IF YOU'RE older than 20, your spine has already started to wear out.
Cheer up, however, for the new director of spinal surgery at Johns Hopkins Hospital is racing to have replacement parts in stock before it finally does.
Sometime this year, Dr. John P. Kostuik and his biomechanics team hope to start human trials on an artificial disc made of titanium and cobalt chrome that may one day provide relief to the 50,000 patients nationwide who develop painful spinal disc problems every year.
Disc ailments are among the many causes of lower back pain, which is at "epidemic" levels in this country, says Dr. Kostuik. And as our population ages, it will become more so.
The silvery new disc has a pair of coiled springs that will absorb skeletal shocks with a lifelike "bounce." It's designed to last 40 years, or 100 million steps, whichever comes first.
If the artificial disc works, it will spare patients much of the pain and risk of spinal fusion surgery, which currently ends in failure 15 percent of the time, Dr. Kostiuk said. The artificial disc implant would be used to alleviate one of the most common lower back injuries, the ruptured or herniated disc.
The natural discs are the tough, jelly-filled capsules between individual vertebrae in the spinal column. Their role is to separate the bony vertebrae, cushion the repeated shocks of walking and running, and to provide flexibility.
Unfortunately, "discs start to wear out in everybody at about 20 years of age," Dr. Kostuik said.
When discs tear or leak, the injury can cause acute pain at the disc itself, or put painful pressure on adjoining nerves. The pain can range from mild and intermittent to severe and chronic.
In spinal fusion surgery, doctors remove the damaged disc, "rough up" the facing surfaces of the adjoining vertebrae and fill the joint with grafts of bone taken from the pelvis. Then the spine is immobilized with screws and plates until the spinal bones simply grow together, or "fuse" into a single unit.
When it works, the procedure eases the pain. But it also reduces the flexibility of the patient's back, and demands six months for healing and three months of rehabilitation.
In 1987 nearly 59,000 spinal fusions were performed in the United States at a cost of about $40,000 each, or $2.36 billion. An estimated 15 percent of the surgeries failed, Dr. Kostuik said, adding $356 million to the costs. By the year 2000, Americans will spend $5.2 billion for 65,000 fusions, and $784 million more to repair the failed ones.
Implantation of a durable artificial disc, he said, would require no bone graft from the pelvis, often the greatest source of pain after fusion surgery. Recuperation would shrink to six weeks, and earlier rehabilitation would speed the patient's return to work.
"Back problems don't get the publicity of heart attacks because they aren't fatal," he said, "but they are actually more expensive to society." That's partly because most occur in our most productive working years.
xTC In 1982, Dr. Kostuik said, the direct costs were estimated at $16 billion, with another $48 million in lost productivity. Those numbers have probably now doubled, and add up to "more than oany other health care issue," he said.
An orthopedic surgeon, Dr. Kostuik began his work on the artificial disc 6 1/2 years ago at the University of Toronto. It's a competitive field. Japanese scientists are working on metal discs for the upper neck. Experimental plastic discs have already been implanted in Europe, but Dr. Kostuik's work suggests those will begin to wear out in four years.
Dr. Kostuik is focusing only on the lower back, because it is the site of most painful back injuries. It's more vulnerable to injury because it bears the heaviest weights, stresses and strains.
As much as possible, he said, "we wanted to replicate the normal function" of the natural disc, as well as its durability.
Duplicating the movement and function of natural discs required detailed studies of their elasticity, and led finally to a design that employs two hinged metal plates separated by a pair of coiled titanium springs. "We spent a year learning to wind wire," Dr. Kostuik said.
In search of materials that will last a lifetime, the researchers eventually settled on hard alloys of titanium and cobalt chrome, and the prototypes were tested at twice normal human spinal loads.
Next, they sought the best ways to attach the device to the adjoining vertebrae.
The initial attachment will be made with screws. But the Canadian-made discs will also have porous metal surfaces that will provide an anchor for the infiltration and growth of natural bone.
Titanium and cobalt chrome have been used successfully in other types of implant. Nevertheless "we are testing well beyond what any implant has been tested before" to ensure the metals are safe, he said. In particular, he is studying how much metal "wear debris" the artificial disc might produce over 40 years, and whether it might be toxic in the body.
After initial human studies beginning this year at Hopkins, Dr. Kostuik hopes to launch a study at 15 clinics around the world. General use of the discs, if all goes well, he said, is probably "four or five years away."
Dr. Kostuik has been given a joint appointment to Hopkins' departments of orthopedic surgery and neurosurgery, with a mandate to create and direct the new Center for Spinal Diseases. The center will focus on the care of adults with spinal problems ranging from simple back pain to ruptured discs, spinal cancer and the correction of severe, life-long spinal deformities.