The spinous processes are located in the very back of the spinal column near the skin surface. In fact, by passing the hand down the center of the low back one is usually able to feel several small prominences. These are the spinous processes. The near proximity of the spinous processes to the skin allows for the implantation of interspinous process spacers with minimal operative intervention and spinal morbidity (unwanted aftereffects of the surgery).
The interspinous process devices are designed to distract (open) the foramen, where the nerve endings pass away from the center of the spinal region and into the legs. It is thought that these devices may also unload the intervertebral disc. They may limit spinal extension (the position the spine takes on when bending backward). This backward bending position may be painful for patients with spinal stenosis because it reduces the space available for the nerve roots in the exiting foraminal openings.
The interspinous devices may be implanted with the patient under a mild sedative and local anesthesia as a day surgery procedure (patient goes home the same day) or under light anesthesia. This may be particularly beneficial for elderly patients for whom more extensive open surgery may present too great of a surgical risk due to less favorable general health and fitness level.
There are several types of interspinous devices. Many of the devices profiled below are still in relatively early stages of development and testing, so data about indications for use, effectiveness, and potential risks and complications are often preliminary and further testing is needed before conclusions may be drawn.
The X-STOP for Lumbar Spinal Stenosis
The X-STOP Interspinous Process Decompression Device, made by Kyphon, Inc., in Sunnyvale, California, is made of two titanium pieces. The first is implanted beside and under the spinous process. The second piece is plate-like and is placed on the opposite side of the spinous process and is then attached to the first.
The X-STOP device recently received FDA approval for use in patients with spinal stenosis in the US. In the study evaluating the device, the primary indications were age of at least 50 years, lower extremity pain (with or without back pain) secondary to a confirmed diagnosis of lumbar spinal stenosis that was relieved with flexion (forward bending), and able to walk at least 50 feet. Patients were not enrolled in the study if they had a fixed motor deficit or more than mild grade spondylolisthesis.
Results of the multi-center trial conducted in the US indicated that among patients with pain arising from neurogenic claudication (a symptom of spinal stenosis), the X-STOP provided significantly greater pain relief than epidural steroid injections (the treatment used as a comparative control).1
The Wallis for Mild to Moderate Degenerative Disc Disease and Spinal Stenosis
The Wallis interspinous device, made by Abbott Spine in Austin, Texas, was developed in 1986. The device's original design used a titanium block inserted between adjacent processes held in place with a flat Dacron cord or ribbon. A clinical trial, conducted outside the United States, was initiated in 1988, involving 300 patients who were all treated for recurrent disc herniation.2 Patients whose surgery incorporated the Wallis device with a second discectomy had significantly better results than those who did not receive the device.
The primary change in the second generation of the Wallis implant was changing the material of the interspinous block to PEEK, polyetheretherketone, a strong plastic-like polymer that has more elasticity and is therefore less rigid that the previously used titanium. Results from a European study including twelve-month follow-up for 137 patients undergoing implantation of the new generation of the device were presented at a 2005 spine conference.3 Patients had significant reduction in pain severity.
The current generation of the Wallis device is being evaluated in a multi-center FDA-regulated clinical trial in the US. It is hypothesized that the Wallis device might be a treatment option for low back pain associated with degenerative disc disease as well as lateral recess and central spinal stenosis. A clinical trial evaluating the efficacy and safety of the Wallis device for treatment of symptomatic degenerative disc disease has recently been started. This trial for degenerative disc disease will compare the Wallis to a total disc replacement. A second clinical trial of the Wallis to treat spinal stenosis trial is being contemplated.
The DIAM for Spinal Stenosis and Degenerative Disc Disease
The DIAM Spinal Stabilization System, a non-rigid interspinous spacer, is made by Medtronic Sofamor Danek, Memphis, Tennessee. A US FDA-regulated clinical trial for the DIAM for patients with lumbar stenosis was initiated in late 2006. Also in 2006, the FDA granted an IDE to Medtronic to study the DIAM in patients with low back pain caused by degenerative disc disease.
The DIAM is an "H" shaped polyester-covered, silicone bumper that is placed between the spinous processes with a mesh band and suture to hold it in place. The device has been used in Europe for a few years. A group from Italy recently reported at a conference that in a series of 912 patients, there was a significant reduction in pain and a high rate of patient satisfaction.4 In their series, there was a 3.8% complication rate including infections, fractures in the spinous processes, and removal of the device combined with fusion.
The Coflex for Lumbar Spinal Stenosis
This interspinous device, made by Paradigm Spine, New York, has been used in Europe. In October 2006, US FDA-regulated clinical trials for the Coflex were initiated, comparing the Coflex with pedicle screw fusion for patients with spinal stenosis. The Coflex device was originally developed as the "interspinous U".
The Coflex™ is based upon the Interspinous U designed in France in 1994. While the height of the device distracts the foraminal opening, the "U" shape is designed to allow controlled movement in forward and backward bending. It has also been used in Europe and favorable results, based on pain and patient satisfaction, were reported at a 2005 spine conference.5,6