Within the knee-joint, on either side, is a cartilage shock absorber called the meniscus. Tears to this structure can cause pain and swelling in the knee and erosion of the meniscus can lead to bone-on-bone joints that abrade the bone and cause further inflammation and osteoarthritis. Because the meniscus is made of cartilage, and since cartilage can be grown in the laboratory, it should be possible, in theory, to make a new meniscus. Researchers at Columbia University Medical Center have succeeded in using 3-D printing to do that just.
The laboratory of Jeremy Mao used made personalized 3-D implants made from a scaffold infused with human growth factors. When implanted into the knee, these growth factors stimulate the body to regenerate the meniscus on its own. Mao and his coworkers successfully tested their treatment strategy in sheep. Their procedure could provide the first effective and long-lasting way to repair of damaged menisci, which occur in millions of Americans each year and can lead to debilitating arthritis. This work from the Mao lab was published in Science Translational Medicine.
“At present, there’s little that orthopedists can do to regenerate a torn knee meniscus,” said Mao, who is the Edwin S. Robinson Professor of Dentistry (in Orthopedic Surgery) at the Medical Center. “Some small tears can be sewn back in place, but larger tears have to be surgically removed. While removal helps reduce pain and swelling, it leaves the knee without the natural shock absorber between the femur and tibia, which greatly increases the risk of arthritis.”
Heavily damaged menisci can be replaced with a meniscal transplant that utilizes tissue from other parts of the body or from cadavers. Such transplants, however, have a low success rate and carries significant risks. Approximately one million meniscus surgeries are performed in the United States each year.
Mao and his colleagues began with MRI scans of the intact meniscus in the undamaged knee. Special computer software then converts these high-resolution scans in to a 3D image. Data from these images are then used to drive a 3D printer, which produces a scaffold in the exact shape of the meniscus, all the way down to a resolution of 10 microns, which is less than the width of a human hair. The scaffold takes about 30 minutes to print and is made from an organic polymer called polycaprolactone, which is the same biodegradable polymer used to make surgical sutures.
The printed scaffold is infused with two recombinant human proteins: connective growth factor (CTGF) and transforming growth factor β3 (TGFβ3). In earlier work, Mao’s team discovered that sequential delivery of these two proteins attracts resident stem cells from the body and induces them to form meniscal tissue.
In order for a meniscus to properly form, these growth factors must be released from specific areas of the scaffold and in a specific order. To accomplish this, the growth factors were encapsulated in two types of slow-dissolving polymeric microspheres. The first of these microspheres released CTGF, which stimulates the production of the outer meniscus. The second microspheres release TGFβ3, which induces the production of the inner meniscus. Finally, this protein-infused scaffold is inserted into the knee so that it can direct the generation of a new meniscus. When these printed, growth factor-infused scaffolds were implanted into the knees of sheep, the meniscus regenerated in approximately four to six weeks. The implanted, biodegradable scaffold eventually disintegrates.
“This is a departure from classic tissue engineering, in which stems cells are harvested from the body, manipulated in the laboratory, and then returned to the patient—an approach that has met with limited success,” said Mao. “In contrast, we’re jumpstarting the process within the body, using factors that promote endogenous stem cells for tissue regeneration.”
“This research, although preliminary, demonstrates the potential for an innovative approach to meniscus regeneration,” said co-author Scott Rodeo, sports medicine orthopedic surgeon and researcher at Hospital for Special Surgery in New York City. “This would potentially be applicable to the many patients who undergo meniscus removal each year.”
Mao and others tested their procedure in 11 sheep. Even though they are four-legged creatures, sheep knees closely resemble that of humans, and therefore, as an excellent model system for orthopedic research. These animals were randomized to have part of their knee meniscus replaced with a protein-infused 3D scaffold (the treatment group) or a 3D scaffold that was not infused with growth factors (the nontreatment group). After three months, the treated animals all walked normally. A postmortem analysis of the treated animals demonstrated that the regenerated meniscus in the treatment group had structural and mechanical properties very similar to those of natural meniscus. Mao’s laboratory is now conducting studies to determine whether the regenerated tissue is long-lasting.
“We envision that personalized meniscus scaffolds, from initial MRI to 3D printing, could be completed within days,” said Mao. The personalized scaffolds will then be shipped to clinics and hospitals within a week. The researchers hope to begin clinical trials once funding is in place.
“These studies provide clinically valuable information on the use of meniscal regeneration in the knees of patients with torn or degenerate menisci,” said co-author Lisa Ann Fortier, professor of large animal surgery at Cornell University College of Veterinary Medicine in Ithaca, N.Y. “As a veterinary orthopedic surgeon-scientist on this multi-disciplinary team, I foresee the added bonus of having new techniques for treating veterinary patients with torn knee meniscus.”