At least once a week a patient will say to me: “I’m not getting spinal surgery because I heard that once you have spinal surgery your spine is never the same and you’ll only need more spinal surgery!” Well, to some extent there is some truth to this statement. Whenever a patient undergoes spinal surgery, a well-known long-term side effect is that the level above or below the surgery can degenerate. This is called adjacent segment degeneration or ASD.
Why does ASD happen? ASD can occur whenever the normal anatomy of the spine is disrupted and as a result, a segment of the spine has to handle more stress than it’s used to. This can happen after even the most minor spinal procedure but more commonly happens after fusions. (see figure 1) Any length of spinal fusion can lead to ASD. However, there has been some evidence to suggest that the more levels fused, the higher the risk of ASD. This is because the longer fused segment acts as a longer lever arm and causes more stress on the disc and facet joints above or below the segment fused. Of course, sometimes long fusion segments are mandatory (as in deformity or trauma surgery) but whenever possible, the number of levels fused should be minimized.
Figure 1: Image on left shows ASD at L4/5 after an open fusion at L5/S1. The L4/5 disc is now so degenerated that it allows the L4 body to slip forward in relation to L5 (a condition known as spondylolisthesis). The image on the right is of a patient with severe ASD at L2/3 several years after she underwent an open L3-5 laminectomy.
There is some controversy as to what actually causes ASD. Here’s what we know for certain: ASD is a common long-term complication of spinal fusions. If you look at the rates of ASD across the literature, the rate of symptomatic ASD (i.e. that requiring additional surgery) after anterior cervical discectomy and fusion, or ACDF, is anywhere from 9-25% (Yang et al, 2012). In the lumbar spine, the rate of symptomatic ASD after fusion is as high as 30% (Cheh et al, 2007). Clearly any patient who undergoes a spinal fusion has to accept the risk that at some point later in their life they may need more spinal surgery.
ASD (and this is where it gets controversial) may also be seen more commonly after traditional, open spinal surgery vs. after minimally-invasive surgery. As you have seen in some of my previous posts, open spinal surgery can be quite destructive. These procedures utilize long midline incisions that strip the supporting muscles and ligaments off of the spine. I equate these muscles and ligaments to the cables of a suspension bridge: if you disrupt these structures at one level you affect the functional stability at multiple levels. Minimally-invasive procedures in spinal surgery are still fairly new and only recently has long-term data on these procedures become available. One recent study (Park et al, 2011) followed patients who underwent minimally-invasive lumbar fusions for an average of 36 months and only 2 out of 66 (3%) patients developed ASD. Compare that to the 30% rate of ASD for open fusions. Recall that minimally-invasive procedures spare the normal supporting structures of the spine and theoretically prevent the “collateral damage” that leads the ASD. In my opinion the Park study is just the first of many studies that will prove that minimally-invasive procedures have a much lower rate of ASD when compared to open spinal procedures.