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William T. Couldwell, MD
NS Innovations editor
william.couldwell@ hsc.utah.edu
Approximately 85 percent of Americans will experience a significant episode of neck or back pain at least once during their lifetimes. For many, transient pain may lead to 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. Thought to be genetic in some cases or acquired through activity, obesity, or tobacco use in others, the diagnosis often is difficult to make. Symptomatic degeneration may be both difficult to identify and to treat, often entailing repetitive examinations and provocative testing such as discography. The most promising surgical option currently in development to combat the pain associated with degenerative disc disease is artificial disc technology for both the cervical and lumbar spine.
Arthroplasty for cervical and lumbar degenerative disc disease has reached the point of clinical trials in the United States and is still considered investigational by the U.S. Food and Drug Administration. Similar to the now common and successful replacement of worn-out hip and knee joints with combination metal-and-plastic artificial joints, the new spinal arthroplasty techniques replace damaged, painful, and incompetent intervertebral discs. The prosthesis is designed to restore normal disc height, lordosis, and function. Spinal disc replacement was first attempted 40 years ago when a surgeon implanted stainless steel balls into the disc spaces of more than 100 patients. Although this pioneering effort seems crude, in the past decade significant research has explored the degenerative processes of the spine, spinal biomechanics and biomaterials.
Using arthroplasty techniques, the loss of height and lordosis associated with desiccation and microinstability resulting from the loss of annular tension can be corrected without destroying the function of the joint. Arthroplasty compares favorably with surgical fusion, a treatment that 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, a number of problems are generated by such procedures. The loss of mobility from fusion of long segments may result in stiffness and loss of functional capacity. Further, the transfer of stress from the fused areas to the bordering nonfused areas may result in a phenomenon known as adjacent segment degeneration in up to 30 percent of patients in the decade following surgery. Arthroplasty alternatives are designed to preserve motion, minimize the risk of facet damage, and limit associated adjacent segment breakdown. Additionally, arthroplasty has the capability of restoring motion to degenerative segments that essentially have lost normal function.
In many patients with multiple levels of mild degenerative disease, surgery to correct all degenerative segments often would be too extensive and disabling. Percutaneous injection techniques, such as facet blocks or discography, may allow identification of a specific pain generator. Ideally such testing would isolate a problematic segment for arthroplasty, or possibly allow a less aggressive intervention in the patient with degenerative pain syndrome wwho otherwise would have required more extensive surgery.
Lumbar Arthroplasty
Arthroplasty iin the lumbar spine represents a significant challenge secondary to both the mechanical strain placed upon the prosthesis and the wide range of normal spinal motion. The device must be strong enough to support axial loading and maintain normal intervertebral height. It must be flexible enough to allow for the rotation, flexion, extension, lateral bending, and translation expected of a normal disc. The prosthesis generally relies upon intact facet joints and ligaments to resist unstable motion. The device must be easily customized to a patient’s size, degree of spinal lordosis, and normal disc space height. Like a natural disc, the artificial disc may need to act as a shock absorber, however this function has been difficult to reproduce using synthetic materials. Finally, the artificial disc must be extremely durable. The mean age of a patient requiring a lumbar disc replacement is approximately 35 years. Given the expectation that the average 35-year old-patient will live another 40 to 50 years, the disc must last at least that long to avoid the need for a challenging revision surgery. Moreover, it has been estimated that every year an individual takes more than 2 million steps and bends 125,000 times. Over a 50-year life span, such basic activities may translate into more than 100 million motion cycles.
The choice of biomaterial used to manufacture the prosthesis is as important as the overall design of the device. Unfortunately, there seems to be no consensus among surgeons and device designers as to the best shape or most appropriate material. The material must be safe for implantation into the human body as well as relatively inert to avoid inciting a destructive inflammatory response at the interface between the vertebral body and the device. Ideally, it should be radiolucent or allow some consistent means of identification to monitor its 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 that may ultimately lead to premature failure of the device.
Types of Lumbar Prostheses
Currently, there are four 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, manufactured by Waldemar Link GmbH, Hamburg, Germany. 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. The device has been implanted in more than 1,000 European patients.
The Prodisc, manufactured by Spine Solutions, New York, N.Y., 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 theoretically allows normal spinal motion.
Hydraulic Discs Hydraulic implants 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. The hydrophilic core absorbs fluid and expands. Most of the expansion takes place in the first 24 hours after surgery, although it takes approximately four to five 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.
Elastic Discs Elastic artificial discs such as the Acroflex disc, made by Johnson and Johnson//Depuyy Acromed, Raynham, Mass., 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 who received this type of artificial disc were evaluated after a minimum of three years. Their preliminary outcomes were graded as follows: two, excellent; one, good; one, fair; and two, poor. One of the elastic discs in a patient who experienced a poor result developed a tear in the rubber. Since that trial, a second-generation elastic disc made of silicone rather than rubber has been approved for more extensive testing.
Mechanical Discs Several “pivot” or “ball” 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, the Maverick by Medtronic Sofamor Danek, USA, Inc., Memphis, Tenn., currently is being tested in a 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 the thinking about the remedies for the degenerative process. Spinal disc replacement not only is possible, but it also holds the potential of providing relief to millions of back pain sufferers. The development of artificial disc technology still has many challenges, but the results to this point seem promising.
Brian R. Subach, MD, is a neurosurgeon at The Virginia Spine Institute, Reston, Va.
This article is adapted from the original at www.spineuniverse.com, and it appears with the permission of SpineUnivers.