CT evaluation of lumbar interbody fusion: current concepts. (2005)

Operative techniques for fusing an unstable portion of the lumbar spine or immobilizing a painful vertebral motion segment have been in use for many years. Lumbar fusion procedures initially were used to treat patients with infections (tuberculous spondylitis) or misaligned spines (isthmic spondylolisthesis) (1–8). Early techniques involving the use of harvested autogenous bone graft without instrumentation were largely unsuccessful because of the lack of structural integrity and the undesirable side effects of the harvesting procedure. Because of the high failure rates associated with early fusion procedures using bone graft or posterior pedicle screws, an eventual transition to more successful approaches to disk height maintenance using a structural graft took place. The evolution of construct design from femoral ring allograft, to threaded cortical bone dowels, to cylindrical metal fusion cages, and ultimately to tapered fusion cages (metal and composite) proceeded with the knowledge that fusion success requires, in part, both mechanical stability and adequate graft material to provide a favorable biologic environment in which fusion can occur (9). Increasingly, lumbar interbody fusions have been performed in patients with degenerative disk disease and discogenic pain syndromes (10–11). It is estimated that more than 300,000 lumbar spine fusion procedures are performed each year in the United States (12). The radiologist should be familiar with the various techniques, devices, and potential complications associated with these procedures. Since the first posterior lumbar interbody fusion performed by Cloward in 1940, spinal interbody fusion techniques have continued to evolve with an increasing number of interbody fusion devices available for use (9, 11–18). These devices have varying geometric configurations and wall thicknesses and are made of various materials, such as titanium, PEEK (polyether etherketone), and other polymers. In addition, human allograft bone dowels and allograft bone spacers are also used in interbody fusion procedures. The choice of a particular interbody fusion device affects the ability of surgeons and radiologists to assess fusion progression on dynamic radiographs and CT scans. Interbody Fusion Techniques Interbody fusion, or fusion across the disk space, can be performed by using several surgical approaches. Posterior lumbar interbody fusion, anterior lumbar interbody fusion, and transforaminal lumbar interbody fusion have unique advantages and potential complications associated with each approach. Each technique can stand alone or can be accompanied by supplemental segmental posterior instrumentation (posterior rods and pedicle screws most commonly). The purpose of all interbody fusion devices is to restore and maintain disk space height and normal sagittal contours (lordosis) and to increase the stability of the operated segment or segments (13). Stability and lordosis are obtained by stretching the annulus and supporting ligaments via distraction of the disk space. This stretching of the motion segment is termed “ligamentotaxis” and provides a biomechanically stable construct that will limit motion and permit fusion to develop. Distraction of the disk space also results in indirect decompression of the foramina (13).