply to back pain, abdominal pain and pelvic pain, each of which was moderately associated with TMD, as seen in other studies.38,40,49,60,71,76
The strength of association with TMD was greater for sets of 2 or more of these “functional conditions” (OR = 6.7) than for individual functional conditions, suggesting that their co-occurrence has additive effects on the association with TMD and indicative perhaps of a shared set of pain regulatory pathways that have become dysregulated.18
Self-reported clinical status differed markedly between cases and controls. Consistent with clinical samples,50
1 in 4 TMD cases had facial pain-related disability, a proportion that was in stark contrast to controls (<1%). While some degree of facial pain, dysfunction and disability is an intrinsic consequence of TMD, the results are illuminating given that some of the facial pain symptoms are reported by at least some controls. Whether such symptoms are significant, however, might be usefully distinguished on the basis of impact to the individual; while the study’s enrollment criteria permitted some history of TMD symptomatology among controls, people who reported having been diagnosed with TMD were excluded as controls.82
Consequently, prior symptoms in controls, if present, would most likely qualify as subclinical in nature. In a related finding, the mean of 15 (0–100 scale) was not trivial amongst the 15% of the controls who responded to the 3 characteristic pain intensity questions based on the lifetime occurrence of regional but not diagnosed pain. A relevant methodological observation is that detailed questionnaires, such as the CPSQ, can elicit reports of symptoms even among people who, during an earlier telephone interview, reported no facial pain for the past 30 days. One explanation is that the questionnaire triggers state-specific memory.37,43
Notably, only 53 individuals (3.2%) reported any regional pain in the past 30 days, and by examination criteria such pain occurred on fewer than 5 days.
Modifying factors that improve or worsen pain might also be intrinsic to musculoskeletal disorders. The large majority of TMD cases reported multiple modifying factors, suggesting that their presence may be important for disorder progression. In fact, rather than curtailing modifying behaviors, engaging in the behaviors might be a benefical part of TMD treatment.30
Similarly, in the case of back pain, studies suggest that avoidance behaviors might contribute to progression of that condition.46,81,93
The expected finding that TMD cases had greater jaw functional limitation than controls is consistent with the disablement model.65
While the mean levels of jaw functional limitation in OPPERA TMD cases was lower than a clinical sample,68
mean levels in OPPERA controls were very low, yielding a strong association. It is worth noting that, among cases, limitation may also be part of avoidance behavior.46
TMJ noises were self-reported by nearly all TMD cases (89%) and only 19% of controls, consistent with the widely held view that such noises are an important part of TMD when combined with clinically significant pain and dysfunction. TMJ noises were identified during the examination in a majority of cases but also in 30% of controls. Given that 73% of OPPERA cases reported having consulted someone for their pain,82
health-care services interaction could represent a possible source of reporting bias, resulting in elevated rate of self-reported noises by the cases relative to the noises detected by examination. Otherwise, individuals are often unaware of noises that clinicians readily detect,86
perhaps accounting for the differential in self-report versus examiner detected among the controls. Other research indicates similar findings: 86% of cases self-reporting clicks,96
48% or 61% of cases with noises detected by examination,73,96
and 37% of general dental practice patients (not screened for TMD) exhibiting TMJ noises during examination.36
Collectively, these findings—self-report versus examination and cases versus controls—from 3 other studies are compatible with those found in this study, suggesting that bias due to participant recruitment methods or due to data collection methods is not a likely source of these statistics.
Similarly, reported locking was also significantly more common among TMD cases than controls. When TMJ noises and locking are prominent features in the clinical history, it is common for substantial muscular dysfunction and pain to also be present, an explanation that is also consistent with high levels of parafunction observed in TMD cases.11
Moreover, longitudinal studies show that clicking and locking often resolve over time with minimal intervention.16
Multivariate and longitudinal analyses will be needed to understand the significance of joint noises and locking in OPPERA’s cases and controls.
