Chapter 26: Bone Pathology

Bone Pathology on bone pathology provides an in-depth review of the skeletal system's normal physiology, hereditary disorders, metabolic conditions, and neoplasms, tailored for medical licensing exam preparation. The text begins by establishing the composition of normal bone, distinguishing between the organic matrix dominated by type I collagen and the inorganic matrix composed of minerals like calcium hydroxyapatite. It delineates the functional roles of bone cells, describing how osteoblasts synthesize osteoid, osteocytes maintain the matrix, and osteoclasts execute bone resorption. A significant portion of the chapter contrasts hereditary pathologies, such as achondroplasia (a common dwarfism linked to FGFR3 mutations), osteogenesis imperfecta (brittle bone disease characterized by blue sclera and collagen defects), and osteopetrosis (marble bone disease caused by osteoclast dysfunction). The summary details metabolic and remodeling disorders, specifically differentiating osteoporosis, defined by reduced bone mass and fracture risk in postmenopausal women and the elderly, from osteomalacia and rickets, which involve defective mineralization due to vitamin D deficiency. Paget disease of bone is explored as a disordered remodeling process involving lytic and sclerotic phases that result in a mosaic pattern of weak, thickened bone. Infectious and necrotic conditions are examined, including pyogenic osteomyelitis, often caused by Staphylococcus aureus, and avascular necrosis of the femoral head. The chapter concludes with a classification of bone tumors, separating benign lesions like osteoid osteoma (known for nocturnal pain relieved by aspirin), osteochondroma, and giant cell tumors with their characteristic soap bubble radiographic appearance, from malignant entities. Primary malignancies discussed include osteosarcoma, presenting with a sunburst pattern and Codman triangle, chondrosarcoma, and Ewing sarcoma, a small blue cell tumor associated with the t(11;22) translocation, while noting that metastatic disease from organs like the prostate and breast remains the most common form of bone cancer.