An adjustment assembly for an adjustable prosthetic valve device, comprising: a first gearbox sleeve and a second gearbox sleeve, each gearbox sleeve forming an internal cavity (210A, 210B) therein, the first and second gearbox sleeves each having at least one opening (202A, 203A, 202B, 203B) adjacent a first end (204A, 204B) thereof; a gearbox housing comprising a first gearbox shell and a second gearbox shell, the first and second gearbox shells each including a center portion (104A, 104B) having an opening (116A, 116B) therein, a first lateral portion (106A, 106B)) and a second lateral portion (108A, 108B); a center gear arranged at least partially within the gearbox housing, the center gear including a gear portion and a shaft portion, the shaft portion being adapted to engage an adjustment tool inserted through one of the openings in the gearbox housing; and a first driven gear and a second driven gear each arranged at least partially within the gearbox housing, the first and second driven gears each including a gear portion (184A, 184B) that engages the gear portion of the center gear, and a shaft portion (182A, 182B); wherein the shaft portion of the first driven gear extends at least partially within an opening formed by the first lateral portions of the gearbox shells and the shaft portion of the second driven gear extends at least partially within an opening formed by the second lateral portions of the gearbox shells characterized in that the first gearbox shell and the second gearbox shell are adapted to at least partially interlock with one another, and at least one of the first and second lateral portions of each gearbox shell has a projection (107A, 109A, 107B, 109B) on an exterior surface thereof, each projection adapted to engage a respective opening formed adjacent the first end of one of the gearbox sleeves when the first and second lateral portions are at least partially inserted within the internal cavities of the first and second gearbox sleeves, whereby the first and second gearbox shells are prevented from separating from one another. a first gearbox sleeve and a second gearbox sleeve, each gearbox sleeve forming an internal cavity (210A, 210B) therein, the first and second gearbox sleeves each having at least one opening (202A, 203A, 202B, 203B) adjacent a first end (204A, 204B) thereof; a gearbox housing comprising a first gearbox shell and a second gearbox shell, the first and second gearbox shells each including a center portion (104A, 104B) having an opening (116A, 116B) therein, a first lateral portion (106A, 106B)) and a second lateral portion (108A, 108B); a center gear arranged at least partially within the gearbox housing, the center gear including a gear portion and a shaft portion, the shaft portion being adapted to engage an adjustment tool inserted through one of the openings in the gearbox housing; and a first driven gear and a second driven gear each arranged at least partially within the gearbox housing, the first and second driven gears each including a gear portion (184A, 184B) that engages the gear portion of the center gear, and a shaft portion (182A, 182B); wherein the shaft portion of the first driven gear extends at least partially within an opening formed by the first lateral portions of the gearbox shells and the shaft portion of the second driven gear extends at least partially within an opening formed by the second lateral portions of the gearbox shells characterized in that the first gearbox shell and the second gearbox shell are adapted to at least partially interlock with one another, and at least one of the first and second lateral portions of each gearbox shell has a projection (107A, 109A, 107B, 109B) on an exterior surface thereof, each projection adapted to engage a respective opening formed adjacent the first end of one of the gearbox sleeves when the first and second lateral portions are at least partially inserted within the internal cavities of the first and second gearbox sleeves, whereby the first and second gearbox shells are prevented from separating from one another.