Fig. 23: Top view of facet joint replacement
(larger view [4])
Fig. 24: Side view of facet joint replacement
(larger view [5])
Facet Technologies
Facet arthritis (degeneration of the facet joint) is currently a contraindication for any type of disc replacement. Proper spinal motion requires normal function of both the disc and facet joints. Therefore, there is an opportunity for facet technologies to provide an adjunct to anterior column disc replacement, or as a stand-alone treatment for patients with isolated posterior column disease.
Facet joint degeneration and disc degeneration frequently occur together, although one may be the primary problem and the other a secondary phenomenon due to altered mechanics of the spine. Central and lateral spinal stenosis, degenerative spondylolisthesis, and degenerative scoliosis may all result from the abnormal mechanical relationship between the anterior and posterior column structures.
Cycle of facet degeneration and disc degeneration
Currently, surgical approaches for spinal stenosis do not restore normal function. In fact, decompression with removal of soft tissue restraints and portions of the facet joints may actually cause instability, or at the least alter normal mechanics. Iatrogenic instability (instability that has inadvertently been induced by medical treatment) can lead to further degeneration and pain.
Spinal fusion puts stress on adjacent structures, and accelerates transitional degeneration and may cause stenosis at the adjacent segment. Secondary operations for hardware removal are occasionally required, and bone graft donor site pain can be a real problem for many patients.
A facet joint replacement would address many of the problems without creating the additional ones cited above. Spinal alignment and mobility could be preserved, there would be less stress placed on adjacent levels, and normal anatomic structures (lamina, spinous process, ligaments) could be preserved. Foraminal height could be restored and there would be no donor site morbidity (unwanted aftereffects from taking a bone graft from the hip).
Figure 23 [6]: Top view of facet joint replacement
Figure 24 [7]: Side view of facet joint replacement
Some facet joint implants have already been designed and patented. They are undergoing mechanical testing but have not yet started clinical trials.
Another approach to altering the mechanics of the posterior column (back of the spine) uses extra-ligamentous implants such as elastic bands to redistribute loads in extension. These devices are still in the early investigative stages.