Many patients have multiple problems as their spines progress through various stages of a degenerative process. The disc sometimes becomes painful due to tears within the disc initiating a series of degenerative events, including disc dehydration. This process results in narrowing of the disc space. As the space narrows, it alters the loads on the facet joints, which in turn may become painful or respond by building additional bone around the joint. This additional bone may cause narrowing of the foramen, limiting the space available for the nerve roots to pass, making them more vulnerable to compression and producing pain or numbness in the lower extremities. Thus painful degenerative problems are not limited exclusively to the anterior or posterior portions of the spine, but may be present in both.
In the past, many patients with chronic, severe back pain (and/or leg pain) from degenerative problems both in the front and back of the spine were typically treated with a combined anterior and posterior fusion (fusing both the front and back of the spine). With the ongoing development of dynamic spinal devices, in the future it is thought that various posterior stabilization devices may be combined with other technologies that treat the disc in the front of the spine, possibly including total disc replacement, disc nucleus replacement, or injections to stimulate the growth of healthy disc tissue. Currently, it is thought that these combined treatment approaches may have two primary applications. In some patients, they will allow addressing pain from the disc with a disc replacement (in the front of the spine) as well as addressing spinal stenosis posteriorly (in the back of the spine).
Due to the data supporting that some posterior dynamic devices unload the disc, it seems reasonable to think that an injection to stimulate the growth and healing of disc tissue may be combined with a posterior device to unload the disc, providing the optimal environment for disc tissue to grow. For example, it has been reported that in several post-operative MRI scans the Wallis device has shown a more normal appearance of the nucleus on what is called the T2 weighted image. As another example, this finding has also been reported with the Dynesys anecdotally, and may also be a factor with a number of other devices. It is not yet known what this will mean physiologically.
Although theoretically promising, such combined treatments have not yet been tried and are likely years away from clinical application.
Disc Regeneration
While posterior dynamic stabilization devices have primarily been designed for use to address problems in the back of the spine, some may also provide the potential for disc regeneration. This is because the unloading of the disc theoretically relieves pain as well as potentially allows regeneration of the disc in a manner similar to unloading an injured knee or ankle while still allowing motion in the joint with the use of a dynamic brace. This unloading may allow a less stressed regenerative process for the spinal disc. In the future, it is thought that such treatments may be combined with tissue engineering to assist with disc tissue regeneration. At the time of this article, all of this is still theoretical and further research is needed to assess whether this assumption is valid or not.
Summary
There are a wide variety of posterior motion preservation devices in various stages of development and clinical investigation. As always, the key to optimal patient outcomes is to identify the cause of the patient's symptoms as comprehensively and accurately as possible. This knowledge can drive decision making as to which device best matches the patient's structural problem(s) and underlying cause of his or her pain.
Only carefully performed clinical studies and long-term follow-up will reveal which of these posterior motion preservation devices provide the maximum benefit for specific painful spinal conditions. Also, refinement of indications, allowing the most appropriate matching of device to each individual patient's problem, will evolve with time and clinical use. For many patients, spinal fusion surgery will continue to be the surgery of choice.
Note: Dr. Hochschuler is a principal investigator for the Wallis device.
