A distractible intervertebral body fusion device (100, 500, 600, 700, 800, 900) adapted for implantation into an intervertebral disc space of a patient having a first bearing surface configured to interface with an end plate of a superior vertebra of the intervertebral disc space and a second bearing surface configured to interface with an end plate of an inferior vertebra of the intervertebral disc space, where the device includes: at least one coaxial screw gear sleeve mechanism disposed between the first bearing surface and the second bearing surface, characterized in that the coaxial screw gear sleeve mechanism includes: a post (111,112,511,621) having a threaded exterior surface (113,115,911) on a portion of the post projecting inwardly from one of the first bearing surface and the second bearing surface; and a corresponding sleeve (120,130,520,620,720,920) configured to surround the post and having: a threaded interior surface (122,132) on a portion of the sleeve projecting inwardly from the other of the first bearing surface and the second bearing surface and configured to interface with the threaded exterior surface of the post; and a geared exterior surface (124,134,922) on a portion of the sleeve, the geared exterior surface defined on a thread (121,131,921) extending in a helical pattern along a height of the sleeve; and a drive mechanism having a surface configured to interface with and drive the geared exterior surface of the sleeve, such that selective operation of the drive mechanism causes a distraction of the first bearing surface and the second bearing surface with respect to the superior vertebra and the inferior vertebra of the intervertebral disc space due to a telescoping expansion resulting from the sleeve translating relative to one of the first bearing surface and the second bearing surface simultaneously with the post translating relative to the sleeve. at least one coaxial screw gear sleeve mechanism disposed between the first bearing surface and the second bearing surface, characterized in that the coaxial screw gear sleeve mechanism includes: a post (111,112,511,621) having a threaded exterior surface (113,115,911) on a portion of the post projecting inwardly from one of the first bearing surface and the second bearing surface; and a corresponding sleeve (120,130,520,620,720,920) configured to surround the post and having: a threaded interior surface (122,132) on a portion of the sleeve projecting inwardly from the other of the first bearing surface and the second bearing surface and configured to interface with the threaded exterior surface of the post; and a geared exterior surface (124,134,922) on a portion of the sleeve, the geared exterior surface defined on a thread (121,131,921) extending in a helical pattern along a height of the sleeve; and a drive mechanism having a surface configured to interface with and drive the geared exterior surface of the sleeve, a post (111,112,511,621) having a threaded exterior surface (113,115,911) on a portion of the post projecting inwardly from one of the first bearing surface and the second bearing surface; and a corresponding sleeve (120,130,520,620,720,920) configured to surround the post and having: a threaded interior surface (122,132) on a portion of the sleeve projecting inwardly from the other of the first bearing surface and the second bearing surface and configured to interface with the threaded exterior surface of the post; and a geared exterior surface (124,134,922) on a portion of the sleeve, the geared exterior surface defined on a thread (121,131,921) extending in a helical pattern along a height of the sleeve; and a threaded interior surface (122,132) on a portion of the sleeve projecting inwardly from the other of the first bearing surface and the second bearing surface and configured to interface with the threaded exterior surface of the post; and a geared exterior surface (124,134,922) on a portion of the sleeve, the geared exterior surface defined on a thread (121,131,921) extending in a helical pattern along a height of the sleeve; and a drive mechanism having a surface configured to interface with and drive the geared exterior surface of the sleeve, such that selective operation of the drive mechanism causes a distraction of the first bearing surface and the second bearing surface with respect to the superior vertebra and the inferior vertebra of the intervertebral disc space due to a telescoping expansion resulting from the sleeve translating relative to one of the first bearing surface and the second bearing surface simultaneously with the post translating relative to the sleeve.