Paget’s disease, or osteitis deformans, is a chronic metabolic disorder characterized by increased bone turnover and breakdown secondary to excessive and disorganized osteoclastic and osteoblastic activity ( Fig. 48.1 ). The precise etiology is still unclear. The excessive breakdown of bone is followed by the formation of weak and abnormal bone. Over time this robust and disordered bone metabolism can result in enlarged and/or deformed bones, leading to a variety of complications. An understanding of the evolution of this disease process is essential to accurately identify a variety of imaging appearances. Sources of confusion arise due to the distribution and wide spectrum of manifestations related to the pathologic phase of disease. Involvement may be widespread, monostotic, or polyostotic ( Figs. 48.2 and 48.3 ). Paget’s disease of bone is the second most common bone disease affecting the elderly population of the United States. It is estimated to affect 1% to 3% of the population over 50 years of age, although this has recently been declining.
Temporal Evolution: Overview
Paget’s disease of bone can demonstrate a variety of patterns of appearance related to the pathologic state of disease progression. Progression is generally slow and manifests with subtle changes over time. The cardinal and classic imaging features of the disease have been referred to as a “caricature of bone” and include advancing osteolysis, coarsened/thickened bone trabeculations, cortical thickening, and osseous expansion ( Figs. 48.4 and 48.5 ).
In order to understand the evolution of Paget’s disease, one must appreciate the rate of progression; recognize the presence of three distinct phases, which in fact represent a continuum; and be aware of the marked variability of disease.
Progression: Paget’s disease is characterized by a gradual progression that evolves through various phases of activity. There is considerable variability in disease severity.
Three phases: Three distinct phases of disease progression exist. Although the three phases have been described as discrete, they represent a continuum ( Fig. 48.6 ).
Lytic phase: characterized by osteoclastic resorption seen as osteolysis on imaging. So-called osteoporosis circumscripta in the skull (described in further detail later).
Mixed phase: characterized by osteoclastic and osteoblastic hyperplasia, although ostoblastic activity predominates. All the classic features may be seen in this phase, including advancing osteolysis, coarsened/thickened bone trabeculations, cortical thickening, and osseous expansion (described in further detail later).
Blastic phase: characterized by a gradual decline in osteoblastic activity seen as osteosclerosis (described in further detail later).
There is prominent variability in the rate of disease progression in comparing different patients and comparing different sites of involvement within the same patient ( Figs. 48.7 and 48.8 ).