Some doctors say the current generation of artificial spinal discs is based on fatally flawed science.
Researchers at the University of California atIrvine say they believe manufacturers, including
Johnson & Johnson
,have made a common but crucial mistake in designingtheir devices.
The doctors say this error stems from amisunderstanding of spinal biomechanics. Thiscrucial miscalculation, they add, explains why thecurrent generation of artificial lumbar discs hasexperienced repeated failures -- with oftencatastrophic consequences for patients.
"They've based their center of rotation on a discspace that is in front of the spinal canal," saysCharles Rosen, a Cal-Irvine spine surgeon who hascriticized J&J's Charite lumbar discs. "Our model,which takes into account the complications that haveoccurred with these discs, suggests that the center ofrotation is in the back of the spinal canal instead.So their design is wrong, period."
The companies disagree. J&J defends its product and criticizes Rosen'stheory, citing peer-reviewed literature. Rosen'shypothesis has been skeptically received by otherexperts in the field as well. Medtronic believes its disc will work even better than discs currently being implanted. (
Click here for a story on the company's Maverick disc.)
In the end, device makers hope to win over patients and shareholders alike. With their cardiac-implant businesses struggling, both J&J and Medtronic could use a blockbuster device right now. And with millions of people suffering from lower-back pain -- including many who clamor for new treatments -- artificial discs have looked, to some, like an outright miracle cure.
But Rosen sees a huge mistake instead.
Rosen first shared his radical theory at apresentation for fellow orthopedic surgeons held inCalifornia last month. He and his partner, Douglas Kiester, came armedwith numerous diagrams and X-rays in an effort to showhow the spine actually works and, in turn, whyartificial lumbar discs are doomed to fail. Theyconcluded that device makers had overlooked thespine's normal function by creating an "artificialcenter of rotation" in a disc space that lies in frontof the spinal canal -- rather than behind it -- andhad compromised the body in the process.
"You have a center of rotation that's normallythere, and they falsely impose another," Rosen says."You can't have two because they will neutralize eachother. Something has to give.
"To give, either the back part of the devicebreaks or the front part dislocates," he says. "Thenyou have a failure."
Rosen has seen both Charites and ProDiscs, nowbeing promoted by Swiss device maker Synthes,regularly fail in these very ways. Indeed, he says,many of the discs that actually work seem to do soonly after "auto-fusing" bones together -- simulatingfusion surgery and ruining the promise of naturalmotion that makes the devices so appealing in thefirst place.
Synthes claims that its ProDisc features "motion similar to that of the normal disc" instead. The company feels that the ProDisc's results, long familiar to leaders in the spinal community, strongly speak for themselves.
"Our positive track record in Europe of more than 15 years confirms the opposite of the claims" made by Rosen, Synthes spokesman Peter Fehlmann says. "Patients leading a happy and normal life for one-and-a-half decades with ProDisc don't suggest that anything is wrong" with either the function or the design of the device.
Still, Rosen has pored over the medical literature in aneffort to determine how device makers might have gonewrong. He says past studies seemed to average values for bothaxes of the spine -- instead of considering themseparately -- in a manner that could incorrectly placethe center of rotation in front of, rather thanbehind, the spinal canal.
J&J disagrees, saying that past research shows "a posterior center of rotation indicates spinal instability." Ultimately, the company insists that artificial discs can -- and do -- work.
"They do not replicate natural motion exactly, and man-made materials are not, by definition, as versatile as those found in the body," J&J conceded in a statement sent to
this month. "But that is not to say that the inherent limitations of artificial discs are somehow evidence of their inadequacy, or that they somehow represent a defect in design.
"Experts at the FDA heard such arguments against (total disc replacement) -- presented in detail and at length by the technology's detractors -- and concluded, nevertheless, that, based on all the available evidence, the Charite should be made available to selected patients as an option in the treatment of degenerative-disc disease."
Still, some experts want more proof.
The orthopedics biomechanics laboratory at the VALong Beach Healthcare System, which is affiliated withUCI, has decided to take a close look of its own. Thelab, best known for its shoulder and knee studies,recently added spine research as its newest area offocus. As its first project in this specialty, the labhopes to bring a fresh perspective to the field byfirst investigating the biomechanical parametersinvolved in normal spine function and then testing the potential advantages and disadvantages of artificial discs.
"Other spine labs are looking at this, too,"admits Thay Lee, founding director of the lab, whichis primarily funded by the government through theDepartment of Veterans Affairs. "But you have to lookat where their funding comes from because that tendsto bias things a lot in our field.
"The first thing we're going to try to see iswhether the theories used by the spinal-discmanufacturers are correct," he adds. "Personally, Ithink that Dr. Rosen is barking up the right tree."
Shades of Gray
But John Peloza -- who is himself a critic of theCharite -- questions Rosen's approach.
"He's trying to make everything black and white,"Peloza says. "It's just not that way. ... The conceptof motion devices is very, very valid. They've beenvery successful."
Of course, Peloza claims, some artificial discswork better than others. Peloza actually testifiedagainst approval of the Charite disc in a hearinghosted by the Food and Drug Administration two yearsago. He found problems with the study, which measuredthe Charite against a failed operation, and citednumerous cases that had left patients in excruciatingbinds.
Ultimately, he feared that problems with theCharite would taint other discs that followed thedevice onto the market. Since then, many insurers havedecided against covering the Charite and could go onto treat competing discs in the same manner.
Peloza has fretted about just that sort ofbacklash.
"The Charite has really hurt the market," Pelozasays. "Unfortunately, I don't know if we can turn this situation around. ... I hope so.I think so. But I can't say so for a fact."
Peloza has learned some important lessons in themeantime. He has personally studied failures involvingthe Charite -- which he believes are sorelyunderreported -- and co-authored scientific researchabout why the disc has malfunctioned. He has examinedthe ProDisc, as well, and considers it a far betterdevice.
But he holds out higher hopes still for theMaverick, which he personally tested in clinicaltrials sponsored by Medtronic ahead of the device'sexpected launch next year.
Peloza, who currently has no financial ties toMedtronic, claims that his patients enjoyed"astronomical" improvement after receiving theirMaverick discs.
"We had several people in the Maverick trial whowound up with no pain at all," he marvels. "That'ssomething that's never seen in the fusion group. ...We do not generally say that 'pain-free' is even an expectation" when it comes to spinal surgery.
Twist and Spin
Still, Rosen suspects that even the Maverick will malfunction in the end. Rosen points out that the Maverick -- like competing lumbar discs -- winds up in front of the spinal canal and, moreover, fails to act like the natural shock absorber that it is intended to replace.
That said, however, Rosen admits that he has yetto see a Maverick fail.
Peloza has learned of only two Mavericks that havebeen removed from patients so far. And he says thatneither disc failed mechanically.
Indeed, Peloza and his colleagues have spent awhole lot more time studying failed Charites. However,he says, they keep encountering the same problem.
"So many of these patients are already part of alawsuit," Peloza says. We "don't want anything to dowith litigation. It could complicate scientificresearch. ... All the facts get twisted upside-down."