intermediate phase of Paget's disease is characterized by an alteration
in osteoclastic activity, that results in an accelerated rate of bone
resorption followed by greatly increased osteoblastic activity. As
the disease advances, the cortices become coarsely trabeculated and
thickened, and the bone broadens and bows. In the late, "sclerotic"
phase of Paget's disease, seen here in the skull, bone resorption
continues, but overgrowth of bone predominates. The late stage is
characterized by a tremendous thickening of the cranial vault with
chaotic bone structure. Areas of lytic activity can be noted toward
the posterior aspect of the skull. In weight-bearing long bones, this
process usually results in structurally weakened bone that has an
increased tendency to deform or fracture.