Temporomandibular disorders (TMDs) are complex musculoskeletal pain conditions that affect the orofacial region. Injury, compared to other known risk factors, is closely related to pain through a nociceptive pathway that starts with detection of real or potential tissue damage and may progress to pain. The association between injury and TMD is largely based on cross-sectional studies which limit causal inference about the role of jaw injury in TMD. The few prospective studies assessed jaw injuries from different settings and have a narrow focus regarding type of jaw injury, potentially limiting counts of injury and affecting internal validity; thus, no prospective study has comprehensively assessed injury to the jaw from various traumatic events in order to investigate the association between incident injury and TMD. In addition, psychological states and pain sensitivity are well-known risk factors for TMD, of which psychological stress has well-established putative mechanisms for affecting pain and TMD. Specifically, stress associated with TMD may potentially exacerbate nociceptive pain episodes once an individual is injured. However, no prospective study has assessed whether injury can modify the association between stress and TMD, and the mechanism underlying the interplay between stress and injury on increased TMD incidence remains unclear.

The overarching goal of this study was to assess the independent and joint associations of jaw injury and stress with development of incident TMD. Aim 1 assessed the association of jaw injury on the risk of developing incident clinically diagnosed TMD in the OPPERA study, and explored whether risk of developing TMD varied according to types of jaw injury, and according to number of injuries when compared to TMD risk in those without jaw injury. Aim 2 assessed the overall association of psychological stress, as measured at baseline and risk of developing clinically diagnosed TMD; and, whether injury measured during OPPERA follow-up was an effect modifier in the association between stress and TMD. In addition among participants who reported having experienced jaw injury during follow-up, concurrent psychological stress, measured during the same three-monthly period that the first jaw injury was reported, was assessed and related with the risk of developing TMD, and whether concurrent psychological stress, adjusted for baseline stress, increased risk of incident TMD.

Both Aim 1 and Aim 2 used data from the OPPERA (Orofacial Pain Prospective Evaluation and Risk Assessment) study that assessed injury, stress, and pain density at 3-monthly intervals using quarterly health updates in individuals who had no clinically diagnosed TMD and no recent injury at enrollment, over a 5-year period from 2006 to 2011. For Aim 1, in order to address the complex analysis requirements related to multiple injuries we simplified the analysis and assessed the association of first jaw injury that occurred during the observation period and TMD. Time-dependent Cox models were used to estimate hazard ratios (HR) and 95% confidence intervals (CI) in both the crude and multivariable adjusted models. Comparisons between types of jaw injury and between number of jaw injuries on risk of TMD were explored using Woolf's test. The OPPERA study also collected data on perceived stress with and without concurrent assessments of injury. In Aim 2, we assessed the independent and joint associations of jaw injury and stress in the development of TMD using Cox proportional hazards. Potential incident TMD cases were identified using a pain density algorithm; potential cases who met the pain threshold were then confirmed using the RDC/TMD protocol for clinical examination by trained and calibrated clinical examiners.

Of the 3,258 enrolled OPPERA study participants, 1,729 participants had no more than 2 missing quarterly health updates during the OPPERA follow-up period of a median of 2.8 (+/-1.2) years, of which 434 (25.1%) experienced jaw injury, and 175 (10.1%) were diagnosed with TMD. Findings from Aim 1 showed that self-reported jaw injury was associated with increased risk of TMD after adjusting for study site, age, gender, and race (HR=1.24, 95% CI=1.07--1.45). There was no difference in risk of TMD by types of injury (p-value=0.91) and a single injury was strongly associated with risk of TMD compared to multiple subsequent injuries (p-value<0.01).

Findings from Aim 2 showed that though incident TMD increased by 32% for every 1-SD increase in stress at enrollment (HR=1.32, 95%CI=1.14--1.52) after adjustment for study site, age, gender and race, injury did not modify this association (p-value for interaction=0.93). (Abstract shortened by ProQuest.).