Approximately 80% of Americans will experience a significant episode of neck or back pain at least once in their lifetime. For many, such initial pain may rapidly become a lifetime of discomfort with associated loss of productivity and functional capacity. One of the major causes of such spinal pain is degeneration of the intervertebral discs. Such degeneration may be both difficult to diagnose and treat. One of the most promising surgical options currently being developed to combat spinal disc degeneration is the use of artificial disc technology for degenerative disease of the cervical and lumbar spine.

Arthroplasty for cervical and lumbar degenerative disc disease has reached the point of clinical trials in the United States. Similar to the now common and successful replacement of worn out hip and knee joints with metallic and plastic artificial joint constructs, the new spinal arthroplasty techniques replace damaged, painful, incompetent intervertebral discs with a prosthesis designed to restore normal disc height, lordosis, and function. The idea of spinal disc replacement is not new. It was first attempted 40 years ago when a surgeon first implanted stainless steel balls into the disc spaces of over 100 patients. Although these pioneering efforts seem crude, over the past decade there has been more significant research into the degenerative processes of the spine, spinal biomechanics, and biomaterials. Today, artificial disc replacement is considered experimental by the Food and Drug Administration (FDA), but is becoming an increasingly more common intervention for patients. The goal remains to develop a device that will eliminate the pain caused by disc degeneration while maintaining mobility and function of the spine.

The loss of height and lordosis associated with desiccation and the micro-instability resulting from loss of annular tension can be corrected without destroying the function of the joint. As a treatment, surgical fusion purposely impairs normal motion by disrupting articular surfaces and by instrumenting across previously mobile segments. Although fusion may be considered the standard of care in many instances, there are a number of problems generated by such procedures. First, the loss of mobility from long segment fusions may result in stiffness and loss of functional capacity. The transfer of stress from the fused areas to the bordering non-fused areas may result in a phenomenon known as adjacent segment degeneration in up to 30% of patients over the decade following surgery. The arthroplasty alternatives are designed to preserve motion segments, minimize the risk of facet damage, and limit associated adjacent segment breakdown.

In many patients with multiple levels of mild degenerative disease, surgery to correct all degenerative segments would often be too extensive and disabling. Percutaneous injection techniques, such as facet blocks or discography, may allow for identification of a specific pain generator. Ideally such testing would isolate a problematic segment for arthroplasty, while possibly allowing a less aggressive intervention in the patient with a degenerative pain syndrome.

Cervical Arthroplasty
In the cervical spine, the patient with a single level of symptomatic degenerative disc disease is very common. Anterior cervical decompression utilizing discectomy and fusion is considered a reasonable treatment measure for radiculopathy, myelopathy, and persistent neck pain. The incidence of stress transfer to adjacent levels after fusion is significant and may result in progressive cervical degeneration.

Attempts at arthroplasty in the cervical spine have resulted in two prototypical discs. The Bryan® Cervical Disc System is a composite-type artificial disc comprised of a low friction, wear resistant, elastic polymer nucleus with two anatomically shaped metal endplates. A flexible membrane forms a sealed space containing a lubricant to reduce internal friction and wear. After implantation, the prosthesis allows for a nearly normal range of cervical motion. At this point, the Bryan® cervical disc is available in Australia, Europe and South Africa. It is currently part of a randomized 12-center FDA trial throughout the United States and should be available outside the trial in the United States shortly.

Another artificial cervical disc, known as the Prestige® disc, was originally developed in Bristol, England. The prosthesis is a ball and socket type device made of stainless steel. It is secured to the anterior aspect of the vertebrae with screws much like ordinary cervical plate fixation. The early clinical results with this device have been promising and additional clinical studies are being conducted in the United States, Europe, and Australia to determine long term outcome data.

The main issue with artificial discs in general is our failure to understand the degenerative process. Knee and hip replacements typically wear out in 10-15 years, so it is unlikely that a cervical prosthesis will withstand a lifetime of use. The Bryan® disc, for example, has been tested under laboratory conditions to a total of 45 human equivalent years of neck movement with little wear noted. The testing is impressive, however, the effects of normal cervical motion and time in a human spine will not be known for a number of years.

