Imaging Musculoskeletal Infection

Radiographs. Five classic signs, and the DRIPS mnemonic below carries them:

• Destruction of cortical and cancellous bone: lytic change needs 30-50% bone loss before it is visible
• Rarefaction: regional osteopaenia around the infected area from hyperaemic bone resorption, and it may be the earliest radiographic sign
• Involucrum: the shell of new bone that forms around dead bone in chronic osteomyelitis, encasing the sequestrum
• Periosteal reaction: periosteal new bone, lamellated (onion-skin) or solid; aggressive patterns mimic malignancy
• Sequestrum: dead, devascularised bone seen as a dense fragment within a lytic cavity, which requires surgical removal

Chronic osteomyelitis. The sequestrum, the involucrum and the cloaca are the anatomy of the chronic picture, and the cutaway below shows how they relate. Adult chronic osteomyelitis is classified by Cierny-Mader, which combines the anatomical type (medullary, superficial, localised, diffuse) with host status (A healthy, B compromised, C treatment worse than the disease).

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MRI accuracy depends on the setting. MRI with gadolinium contrast is the imaging modality of choice for osteomyelitis, but quote its accuracy with the setting attached, because MRI is not one test with one number. In acute osteomyelitis and the diabetic foot it performs well: pooled across 16 studies of foot osteomyelitis the diagnostic odds ratio was 42.1, with specificity 82.5% at a 90% sensitivity cut point, enough to rule the diagnosis both in and out and far ahead of bone scanning, radiography or white-cell studies.

In chronic osteomyelitis MRI degrades badly. Pooled across 23 studies its sensitivity was 84% but its specificity only 60% (95% CI 38-78%). Previous surgery, healed infection, metalwork and remodelled marrow all produce the same signal changes, so a positive MRI in a chronically infected limb means much less than the same scan in a previously untouched bone.

Where FDG-PET comes in. That collapse in specificity is the whole reason FDG-PET exists in this algorithm: in the same meta-analysis it achieved 96% sensitivity and 91% specificity for chronic osteomyelitis, the best of any single modality. These are level 1 evidence, from meta-analyses of diagnostic accuracy studies.

Marrow oedema. The most sensitive and least specific MRI finding. On T1 the normal bright fatty marrow is replaced by dark signal; on STIR or T2 fat-sat the marrow is bright. The same pattern is seen in tumour, stress fracture and contusion, so clinical correlation is essential.

Abscess and phlegmon. On contrast-enhanced T1 fat-sat images an abscess is a collection with ring enhancement, the wall enhancing and the centre not; phlegmon shows diffuse, homogeneous enhancement. The distinction is critical for surgical planning, because abscesses typically require drainage while phlegmon may respond to antibiotics alone.

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The penumbra sign. A thin rim of T1 hyperintensity, 1-2 mm wide, immediately surrounding an intraosseous abscess, the Brodie abscess (subacute staphylococcal osteomyelitis). The rim is granulation tissue at the interface between abscess and viable bone. It is specific for infection, and it is how a Brodie abscess is separated from a bone tumour. The three images that follow are the same case across modalities.

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Cortical erosion and sinus tract. Cortical destruction, an interrupted cortex with surrounding soft-tissue oedema and enhancement, is best assessed on contrast-enhanced T1 fat-sat images. A sinus tract is a linear enhancing track from the infected bone through the soft tissues to the skin surface, and it is specific for chronic osteomyelitis.

Children under 18 months. The growth plate still carries transphyseal blood vessels, which allow infection to cross from the metaphysis to the epiphysis and into the adjacent joint, so osteomyelitis and septic arthritis can occur together. After 18 months the growth plate acts as a barrier: the same infection in an older child or adult tends to spare the joint unless it ruptures through the cortex. A toddler with osteomyelitis needs the neighbouring joint examined.

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PJI imaging is challenging because metalwork creates significant artefact on MRI and the distinction between aseptic loosening and infection is often unclear on radiographs.

