Why Preserve Motion in the Spine?
Simple question, we suppose. But illogical.
Nature imbued every spine with the ability to move. Both natural and unnatural forces (like trauma or spine fusion surgery) conspire to rob the spine of movement. If motion is good enough for nature, why do we ask this particular question?
Actually, asking this question implies that, maybe, motion preservation of the spine might NOT be healthy, beneficial or necessary for the otherwise traumatized or deteriorated spine.
Maybe Mother Nature made a mistake. Could CMS (Centers for Medicare and Medicaid—which has roundly criticized motion preservation), the FDA (which is increasingly adversarial to any motion preservation spinal implant), Deyo (who is one of the leading critics of spine motion preservation) and other critics know better?
At this past week’s Preservation of Motion in the Spine meeting at Duck Key, organizer Dr. Antonio Castellvi opened the meeting by asking; Why preserve motion in the spine? Not a single surgeon in the audience laughed or said, are you kidding?
We do live in strange times.
Let’s start with a case study. The patient is a young military man who is also a fairly heavy smoker. He began to experience low back pain and right leg pain four years ago. Conservative care did not stop his pain. An MRI showed that he had a right lateral herniated nucleus pulposus at L5-S1.
Dr. Castellvi put up some of the X-rays for this case and told the assembled surgeons that he performed a laminotomy on this patient after conservative care failed (about seven months later). Leg pain and weakness went away.
A year and a half later the patient returned complaining of new and excruciating low back pain. Unlike the first time he crossed Castellvi’s threshold, this time the pathology was at three levels. Pain generators now were at the disc level AND down the leg. Dr. Castellvi slapped up more MRI and CT scans. “Now this patient was suffering from three level discogenic and concordant pain. Do we do a three level fusion?” he asked his surgeon audience. The answer came back quickly—“No!”
Ok, what do the experts say?
Here is a table from the JBJS (Journal of Bone and Joint Surgery) which could be considered a road map for most spine surgeons.
Notice that the chart splits into two sections: Leg Pain and Back Pain. This patient has both. None of the resulting decision branches addresses three-level deterioration. The closest paradigm we see is under back pain where the chart, in parenthesis, says 1-2 level.
This patient has already been treated with laminectomy. What now? This may be uncharted territory, but it is by no means unfamiliar to busy spine surgeons. Should surgeons do a two-level fusion and then top off the third level? Should surgeons do the exceedingly difficult and biomechanically risky three-level fusion surgery?
Leaving his audience to ponder the conundrum, Castellvi pulled his discussion back to the biologic basics. The spine holds the body erect and 80% of the daily pounding is absorbed by those 22 spongy shock absorbers known as the nucleus pulposus. When the system begins to break down, it usually starts in the nucleus.
Surgeons know the nucleus is breaking down when it changes shape, loses fluid, shrinks and gets less spongy. Then the nucleus starts to shift out of alignment. When the nucleus is out of alignment, the other connected structures (vertebrae, facet joints, adjacent levels) also shift.
When any segment of the spine begins to move out of alignment the spine’s ability to carry the body’s load also changes. In the healthy spine, loads are carried well when they, essentially, line up from top to bottom. It’s like a beautifully curved plumb line for the body. Not only is that center line critical for healthy load bearing, but it also forms a center of rotation for the spine.
Oh yes, the spine must rotate. And flex. And extend.
When everything lines up along the center of rotation, then loads are absorbed and transferred around the spine brilliantly. The spine is, if nothing else, a masterpiece of engineering. Degeneration of the nucleus changes the spine’s ecology and starts to transfer loads in directions that move the center of rotation out of alignment—nerves are impinged and the body’s warning system (pain) kicks in with a vengeance.
Spine fusion works because it puts the spine back into alignment and removes the pain generator. However, it does reduce the spine’s ability to rotate, flex and extend. In some segments, like L5-S1, that’s not such a big deal. In the cervical spine (neck) it’s a bigger deal.
