An expandable rod system (310) includes a first rod portion (340) having an internal cavity, a second rod portion (320) sealingly positioned within the internal cavity of the first rod portion and moveable in a first axial direction relative to the first rod portion, and an osmotic chamber (374) for receiving an osmotic agent to facilitate movement of the second rod portion to expand the rod system. The rod system includes a lock assembly (356) engageable with the second rod portion to prevent the second rod portion from moving in a second axial direction. The lock assembly includes a tapered ramped portion and a bearing member that is moveable within the tapered ramped portion such that when compression forces are imparted on the second rod portion, the bearing member becomes wedged in the tapered ramped portion.
