Friday, October 26, 2012
Adjacent Segment Pathology: Natural History or Iatrogenesis?
For decades, spine surgeons have debated whether adjacent segment pathology (ASP) is a result of the natural history of spinal degeneration or due to altered biomechanics following fusion of the adjacent segment. Despite the importance of this question, no definitive studies have come forth to settle the issue. In an attempt to better evaluate this topic, a Spine focus issue was dedicated to this topic. In this issue, Dr. Lee and his colleagues from the University of Washington performed a systematic review to determine: 1) what is the natural history of cervical and lumbar degeneration in the normal population? 2) what are the rates of ASP following cervical or lumbar fusion?, and 3) what are the rates of ASP in candidates for cervical or lumbar fusion treated nonoperatively? Given the lack of a definitive head to head study comparing ASP in patients treated with fusion or nonoperatively, the authors hoped that by defining the rates in non-comparative studies that they could then draw a conclusion based on the data aggregated across different studies. They reviewed multiple longitudinal, population-based studies demonstrating that over 30% of patients had an increase in degenerative changes in the lumbar spine, while almost 50% of patients with cervical degeneration had increased degeneration over 6 years. Many studies also evaluated the rate of radiographic ASP (RASP) after cervical and lumbar fusion, with reported rates between 6.3% and 44%, depending on the length of follow-up and definition of RASP. Not surprisingly, only one study evaluated the natural history of RASP in patients eligible for fusion but who were treated nonoperatively.1 This study was an RCT that compared RASP between patients randomized to fusion or nonoperative treatment for isthmic spondylolisthesis. At an average follow-up of 12 years, patients treated surgically had a higher rate of RASP on some measures than those treated nonoperatively, but 50% of the nonoperative group was lost to follow-up compared to less than 20% of the operative group. Additionally, on the most reliable computer generated measurements, there were no differences in change in disk height or translation at the adjacent level in those treated with or without surgery. Despite their efforts to aggregate studies in order to determine the cause of ASP (i.e. natural history vs. iatrogenic), the authors could only make a weak conclusion that fusion might accelerate ASP.
This paper highlights how difficult it is to answer the ASP question. Despite surgeons being concerned about ASP for decades, we still do not know how much fusion accelerates this process. To answer this question would require an RCT comparing fusion to nonoperative treatment that includes long-term radiographic and clinical follow-up. Very few such studies have been done, and in the case of the trial by Ekman et al. discussed above, 50% of the nonoperative patients were lost to follow-up.1 The Spine Patient Outcomes Research Trial (SPORT) did follow patients treated surgically and nonoperatively for degenerative spondylolisthesis, but the nonoperative patients were not followed with routine radiographs. As this paper points out, the question about the etiology ASP remains unanswered. It is probably reasonable to assume that fusion has some negative effect on adjacent segments, but the available data from motion sparing technology does not suggest that there is anything we can do about it with currently available technology. As long as that remains the case, the question about the etiology of ASP will probably remain academic.
Please read Dr. Lee’s paper and accompanying commentary. Does this systematic review affect the way you view the etiology of ASP? Let us know by leaving a comment on The Spine Blog.
Adam Pearson, MD, MS
Associate Web Editor
1. Ekman P, Moller H, Shalabi A, Yu YX, Hedlund R. A prospective randomised study on the long-term effect of lumbar fusion on adjacent disc degeneration. Eur Spine J 2009;18:1175-86.