involucrum

An involucrum is a layer of new bone growth formed outside existing bone, and it is seen in pyogenic osteomyelitis. Its formation follows a specific sequence: pus accumulating within the bone strips the periosteum away from the underlying cortex, and the lifted periosteum then lays down new bone around the infected segment. The result is a sheath of reactive bone enclosing the original cortex. Because it arises only as a response to infection, the involucrum is a pathological rather than a normal skeletal feature, and its presence is a marker of the process that produced it. It is classified within the skeletal system as a part of an organ rather than as a bone in its own right.