Fusion has become the standard of care for numerous pathologic conditions of the spine over the past 50 years. Instrumented thoracolumbar fusion for adolescent and adult spinal deformity has enjoyed great success in arresting the progression of and correcting scoliotic deformity. In the cervical spine, decompression and fusion have provided a greater than 90% likelihood of relieving radicular symptoms and stabilizing or improving myelopathy. (1,2) Decompression and posterolateral fusion of the lumbar spine have been shown to be the superior form of surgical management for degenerative spondylolisthesis in prospective randomized studies. (3) Given the high clinical success recognized in using fusion for deformity and stenosis of the cervical and lumbar spine, the long-term sequelae of these procedures has been considered of secondary importance. However, as spinal fusion is being performed on younger patients and as the rates of cervical and lumbar spine surgery have increased over the past two decades, (4) concern regarding the effect on adjacent motion segments has been increasing. Furthermore, as indications for fusion have expanded to include mechanical back and neck pain with more variable success rates, concern for adjacent segment degeneration has been amplified.

Adjacent level degeneration, following cervical, lumbar, and lumbosacral fusions, has been well documented in recent literature, yet there remains considerable controversy as to when adjacent level radiographic degeneration becomes clinically relevant. (5-9) It has also become evident that maintenance of sagittal alignment of the spine has important clinical implications when treating and preventing adjacent segment degeneration and flat back syndrome. (10,11) Numerous studies have demonstrated in properly selected patients that decompression and fusion can yield marked improvement with respect to pain and outlook for various conditions of the spine. (12-16) However, there are also numerous articles that suggest otherwise, with the results of spinal fusion described as unpredictable, if not unsatisfactory, and often precipitating further surgical intervention. (17,18) It is important, therefore, to understand the long-term consequences of cervical, lumbar, and lumbosacral fusions, and their impact on adjacent segment degeneration.

Cervical Spine

Clinical Evidence of Adjacent Segment Disease

Radiographic evidence of adjacent level degeneration in the cervical spine has been documented. Katsuura and colleagues noted degenerative changes evident on radiological examination in levels adjacent to a fused segment in 21 of 42 (50%) patients, following anterior cervical interbody fusion, at 9.8 years average follow-up. (19) Goffin and associates reported on 181 patients treated by anterior cervical interbody fusion with an average follow-up of greater than 8 years. Patients were clinically and radiologically examined by independent investigators who reported mild radiologic deterioration at adjacent levels in 92% of cases studied and moderate to severe changes in 43% of cases. (20) The severity of radiographic changes was associated with the time interval since surgery. However, radiographic adjacent segment degeneration was not correlated with clinical symptoms.

Herkowitz and coworkers studied 44 patients with cervical radiculopathy randomized to anterior discectomy and fusion versus posterior foraminotomy with 4.5 years follow-up. Forty-one percent of those undergoing anterior fusion developed adjacent level radiographic degeneration. Interestingly, 50% of those...