ARTICLE IN BRIEF
✓ Orthopedic and neurosurgical spine surgeons debate the solidity of evidence for a new FDA-approved device for relieving back pain related to degenerative disc disease.
Will the second artificial disc to be approved by the Food and Drug Administration (FDA) offer real relief to patients suffering from degenerative disc disease? Or is the ProDisc-L Total Disc Replacement, approved by the FDA in August, destined to be another in a long line of biomechanical false hopes?
Orthopedic and neurosurgical spine surgeons who spoke with Neurology Today covered the waterfront of opinions about the new device and its prospects for relieving back pain related to degenerative disc disease. But all agree that ProDisc will not be the last artificial disc to reach the market; several other device manufacturers are expected to join the competition in what is expected to be a burgeoning new market for orthopedics.
The ProDisc, manufactured by Synthes Spine, Inc., is an artificial intervertebral disc made from metal and plastic that is implanted during arthroplasty to replace a diseased or damaged intervertebral disc. It consists of two metal endplates that are anchored to the top and bottom surfaces of the vertebrae, and a plastic inlay that fits between the two endplates; the top, or superior, endplate can slide over the domed part of the inlay, allowing movement at the level of implantation.
The first artificial disc, the Charite, was approved by the FDA in October 2004 and is manufactured by DePuy Spine, Inc., a Johnson and Johnson company.
BIOMECHANICAL SOLUTIONS TO BACK PAIN
But neurosurgeon John Loeser, MD, Professor of Neurosurgery at the University of Washington in Seattle, cast doubt on the entire enterprise of biomechanical solutions to back pain, including the practice of spinal fusion.
“This is one more attempt to solve a human problem with a biomechanical device, and I predict it will be seen as a not very satisfactory solution,” he said.
Dr. Loeser no longer performs spinal surgery and has not used the new ProDisc, but he says he cares for a large number of patients who have undergone unsuccessful surgery for back pain. He traced a history of dashed hopes in this area back to a 1934 paper in the New England Journal of Medicine, in which W.J. Mixter and J.S. Barr first described “ruptured disc” as a cause of back pain.
“This was the beginning of the age of the disc,” he said. “There was an explosion of surgeries to treat ruptured discs, and it was quickly learned that although some people got better, large numbers of patients experienced new or persisting problems. Disc surgery was not the panacea it appeared to be.”
This disillusionment was followed by the development of lumbar fusion therapy, the effort to reduce pain by stabilizing the spine and reducing motion; later, fusion was augmented by the implantation of pedicles screws and other hardware designed to make the fusion more durable.
[In a February 2005 Neurology Today article, several prominent neurosurgeons and neurologists said too many spinal fusion therapies were performed without the benefit of solid research.]
“The latest theory is that, well, since no motion is bad and too much motion is also bad, maybe the answer is an artificial disc that will control the amount of motion, giving you some motion but not an abnormal amount,” Dr. Loeser said.
“The problem is that there has never been any evidence that motion is an important variable at all,” he continued. “I predict we will end up with large numbers of surgeries using an expensive device that has significant surgical hazards, and we will find persisting back or leg problems in spite of the fact that these patients have a disc replacement.”
APPROVAL STUDIES CRITICIZED
Dr. Loeser is critical of the studies accepted by FDA for both artificial disc products, studies that in both cases were funded by the manufacturing companies.
“If you look carefully at the research study that was accepted by the FDA for the Charite disc, they allowed the company to run a controlled study against a stand-alone anterior fusion, which is well known to be an inadequate operation that is not done anymore by anyone who knows anything about back pain,” he said. The study was published last year in Spine (2005;30:1576–1583).
Dr. Loeser said he has not seen the evidence for ProDisc, but remains skeptical nevertheless. “Each manufacturer argues that its device is better than another one without any good evidence,” Dr. Loeser said. “The idea that someone has an artificial disc that will solve the problem that someone else's artificial disc did not solve – well, I'm waiting to see that.”
Ultimately, he believes the causes of much back pain are not the kind that can be ameliorated with biomechanical gadgets. These include psychological factors such as depression and fears and anxieties about the meaning of back pain, as well as a patient's feelings about returning to work.
