There has been a virtual “cage explosion” in spine surgery in the United States over the past two years. Cages are devices placed between adjacent vertebrae to facilitate a fusion of the instrumented interspace. The cages are typically made of titanium, although allograft bone spacers can be used for the same purpose. The overwhelming majority of cases involving cages are performed in the lumbar spine.

Lumbar interbody fusions have been performed for many years. Posterior lumbar interbody fusions (PLIF’s) were popularized by orthopedic surgeons for the treatment of back pain resulting from degenerative disc disease. Typically, most of these surgeries occurred at the caudal motion segments (either L4-L5 or L5-S1). PLIF’s have been performed for more than fifty years. Anterior lumbar interbody fusions (ALIF’s) have enjoyed less widespread popularity; however, ALIF’s also have been performed for more than fifty years.

The rapid increase in the number of interbody fusions has occurred for multiple reasons. First, biomechanical studies have shown that the instantaneous axis of rotation of the lumbar spine is located in the vertebral body. As such, the most biomechanically desirable location to support the degenerated spine is between the vertebral bodies. This is most easily accomplished by performing an interbody fusion. Second, the continuing controversy over the use of bone screws in the pedicles of the lumbar vertebrae (pedicle screws), due to the Food and Drug Administration (FDA) refusing to grant unrestricted approval of these devices, has led some spine surgeons to seek other methods of stabilizing the lumbar spine.

Significant controversy has been raised over the indications for interbody fusions (either PLIF or ALIF). Classically, degenerative disc disease, which causes mechanical low back pain, has been the most frequent surgical indication. Discogenic back pain causes back pain to a much greater degree than leg (radicular) pain. The corroborating radiographic imaging studies also have been challenged. Magnetic resonance (MR) imaging studies will show a black disc on T2 – weighted sequences; however, disc abnormalities have been demonstrated on MR studies in up to one-third of normal volunteers who do not have back pain. Provocative discography involves performing a discogram in an alert patient where infusion of dye into the disc space at the involved segment will re-create the patient’s typical back pain. In addition, dye injected into other discs will not cause the same type of pain pattern.