Surface lesions of bone refer to lesions arising external to the medullary cavity. These surface lesions of bone may arise from the cortex, the periosteum, or the soft tissues immediately adjacent to the periosteum. The lesions are designated according to their origin such as cortical, subperiosteal, periosteal, and parosteal. Cortical lesions arise from the cortex and can be further subdivided into endosteal lesions that arise from the inner cortex and intracortical lesions that are centered within the cortex; subperiosteal lesions arise between the cortex and the periosteum; periosteal lesions arise within the periosteum; and parosteal lesions arise from fibroblastic tissue external to the periosteum. This terminology is not absolute because in many cases it is difficult to determine the exact site of origin of the lesion either radiographically or histologically. The term paracortical has been used when the site of origin of a lesion on the surface of bone is not clear.