Taking a Closer Look at Neck Motion After Fusion Versus Disc Replacement

» Taking a Closer Look at Neck Motion After Fusion Versus Disc Replacement
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For people with severe neck pain from disc degeneration, it is possible to have a disc replacement now instead of a neck fusion procedure. It's a new enough treatment that surgeons are continuing to conduct studies of fusion versus disc replacement.

There is particular interest in what happens to the adjacent spinal level. In other words, does the vertebral unit above and/or below the fusion develop problems because of faster degeneration? Does using a disc replacement instead of a fusion procedure improve outcomes with less adjacent segment disease? What happens to neck motion if a fusion is reversed (removed and replace with a disc replacement)?

In this study, cadavers (human bodies preserved after death) were used. The surgeon fused some necks and compared cervical spine (neck) movement with other necks that had a one-level fusion plus a one-level disc replacement. The fusion and disc replacement were at adjacent levels.

The combination fusion-disc replacement is referred to as a hybrid. As part of the study, they did perform a fusion at one level and then reversed the procedure. This involved removing the fusion and replacing it with a disc replacement.

There were three significant findings from this study:

  • Neck motion is the same after a disc replacement when compared with a hybrid (disc replacement at one level with fusion at the next level)
  • There was no difference in neck motion regardless of the location of the fusion (above or below the disc replacement) in the hybrid procedure
  • Results after the hybrid procedure (again measured in terms of neck motion) werebetter than with a two-level fusion.

The authors report that the muscles of the neck have to work harder to restore motion when there is a two-level fusion compared with a hybrid. The human body tries to restore normal neck range-of-motion as much as possible, especially after a fusion. That may mean increasing motion at other segments to the point of becoming hypermobile (too much motion).

It is thought that this hypermobility contributes significantly to adjacent disease (i.e., the segment next to the disc fusion starts to deteriorate). Hypermobility combined with increased load transferred to the adjacent segments affects more than just those levels.

Based on the results of this study, the authors suggest that there are more biomechanical effects on the entire cervical spine after a two-level fusion compared with either one-level fusion or a hybrid (one-level fusion plus one-level disc replacement). Greater muscular effort in an attempt to maintain full motion after a two-level fusion may be the reason patients develop muscle fatigue and pain.

There are some limitations to this study. Using cadavers is helpful but still not the same as a live human. So the effects observed may not truly reflect what is happening inside the human body under the same conditions. And cadaver studies do not allow for observation of long-term effects.

Further study is needed to evaluate the value of disc replacement versus disc replacement combined with fusion versus fusion alone. Additionally, there is a need to look at the results of these comparisons at one-level versus two-level procedures.

At least for the moment, it looks like the hybrid approach may be a better choice than a two-level fusion. Reversal of vertebral fusion and replacement with a disc implant may be beneficial but will also require some additional study before becoming a routine procedure.

Reference: Michael J. Lee, MD, et al. Disc Replacement Adjacent to Cervical Fusion. In Spine. November 1, 2011. Vol. 36. No. 23. Pp. 1932-1939.

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