**13. Tongue-retaining devices (TRDs)**

Tongue-retaining devices were first described by Cartwright and Samelson in 1982 [28]. During sleeping and in supine position, the tongue and all gravity-dependent tissues tend to fall posteriorly. With TRDs, the tongue is prevented from dropping posteriorly by suction created when the patient forces the tongue into a hollow bulb built into the device. The forward position of the tongue increases the volume and decreases the resistance of upper airway. The superiority of TRDs over MADs is that they can be used for edentulous patients [10–12, 16, 17, 19, 20, 23, 25, 31].
