A set of fenestration burrs, for fenestrating otic capsule bone (34), includes an initial burr (150) and a sequence of fenestration polishing burrs (180). A polishing burr (152, 1521), of each of the burrs (150, 180), carries at least one spiraling flute (166, 166'). Fenestrations (36) piercing the bone (34) formed using the burrs (150, 180) exhibit uniform diameters while excluding bone dust from the inner ear. An implantable casing (72) includes a hollow collar (76) from which projects a hollow sleeve (74) receivable into the fenestration (36). The casing (72) is secured there by at least one prong (92, 102) jutting from the sleeve (74). A therapeutic appliance (134) is insertable into the casing (72). A flange (116) extending from one end of the sleeve (74) carries at least one L-shaped slot (122) open at one end and extending circumferentially around the flange (116).