Pyogenic osteomyelitis in a tibia. The sliver of necrotic bone is called a sequestrum. The site of infection was filled with pus and drained from the skin. Osteoclasts and osteolytic enzymes in the pus eroded the surface of the bone. Blood flow in infection was unbalanced and disrupted and was initiated in this case by a traumatic event. Active hyperemic forces in the capillaries contributed to bone erosion and destruction while passive hyperemia, venous congestion and protein leakage favored woven bone formation in the osteogenic cells of the periosteum. Sometimes periosteal new bone formation creates a sleeve of woven bone around the site of infection which is known as an involucrum.