You almost certainly know them: Someone who had to retire from soccer because of a hip problem. A grandmother who can't lift her arm to comb her hair because of shoulder pain. A coworker who had a knee replacement. So often, the cause is osteoarthritis, a wear and tear of the joints that affects one in six people over the age of 30. Osteoarthritis has no cure, and the only remedies are the implantation of a prosthesis or some treatment for pain. There is reason for optimism, however, because an agency in the US Department of Health and Human Services has allocated millions of dollars to various initiatives investigating a cure for this disease. That agency is the Advanced Research Projects Agency for Health (ARPA-H), and the project that seeks to eradicate osteoarthritis is called NITRO, or Novel Innovations for Tissue Regeneration in Osteoarthritis.…