We have shown using a comparative cross-sectional multicentre design that around one third of the participants with JIA in this cohort had TMD. Half of children and adolescents with JIA reported pain during jaw movements and pain on palpation of the masticatory muscles and TMJs as compared to one fourth of their healthy peers, palpatory pain was associated with sDMARDs and bDMARDs treatment, and children and adolescents with JIA had a significantly lower mean vertical jaw movement. Moreover, TMJ-related clinical signs and vertical jaw movement ≤40mm had the highest association in the JIA group.
The reported prevalence of TMD in children with JIA varies between 38% and 83% according to the definitions and methods of ascertainment used, to the cohort examined, and to differences in populations (15, 32–35). Ferraz and colleagues, in their study of 15 children with JIA ranging in age from 6 to 28 years (mean age 16.3 years), reported a high prevalence of 83%. Still, they did not describe the method of ascertainment, i.e., whether the figures were based on self-reporting or on clinical examination . A previous study from Rongo and colleagues based on 50 participants with JIA aged 9–16 years found a prevalence of TMJ damage from 100 joints to be 74% as assessed by MRI . Others have reported a prevalence of 55% based on a questionnaire  and of 72% based on clinical signs . However, none of those studies were based on the research diagnostic criteria RDC/TMD, and the children were older than those in our study. In contrast, a longitudinal study by Zwir et al., including 75 children, revealed a prevalence of 38% based on symptoms and 47% based on clinical examination . Their results are in line with ours.
In our study, the prevalence of TMD, either based on symptoms or clinical signs, in the healthy peers, were quite high at 28% and 29%, respectively. This was higher than in earlier studies among adolescents reported by Graue and colleagues (7% and 12%, respectively) and Østensjø and colleagues (7%) [8, 9]. The combination of both symptoms and clinical signs for TMD was more in line with the mentioned studies. Studies from Finland and Brazil confirm our results with a high prevalence of TMD in the normal population. Vierola et al.  reported a TMD prevalence of 35% (mean age 7.9 years) and de Paiva Bertoli reported a TMD prevalence of 34% (mean age 11.0 years) . The difference in TMD prevalence in the normal population of children and adolescents is probably due to the use of different diagnostic tools, different numbers of participants, different ages of the studied populations, different countries, and different study designs. In studies from Norway, Graue and colleagues  used two screening questions for pain related to TMD  and DC/TMD  for symptoms and clinical signs in a population of 210 children and adolescents aged 12–19 years. Østensjø et al.  used the same two screening questions of TMD symptoms  for screening a population of 560 adolescents aged 13–19 years. Then a modified RDC/TMD examination  was used for those who answered yes to 1) having pain in the temples, face, TMJ, or jaws once a week or more and 2) having pain once a week or more when opening the mouth wide or chewing. The Finnish group  used the RDC/TMD  for clinical signs in 483 children aged 6–8 years, and the Brazilian group  used the American Academy of Orofacial Pain  form for screening and the RDC/TMD  for clinical examination in a population of 934 children aged 10–14 years. Thus it is clear that it can be challenging to get an exact figure on the prevalence of TMD in the normal population. A previous meta-analysis conducted by da Silva and colleagues showed the overall prevalence of intra-articular joint disorder to be 16% .
In our study, approximately half of the JIA subjects had clinical findings consistent with TMD, with no differences according to JIA category. Because the numbers for three of the categories – systemic arthritis, rheumatoid factor positive polyarthritis, and psoriatic arthritis – were relatively low, these results should be interpreted with caution.
he sensitivity and specificity of the clinical orofacial examination in relation to TMJ has been debated because displacement of the disc, although eliciting a clicking sound, might be asymptomatic [39–41]. Based on the DC/TMD criteria, asymptomatic TMJ clicking is still defined as TMD. However, several studies have shown that pain-free clicking represents a normal variant, typically seen in girls during puberty . Recently, a clinical examination protocol for JIA was developed by the Temporomandibular Joint Juvenile Arthritis Working Group (TMJaw). This examination protocol focuses on three general items, namely TMJ symptoms, TMJ dysfunction, and dentofacial deformity in JIA, and it shows acceptable reliability and validity .
We found, in accordance with other studies, that the TMJ area and the masseter muscle region were common locations for pain in JIA . However, a recent study from Koos and colleagues reported a lower frequency of masticatory pain on palpation , and Kristensen and colleagues stated that masticatory pain complaints could develop over time . In the present study, more than half of the participants with JIA showed clinical signs in the TMJ region and the masseter region, and more than one-fourth of the participants with JIA had TMD. A longitudinal multicentre approach might elucidate the development of masticatory muscle pain, as Kristensen and colleagues have suggested .
The vertical unassisted jaw movement has been widely used as a valid marker for TMJ arthritis . We showed that participants with JIA had lower vertical movements compared to their healthy peers, but the differences were relatively small, thus questioning its clinical significance. Viewed differently, for children and adolescents aged <11 years, the cut-off value of 40 mm was within the range of normal vertical jaw movement . Further, our findings suggest that lateral movement did not differ significantly between the two groups, which is in line with the results of Twilt and colleagues  and Küseler and colleagues (22). In the latter study of 15 children with JIA with a mean age of 12 years, the recorded decreased lateral movements were ≤5 mm with no significant relevance .
We found no statistically significant differences in the presence of TMD according to JIA categories. However, we found a significantly higher occurrence of clinical signs in participants with JIA currently on DMARDs medication (whether synthetic or biologic) compared to those not taking such medication. A high risk of developing clinical signs of TMD was associated with a severe disease course, as indicated by the use of DMARDs.
The strengths of this study are the relatively large number of participants, in which the study groups were well matched, and the meticulous standardisation of the clinical TMJ assessment performed prior to and during the study period. However, the large number of participants should not hide the fact that we are dealing with an underpowered sample size that was lacking 75 participants. An additional limitation is that the overall response rate of 63%, although considered acceptable, might have influenced the results because the group that did not participate was, on average, slightly younger and had a somewhat lower proportion of girls. Also, the shortened version of the DC/TMD used in this study is not directly comparable with studies having used the full DC/TMD score. In the present study, children and adolescents with JIA with TMD involvement were defined based on clinical examination, and both self-reported and parent-reported pain. Further studies will focus on the role of imaging on the diagnosis of TMJ arthritis in children with JIA. Clinical orofacial examination may not be reliable for diagnosing disc displacement without reduction . Imaging diagnosis is particularly important in JIA with non-symptomatic TMJ involvement because hard tissue loss in the condyle might hinder the growth of the mandible and subsequently affect chewing function and cause aesthetic problems .