Artificial Disc Replacement
Individuals with degenerated discs in the lower (lumbar) spine may suffer from chronic low back pain. Most patients with symptomatic degenerative conditions in the spine are treated non-surgically with anti-inflammatory medications such as NSAIDS, physical therapy and corticosteroid injections. Many of these patients have a positive response to non-surgical methods of treatment, but there are a number of patients who continue to experience “chonic” and disabling pain. The chronic nature of back pain may interfere with the patient’s ability to work and participate in regular daily activities. As a result, patient may turn to surgery to alleviate his symptoms.
There are many different conservative and minimally invasive surgical treatment options available to manage degenerative disc disease. In most cases when surgery is recommended, the treatment of choice has traditionally been lumbar spinal fusion. As with any surgery, spinal fusion also has a number of drawbacks. First, the ability of the bone to heal or “fuse” varies from patient to patient. 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.”
Anatomy of the Spine:
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.
Early research in the development of artificial disc replacement began in the 1960’s however, it wasn’t until the 1990’s when the interest flourished for the development of the artificial disc. 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.
At this time, artificial disc replacement is still considered experimental and has not yet been approved by the (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).
