Nested interbody spinal fusion implants

An interbody spinal fusion implant for surgical implantation at least in part within a disc space between two adjacent vertebral bodies in a segment of a human spine, said implant comprising:

• opposed upper and lower arcuate portions for contacting each of the adjacent vertebral bodies when positioned therein, each of said upper and lower arcuate portions having at least one opening adapted to communicate with one of the adjacent vertebral bodies, said openings of said upper and lower portions being in communication with one another and adapted for permitting for the growth of bone from adjacent vertebral body to adjacent vertebral body through said implant;
• at least a portion of a helical thread formed on the exterior of each of said opposed arcuate portions for penetrably engaging the adjacent vertebral bodies and to facilitate securing said implant into the spine by at least in part rotating said implant about a longitudinal axis of said implant, said thread having a pitch;
• an insertion end for entry into the spine, a trailing end opposite said insertion end, and a length therebetween, said trailing end having a perimeter with opposed sides, said trailing end having an exterior surface spaced apart from said thread, said exterior surface extending from one of said opposed sides to another of said opposed sides of said perimeter;
• and
• a characteristic on at least said exterior surface of said trailing end of said implant, said characteristic being spaced apart from said thread, said characteristic being adapted to indicate a rotational orientation of said upper and lower arcuate portions relative to the adjacent vertebral bodies into which said implant is adapted to be inserted, the length and pitch of said implant having a ratio relative to one another such that the length of said implant when divided by the pitch of said thread equals a number that when divided by one of 0.25 and 0.5 equals an integer, said characteristic being configured to indicate an initial insertion orientation of said implant, said thread having a starting point proximate said insertion end, said starting point of said thread being indexed such that insertion of said implant in said initial insertion orientation positions said upper and lower arcuate portions toward the adjacent vertebral bodies upon one of a predetermined number of a quarter and a half rotations of said implant.