Periosteal Pathologic Conditions: Imaging Findings and Pathophysiology.

The periosteum plays a key role in bone health and is a primary means by which the skeleton responds to a wide range of insults, both benign and malignant. Developmental and histologic features of normal periosteum explain some of the characteristic imaging findings seen in the setting of bone abnormalities. Patterns of periosteal reaction, both in morphology and distribution, are key to distinguishing benign or physiologic periosteal reaction from types of periosteal reaction that warrant further evaluation. The authors review the histologic features, distribution, and development of normal periosteum. Nonaggressive and aggressive types of periosteal reaction are presented with key associations for each. The presence of nonaggressive types of periosteal reaction implies that the underlying process affecting the bone is slow enough that the periosteum is able to heal it or contain it in an organized manner. In contrast, aggressive types of periosteal reaction are seen when the underlying bone insult outpaces the ability of the periosteum to contain it. Image-guided biopsies of lesions with periosteal reaction should be used to sample the site of the most aggressive pattern, as this approach can aid in accurate histologic grading and in detection of tumor cells and bone matrix. The distribution of periosteal abnormalities is as important as the morphology, with a diffuse periosteal reaction favoring systemic causes such as rheumatologic, metabolic, and hematologic conditions compared with a more localized periosteal reaction. Important causes of localized and diffuse periosteal reaction are discussed in a systems-based format, with an emphasis on clinically important causes. © RSNA, 2022.