The comorbidity of the two conditions is high. Approximately one in four patients with clinically diagnosed TMD has OSA diagnosed by polysomnogram (PSG). This comorbidity poses a problem for management because neither appliance is considered effective in treating both conditions. In fact, mandible advancement is assumed to worsen TMD pain. However this assumption is not well supported by scientific evidence, with some studies finding that mandibular advancement actually reduces TMD pain, albeit after a transient increase in TMD pain in some patients. Furthermore, anecdotal evidence suggests that mandibular advancement reduces TMD pain. It is believed that by maintaining an open airway with the oral appliance, patients no longer have to clench their teeth in an effort to prevent the airway from collapsing during sleep.