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Artificial Disc Explained

Approximately 80% of Australians experience significant back pain at least once in their lifetime, and for many people, spinal disorders are a lifelong problem. The personal and monetary costs associated with this widespread problem are staggering. One of the main causes of back pain and spinal disorders is the degeneration of spinal discs. Disc degeneration is painful and often difficult to treat. One of the most promising surgical options currently being developed is the use of artificial discs.

What is a Spinal Disc?Lumbar Spine
The spaces between each vertebra in your spine (spinal discs) serve as shock absorbers for your spine. As we age, these discs can deteriorate. When this happens, they lose their shock absorbing abilities and cause pain and wear and tear on the vertebra.

Current Treatment Options
Non-surgical options for people with neck and back pain include rest, heat, pain medications, physical therapy and chiropractic manipulation. Unfortunately, these treatments fail in a significant number of patients.

When non-surgical treatment options fail, surgery is often the next step. This usually means spinal fusion surgery. Unfortunately, there are a number of drawbacks to undergoing a spinal fusion. First, the bone does not always heal or "fuse" correctly. In fact, the overall success rates for these procedures range from 48% to 89%. Second, a spinal fusion at one or more levels causes stiffness and decreased motion of the spine. Third, spinal fusion at one or more levels increases stress to the rest of the spine. This transferred stress may cause new problems to develop at the other levels, which may lead to additional back surgery. Clearly, an alternative treatment option is needed.

Artificial Discs - Why Are They Important?
We have all heard of hip and knee replacement surgeries. These procedures have provided much relief for people who suffer from pain in those areas. But what about spinal disc replacement? Is this the option millions of back pain sufferers have been looking for?

The idea of spinal disc replacement is not new. It was first attempted 40 years ago when a surgeon implanted stainless steel balls in the disc spaces of over 100 patients. These pioneering efforts were followed by more than a decade of research on the degenerative processes of the spine, spinal biomechanics and biomaterials before serious efforts to produce an artificial disc resumed. Today, artificial disc replacement is considered experimental and is not yet approved by the Food and Drug Administration (FDA), but the goal is to develop a device that will eliminate the pain caused by disc degeneration while maintaining mobility and function.

Artificial Discs - Key Design Features
There are a variety of factors designers must keep in mind as they develop an artificial disc. The device must be able to maintain proper intervertebral spacing, allow for the full range of motion and provide stability. It must also come in a variety of sizes to accommodate patient height and spacing needs. 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 very durable. The average age of a patient needing a lumbar disc replacement is about 35 years. This means that to avoid the need for revision surgery, the artificial disc must last at least 50 years. It has been estimated that an individual takes 2 million steps per year and bends 125,000 times; therefore, over the 50-year life expectancy of the artificial disc, there would be over 106 million cycles. This estimate does not even include the subtle disc motion that occurs with the 6 million breaths we each take per year!

What an artificial disc is made from is also an important factor in the development of this technology. It must be made of materials that are safe to be implanted in the human body, not cause allergic reactions and not damage other parts of the spine. Also, it would be ideal if the artificial disc were made of a material that could easily be seen on an x-ray or other imaging test. This would make it easier for the surgeon to monitor the effectiveness of the artificial disc over time.

Artificial Discs - What's on the Horizon?

A prospective, randomised, controlled IDE clinical study performed in the US consisted of 541 patients. There were 276 enrolled in the investigational group (Prestige Disc Replacement), with the 265 control patient's receiving a bone and plate fusion.The Prestige group had statistically lower rates of secondary surgical procedures related to revisions and supplemental fixations. Their neurological success rate was statistically higher than the control group.Of great interest is the fact patients returned to work more quickly in the investigational group, with a median time of 45 days (16 days earlier than the control group). The full report of this IDE study and the determination of the safety and effectiveness of the Prestige Device can be found on the FDA website. Click here to see the FDA report.

Today, there are 4 "types" of artificial disc being studied and tested. They include composite, hydraulic, elastic and mechanical discs. The following is a brief description of each.

Composite Discs
The most widely implanted disc to date is a composite disc called the Link SB Charité disc (now the CHARITÉ™ Artificial Disc, DePuy Spine, Inc.). 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.Artificial Disc

Photo: CHARITÉ™ Artificial Disc Courtesy of DePuy Spine, Inc.

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.

 

 

There have also been several reports on results from an artificial cervical disc, which was originally developed in Bristol, England. This disc is a ball and socket type device made of stainless steel. It is secured to the vertebra with screws. The results of this device have been good and additional clinical studies are being conducted in Europe and Australia.

Artificial DiscThe Bryan Cervical Disc System is another composite type artificial disc designed with a low friction, wear resistant, elastic nucleus with two anatomically shaped metal plates. A flexible membrane forms a sealed space and contains a lubricant to reduce friction and wear and tear. The implant allows for normal range of motion and comes in five sizes. The initial clinical experience with the Bryan Total Cervical Disc Prosthesis has been promising.

 

Photo: Bryan Cervical Disc Prosthesis

 

Hydraulic Artificial Discs

Artificial DiscHydraulic artificial discs have a gel-like core covered with a tightly woven polyethylene "jacket". Before it is implanted, the pellet-shaped hydrogel core is compressed and dehydrated to minimize its 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 about 4-5 days for the hydrogel core to reach maximum expansion. 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.

Photo: PDN Prosthetic Disc Nucleus

This type of artificial disc has been extensively tested, and the results have been good. Currently, further clinical evaluation is being conducted in Europe, South Africa and the United States.

Elastic Discs

Artificial DiscElastic type artificial discs are made of a rubber core bonded to two titanium endplates. The results of testing have been mixed. 6 patients who received this type of artificial disc were evaluated after a minimum of 3 years, at which time the results 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 testing.

Photo: Acroflex Disc

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 a polyethylene core and metal endplates with two vertical wings. Patients who received this implant reported good to excellent results.

The Future is Bright
Artificial spinal disc replacement is not only possible but it has the potential of revolutionising the treatment of spinal degeneration and providing relief to millions of back pain sufferers. The development of an artificial disc still has many challenges, but the results from initial efforts have been promising. Stay tuned…the future is bright.

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