An implantable device for mounting to a patient's bone includes a housing and at least one osseointegrating protuberance. The housing includes a surface having an abutting portion configured to abut the bone when the housing is implanted in the patient, the abutting portion defining a housing axis orthogonal to the surface. The at least one osseointegrating protuberance: extends from the surface of the housing; is adapted to abut the patient's bone; and has a substantially smooth shaft.