The periosteum is a membrane that covers the majority of bone except at locations at and near cartilage. How periosteum responds to stimuli (e.g., trauma, infection, metabolic process, and neoplasm) can often give clues to the etiology of the underlying stimulus. An aggressive or destructive process will often greatly alter the periosteum, whereas a nonaggressive process usually gives the periosteum the opportunity to remodel with a more orderly architecture. The distribution of periosteal reaction (focal or diffuse) may give clues to the underlying process. Periosteal reaction may also be grouped into several patterns: smooth, solid or thick, and aggressive (laminar, sunburst, Codman triangle, and cloaking).
Smooth periosteal reaction consists of one or multiple unbroken layers of ossified periosteum along the cortical surface. Smooth periosteal reaction indicates orderly new bone formation; the bone has had time to heal or remodel toward its original architecture. As a rule, the unbroken lamellar reaction suggests a process that is not very aggressive.
Aggressive periosteal reaction is likely the result of disorganized new bone in response to a destructive process. New bone formation cannot keep pace with the accelerated rate of bone destruction. In response to a rapidly destructive process, new bone formation along the periosteum produces several characteristic patterns. Lamellar (onion peel) periosteal reaction appears as successive layers of new bone arising from the periosteal surface. Spiculation is caused by calcification of the periosteal Sharpey fibers, which are oriented perpendicular to the cortical surface. The radial sunburst pattern is a classic sign of osteosarcoma and affects mainly mineralized osteoid. However, the sunburst pattern may also be seen in hemolytic anemias, thalassemia, and sickle cell anemia. The Codman triangle is another sign indicating an aggressive process. The Codman triangle is an acutely-angled triangle of periosteal reaction along the cortex surface and indicates the transition between the periosteum of the aggressive disease process and the uninvolved bone. No tumor cells are found in the Codman triangle. Just as the physis will act as a temporary barrier to the spread of tumor or infection, the periosteal collar will remain intact.
Periosteal cloaking occurs when a long segment of periosteal membrane is altered by disease and results in a long segment of exuberant thickening. Rapidly growing bones appear to be predisposed to producing cloaking in response to injury. Trauma and infection are the most common causes of cloaking.