Vertebral fusion and disc replacement are comparable, but shorter recovery time for disc replacement makes it attractive.
Back pain, difficult to treat due to its complex etiology, has drawn the interest of managed care and organized medicine. A number of new technologies have been developed including implantable spinal cord stimulators, implantable drug delivery devices, and spinal cages to promote fusion surgical results, but no technology eluded scientists as long as replacement disc technology.
Recently, though, the FDA approved the Charité Artificial Disc (Dupuy Spine, a Johnson & Johnson company). This disc, which had been approved in 24 countries before it received U.S. approval, is composed of two cobalt chromium endplates and a polyethylene core that moves between them. The core incorporates a radio-opaque cobalt/chromium alloy, allows movement, and provides the needed separation of the vertebrae. The device is available in five sizes, each one in four shapes. These combinations allow for a customized response to the many different sizes and shapes of the individuals seeking surgical replacement. Total cost of the device is $11,500. This includes two endplates ($5,400 each) and the center core ($700).
Artificial replacement discs were first contemplated over 40 years ago. Research started in the 1960s, just after the revolutionary arrival of the total hip replacement. Without a replacement disc, back surgery has been little more than salvage surgery, not reconstructive surgery.
And back surgery is common — nearly three quarters of a million procedures, 200,000 of which are fusions, are done yearly in the United States with costs of back pain treatment reaching over $50 billion. Existing surgical techniques, discectomy, and fusion — even with the improved techniques of today — are merely stopgap measures which actually worsen the mechanical issues of disc disease and result in incomplete pain relief and numerous potential secondary complications, such as spinal stenosis and facet pain. In fact, the resulting mechanical problems have actually created a new syndrome, termed Transition Syndrome.
Entrapped pain fibers
These complications occur because the space the disc occupies in the spinal column is important to the maintenance of normal mechanical function. Remove the disc and the space closes, thus entrapping pain fibers. The surgical fusion technique can cause extra bone formation, resulting in, ironically, pain in a portion of patients. The resulting mechanical changes to the spinal column also produce added physical stress on the adjacent discs. Because of the structural complexity of the normal disc, replacing the diseased disc with an artificial one has been likened to finding the Holy Grail.
An individual vertebra is made up three components. A hard outer layer is made up of collagen fibers running at 30-degree angles to each other. This structural formation provides angular as well as compression strength. The middle layer, a seemingly simple cushiony material located within individual vertebrae, is actually an engineering marvel. The actual cushion material is made up of a glycosaminoglycan that is capable of absorbing a considerable amount of water, which acts as shock absorber. The final component is an end plate that allows nutrients to pass into the disc. All of these components must be healthy to allow normal functioning of the disc.
The complex bones of the back and the shock-absorbing soft component of discs provide the flexibility and stability required for a healthy back. In addition, the discs provide the very large spinal nerves with protection from the movement and weight-bearing actions of the bones of the body. Reproducing this is a very complicated engineering feat.
Replacement disc benefits
First and foremost, recovery from surgery is more rapid with a replacement disc than with the fusion technique — 12 weeks instead of nearly 6 months. In addition, most patients can leave the hospital earlier after a replacement disc has been implanted than with surgical fusion.
The surgery is started in the abdomen and requires both an abdominal surgeon and a spine surgeon. The procedure takes about two hours and is typically followed by a four-day hospital stay.
This procedure will not work if a proper diagnosis of the source of pain is not pursued, so expert diagnostic work is mandatory. In addition, the FDA indications for this product are numerous: skeletally mature patients with degenerative disc disease at one level in the lumbar spine (from L4-S1), and no more than 3 mm of spondylolisthesis at the involved level. Patients must not have had relief from pain after at least six months of nonsurgical treatment.
Clinical trials, using a noninferiority method evaluated over 24 months, compared the Charité Artificial Disc with a commercially available spinal fusion cage. The trials resulted in a 5.4 percent failure rate in the treatment population versus an 8.1 percent failure rate in the control population.
The Charité Artificial Disc should not be used for patients with active systemic infection or infection localized to the site of implantation; osteoporosis; osteopenia; bony lumbar stenosis; allergy or sensitivity to implant materials; or isolated radicular compression syndromes due to disc herniation and pars defects.
Overall, the cost of this surgical solution is probably the same as for a fusion cage and surgery, including hospital, professional, and durable medical device costs. The outcomes are no better and no worse. Long-term results remain to be determined, but from a nonengineering perspective, the approach is more sound than the alternatives.
Use with care
Even so, the device is not a panacea. It must be used in the appropriate patient. Therefore, managed care decision-makers must set up a screening method that prevents this approach from being used in marginal patients. Also, the learning curve is not inconsequential. A centers-of-excellence approach to this device would be a wise move. Competitors are soon to enter the market, giving managed care a more complicated, but potentially competitive, bargaining position.
The FDA also has required a post-approval study to follow subjects for five years on a variety of endpoints, including maintenance or improvement of neurological status versus baseline, no permanent neurological deficits compared to baseline, absence of major complications (defined as major vessel injury or major neurological deterioration), and device failures requiring revision, reoperation or removal.
Overall, this device offers an alternative to traditional disc removal and fusion for severe pain patients. It seeks to offer a mechanical substitute for the natural disc, and initial indications appear promising.