An expandable prosthetic implant for engagement between vertebrae, comprising: - an inner member having a hollow interior portion and a threaded external portion and including a first end portion configured to engage a first vertebral body; - an outer member having a hollow interior portion configured to coaxially receive the inner member therein and including a second end portion configured to engage a second vertebral body, wherein the inner and outer members (12, 14) are moveable relative to each other along a longitudinal axis; - a gear member positioned coaxial to the inner member and outer member and axially fixed to the outer member and freely rotatable with respect to the outer member, - wherein the gear member threadedly engages the threaded portion of the inner member, wherein the first and second end portions (20, 62) are configured to respectively engage first and second vertebrae, - wherein the implant is configured to engage the vertebrae such that first and second end portions (20, 62) are oriented in a predetermined alignment with respect to the first and second vertebral bodies, and - wherein the gear member includes gear teeth extending around the perimeter of the gear member, the gear teeth extending from a proximal end of the gear member to a distal end of the gear member, - a locking member received in the outer member, the locking member having an engagement member; characterised in that the locking member has at least one flexible arm (86, 88) and a through-hole; - the gear member includes a plurality of cutouts for engaging the engagement member of the locking member, - and the locking member is movable between a first position in which at least one flexible arm (86, 88) is unflexed and the engagement member of the locking member engages the cutouts of the gear member preventing the gear member from rotating and a second position in which the at least one flexible arm (86, 88) is flexed and the engagement member of the locking member is disengaged from cutout of the gear member allowing the gear member to rotate. - an inner member having a hollow interior portion and a threaded external portion and including a first end portion configured to engage a first vertebral body; - an outer member having a hollow interior portion configured to coaxially receive the inner member therein and including a second end portion configured to engage a second vertebral body, wherein the inner and outer members (12, 14) are moveable relative to each other along a longitudinal axis; - a gear member positioned coaxial to the inner member and outer member and axially fixed to the outer member and freely rotatable with respect to the outer member, - wherein the gear member threadedly engages the threaded portion of the inner member, wherein the first and second end portions (20, 62) are configured to respectively engage first and second vertebrae, - wherein the implant is configured to engage the vertebrae such that first and second end portions (20, 62) are oriented in a predetermined alignment with respect to the first and second vertebral bodies, and - wherein the gear member includes gear teeth extending around the perimeter of the gear member, the gear teeth extending from a proximal end of the gear member to a distal end of the gear member, - a locking member received in the outer member, the locking member having an engagement member; - the gear member includes a plurality of cutouts for engaging the engagement member of the locking member, - and the locking member is movable between a first position in which at least one flexible arm (86, 88) is unflexed and the engagement member of the locking member engages the cutouts of the gear member preventing the gear member from rotating and a second position in which the at least one flexible arm (86, 88) is flexed and the engagement member of the locking member is disengaged from cutout of the gear member allowing the gear member to rotate.