Using radiocarbon dating as a forensic tool, researchers have found that human cartilage rarely renews in adulthood, suggesting that joint diseases may be harder to treat than previously thought. The technique, which dates tissues by tracing radioactive carbon and measuring it against levels of carbon-14 in the atmosphere from the nuclear bomb testing in the 1950's and 1960's, reveals that cartilage is an essentially permanent tissue in healthy and osteoarthritic adults alike. The findings may help explain the limited success of cartilage transplant and stem cell therapy for osteoarthritis, and may redirect treatment efforts to preventing cartilage disease and protecting joints from further damage. Whether cartilage, the tissue lining the surface of joints, regenerates or remains "fixed" throughout life is a subject of debate. Less still is known about the effects of joint diseases on cartilage turnover. Katja Heinemeier and colleagues turned to the bomb pulse method, which exploits the fact that all living things through their diet incorporate carbon-14 from the atmosphere. During the Cold War, atmospheric levels of this carbon isotope spiked due to the testing of nuclear bombs, leaving a detectable imprint in all organisms living at the time. The technique has been used to estimate the age of fat, muscle, the eye lens, and other tissues. Heinemeier et al. now apply it to cartilage in knee joints from eight healthy and 15 osteoarthritic individuals born between 1935 and 1997. Across all individuals, the researchers detected virtually no formation of new collagen in cartilage, even in disease or under high loads, suggesting that the tissue is an essentially permanent structure. The findings help explain why human cartilage has poor healing capacity after injury and present new challenges for treating osteoarthritis and other joint diseases.