“In the rush to new hardware, these factors are often ignored, so you end up with a technically elegant solution to a problem without changing the outcome for the patient,” he said. “I worry that we are going to fall into the technological imperative. This is an expensive technology and it is being marketed with tremendous hype. Some patients have structural problems that can be ameliorated by surgery, but there is a huge group of patients who probably don't need it.”
SUPPORT FOR THE REPLACEMENT DISK APPROACH
On the other side of the fence is orthopedic surgeon James J. Yue, MD, who has performed approximately 150 disc arthroplasty surgeries using ProDisc and participated in the multi-center study of the device for FDA approval. A summary of the study is online at www.fda.gov/cdrh/pdf5/p050010b.pdf.
He is an enthusiastic supporter of disc replacement using ProDisc as an alternative to fusion.
“There are probably just five or six percent of patients who don't get complete relief of pain,” said Dr. Yue, Co-chief of the Division of Spine Surgery and Director of the Yale Spine Fellowship at Yale University School of Medicine. “Compared to fusion I have had consistently satisfactory results. The return-to-work rate is much faster and the adjacent level changes are less. When you do a replacement at one level, the chance of doing a second surgery at an adjacent level is lower than with fusion therapy.”
Dr. Yue said the ProDisc has a thickness of just 10mm and can therefore be implanted in patients with smaller disc spaces. But he acknowledged that the new disc does not come in as many footprint sizes as the Charite, which is manufactured in as many as 12 sizes.
Dr. Yue contends that outside of a diagnosis of sciatica, spinal surgery for back pain has not been received with open arms by his neurosurgical colleagues. However, he believes that “if the disc is the source of back pain, there is now a better chance that patients can get better [with arthroplasty].”
He has published extensively on disc replacement with ProDisc in a variety of patients – smokers and non-smokers – and with single and multilevel discogenic low back pain.
In a 2005 paper in Spine (30(19):2192–2199), Dr. Yue and colleagues reported a prospective study with at least a two-year follow-up of 25 patients treated with multilevel lumbar ProDisc total disc arthroplasty.
Pain and patient satisfaction scores were significantly reduced at 3- and 48-month follow-up. Radiographic analysis revealed disc height increases of between five and 12 mm, and disc motion from three-to-seven degrees, with no change in adjacent level disc heights.
CANDIDATES FOR DISC REPLACEMENT
So how does a clinician determine who is a good candidate for disc replacement? “It takes time and depends on the art of listening to the patient and determining what it is that makes their pain worse or better, where their pain is, and how affected they are,” Dr. Yue said. “These complaints must be complemented by appropriate imaging studies such as an MRI. If you pick the wrong patient, for instance a patient with facet arthritis or someone who has had excessive and long-term disc height loss, they may not do as well. It really is an issue of the surgeon driving the car in the right direction and in the proper way.”
Dr. Yue also believes that in the hands of a trained neurosurgeon, disc replacement can be far less risky than fusion therapy, particularly for certain patients. “Smokers are terrible candidates for fusion but really good candidates for disc replacement,” he noted.
Between the optimism of Dr. Yue and the skepticism of Dr. Loeser, neurosurgeon Michael Wang, MD, strikes a note of cautious expectation for disc replacement using ProDisc, with the possibility of real improvement over fusion for a select group of patients.
Dr. Wang, Assistant Professor and Spine Director in the Department of Neurological Surgery at the University of Southern California School of Medicine, claims no strong bias for or against the PRODISC, but says the device is the tip of a biotechnological movement toward “motion preservation” as an answer to low back pain that has enormous financial backing from the device manufacturing industry. And he said the competition that will ensue from having more than one device on the market can only be a good thing, bringing down the price of an expensive surgery and potentially changing the attitude of insurers who have thus far refused to cover disc replacement.
Dr. Wang calls fusion therapy the “gold standard,” but acknowledges obvious problems with the procedure. And he said that in comparison with fusion therapy, the breakdown among patients who get better, stay the same, or get worse following treatment is liable to be roughly the same with disc replacement: roughly 75 percent will get better; 20–22 percent will stay the same; and a small percentage will get worse.
But he adds a critical caveat: In the category of patients who get better, a small number of those who receive disc replacement experience nearly total amelioration of pain that is rarely or never seen with fusion therapy.
“I have never seen a fusion patient come back and say, ‘I'm like I was 18 years ago.’ But I've had arthroplasty patients who go from being disabled to nearly pain-free,” he said.*