With each step, cartilage in your knee absorbs impact, reduces friction, and keeps everything functioning smoothly. After millions of steps, it may be no surprise when that cartilage begins to wear out.

Cartilage degeneration often results in arthritis, which can reduce mobility and cause chronic pain. It cannot be repaired, but research on replacement tissues may soon offer a viable alternative. “If you have arthritis in your hip or knee, we don’t just have to fix a little pothole, we can resurface the entire joint,” said Dr. Farshid Guilak, a Duke University researcher whose team is working on a technique to replace large swaths of existing cartilage with new tissue grown from your own stem cells. The team is currently testing their approach on large animals, and Dr. Guilak expects to begin human testing within four to five years, if not sooner. Cartilage is a strong, flexible, fibrous tissue that exists in a few different forms throughout your body. Articular cartilage, the focus of this work, coats the ends of bones in your major weight-bearing joints including your knees and hips. It consists of cells called chondrocytes that produce large amounts of a tough, resilient matrix made mostly of collagen fibers.

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Articular cartilage continually withstands huge amounts of force—three to five times your body weight during normal activity and ten times your body weight during exercise. It absorbs impact and reduces friction between bones as you sit, walk, run and jump. But over time, that repetitive force can cause it to break down. “Cartilage is a tissue that has no blood supply, no nerves, no lymphatic system, so it actually has a very poor ability to repair itself,” Dr. Guilak said. Today, he said, more young people are dealing with degenerating cartilage due to sports injuries and obesity, and people in their 40s and 50s with damaged cartilage must decide between limping around for decades or opting for a knee replacement likely to wear out within 20 years, well before the end of their lives. Dr. Guilak and his team have developed a three-dimensional woven scaffold—a dense material made of woven fibers that is strong and flexible—that’s similar to cartilage but is also porous. They print the material on a 3D weaver, which is similar to a 3D printer, but can produce a fiber reinforced structure. Stem cells are harvested from a patient’s bone marrow or liposuction waste, where they are plentiful and easy to obtain. The cells are exposed to a cocktail of growth factors, hormones and vitamins that converts them into cartilage cells and they can then be injected into the woven scaffold. The scaffold, now full of cartilage cells, is covered with a tough, gel-like material designed to reduce friction and withstand wear, and the entire synthetic cartilage structure is placed inside the patient. The synthetic cartilage functions like the real thing, and over the next year or two, the cartilage cells begin to produce their own matrix, which replaces the synthetic parts of the structure. The end result is new, completely natural cartilage that was developed from your own cells.

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If the current trials in large animals are successful, the researchers will move on to a long term animal study followed by human trials to test for safety and efficacy, Dr. Guilak said. Other groups are also involved in cartilage research, including one led by Dr. Eben Alsberg at Case Western Reserve University and Dr. Ali Khademhosseini at Harvard University. Dr. Alsberg called the work at Duke “excellent,” but pointed out that he and his team are taking a slightly different approach. They plan to inject stem cells into water-based gels called hydrogels and then investigate how different factors affect cartilage development. They will adjust factors that influence gel stiffness, cell growth and attachment, and mechanical compression until they find the optimal combination to promote cartilage formation. “The big goal is to take cells…and allow them to do what they’re evolved to do—make tissues,” Dr. Khademhosseini said.