MRI — Gold Standard
T1: marrow replacement (low signal replacing normal bright fatty marrow — most sensitive finding). STIR/T2: hyperintense edema in marrow and surrounding soft tissues. Post-contrast: abnormal marrow enhancement, periosteal enhancement, soft tissue/joint enhancement.
Periosteal reaction on radiograph is delayed 7–10 days after onset — a normal XR does not exclude osteomyelitis. Three-phase bone scan is useful when MRI is contraindicated.
Septic Arthritis — Urgent
Joint space widening, periarticular edema, and joint effusion on MRI. Cartilage destruction is rapid (within 24–48 hours of untreated infection). Requires urgent joint aspiration (synovial fluid WBC >50,000, predominant PMNs) and surgical washout. Do not delay aspiration for MRI if clinical suspicion is high.
Common organisms: S. aureus (all ages), N. gonorrhoeae (sexually active adults), Group B Strep (neonates).
Comparison of Osteomyelitis vs. Septic Arthritis
| Feature | Osteomyelitis | Septic Arthritis |
|---|---|---|
| Primary MRI sequence | T1 (marrow replacement); STIR (edema); post-contrast T1 fat-sat (enhancement, abscess rim) | T2/PD fat-sat (effusion, synovitis); post-contrast T1 fat-sat (synovial enhancement, pannus); STIR (periarticular edema) |
| Key MRI findings | T1 low signal replacing normal marrow fat; STIR/T2 marrow hyperintensity; periosteal reaction; subperiosteal abscess (rim-enhancing fluid collection lifting periosteum); Brodie abscess (focal T2 bright lesion with penumbra sign) | Joint effusion (T2 bright); synovial thickening and enhancement; periarticular bone marrow edema (reactive); cartilage thinning/destruction (late); joint space narrowing |
| Periosteal reaction | Delayed 7–10 days on XR; aggressive (permeative, onion-skin, Codman triangle) in acute hematogenous; visible earlier on MRI as periosteal T2 hyperintensity and enhancement | Not primary feature; periarticular osteopenia on XR; erosions in chronic/RA; subchondral changes late |
| Joint involvement | Secondary involvement via direct extension from metaphysis (children: metaphysis vascular, adjacent to physis); septic arthritis can coexist especially in hip/shoulder where metaphysis is intracapsular | Primary joint space involvement; metaphyseal osteomyelitis can seed joint; always image adjacent bone when septic arthritis diagnosed |
| Common organisms | S. aureus (most common all ages); S. epidermidis (post-op/implant); Salmonella (sickle cell); Pseudomonas (IV drug use, diabetic foot); MRSA increasingly prevalent | S. aureus (most common); N. gonorrhoeae (young adults); Group B Strep/gram-negative (neonates); Kingella kingae (children <5); Strep pneumoniae |
Reference
Pineda C et al. Radiographic imaging in osteomyelitis. Semin Plast Surg. 2009;23(2):80–9.