Artificial Disc Replacement
Individuals with degenerated discs in the lower (lumbar) spine sometimes suffer from disabling, chronic low back pain. Most patients with symptomatic degenerative conditions in the spine are treated non-surgically with anti-inflammatory medications, physical therapy and injections. Most of these individuals will favorably respond to non-surgical methods of treatment, but a subset of individuals will continue to experience pain. The chronic nature of back pain often interferes with the ability to work and participate in regular daily activities. As a result, surgical treatment may become necessary.
There are multiple conservative and minimally invasive treatment options available to manage symptomatic degenerative disc disease. However, if surgery is indicated the surgical treatment of choice has traditionally consisted of a lumbar spinal fusion.
Unfortunately, there are a number of drawbacks to undergoing a spinal fusion. First, the ability of the bone to heal or fuse varies. The average success rate of a lumbar spinal fusion is approximately 75%-80%. Failure of the fusion to heal may be associated with continued symptoms. Second, a spinal fusion at one or more levels will cause stiffness and decreased motion of the spine. Third, having a spinal fusion at one or more levels will cause more stress to be transferred to adjacent levels. The problem with the transferred stress is that it may cause new problems to develop at the other levels, which may also lead to additional back surgery.
For these reasons, neurosurgeons and orthopedic surgeons have engaged in research to offer an alternative to lumbar spinal fusion surgery. One promising area of research includes the development of an artificial spinal disc. In order to better appreciate the advantages and disadvantages of artificial spinal disc replacement, it is important to have a basic understanding of normal spinal anatomy, including the function of the spinal discs.
The spine is a column that is made of up bones and discs. The blocks of bone (or vertebrae) provide the anterior support and structure of the spine. Posteriorly, the two facet joints at each level provide stability and movement of the motion segment. The spinal discs are in between the bones and act like a cushion or shock absorber between the vertebrae. The discs also contribute to the flexibility and motion of the spinal column.
The discs are made up of two parts: 1) the inner portion of the disc is called the nucleus pulposus and is a jelly-like material 2) the outer part of the disc is a stronger, more fibrous, material called the annulus fibrosus. The annulus fibrosus surrounds and supports the inner jelly material. The annulus is also in contact with nerve fibers or pain receptors, called nociceptors.
Disc material is primarily composed of water and other proteins. As a normal part of aging, the water content gradually diminishes which can cause the disc to flatten out and even develop tears or cracks throughout the annulus fibrosus. These discs are often referred to as degenerative discs and may or may not cause pain.
In the case of a degenerative disc, the inner jelly material can bulge out and press up against the annulus fibrosus. This can stimulate the pain receptors causing back pain to occur. The cracks or tears that develop within the annulus fibrosus can also become a source of pain. Back pain that is caused by the spinal disc is often referred to as discogenic low back pain.
Sir John Charley revolutionized orthopedics in the 1960's with the advent of the total hip replacement. At that time, early research in the development of artificial disc replacement began as well. Despite the early interest, lumbar spinal fusion remained the gold standard treatment for back pain. Because of the complications associated with lumbar spinal fusion, a renewed interest in artificial disc replacement resurfaced in the 1990's. The purpose and advantage of artificial disc replacement is to replace the worn out disc, while preserving the motion at the operated spinal level. This could potentially not only treat the underlying back pain, but also protect patients from developing problems at an adjacent level of the spine.
Figure 1. CHARITÉ Artificial Disc (DePuy Spine, Inc.)
Photograph Courtesy of DePuy Spine, Inc.
The Food and Drug Administration (FDA) has approved the CHARITÉ Artificial Disc (DePuy Spine, Inc. of Raynham, MA) for use in treating pain associated with degenerative disc disease. The device was approved for use at one level in the lumbar spine (from L4-S1) for patients who have had no relief from low back pain after at least six months of non-surgical treatment.
Currently, artificial disc replacement is considered experimental and is not approved by the Food and Drug Administration (FDA). Most of the research that has been conducted on artificial discs has been carried out in Europe. Different models have been developed, but the most widely used and known artificial disc is the LINK SB Charite III prosthesis made by Waldemar Link GmbH & Company, Hamburg, Germany. The model consists of two metal metallic plates that have teeth to anchor the implant between the bones or vertebral bodies. Between the two plates is a rubber core made up of polyethylene that allows for motion (See Figure 1). A metal ring surrounds the outside of the rubber core so that it can be located on x-ray (See Figure 2).
In order to avoid complications that may arise from artificial disc replacement surgery, careful selection of patients by the surgeon is critical. At present, it is thought that the best candidates for spinal disc replacement are adults with a one level symptomatic degenerative disc. Patients whose bone may not be as strong due to aging, or some other bone disorder, may develop problems if the implant settles into the soft bone. Therefore, these individuals are not considered optimal candidates for this type of procedure. Since there can be movement of the implant, patients with a slippage of one vertebra on another (termed spondylolisthesis) are also not considered candidates for artificial disc replacement. Based on the current research, the clinical diagnoses that seem the most fitting for artificial disc replacement include symptomatic degenerative disc disease and post-discectomy syndrome. Post-discectomy syndrome is persistent back pain following previous surgery to remove a herniated disc.
In addition to the potential complications associated with undergoing surgery and general anesthesia, the complications associated with artificial disc replacement may include breakage of the metal plate, dislocation of the implant, and infection.
To help minimize complications associated with the implant itself, proper selection of patients and size of implant is very important. Patients may also not improve following the procedure and may require additional surgery. Finally, like joint replacement surgery, artificial implants may fail over time due to wear of the materials and loosening of the implants. Therefore, long term studies that track the life span of the implants are needed.
Another artificial disc device that is similar to the metal implant is the prosthetic disc nucleus (PDN). Like the metal implant, the purpose of it is to restore disc height and allow for normal spinal motion. However, instead of the whole disc being removed, with the prosthetic disc nucleus, only the inner jelly material, or nucleus, is removed and is then replaced with two mini "pillows" (See Figure 3).
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