Studying the causes and ways to avoid adjacent segment disease (ASD) of the cervical (neck) and lumbar (low back) spine has become a major focus of research these days. But with the ever changing surgical tools and techniques, it is difficult to compare studies from 10 years ago with current studies. And that dilemma will continue into the future as treatment strategies change with evolving technology.
Even so, efforts are being made to study the problem of adjacent segment disease (ASD) and ways to avoid it. Adjacent segment disease refers to breakdown of the vertebrae next to a spinal fusion or disc replacement implant. Focusing on the causes of the problem may help surgeons identify avoidance strategies.
Currently, there are three known causes of adjacent segment disease. They are: 1) the natural history (what normally happens) in the disc at the next level, 2) the biomechanical stress and increased motion placed on the next level because of the fusion, and 3) anatomical disruption of the vertebral segment at the next level from the surgery.
In this review article, surgeons from Walter Reed Military Medical Center and Thomas Jefferson University team up together to present the latest evidence on adjacent segment disease (ASD) and how to avoid it. They say that strategies to avoid ASD seem to be working better for the cervical spine than for the lumbar spine. And ASD may be less likely to develop after disc replacement compared with spinal fusion.
Matching strategies with causes isn't easy. For example, to know for sure if natural history (the natural tendency for the spine to degenerate over time without surgery) is the real problem is difficult to study. Providing needed surgery for one group of patients while comparing them to a control group (those who don't have the surgery) isn't ethical. So the true incidence of ASD linked with natural progression of disc degeneration is likely to remain unknown.
Studying the biomechanical changes at the adjacent levels is a bit easier. Researchers can measure the increased motion at the next vertebral levels and measure the increased pressure on the disc in between the vertebrae.
These kinds of changes are more likely after fusion (because of loss of motion) than with disc replacement where motion is preserved. Determining the exact biomechanical changes that contribute to ASD and finding ways to change that relationship are the focus of many current studies. To date, there has been a wide range of conclusions about this variable. Some studies show that the natural tendency for discs to degenerate over time is the main cause of the problem while others point to the change in biomechanics as the most important factor.
But there's no mistaking the fact that the surgery has an effect as well. Whether fusion or disc replacement, the structures are changed, the normal anatomy is altered, and the result could be aggravation of the adjacent levels. Studies are needed to show what anatomic parts must be preserved and not disrupted during surgery to prevent adjacent segment disease. Likewise, determining technical strategies during the procedure that might help avoid adjacent segmental disease will be important.
In summary, studies of adjacent segment disease (ASD) for the cervical and lumbar spines continue to report a wide range of variable results. Working to find avoidance strategies based on known causes of ASD will remain the focus of ongoing research. Long-term results will be difficult to judge given the ongoing changes in technology and surgical techniques used in these procedures. But this should not deter surgeons from trying to develop effective avoidance strategies.
Reference: Melvin D. Helgeson, MD, et al. Update on the evidence for adjacent segment degeneration and disease. In The Spine Journal. March 2013. Vol. 13. No. 3. Pp. 342-351.