Differences in clinical status by examination between cases and controls were also in part a consequence of case-classification, yielding expected differences in all clinical parameters. One notable finding concerns the 17% of controls (n = 262) who reported pain after terminating the assisted mouth-opening procedure, contrasting with the 11% of TMD cases (n = 20) who did not terminate the procedure and furthermore said it was not painful. The former may be a group at high risk for first-onset TMD, while the latter group may represent individuals coping well with TMD.
The number of neck and body sites that were painful to palpation was conspicuously greater in TMD cases than controls. This is the somatic equivalent of case-control differences in self-reported pain disorder symptoms and supports growing evidence for the overlap among pain disorders.1–4,7,8,74,97
Meanwhile, positive clinical findings occurred at a substantial rate among the controls (–). Whether it represents variation of a normal phenotype, as traditionally viewed, or a marker for elevated risk of TMD onset remains to be discovered in our prospective cohort study.
There are reasonable grounds to compare anthropometric measures between cases and controls. The condition might limit mastication, thereby affecting diet and weight. Another proxy, given the greater prevalence of TMD in women, is gonadal hormones in TMD.47,48
Multiple lines of evidence suggest this sexually dimorphic index reflects prenatal exposure to estrogens versus androgens, such that lower 2D:4D ratios reflect higher androgen exposure, while lower ratios indicate greater prenatal estrogen52,58
and may thereby account for greater occurrence of TMD in women than men. However, none of the anthropometric measures in OPPERA were associated with TMD after adjusting for demographic variables.
Health status revealed generally poorer health in TMD cases than controls, consistent with other studies.10,39
Relative to controls, TMD cases reported more neural/ sensory conditions, respiratory conditions, and medication usage, and they were more likely to rate their health as fair or poor. Of note, these associations were not confounded by demographic characteristics.
Several potential limitations of the present study warrant highlighting. One limitation is that many self-reported clinical measures were assessed using single-item questions. Although widely used in clinical practice and ubiquitous in medical research, such measures rarely have supporting evidence of reliability or validity. Multiple items are almost always needed for assessing latent variables (such as complex mood states), whereas single items are often considered sufficient for assessing straight-forward concepts, such as “do you have pain?” For some conditions, such as obstructive sleep apnea, validated multi-item instruments are available, although it was not feasible to add them to the already large number of questions asked in OPPERA. For other areas, such as perception of TMJ clicking, the face validity and presumably content validity of the single item are strong, and, assuming clear operationalization, at issue is the items’ reliability which is directly testable. Two of the self-report instruments, the CPSQ and the medical history, ask multiple questions about health status using single-item measures, although we are only now in the process of evaluating their reliability.
Another caveat is that the case-control study design does not allow us to establish whether the characteristic developed before or after onset of TMD, thereby hindering conclusions about possible causal effects. A third limitation is that the analyses consider each measure independently of other clinical measures. Odds ratios likely will be attenuated when multiple clinical variables, and other characteristics assessed in OPPERA,28,35,55
are considered simultaneously using a multivariate approach.
In conclusion, TMD cases and controls differed on most (59 of 71) of the measures from among this broad array of self-reported and clinical characteristics. Most of the findings confirmed results from other studies. Many odds ratios exceeded 5, signifying very strong associations, and most remained stable after adjustment for demographic characteristics. For several sets of clinical characteristics, we believe marked associations with TMD occurred because they are implicitly part of the condition. Yet many of those same characteristics were found in at least some controls. Consequently, we do not regard large ORs as prima facie evidence of causality; instead, we regard larger ORs as suggestive of more complex patterns of interaction, perhaps iterative over time, between pain, the person, and the variable. The importance of these findings lies in the comprehensiveness of the collected data and their relevance for the clinical domain which primarily serve as a reference against which to compare findings reported in the other papers in this series. Because this study is of moderate power, limited by selection of only 185 cases, observed findings should also be considered as a base for developing additional hypotheses. A future goal of the OPPERA prospective cohort study is to determine whether those characteristics represent early signs of TMD, useful markers signifying elevated risk of first-onset TMD, or variation in normal phenotypes.29