Effects of thermoplastic retainers on occlusal contacts.

The aim of this prospective study was to evaluate the number of contacts in centric occlusion during retention with thermoplastic retainers (Essix retainers) and in the long term. After four premolar extractions and active orthodontic treatment of 15 Class I (10 females, 5 males; mean age 17.20 +/- 1.7 years), thermoplastic retainers were used. Occlusal contacts were determined from occlusal registrations taken in centric occlusion at the beginning (T0), end (9 months of retention; T1), and after 2.5 years (T2). The occlusal contacts determined in these patients were compared with the values of 15 'normal' Class I subjects (9 females, 6 males; mean age 17.10 +/- 1.60 years) who had not undergone orthodontic treatment. Wilcoxon and a Mann-Whitney U-tests were used to evaluate intra- and intergroup differences. No significant change was observed in the number of posterior contacts during T1, whereas a significant increase was found at T2 (P < 0.01) for the second premolars (P < 0.01) and second molars (P < 0.05). Both 'ideal' and 'non-ideal' contacts increased significantly but only at T2 (P < 0.05). The number of ideally located contacts on the posterior teeth at all three periods were lower than normal values (P < 0.01); while non-ideal contacts at T1 (P < 0.05) and T2 (P < 0.01) were found more often when compared with the values of normal subjects. Only the increased number of premolar contacts at T2 was more than normal values (P < 0.01). There was no expected increase in occlusal contacts at T2; however, posterior occlusal contacts were increased at T3.