Codman’s triangle (figure 2) is a radiographic sign that occurs in other conditions such as osteomyelitis, eosinophilic granuloma, Ewing’s sarcoma and other tumors. It can appear whenever a lesion occurs inside a bone and grows outside of it. When the lesion  (or pus in osteomyelitis)  surpasses the cortical bone, it causes detachment of the periosteum that was attached to the cortical bone. The lesion therefore grows and this detachment stimulates the periosteum to produce bone. In this way, a layer of mineralization occurs as if a neocortical was formed. This process can occur successively, forming true layers (lamellae) (figures 3a and 3b). These layers can be thin  (thin lamellar periosteal reaction – characterizing the growth speed of the lesion)  or thick   (thick lamellar periosteal reaction – characterizing slow growth) . As the lesion progresses, this attempted “barrier” ruptures and a triangle appears radiographically delimited  externally by the periosteum  (which was successively detached and removed from the bone cortex),  internally by the  bone cortex and at the  base by the  expanding tumor.

         The imaging study of the lesion can be more detailed using tomography (figure 3c), which allows the evaluation of erosion of the bone cortex and extra-cortical involvement. Magnetic resonance imaging is important to define the extent of the tumor, both in the medullary and extracortical areas (figures 4d, 4e and 4f). This exam also allows you to evaluate the relationship between the tumor and the soft parts (muscles, vessels and nerves) and is essential for surgical planning.