The last group of posterior motion preservation devices includes total or partial posterior element replacements. These devices are primarily designed to address facet pain or spinal stenosis (narrowing of the passageways for the nerves). For many patients, spinal stenosis is due to degeneration of the facet joints. These devices replace the facet joints in the back of the spine (or the total posterior elements of the spine) and are designed to maintain movement of the spine.
Anatomic Facet Replacement System (AFRS) for Lumbar Spinal Stenosis
The AFRS is manufactured by Facet Solutions in Logan, Utah. The implant is made from a super-alloy with highly polished articulating surfaces. The backing of the implant that interfaces with the bone is coated with material to promote bony in-growth. The implant is secured to the bone with conventional pedicle screws. Separate implants are used for both the top and bottom facets. The US FDA granted an IDE approval in May 2006 to conduct a study for the AFRS, and a US FDA clinical trial for the AFRS is currently underway to study facet joint replacement for the treatment of lumbar spinal stenosis.
Total Facet Arthroplasty System (TFAS) for Lumbar Spinal Stenosis
The TFAS is manufactured by Archus Orthopedics, Inc., in Redmond, Washington. The TFAS is a facet joint replacement device. It is anchored by pegs passing into the vertebral body along the same course as pedicle screws and cemented (using bone cement) into the vertebral body. Motion is achieved by a sphere sliding along a curved plate. Preliminary results of 16 patients were presented at a 2007 spine conference.1 Results indicated clinically significant improvement in ZCQ symptom scores (patient-reported symptom severity, physical function, patient satisfaction) in 12 out of 16 patients, and improvement in leg pain and back pain VAS scores (patient-reported pain severity) in 14 out of 16 patients. The FDA-regulated trial for this device is underway and open for patient enrollment for patients with moderate to severe lumbar spinal stenosis.
Total Posterior System (TOPS) for Lumbar Spinal Stenosis
A total posterior element replacement system, the TOPS, is manufactured by Impliant. This system is anchored by devices much like pedicle screws. The entire posterior elements are removed and the device implanted. Within a plastic-like cover are "bumpers" that allow, but limit, the extent of rotation and extension. Preliminary results of eight patients were presented at a 2005 spine conference.2 The results indicated improvement in pain. A US FDA clinical trial for the TOPS system is underway for patients with moderate to severe spinal stenosis of the lumbar spine.