Abstract: A microactuator (32) of an implantable hearing aid system (10) is secured within a casing (50) implanted into a fenestration (52) that pierces the promontory (18) of the otic capsule bone (31). The casing (50) includes a hollow sleeve (62) that has an outer surface (64) and a first end (66) that is received into the fenestration (52). The sleeve (62) also includes an inner surface (68) adapted to receive a barrel (74) of the microactuator (32). The casing (62) also includes a flange (76) that is integral with the sleeve (62) and projects outward from the outer surface (64) of the sleeve (62) about a second end (78) of the sleeve (62). Various means secure the sleeve (62) within the fenestration (52) such as screwing into the promontory (18) or clamping to the promontory (18). The casing may fasten the microactuator (32) to the casing (50) by a threaded attachment, with screws, with button-and-socket snap fasteners, or with a slotted tongue-and-groove lock.