The primary feature is one of active remodeling.
The bone matrix is abundant and dense in the early stages, but with
increased numbers of osteoblasts lining the bone trabeculae and focal
collections of multinucleated osteoclasts at areas of active resorption. As
the disease progresses, the matrix becomes scantier with osteoblastic
activity becoming intense in advanced cases. Subsequently, there is a
reduction in the amount of bone matrix associated with an increasingly
disorganized arrangement of bone trabeculae. There is a corresponding
increase in the prominence of fibrous tissue with more extensive
vascularity. In the end stage of the disease, there is almost complete fatty
replacement of the bone, with few remaining features of the original nature
of the tissue.