Dynamic Stabilization's Use in Treating Low Back (Lumbar) Disorders
Traditionally, spinal fusion has been offered to patients who suffer from lumbar (low back) degenerative disc disease, spinal stenosis or spondylolisthesis. Early fusion procedures performed without instrumentation (e.g. rods) proved to be more prone to failed fusion (pseudofusion) than instrumented fusion. As metal implants were developed and used, failure rates declined. However, despite this improvement, fusion is known to significantly alter spinal biomechanics, which is a concern for causing premature disc degeneration at levels neighboring fusion, especially in young patients.
When is dynamic stabilization indicated?
Potentially, there are several indications for dynamic stabilization.
1. When used with fusion, the goal is to adequately brace the spine so fusion occurs but not with the excessive rigidity that metal rods convey. There is some evidence that interbody grafts fuse at a higher rate when they are compressed, which occurs better with dynamic stabilization than traditional rigid instrumented fusion.
2. To recreate the posterior tension band. The 'posterior tension band' is part of the spine's anatomy found at the back of the spine (posterior). Elements that make up the posterior tension band are the spinous processes and lamina (bony plate that is part of each vertebral body) along with the ligaments that join these. Often, these posterior elements are removed during a spinal decompression procedure, which can potentially destabilize the spine. Dynamic stabilization restores the posterior tension band (posterior support) without the need for spinal fusion.
3. Build a transition zone to bridge a fused segment to a nonfused segment by spreading the forces across multiple spinal levels. The analogy here is akin to the plastic or rubber collar on an electrical cord that spreads forces between the highly flexible cable and rigid plug.
Are there different types of dynamic stabilization devices?
Many dynamic stabilization devices have been introduced during recent decades. These include:
- Dynesys® Dynamic Stabilization System (Newest)
- Bronsard's Ligament
- Graf Ligament Dynamic Stabilization System
- FASS (Fulcrum Assisted Soft Stabilization) System
- Leeds-Keio Ligamentoplasty
- DSS (Dynamic Soft Stabilization) System
- Loop System
Most of these devices have predominately been used throughout Europe with limited use in the United States. The Dynesys® System is the only posterior dynamic stabilization system to receive clearance from the U.S. Food and Drug Administration (FDA) and its clearance is granted only for use as an adjunct to fusion.
Why is the use of these technologies limited?
Many surgeons have been slow to adopt use of these technologies because of their complexity, perceived risk of failure and the lack of long-term follow-up. The longevity of a dynamic stabilization construct in an active adult, with constant motion, is an important consideration. Many of the previously mentioned devices have been abandoned because of failure over time.
Implant Designs Improve: PEEK
Technological advances have led to marked improvement in implant design. Biomechanical testing shows many of the new devices to be well-tolerated long-term.
PEEK (poly-ether-ether-ketone), a polymer with properties similar to bone, is one such improvement. This material has been successfully used as a semi-rigid alternative to nonmalleable stainless steel or titanium interbody grafts (cages). Flexible rods made of PEEK enable surgeons to add a dynamic component to a rigid lumbar pedicle screw construct. Because there are no moving parts and the rod itself flexes, there is less concern over mechanical failure. Biomechanical testing has shown the devices to respond favorably to extensive wear with minimal risk of breakage.
Although disc replacement is a current 'hot topic', it does not treat many common lumbar (low back) problems such as spinal stenosis, spondylosis, or spondylolisthesis. Advances in posterior instrumentation using dynamic stabilization will expand treatment options for many patients who suffer from low back and leg pain.