At 71, Mr. Cevoli, a high-school guidance counselor in Teterboro, N.J., coaches cross country, teaches mogul skiing, surfs and works summers as a lifeguard on Cape Cod. Despite pain in his left hip and knee, he questioned the need for major surgery, worrying it would sideline him for too long. Instead, he tried an increasingly popular treatment, in which stem cells are extracted from a patient's own bone marrow and injected into worn or injured joints to promote healing.