Musculoskeletal Radiology
Core framework
MSK questions fall into trauma, arthritis, tumor, and internal derangement. For fractures, describe location, displacement, angulation, and articular involvement; know Salter-Harris I–V in children and the classic adult fractures (scaphoid, femoral neck, posterior malleolus, Jones, Lisfranc). Occult fractures are best seen on MRI (or CT for cortical detail).
Arthritis patterns
Osteoarthritis: asymmetric joint space loss, subchondral sclerosis, osteophytes, subchondral cysts, no erosions. Rheumatoid arthritis: symmetric, marginal erosions, uniform joint space loss, periarticular osteopenia, atlantoaxial subluxation. Psoriatic arthritis: "pencil-in-cup," periostitis, asymmetric, DIP involvement. Gout: punched-out erosions with overhanging edges, tophi, preserved joint space. CPPD: chondrocalcinosis and patellofemoral/DIP involvement.
Tumors and MRI
Location, age, and margin drive diagnosis. Epiphyseal lesions in children: chondroblastoma, giant cell tumor (after physeal closure). Diaphyseal: Ewing sarcoma, osteoid osteoma. Metaphyseal: osteosarcoma, nonossifying fibroma. MRI of internal derangement: meniscal signal must touch the articular surface to be a tear; ACL tears show discontinuity and bone contusions in the lateral femoral condyle/posterolateral tibial plateau.
Traps and rule of thumb
Traps: interpreting intrasubstance meniscal signal as a tear; missing a posterior malleolus fracture in an ankle series; confusing gout with RA (gout erosions are juxta-articular with overhanging edges, RA erosions are marginal).
Rule of thumb: a bone lesion in the epiphysis of a skeletally immature patient - think chondroblastoma before anything else.