- Clinical science
Paget disease of bone (PDB, or ostlamellar bone to be replaced by weak woven bone. The cause of this common yet underdiagnosed skeletal disease is not known. It predominantly affects individuals over the age of 55 and is characterized by localized pain and bony deformities (such as bowing of long bones). Skeletal x-ray, bone scans, and serum alkaline phosphatase are important tests for diagnosing and monitoring the progression of PDB. Treatment is mainly supportive and involves the use of bisphosphonates to inhibit osteoclastic function.rmans) is a slowly progressive monostotic or polyostotic skeletal disease. It is characterized by increased bone turnover, which causes normal
Paget disease of the bone should not be confused with Paget disease of the nipple (see ) or (see ), which are named after the same physician.
- Second most prevalent skeletal disease after osteoporosis
- Sex: ♂ > ♀ (1.2:1)
- Age of onset: > 55 years
Epidemiological data refers to the US, unless otherwise specified.
- PDB is an idiopathic disease.
- PDB is associated with a high rate of bone remodeling.
Bone remodeling in Paget disease occurs in three phases: 
- Lytic phase: : increased number of osteoclasts, which are larger than usual and have more than the usual number of nuclei, appear in bone → increased osteoclastic activity → increased rate of bone resorption
- Mixed lytic and blastic phase: increased osteoclastic activity; is accompanied by an increased number of osteoblasts; , which retain a normal morphology → increased rate of bone formation with haphazardly laid collagen fibers → abnormal woven bone that is hypervascular
- Sclerotic phase: (quiescent phase): Osteoclastic activity and osteoblastic activity eventually cease.
Disease localization: The pelvis, skull, vertebral column, and long bones of the lower extremities are the most commonly affected sites .
- Monostotic PDB: affects only one bone (∼ ⅓ of cases)
- Polyostotic PDB: affects two or more bones (∼ ⅔ of cases)
- Approximately 70–90% of cases are asymptomatic.
- Bone pain, which may be associated with erythema and elevated skin temperature over the affected bones
- Bony deformities; e.g., bowing of legs (saber shin)
- Skull involvement (in ∼ 40% of cases)
- Pathological fractures: Chalk-stick fractures of long bones 
- Laboratory tests
- Deformed bones with both sclerotic and osteolytic lesions
Thickened cortical bone
- Coarsened trabeculae; expansion or enlargement of a region of the bone.
- Skull x-ray: thickening of the diploe; osteoporosis circumscripta (cotton wool appearance).
- Vertebral x-ray: thickening of the upper and lower plates of the vertebral body gives rise to a “picture frame” appearance; diffuse enlargement of the vertebrae (ivory vertebra)
- Pelvic x-ray: disruption/fusion of sacroiliac joints; thickened iliopectineal line (brim sign)
- Bone scans (skeletal scintigraphy): to test for additional bony lesions
- Osteopetrosis is an inherited, diffuse bone disease that results in increased sclerotic appearance of the skeleton on radiological examination (from Latin: "petrosus" = stony)
- Pathophysiology: impaired osteoclastic resorption of bone with preserved osteoblastic function
- Clinical features
- Bone marrow transplantation may be successful in the case of pancytopenia.
- Fibrous dysplasia is a benign, developmental disorder of bone that causes normal skeletal tissue to be replaced by fibrous tissue.
- Post-zygotically acquired, somatic, gain-of-function mutation in GNAS1 gene on chromosome 20q
- Disease phenotypes
- Fibrous dysplasia accounts for approx. 5% of benign bone lesions
- Most commonly presents during adolescence
- Common to all disease phenotypes
- Additional symptoms (specific disease phenotype)
The differential diagnoses listed here are not exhaustive.
- First-line: bisphosphonates
- Second-line: calcitonin therapy
- Supportive therapy
Medical therapy should be initiated among all patients with active disease (raised ALP levels or active disease foci on skeletal scintigraphy).
Surgical therapy may be indicated in patients with bone deformities and/or pathological fractures.