We found that MADs might be successful in the treatment of OSA as a first-line therapeutic modality, resulting in the improvement of several sleep-related variables. The current study proposes that MAD treatment could be preferentially recommended to OSA patients with tongue base narrowing and soft palate obstruction, and MADs have the potential to provide clinically satisfactory therapeutic outcomes in the reduction of upper airway narrowing.The main pathologic conditions in OSA are upper airway collapse and narrowing, and many anatomical factors at the level of the pharynx may contribute to these pathologic conditions.[6,16] Effective treatment of OSA requires maintenance of upper airway patency during sleep, which can be achieved through several modalities, including invasive and noninvasive interventions.[17–25] For the treatment of OSA, there are several options from nonsurgical approaches such as weight control and intranasal space maintaining to surgical approaches including palatopharyngoplasty.[26] CPAP is the current mainstay of noninvasive therapy for OSA and is an effective treatment that has been shown to reduce apnea or hypopnea, subjective sleep-related symptoms, and attenuate cardiovascular complications.[3,27–29] Despite its clinical effectiveness and high success rate of CPAP for improvement of OSA, adherence to its use is poor and pressure intolerance is a frequent complaint reported by CPAP-nonadherent patients.[30,31] As a nonsurgical approach, MADs are now widely recommended as an alternative treatment strategy for subjects who were diagnosed with mild or moderate OSA and who are unable to tolerate CPAP therapy.[32–34] MADs had been actually used as a device for reducing snoring within the dental field and are often more tolerable for certain OSA subjects with appropriate oropharyngeal anatomy.[34]In 2006, the American Academy of Sleep Medicine (AASM) updated the practice parameters for the treatment of OSA with oral appliances. They stated that oral appliances are indicated for use in mild-to-moderate OSA patients who prefer oral appliances to CPAP, do not respond to CPAP, are not appropriate candidates for CPAP, or have experienced failed treatment attempts with CPAP or behavioral measures such as weight loss and sleep position change.[35] Some evidence suggests a significant improvement in symptoms and sleep study parameters through MAD application based on the correction of upper airway collapse at the retroglossal areas in OSA patients.[36,37] However, oral appliance therapy has generally been thought to be less effective in relieving upper airway obstruction when compared with CPAP, and the positive impact beyond that observed in individual cases of MAD use as first treatment modality has not been systematically investigated.[38] In addition, there has been controversy over the success rate of MAD treatment in controlling OSA, and large variability is observed in the reduction of AHI with MADs.In the present study, we found that 72% of patients achieved a satisfactory outcome with 50% or greater AHI reduction, and an AHI of less than 10 per hour after MAD treatment. We also observed that satisfactory outcomes could be achieved in OSA patients irrespective of OSA severity, and 37% of subjects who showed a satisfactory outcome had severe OSA.It has been reported that MAD treatment is less efficacious than CPAP for improving the sleep parameters of OSA. Only a few studies have reported a therapeutic effect of MAD treatment for the control of OSA in patients with mild sleep-related symptoms.[39–41] Previous studies also have estimated that patients with severe OSA treated with MADs have shown lower success rates, and the results showed the evidence of efficacy of MAD for OSA patients and total success rate of MAD was below 50%.[32,33] Accordingly, MADs have not been recommended as an initial treatment for patients with moderate or severe OSA.However, we determined that MADs could be recommended to OSA patients with more extensive symptoms including moderate or severe OSA. Our study also suggested that MADs may not be inferior to CPAP in achieving successful outcomes and normalizing PSG indices in the treatment for OSA. Therefore, we concentrated on evaluating the clinical characteristics of patients who benefitted from MAD treatments irrespective of OSA severity. Future studies may be useful for identifying sleep parameters or anatomic structures of patients who do benefit from MAD treatment.Based on the current findings, patients wi