A Better Disc for Back Repair
The back pain caused by a damaged intervertebral disc often requires surgery, which means either replacing the disc with a plastic or metal implant or removing the disc and fusing the adjacent ones together. A new type of replacement disc—consisting of a scaffold seeded with living cells—could relieve back pain without many of the side effects caused by existing surgical approaches.
Researchers at the Medical University of South Carolina made a prototype replacement disc by printing an outer scaffold and then seeding the scaffold with living cells. The scaffold closely mimics the intricately layered microstructure of a real intervertebral disc, and is the first step toward making an implant that can perform the same supportive and shock absorbing functions as the original.Compared to the metal and plastic implants used today, an artificial scaffold swathed in living tissue could repair itself, and constant access to blood supply would reduce the risk of infection after surgery.
An intervertebral disc, or IVD, is shaped like a jelly donut, with an soft, elastic center and a tougher, fibrous outer layer. Sandwiched between vertebrae in the spine, the disc defines and supports the spine’s movement, holding bones in place while allowing the spine as a whole to bend and twist. The discs also act as shock absorbers, cushioning impacts to the spine. When a disc becomes worn, pressure along the spine is unevenly distributed, and if the vertebrae shift even slightly, they stretch the nerves circling the spine, causing pain. If exercise and physical therapy offer no relief, surgery may be required.
Spinal fusing, however, restricts bending and twisting in the fused section of the spine, so some surgeons make a strong case for the implant method. “None of us were born with fused spines,” says Barton Sachs, a professor of orthopedics at the Medical College of South Carolina, who routinely performs disc-implant surgery and who was not connected with the new work. Fusing two bones together can increase the pressure on neighboring segments, wearing out other discs, Sachs says. Not only does an implant preserve motion, but the recovery time from implant surgery is shorter. “It works extremely well,” says Sachs. “[Patients] get out of the hospital faster; they get back to their lifestyles faster.”
But the implants currently used do not absorb shock. “You’re putting in materials that look medieval, and that’s the state of current clinical practice,” says Robert Mauck, professor of orthopedic surgery and tissue engineering at the University of Pennsylvania. Mauck is working on a competing improvement to disc implants.
Researchers at the Medical University of South Carolina, led by Xuejun Wen, professor of bioengineering and regenerative medicine at Clemson University and the Medical University of South Carolina, tried to closely mimic the natural architecture of the disc, so that it can perform the same functions as the original.
First, they modeled the complex inner structure of the disc on a computer. Then they extruded dissolved polyurethane through a fine glass micropipette tip onto a platform kept at -4 degrees Celsius. The cool temperature of the base caused each printed layer to solidify quickly and allowed successive layers to stack on and maintain their shape. “If you don’t cool it really quickly, you won’t get the structure you want,” says Wen. Finally, the group seeded bovine cells on the scaffold, to test if the structure supported cell growth. These grew to fill it over 19 days, after which the cells were found to have arranged themselves as they would in a natural disc.
“It’s a clever application of additive manufacturing and an exciting piece of work,” says James Iatridis, professor of orthopedics and neurosurgery at the Mount Sinai School of Medicine, in New York. But Iatridis adds that “several alternate approaches involving biological repair are nearing clinical trials or at more advanced stages of development.”
While the new work by Wen and his group comes closest to replicating the microstructure of a real disc, its performance has yet to be tested. In the next few months, the discs will be tested in rats. “We’re still trying to understand how complicated our engineered solutions need to be in order to restore function,” says Mauck.
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