Purpose: To compare the efficacy of twin-block (i.e., allows mouth opening) and fixed (i.e., maintains mouth closure) mandibular advancement splints (MASs) for the treatment of obstructive sleep apnea–hypopnea syndrome (OSA). Materials and Methods: From 2011 to 2013, 23 patients with OSA in the twin-block group, and from 2013 to 2015, 29 patients in the fixed MAS group were included. All patients underwent polysomnography before and after 3 months of treatment. The two sets of polysomnographic and cephalometric variables were compared. Results: A significant difference (p < 0.001) was observed in the apnea–hypopnea index before and after MAS treatment in both groups (twin-block group: 20.6 ± 11.5 vs. 14.7 ± 9.4; fixed group: 21.4 ± 15.2 vs. 11.2 ± 9.7). In the twin-block group, 5 patients (21.7%) were complete responders, 9 (39.1%) were fair responders, and 9 (39.1%) were nonresponders; the corresponding figures for the fixed group were 14 (48.3%), 9 (31.0%), and 6 (20.7%) patients. A significant between-group difference was observed in the distribution of responders (p = 0.046). The fixed group showed a significant improvement in the snoring index (p = 0.003), arousal index (p = 0.036), and desaturation rate (p = 0.012). Finally, the change in incisal overjet was larger in the fixed group than in the twin-block group (p < 0.001). Conclusions: These results suggest that fixed oral appliances are superior in treating OSA, based on their ability to prevent mouth opening and reduce incisal overjet.
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