Musculoskeletal Radiology
Core concepts
Understand the ABCs of bone lesions: age, location (epiphysis, metaphysis, diaphysis), margin (narrow zone of transition = benign; wide = aggressive), matrix (osteoid, chondroid, fibrous), and periosteal reaction (benign solid vs aggressive spiculated or Codman triangle). Know the classic appearances: osteosarcoma (sunburst, Codman triangle, metaphysis of long bones in adolescents), Ewing sarcoma (diaphysis, onion-skin periosteum), giant cell tumor (epiphyseal, eccentric), osteochondroma (continuity with cortex and medulla), and enchondroma (chondroid matrix, stippled calcification). Master fracture patterns and the Salter-Harris classification. Recognize the signs of osteoarthritis (joint space narrowing, osteophytes, sclerosis, subchondral cysts) versus inflammatory arthritis (erosions, periarticular osteopenia).
Traps
Age matters: a lesion in a child is likely different from the same lesion in an adult. Do not call a growth plate a fracture; use the Salter-Harris system. Beware the aggressive-appearing benign lesion (e.g., osteoblastoma) and the deceptively benign-appearing malignant lesion (e.g., metastasis in older adults). Distinguish osteomyelitis (ill-defined, permeative) from tumor.
Rule of thumb
A narrow zone of transition and solid periosteal reaction favor benignity; a wide zone of transition with aggressive periosteal reaction favors malignancy.
Worked example
A 15-year-old with knee pain: distal femoral metaphyseal lesion with sunburst periosteal reaction and Codman triangle suggests osteosarcoma; biopsy after staging imaging.