compact bone

STRUCTURE: The functional unit is the osteon (Haversian system): concentric lamellae of mineralized matrix around a central Haversian canal (containing blood vessels, nerves, lymphatics). Volkmann canals run perpendicular and connect Haversian canals to each other and to the periosteum and marrow cavity. Osteocytes sit in lacunae and communicate via canaliculi (gap junctions). Interstitial lamellae are remnants of old osteons; circumferential lamellae lie at the outer (periosteal) and inner (endosteal) surfaces. Periosteum covers the outer surface (Sharpey fibers anchor tendons/ligaments); endosteum lines the inner surface.

COMPOSITION: Organic matrix (~35%) is mostly type I collagen (the cause of osteogenesis imperfecta when defective); inorganic (~65%) is hydroxyapatite (calcium phosphate). Cells: osteoblasts (build; produce osteoid, alkaline phosphatase, osteocalcin; express RANKL), osteocytes (mature, mechanosensing; secrete sclerostin), osteoclasts (resorb; derived from monocyte/macrophage lineage; RANK-RANKL activation; acidic H+ secretion and cathepsin K).

DISTRIBUTION: Diaphysis (shaft) of long bones is mostly compact bone; epiphyses and metaphyses are mostly trabecular bone with a thin cortical shell. Flat bones (skull) have inner and outer cortical tables ("diploe" between them).

COMPACT vs TRABECULAR: Compact ~80% of mass, ~20% of surface area, slow turnover, high resistance to bending/torsion. Trabecular ~20% of mass, ~80% of surface area, high turnover, so it is affected first by metabolic bone disease (e.g., postmenopausal osteoporosis causes vertebral compression fractures and distal radius fractures).

CLINICAL CORRELATIONS: - Primary hyperparathyroidism: PTH preferentially resorbs cortical bone (e.g., distal radius, subperiosteal resorption on hand X-ray, osteitis fibrosa cystica with brown tumors), while trabecular bone is relatively preserved; intermittent PTH (teriparatide) is anabolic. - Osteoporosis: Type I (postmenopausal) mostly trabecular loss; Type II (senile) involves both trabecular and cortical loss (hip fractures). - Osteomyelitis: Infection can spread via Volkmann/Haversian canals; sequestrum is necrotic cortical bone; involucrum is new reactive bone formation around it. - Paget disease: Disordered remodeling with a mosaic (jigsaw) lamellar pattern; elevated alkaline phosphatase with normal calcium/phosphate; risk of osteosarcoma, high-output heart failure, hearing loss. - Osteopetrosis: Defective osteoclasts (carbonic anhydrase II or other defects) cause dense but brittle bone, "Erlenmeyer flask" deformity, pancytopenia from marrow crowding, cranial nerve compression. - Osteoid osteoma: Benign tumor in the cortex of long bones with nocturnal pain relieved by NSAIDs (via prostaglandin); shows a radiolucent nidus with surrounding sclerosis. - Ewing sarcoma: Diaphyseal lesion, t(11;22), "onion skin" periosteal reaction. - Osteosarcoma: Metaphysis, "sunburst" pattern and Codman triangle (periosteal elevation). - Fracture healing: Hematoma, soft callus (fibrocartilage), hard callus (woven bone), then remodeling to lamellar bone (Wolff's law: bone remodels in response to mechanical stress). - Vitamin D deficiency: Rickets in children (defective mineralization of growth plate) and osteomalacia in adults (defective osteoid mineralization).

Bone development: Intramembranous ossification (flat skull bones, clavicle) forms bone directly from mesenchyme; endochondral ossification (long bones) uses a cartilage model. Both produce woven bone first, which is remodeled to lamellar compact bone.