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Temporomandibular disorders (TMD) embrace pain and dysfunction in the temporomandibular joint (TMJ) and jaw muscles. TMD is a prevalent condition in the population and constitutes a significant health problem. Knowledge of factors influencing the onset and course of TMD is important in preventive care and development of treatment strategies as well as in clinical decision making. The aim of this thesis was to gain knowledge of whether variations in dental occlusion, bruxism, spinal pain and gender predicted the development and course of TMD.

The study population comprised 371 undergraduate dental students. A questionnaire was used to obtain case histories. Clinical examination included the function of the TMJ, jaw muscles, maximal jaw mobility, the morphological occlusion, and contact patterns in centric and eccentric positions. The examinations were performed at start, and after 12 and 24 months. In total, 280 subjects were examined at all three occasions.

The incidence of TMJ pain and dysfunction was high among both males and females. The course composed onset, recovery and maintenance resulting in a fluctuating pattern. Females were more likely to have persistent TMJ pain and dysfunction than males. The incidence and persistence of jaw muscle signs and symptoms was high and significantly more common in females. A similar fluctuating pattern as for TMJ pain and dysfunction was found. Crossbite predicted onset and persistence of TMJ pain and dysfunction; mandibular instability in centric positions predicted persistent TMJ pain and dysfunction, as well as persistent jaw muscle signs or symptoms. Reported bruxism increased the risk for TMJ pain and dysfunction but did not significantly affect the course of jaw muscle signs and symptoms. Spinal pain at baseline predicted the onset of jaw pain, headaches, and TMD pain. Signs of TMD at baseline predicted the onset of non-pain symptoms of TMD, jaw pain, headaches, and spinal pain.

In conclusion, the results in this thesis show high incidence rates for TMD, headaches, and spinal pain among dental students. Crossbite, mandibular instability, reported bruxism, as well as female gender were identified as contributing risk factors. Spinal pain and TMD mutually predicted each other, indicating common pathophysiological mechanisms and individual vulnerability. The findings support a multidisciplinary approach, and it is recommended that the status and function of the jaw system be considered in patients with spinal pain.

Both spinal pain and temporomandibular disorders (TMD) commonly occur in the general population. Previous studies demonstrate neurophysiologic and biomechanical couplings between the trigeminal and cervical regions. This investigation tested the null hypothesis of no relationship between spinal pain (neck, shoulder and/or low back) and TMD, by using questionnaires and clinical examinations of the jaw function.

In an age- and sex-matched case-control study, the specific aim was to compare the prevalence of signs and symptoms of TMD among cases with long-term spinal pain and controls without spinal pain. The results showed that subjects with spinal pain had signs and symptoms of TMD significantly more often than did controls. The associations remained after excluding all participants with jaw pain. Furthermore, the comorbidity pattern was similar, regardless of location of spinal pain.

In a cross-sectional study, the specific aim was to test whether there is a reciprocal cross-sectional dose-response-like relationship between spinal pain and TMD. Two different designs were used, one with frequency/severity of spinal pain as independent variable, and the other, with frequency/severity of TMD symptoms as independent variable. The analysis showed increasing odds for presence of TMD symptoms with increasing frequency/severity of spinal pain, and increasing odds for presence of spinal pain with increasing frequency/severity of TMD symptoms.

In a case-control study within a 2-year prospective cohort, the specific aim was to test whether there is a reciprocal temporal relationship between signs and symptoms in trigeminally, and symptoms in spinally, innervated areas. Incidence of symptoms in these areas was analyzed in relation to presence of spinal pain, headaches, and signs and symptoms of TMD at baseline. The main findings were that presence of signs of TMD at baseline increased the onset of spinal pain and symptoms in the trigeminal area, and that spinal pain increased the onset of symptoms in the trigeminal area. An augmentation effect between the significant baseline variables was observed for the incidence of headaches and jaw pain.

In conclusion, the investigation demonstrated a cross-sectional and temporal relationship between spinal pain and TMD; thus, the null hypothesis was rejected. The results indicate common pathophysiological mechanisms in the development of spinal pain and TMD. The comorbidity and reciprocal influence that were found call for an integrated and multidimensional approach in the management of individuals with long-term spinal pain and TMD.