By Dov Michaeli
For people of a certain age low back pain is a fact of daily life. Why, of all people, should our venerable seniors suffer from this affliction? The short answer is disk degeneration. The longer answer requires a longer explanation. So bear with me and I’ll walk you through it.
The intervertebral disk
This structure is a marvel of engineering. Now don’t get me wrong, I am not implying that an all knowing intelligent engineer designed it. What is marvelous about it is that an evolutionary process of trial and error lasting many millions years made us, and all vertebrates, well…vertebrates. And probably as important, it allowed us to walk upright, although we had to pay a penalty for the privilege. More on that later.
The picture below shows the basic structure of a disk.
The center of the disk, called nucleus pulposus, is made up of cartilage material, which has the consistency of a gel; imagine a water-filled sponge. If you apply pressure on the sponge from above, what would keep the water from running off to the sides? To contain it the evolutionary engineer designed a ring around the center, called anulus fibrosus, which means fibrous ring. And that’s exactly what it is: a ring of collagen fibers. The picture below shows why those collagen fibers can withstand the tremendous forces exerted of the nucleus every time we pound the pavement, and prevent the water and the gel from leaking out.
The collagen molecule is made up of three helical chains wound about each other. These molecules, laid out next to each other form a fibril, and fibrils are laid out next to each other form fibers. The fibers, in turn, are intertwined with each other to form a strong dense mat. The structure is reminiscent of a rope. Indeed, this is exactly how ropes are structured.
As an aside, did you ever wonder how the cables of the famous suspension bridge of the Golden Gate can carry the huge weights passing over it day and night? The cables are organized exactly like the fibers making up the fibrous ring of the intervertebral disk. Any localized structural weakness in the cable can end up in a big problem. So does a weakness in the anulus fibrosus.
Oh, my aching back
Now we are perfectly equipped to understand the problem afflicting us back-pain sufferers. When the anulus fibrosus weakens for some reason, pressure on the nucleus pulposus would cause the it to bulge when pressure is applied from above (carrying weight) or from below (running). And if the ring is so weakened that it could not withstand the pressure, it would go beyond bulging –it would rupture. Either way, it puts pressure on the adjacent nerves and irritates them. Hence the inflammation and pain.
When pain gets unbearable
Normally, back pain is self-limited and resolves spontaneously. Clinical studies have shown that simply walking would hasten recovery. Stretching exercises help in recovery, as well as in preventing the episodes in the first place.
When all else fails, neurosurgeons and orthopedists would be very happy to use the knife to remove your ailing disk and fuse the two adjacent vertebras. This of course results in limited mobility, but that’s not the worst of it: the track record of this procedure is less than stellar. In fact, it’s pretty dismal.
The device companies rushed into the breach. Using new-fangled plastics or metals and the genius of human ingenuity, these are the prosthetic designs they came up with.
Now, I don’t want to belittle these heroic efforts to replace nature. The intervertebral disk prostheses allowed many sufferers to move around, albeit not with the same freedom and agility. But the failure rate of these devices is just too high, and replacing them can end up in a catastrophe. What’s more, the artificial materials don’t last forever: wear and tear causes erosion of the surface, releasing particles that become a source of chronic inflammation and pain.
Help on the way?
No engineer can outdo the genius of nature, so why not pay the ultimate compliment and mimic it? Indeed, Lawrence Bonassar, a biomedical engineer at Cornell University, and his team reported their invention in the August 1, 2011 issue of the Proceedings of the National Academy of Sciences. They created an artificial scaffold shaped like a disk, with collagen on the outside to provide structural stability and a gel in the center. But that would not be enough: the natural disk is a living, continuously self-renewing structure. In the nucleus pulposus there are specialized cells called chondrocytes that continuously synthesize new cartilage to replace the old one, that gets eroded with time. Likewise, the annulus fibrosus contains cells called fibroblasts that synthesize new collagen to replenish the eroding ring. So they added fibroblasts to the collagen, and chondrocytes, which they seeded into the gel. For 2 weeks, they let the cells grow around the scaffold, creating a living disk and taking over both parts of the artificial scaffold. The bioengineered disk provided as much cushioning space between spinal vertebrae as a typical disk does. Moreover, cells from the implant didn’t just populate the space within the scaffold—they started growing outward into the rest of the spine, as the cells in a normal disk do. The picture below shows the natural disk (left) and the artificial/natural-like disk (right).
All this is nice, but it still doesn’t prove that it actually works in real life. Well, over the 6-month study period, the implanted disk showed no signs of wear. In fact, the disk began to function even better, in terms of the amount of cushioning it provided between vertebrae, as it filled out with living cells and became integrated into the spine. The researchers confirmed that the implant allowed the spine to bear as much weight and move as freely as a normal disk.
Are we there yet?
Not quite. The disk was engineered to fit the tail of a mouse. So now they have to scale it up to fit a human spine. Scientists tend to dismiss this as “just and engineering issue”. In reality, scale-up issues are complex and require a lot of ingenuity. But the problem is not insurmountable.
The other issue is that a four-legged mouse doesn’t bear much weight as a two-legged human; so the bar for the functionality reported by the scientists is quite low. It remains to be seen if the newly created disk can bear the weights carried by a weightlifter doing squats.
I hope it does. I believe it will.