An involucrum (plural involucra) is a layer of new bone growth outside existing bone seen in pyogenic osteomyelitis. It results from the stripping-off of the periosteum by the accumulation of pus within the bone, and new bone growing from the periosteum.
It can be seen radiographically (i.e., with x-rays), although it is seen rarely in developed countries, given that osteomyelitis is rarely left untreated there.
References
Involucrum Wikipedia(Text) CC BY-SA