Removal of an intervertebral disc is often desired if and when the disc degenerates. The disc may be replaced with a device such as a cage or other spacer that restores the height of the disc space and allows bone growth through the device to fuse the adjacent vertebrae. Spacers often do not intimately connect the two vertebral bodies and a combination of plates and screws are often used to obtain the rigidity necessary to enable bone to grow and fuse the adjacent vertebral bodies.
Implants for spinal fusion that are impacted into the disc space and allow growth of bone from adjacent vertebral bodies through the upper and lower surfaces of the implant are known in the art. Such implants are typically provided with a lordotic taper to enable a surgeon to recreate an appropriate lordotic curvature to the motion segment. In order to create the appropriate environment for fusion, fixation hardware is applied to the spinal segment to limit the relative motion between the vertebral bodies to be fused.
Furthermore, interbody implants that feature a screw thread form connected to a central body have been developed, such as the well-known cylindrical threaded spacers. These devices are typically hollow and allow bone growth through fenestrations in the device. Clinically, these devices are associated with the risk of post-operative loss of disc height due to the small surface area available to resist subsidence into the adjacent vertebral body relative to design of the impacted cages.
Attempts to combine the features of the impacted implants with the implants using screw thread forms, provide a greater resistance to subsidence; however, they offer little resistance to anatomic motions where the vertebral bodies move apart from each other, such as is typical in flexion and lateral bending.