The purpose of this study is to compare the cost-effectiveness and effectiveness of mandibular repositioning appliance (MRA) versus Continuous positive airway pressure (CPAP) therapy in patients with moderate Obstructive Sleep Apnea Syndrome (OSAS).

Study design: In a randomized parallel controlled study 86 patients will be randomly assigned to either MRA therapy or CPAP therapy. Group A receives MRA. Group B receives CPAP. The total duration of the study is 12 months. Measurements will be done at baseline, after 3, 6 and 12 months.

Intervention: Group A will be treated with a bibloc MRA (Somnodent). The mandible will be set at 70% of the patient's maximum advancement and will be adjusted to the convenience of the patient. Titration will be continued until symptoms abate or until further advancement causes discomfort.

Group B will be treated with CPAP. Proper CPAP-pressure will be set for each patient separately. Patients are fitted with a comfortable CPAP mask before titration of the CPAP-pressure. For CPAP-titration, patients are instructed to adopt their own typical sleeping habits.

Inclusion Criteria:

• Individuals who have been subjected to polysomnography and are diagnosed as having moderate (AHI 15-30) OSAS;
• Aged ≥ 18 years;

Exclusion Criteria:

Medical and psychological criteria:

• Patients previously treated for OSAS (e.g. CPAP, MRA);
• Morphologic abnormalities of the upper airway (e.g., a compromised nasal passage, enlarged tonsils or adenoids, or upper airway soft-tissue or craniofacial abnormality);
• Reported or documented unstable endocrine dysfunction (hypothyroidism, acromegaly, or pituitary adenoma); Reported or documented severe cardiovascular- or pulmonary co-morbidity
• Clinically concurrent cardiovascular disease (coronary artery disease, heart failure,cardiac arrhythmias)
• CVA within 6 months prior to randomisation
• Daytime respiratory insufficiency
• Severe Chronic Obstructive Pulmonary Disease (COPD) (GOLD 3 or 4; FEV1 / FVC < 70% and FEV1 < 50%);
• Other diseases that may impact the evaluation of the results of the study according to the investigator's judgement.
• Reported or documented psychological condition precluding informed consent (e.g., mental retardation, depression or schizophrenia);

Whether the patient has unstable endocrine dysfunction, severe cardiovascular- or pulmonary co-morbidity or a psychological condition precluding informed consent, will be assessed by evaluating the patient's medical record.

Dental exclusion criteria:

• Extensive periodontal disease or tooth decay;
• Active temporomandibular joint disease (including severe bruxism);
• Restrictions in mouth opening (< 25mm) or advancement of the mandible <5mm);
• Partial or complete edentulism (less than eight teeth in upper or lower jaw).