Two hundred and fifty four females volunteered to take part in the study and completed baseline sensory assessments, of whom 212 (83%) provided a blood sample and written consent for genotyping. Follow-up data about clinical TMD status were obtained from 186 subjects. Sixty two subjects with follow-up data (33.3%) had pain-sensitive haplotypes and the remaining 124 were classified as pain-resistant. Ninety-nine subjects (53.2%) reported a history of orthodontic treatment, 75 (40.3%) said they had not had orthodontic treatment but 12 subjects (6.4%) said they did not know and they were excluded from subsequent analyses.
Among the cohort of 174 people available for analysis, 15 new cases of TMD were diagnosed using RDC criteria during the follow-up period that averaged 30 months (range=8–42 months). The fifteen new cases of TMD represented a cumulative incidence of 8.6% (95% CI = 4.4% – 12.8%). Among subjects with pain-sensitive haplotypes, cumulative incidence was 15.4% compared with 5.7% among subjects classified as pain-resistant (Table 1). This yielded a statistically significant relative risk of 2.68 (95% CI = 1.03 – 7.01) comparing pain-sensitive with pain-resistant subjects. Risk of TMD was three-fold greater among people who reported a history of orthodontic treatment compared with those who did not, although the associated relative risk did not reach statistical significance, as indicated by the 95% confidence interval (0.89 – 10.35) that included unity. Two thirds (67.3%) of subjects with pain-sensitive haplotypes reported a history of orthodontic treatment compared with 52.4% for pain-resistant haplotypes, although the difference in percentages was not statistically significant (Chi-square, 1 df, P=0.07).