diaphysis

**Definition/Anatomy:** The diaphysis is the long, tubular midsection (shaft) of a long bone. Regions of a long bone from center outward: diaphysis (shaft) → metaphysis (flared region containing the growth plate/physis in children) → epiphysis (end of bone, articular surface).

**Structure:** Thick cortical (compact) bone forms the wall; the medullary cavity contains yellow marrow (fatty) in adults (red marrow in children). The periosteum covers the outer surface (Sharpey fibers anchor it), and the endosteum lines the inner surface. The nutrient artery enters through the nutrient foramen and supplies the inner two-thirds of the cortex and marrow. The periosteal vessels supply the outer third.

**Development:** Long bones form by endochondral ossification. The primary ossification center appears in the diaphysis during fetal development (about week 8). Secondary ossification centers appear in the epiphyses, mostly after birth. Longitudinal growth occurs at the physis (growth plate) between the epiphysis and metaphysis. Appositional growth (widening) occurs by periosteal intramembranous bone formation.

**Clinical significance:** - **Diaphyseal fractures:** Common in femoral shaft, tibial shaft, and humeral shaft fractures. Complications include fat embolism (long bone fractures, especially femur → hypoxemia, petechiae, altered mental status), compartment syndrome (tibial shaft), and radial nerve injury (humeral shaft fracture; wrist drop). Treatment is often intramedullary nailing. - **Pediatric fractures:** Greenstick (incomplete, cortex breaks on one side) and buckle (torus) fractures occur because children's bone is more pliable. Spiral diaphyseal fractures in a nonambulatory child raise suspicion for child abuse. - **Osteomyelitis:** In children, hematogenous spread typically seeds the metaphysis (sluggish, looped blood flow). In adults, the epiphysis is more common. Neonates can have spread into the epiphysis and joint through transphyseal vessels. - **Bone tumors by location:** Ewing sarcoma arises in the diaphysis of long bones (onion-skin periosteal reaction, t(11;22) EWS-FLI1, small round blue cells). Osteosarcoma usually involves the metaphysis (Codman triangle, sunburst pattern). Giant cell tumor involves the epiphysis (soap bubble appearance). Chondroblastoma involves the epiphysis. Fibrous dysplasia and adamantinoma can involve the diaphysis. Multiple myeloma and lymphoma can also involve the marrow cavity. - **Paget disease of bone:** Can involve long bone diaphyses, with cortical thickening and increased alkaline phosphatase. - **Rickets/osteomalacia:** Defective mineralization; rickets affects the physis/metaphysis (widening, cupping, fraying) with bowing of the diaphysis in weight-bearing bones. - **Osteogenesis imperfecta:** Type I collagen defect causes thin cortices and fragile diaphyses with recurrent fractures, blue sclerae, hearing loss, and dentinogenesis imperfecta. - **Osteopetrosis:** Failed osteoclast function causes dense but brittle bone, "Erlenmeyer flask" deformity of the metaphysis, and obliteration of the marrow cavity (pancytopenia, extramedullary hematopoiesis).

**High-yield memory aid:** "Diaphysis = Distant from joints / Diameter = shaft." Epiphysis is "Edge" (end); metaphysis is "Middle" between them (flared). Ewing = Diaphysis; Osteosarcoma = Metaphysis; Giant cell tumor = Epiphysis.