Radiographs. May show progressive osteolysis, periosteal reaction, endosteal scalloping or hardware loosening, but these signs are non-specific and may be identical in aseptic loosening. Rapid progressive osteolysis within the first 2 years raises suspicion for infection.

Triple-phase bone scan. Sensitive but not specific: it is positive for any cause of increased bone turnover, including normal post-operative remodelling for up to 12-18 months. Its pooled specificity in chronic osteomyelitis is 25%, which is why it is a screening test at best.

Combined labelled WBC and sulphur colloid scan. The most accurate nuclear medicine combination for PJI. Sulphur colloid is normally taken up by displaced marrow, so WBC accumulation at the prosthesis without corresponding sulphur colloid uptake indicates infection rather than normal marrow distribution. In 59 painful failed hip and knee replacements this achieved sensitivity 100%, specificity 91% and accuracy 95%; those are figures from a single small series, so treat the 100% as "very high" rather than absolute.

FDG-PET around a prosthesis. Do not weld the FDG-PET figures from two different settings into one claim, which is an easy way to lose a viva. The often-quoted 96%/91% pair belongs to chronic osteomyelitis, where FDG-PET is the single most accurate modality. Around a prosthesis it is a different and much weaker test: in the head-to-head comparison against combined leucocyte/marrow imaging, its best accuracy by any interpretive criterion was 71%, against 95% for the leucocyte/marrow study. FDG-PET is the right answer for chronic osteomyelitis and for the axial skeleton, and the wrong answer for confirming or excluding a prosthetic joint infection.

MRI with metal artefact reduction. Newer sequences (MARS, SEMAC, VAT) can partially overcome metalwork artefact. MRI may show periosteal oedema, fluid collections and synovitis, but interpretation requires expertise.

Joint aspiration remains the gold standard for PJI diagnosis; imaging supports but does not replace microbiological assessment. The synovial fluid goes for WBC count, differential and culture, and increasingly for alpha-defensin and leucocyte esterase strip testing.

Septic arthritis is a clinical and microbiological diagnosis confirmed by joint aspiration; imaging supports it and excludes adjacent osteomyelitis but never replaces the aspirate. Its role is to detect the effusion, guide the aspiration and assess for bone involvement.

Radiographs. Often normal early, and useful as a baseline and for late change. The early signs are soft-tissue swelling and an effusion, seen as joint-space widening and capsular distension (a displaced fat pad at the elbow, or a widened medial joint space at the hip in a child). Untreated, juxta-articular osteopenia follows, then joint-space narrowing from cartilage destruction, marginal and subchondral erosions, and ultimately bony ankylosis. Rapid joint-space loss compared with the contralateral side is the key sign.

Ultrasound is first-line. It is the most sensitive test for an effusion, shows synovial thickening and increased Doppler flow (hyperaemia), and guides the aspiration, which is the decisive investigation. It cannot reliably distinguish a septic from a sterile effusion, so the aspirate is essential.

MRI is for suspected bone or soft-tissue involvement. It shows the effusion (often complex or loculated), synovial thickening with avid enhancement after gadolinium, perisynovial soft-tissue oedema, and reactive marrow oedema in the adjacent bones, which may be reactive change or contiguous osteomyelitis; the discriminators in the mimics table apply. Pus may show restricted diffusion on DWI. MRI is the key test for "is there coexisting osteomyelitis?", which changes the surgical plan.

The irritable hip. In a child, ultrasound confirms the effusion and guides aspiration, and MRI helps exclude adjacent proximal-femoral or pelvic osteomyelitis. The Kocher criteria (non-weight-bearing, a history of fever, ESR of at least 40, WCC over 12,000) stratify the probability of septic arthritis against transient synovitis.

MRI evidence supports its role as the gold standard for osteomyelitis diagnosis.

Evidence guides the complex imaging approach to musculoskeletal infection.

Society guidelines anchor imaging selection to microbiological confirmation.