What are the typical histological and radiological appearances and triphasic progression of pagetic bone lesions?
PDB progresses through three phases. First, there is a focal increase in bone resorption due to increased osteoclast activity by numerous enlarged, often multinucleated osteoclasts (lytic phase) . Radiographic findings include bone loss, wedge-shaped areas of resorption in long bones (blade of grass), and circumscribed lytic lesions in the skull (osteoporosis circumscripta). Second, increased osteoblast activity and accelerated bone formation occur, resulting in disorganized collagen architecture (mosaic or woven, instead of the normal lamellar pattern) and a mixed lytic/sclerotic phase. This abnormal bone has impaired strength and is at higher risk for fracture. On x-ray, bones are enlarged and sclerotic. Additionally, bowing deformities, transverse linear radiolucencies (“pseudofractures”), and thickening of the calvarium (cotton wool) and iliopubic/ilioischial lines (brim sign) may be seen ( Fig. 53.1 ). Finally, there is the sclerotic phase where there is reduced bone cell activity, areas of resorbed bone replaced by fibrous tissue, and persistence of abnormal bone architecture including enlarged, sclerotic bones. When atraumatic fractures of long bones occur, they typically are transverse (chalk stick) and not spiral, reflecting the weakened bony microarchitecture of pagetic woven bone.
