A chondroblastoma is a rare type of noncancerous bone tumor that begins in cartilage. This is the specialized, gristly connective tissue from which most bones develop. It plays an important role in the growth process. There are many different types of cartilage in the body. Chondroblastoma most often affects the ends of the long bones, near the growth plate, in the arms at the shoulder, and in the legs at the hip and knee. It is also called Codman’s tumor.