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A palisade type of new bone formation has a ragged margin and resembles a picket fence in appearance. This appearance is associated with a lesion that is at least moderately aggressive. Solid homogeneous periosteal new bone indicates a chronic and probably inactive process. The so-called “sunburst” pattern of new bone formation is relatively uncommon and appears as radiating spicules of poorly defined new bone. The degree of mineralization of new bone is a useful and reliable indicator of the age of a lesion and, to a lesser extent, of the degree of aggression. Poorly mineralized new bone is almost always recently formed. In adult animals, it takes approximately 10 days for mineralized new bone to become radiographically visible and, sometimes, only if using a hot light or by making an underexposed film. In juvenile animals, mineralization may happen more quickly; and new bone formation may be visible 5 to 7 days after an insult. Acute periosteal new bone may be so poorly mineralized that it is not visible without using a hot light. A relatively underexposed film may be helpful in detecting very poorly mineralized new bone. If the degree of mineralization of new bone is comparable to that of the original underlying cortical bone, then a chronic process is indicated. The shape and definition of the margin of the new bone should also be evaluated. A useful rule of thumb is that, if it is impossible to trace the outline of the new bone with a sharp pencil, then it is classified as poorly defined and should be considered recent and active in nature. A well defined edge indicates a chronic process, and a smooth margin indicates a chronic lesion that is completely remodeled and probably inactive. A well-defined irregular margin suggests a lesion that is chronic but probably active and likely to be mildly to moderately aggressive. Productive lesions often include both active and inactive areas of bone formation. In such cases, the lesion should be classified based on the most aggressive new bone. The rate of change of a lesion is a useful indicator of the degree of aggression or malignancy. Serial radiographs obtained at 7- to 14-day intervals can be used to evaluate suspicious lesions, especially when the initial findings are equivocal. Aggressive lesions usually show a rapid rate of change, with clear evidence of progression of osteolysis and bone formation over a relatively short period of time. If serial radiographs demonstrate no change over a period of 4 to 6 weeks, then the lesion is almost certainly nonaggressive. All of these changes are considered in deciding how aggressive a skeletal lesion is. Some lesions have both aggressive and nonaggressive features. A common example is an osteosarcoma, which may have moth-eaten and permeative lysis in the center of the tumor but well-organized, well-mineralized new bone at the periphery where the expanding tumor has lifted the periosteum.

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