Devices in this category can best be described as an internal brace - allowing controlled movement of the affected segment of the spine. Most of these devices tend to be derived from the pedicle screw and rod constructs (used in spinal fusion surgery) of the 1980s and 1990s. Those devices were designed to provide rigid stability to the spine while boney incorporation of the fusion mass was achieved. The goal of the new generation of devices is to allow controlled motion in such as way as to achieve more normal movement of the spine.
Several of these devices have been approved for use as an adjunct to spinal fusion, and are now being tested as a standalone device. These devices are typically used to treat patients with symptomatic spondylolisthesis or degenerative disc disease. Some devices are also being studied or will be studied as stand-alone devices (without fusion), and for the treatment of lumbar spinal stenosis.
Rather than having rigid screws and stiff metallic rods, this group of devices has the following combinations of components:
- Screws and cords
- Screws, cords, and spacers
- Screws and flexible rods
- Screws and rods with movable parts
- Flexible screws and flexible rods
- Screws and inflatable rods
There are many devices of this type. Presented below is an overview representing the various types of pedicle screw based dynamic devices.
One of the earliest of the pedicle screw based devices was the Graf ligament. This system was developed in Europe and uses braided polyester cables looped around the screws to provide stability while allowing motion. There have been several published reports on the clinical results of this device, and the outcomes have been inconsistent.1-4 The variation in results may be due to differences in the patient populations operated on and/or in the outcome assessments used. This device is currently only available in the U.K.
Another system using, manufactured by Zimmer Spine. This device also incorporates a plastic spacer over the cords. The device has been used as a dynamic stabilization device in Europe with mixed results reported.5,6 The Dynesys System is dynamic stabilization system that has received clearance from the US FDA.
The FDA clearance for the Dynesys system is limited to use as an adjunct to spinal fusion of the thoracic, lumbar, and sacral spine for degenerative spondylolisthesis with neurologic impairment, and for a prior failed spinal fusion (pseudoarthrosis). When used as a pedicle screw fixation system, the Dynesys Spinal System is indicated for use in patients who are receiving fusion of the lumbar or sacral spine with autogenous graft only, and who are having the device removed after development of a solid fusion mass.
Clinical trials are ongoing for use of the Dynesys system as a stand-alone device in the absence of a spinal fusion.
Some devices have maintained the original concept of metal rods, but have designed a mobile joint within the rod. One such device is the IsoBar, which is manufactured by Scient'x, in Guyancourt France. At the time of this article, no published reports on the use of this device were found. The IsoBar is in use in Europe.
Dynamic Soft Stabilization system
The Dynamic Soft Stabilization system uses pedicle screw anchoring rather than traditional straight rods, and incorporates an elliptical metal coil connected to the screws. This device is not approved for use in the US, but has been used in other parts of the world for preliminary evaluation. Twelve-month follow-up of 16 patients indicated favorable outcome with significant reductions in pain.7
The Stabilimax NZ Dynamic Spine Stabilization System, made by Applied Spine Technologies, New Haven, Connecticut, received FDA approval in January 2007 to commence US clinical trials. The Stabilimax NZ will be compared to traditional spinal fusion for patients with lumbar spinal stenosis.
In addition to the above-described technologies, many other dynamic stabilization devices are currently in various stages of development. In addition, some companies are developing technologies that would allow a combination of posterior dynamic stabilization devices and total disc replacement as an alternative to spinal fusion.