A helpful way to visualize the importance of the center of rotation to spine health is the following graphic that shows the relationship of the spine’s neutral zone to back pain.
The neutral zone is nature’s sweet spot where the spine is lined up and is handling its load bearing job well. Think of the neutral zone as a ball in a bowl. When the spine is healthy, the neutral zone’s ball sits at the bottom of a soup bowl and can move within a narrow range as the spine rotates or flexes and extends. That’s a health, pain free spine.
When the nucleus begins to wear out, the vertebrae and other structures shift out of that healthy curved plumb line which stretches our imaginary bowl out and the center of rotation starts to swing too far out like, in effect, a marble in a shallow salad bowl. The result is pain.
What does fusion do to our imaginary bowl? It turns it into, essentially, a shot glass which stops the neutral zone from moving. Yes, the patient may be pain free, but rotation, flexion and extension are almost eliminated at that segment.
If it were possible to introduce some amount of motion with stabilization, then the spine could return to its proper alignment but still allow for a center of rotation to move within a narrow range.
Fusion alone has two big concerns for surgeons. First, fusion concentrates the load bearing of the spine and, since fused segments can’t move they can’t transfer the loads in a healthy way. What happens next? The sections of the spine that are now absorbing new loads begin to also change—sometimes painfully. In some patients, it begins a new cycle of deterioration.
Many studies have been conducted to measure the amount of new pressures that are created by fusion. They all show that intradiscal pressures increase at the levels adjacent to the fusion—on average by about 45%. And those pressure changes have a global effect on the entire spine.
Over the last two decades spine surgeons have become expert at figuring out how to use fusion to minimize these problems providing the disease is at one level and the spine is otherwise healthy. Even two-level fusions are routinely managed well—assuming an otherwise healthy, non-smoking, non-osteoporotic, non-diabetic, or non-obese patient.
Ok, fine. But we have a young smoker with a three-level deterioration and leg pain and disc pain. Again, when Castellvi asked his audience if they would fuse three levels, the surgeons said “no”.
Again, three-level spine deterioration is not uncommon. As far back as the mid-20th century, researchers were pondering treatments for these patients. Ulf Fernstrom offered his suggestions in a 1966 paper titled “Arthroplasty with intra-corporeal endoprosthesis in the herniated disc and painful degenerative disc” Acta Scandanavia (supplement) 1966. He proposed the very simple idea of implanting a metal ball in the disc space, like a marble in the bowl. Here is an X-ray of implanted Fernstrom balls.
One hundred and ninety-one Fernstrom balls were implanted in 105 patients between 1962 and 1964 (including President John F. Kennedy). Only one ball displaced backwards, hit a nerve root causing temporary palsy which cleared upon removal of the ball. These are phenomenal results.
On the strength of these outcomes and, again, understanding that the spine is suppose to rotate, flex and extend in order to carry the body well, more than 60 motion preserving implants were invented in the ensuing thirty years to 1994.
1994. Sixteen years ago. And how many motion-preserving devices are both approved in the U.S. and paid for by reimbursing agencies today? Three motion preserving implants—at best.
Outside the U.S. surgeons are now on the fourth and fifth generation of motion preserving implants and the range of choices is far beyond any available in the U.S. Here is a picture of some of the motion preservation devices that are facing a decidedly uncertain future at the hands of an adversarial FDA and CMS.
What happened to the young military veteran with three-level disease and both discogenic and leg pain? He went to Germany for treatment.
In this country CMS has repeatedly denied reimbursement for motion preserving devices. In both its written and oral commentary to the industry CMS has denigrated the value of motion preservation in the spine. The FDA, which has approved a handful of motion preserving implants, is by its own admission a “broken system.”
So spine surgeons ask the illogical question with a straight face and, since they do truly care about patient outcomes, a heavy heart: Why preserve motion in the spine?