Figure 3. Aneurysmal bone cyst (ABC) proximal tibial metaphysis. The marked expansion has thinned the posterior and medial cortex such that it is no longer radiographically visible. The interface with preserved tibia is well-defined with some sclerosis (a, arrow). A multilamellated periosteal response can be seen at the inferior margin of the lesion—a periosteal buttress (b, arrow)—often identified in this condition.