Lumbar Arthroplasty
Arthroplasty of the lumbar spine represents a significant challenge secondary to both the significant mechanical strain and the wide range of normal spinal motion. The device must be strong enough to support axial loading and maintain normal intervertebral distance. It must be flexible enough to allow for the rotation, flexion, extension, and translation, while resisting instability. The device must be easily customized to patient size, spinal lordosis, and disc space height. Like a natural disc, the artificial disc must act as a shock absorber, especially if it is going to be used in several levels of the spine at one time. Finally, the artificial disc must be extremely durable. The mean age of a patient needing a lumbar disc replacement is approximately 35 years. With the expected lifespan of a 35 year old patient averaging 40 to 50 years, the disc must last at least that long to avoid the need for a challenging revision surgery. It has been estimated that an individual takes some 2 million steps per year and bends 125,000 times. Over a possible 50 year span that translates into more than 106 million motion cycles. This estimate does not even include the subtle disc motion which occurs with the millions of breaths we take each year. The specific material from which the prosthesis is constructed is extremely important. Unfortunately, there is no consensus among surgeon-designers as to the best construct or most appropriate material. The material must be safe for implantation into the human body and must be relatively inert to avoid inciting an inflammatory response. It should ideally be radiolucent or cause minimal imaging artifact, yet it should have some consistent means of identification to monitor position and relationship to the bordering endplates. It should not produce wear debris which may cause injury or scar tissue formation around neighboring neural structures.

TYPES OF LUMBAR PROSTHESES
Currently, there are 4 different subtypes of artificial disc undergoing evaluation. These basic subtypes include composite, hydraulic, elastic, and mechanical discs.

Composite Discs
The most widely implanted disc to date is a composite disc called the Link SB Charite® disc. This device is made of a polyethylene spacer and two separate metal endplates and comes in different sizes. It also has a ring around it to make it visible on an x-ray.

This device has been implanted in over a thousand European patients with relatively good results. Additional clinical trials using this device are ongoing in Europe, the United States, Argentina, China, Korea and Australia.

Tthe Prodisc® is a three-piece construct. The superior and inferior pieces are made of rough titanium designed to encourage bone growth from the vertebral body into the prosthesis. The central nuclear part is made of ultra high molecular weight polyethylene with an extremely low coefficient of friction, which hopefully allows normal spinal motion.

Hydraulic Artificial Discs
Hydraulic artificial discs have a gel-like core covered with a tightly woven polyethylene "jacket". Before implantation, the pellet-shaped hydrogel core is compressed and dehydrated to its minimum size. Once it is implanted, the outer woven covering allows fluid to pass through to the core, which immediately begins to absorb fluid and expand. Most of the expansion takes place in the first 24 hours after surgery, although it takes approximately 4-5 days for the hydrogel core to reach its maximum size. Placement of two hydraulic implants within the disc space generally provides the lift that is necessary to restore and maintain disc space height in most patients.

This type of artificial disc has been extensively tested, and the preliminary results have been good. Currently, further clinical evaluations are ongoing in Europe, South Africa and the United States to determine long-term efficacy.

Elastic Discs
Elastic type artificial discs, such as the Acroflex® disc, are made of a rubber core bonded to two titanium endplates. The results of testing have been somewhat mixed. Recently, a small series of patients received this type of artificial disc were evaluated after a minimum of 3 years. Their preliminary outcomes were graded as follows: 2 excellent, 1 good, 1 fair, and 2 poor. One of the elastic discs in a patient with a poor result developed a tear in the rubber. Since that time, a second-generation elastic disc made of silicone rather than rubber has been approved for more extensive testing.

Mechanical Discs
Several pivot or ball type artificial discs have been developed for the lumbar spine. One device, made of metal-hinged plates with an interposed spring, has been tested on sheep with good results. Another device has metal endplates in a ball and socket design with two vertical stabilizing wings. This device, called the Maverick® , is currently in an ongoing randomized U.S. trial. Preliminary results appear to be promising.

LOOK TO THE FUTURE
Although this article is simply an overview, clearly arthroplasty for spinal disc disease has revolutionized our thinking about the degenerative process. Spinal disc replacement is not only possible but it has the potential of providing relief to millions of back pain sufferers. The development of artificial disc technology still has many challenges, but the results from initial efforts to this point seem promising.

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