Described are two cylindrical spinal fusion implants. Typically, implant (450a) is inserted into the implantation space first, and then implant (450b) is inserted into the same implantation space behind, and preferrably coaxial to, implant (450a) in a "box car" arrangement. Trailing end of implant (450a) is configured to be placed in contact with leading end of implant (450b), and preferably complementary engage the leading end. For example, the trailing end may include raised portions that cooperatively engage raised portions of the leading end of implant (450b). When implants (450a and 450b) are in contact, it is possible to impart movement of implant (450a) within the implantation space by movement of implant (450b). In this manner, it is possible to fine tune the depth of insertion of implant (450a) without removing implant (450b). The ability to move implant (450a) in this manner also prevents stripping of implant (450b) due to the failure of movement of implant (450a).
