A. IMAGING FINDINGS

Given the potential lack of specific symptoms and laboratory test results, diagnostic imaging plays a crucial role in many patients. Indications and disadvantages of the different imaging techniques are summarized in Table 1. 

Table 1: Table 1. Indications and disadvantages of the different imaging techniques to...

1. Conventional radiography (CR)

The technique of choice for initial evaluation is conventional radiography. Radiographs lag behind onset of infection by 10-14 days; during this period, only soft tissue swelling and osteoporosis can be seen. (Figure 5)

Tip: The first step is to consider the differential diagnosis of fracture or bone tumor.

Trick: CR are useful for assessing the progression of the disease, by comparing follow-up films with the initial radiograph.

Fig 5: Figure 5. Osteomyelitis in Conventional Radiography. (A) A 55-year-old woman...

2. Ultrasound (US)

Its use is limited as it cannot assess bone properly. However, on infants and children, it has high sensitivity for detection of periosteal elevation, abscesses, sinus tracts and deep venous thrombosis. (Figure 6)

Tip: Soft tissue edema is seen as areas of hypervascularity around the affected bone on color Doppler.

Trick: Useful for US-guided biopsy/aspiration (Analysis of joint effusion, abscess drainage).

Fig 6: Figure 6. Osteomyelitis in Ultrasound. (A) A 4-year-old boy with fewer and...

3. Computed Tomography (CT)

CT has superior bony resolution to MRI and is better at demonstrating osseous changes and intraosseous gas. (Figure 7)

Fig 7: Figure 7. Osteomyelitis in CT. (A) 11-year-old girl with leg pain after acute...

Findings: Blurring of fat planes, increased density of fatty marrow, periosteal reaction, cortical erosion/destruction, sequestrum, involucrum, intraosseous gas. 

Tip: Dual-energy CT with virtual non-calcium (absence of suppression) and Z effective maps (reduced effective atomic number) can aid in the detection of bone marrow edema. (Figure 8)

Fig 8: Figure 8. Osteomyelitis in Dual-CT. 14-year-old boy with leg pain and fever. CT...

4. Magnetic Resonance (MR)

MRI is the modality of choice for bone and soft tissue evaluation. Use of T1 and T2 FS or STIR sequences is recommended. 

4.1 Acute/Subacute OM

Bone marrow edema (Low T1W, High T2W) is the earliest feature of acute OM (can be detected 1 to 2 days after the onset) (Figure 9). We can also see abscesses (intraosseous, subperiosteal or soft tissue) which could have a thin rim of intermediate T1W surrounding the abscess (hypervascular granulation tissue) with enhancement after contrast administration (Penumbra sign). When intraosseous and in subacute OM, it´s called Brodie’s abscess (more frequent at the metaphysis of long bones). (Figure 10)

Findings: Bone marrow edema, heterogeneous enhancement, periostitis, intraosseous, subperiosteal and soft tissue abscess.

Tip: The penumbra sign and can help differentiate an abscess (drainable) from a phlegmon. 

Trick: Comparison with normal marrow signal (contralateral bones) can be useful for detecting edema.

Fig 9: Figure 9. Acute osteomyelitis in MRI. (A) A 13-year-old boy with sickle cell...

Fig 10: Figure 10. Subacute osteomyelitis in MRI. (A) A 4-year-old boy with fewer and...

4.2 Chronic OM

In chronic OM the key findings are sequestrum and involucrum. The disruption of the intraosseous and periosteal blood supply leads to formation of a necrotic bone fragment, known as a sequestrum, which is surrounded by pus and granulation tissue. The involucrum is reactive new bone formed around the sequestrum. A cloaca (seen on both acute and chronic OM) is a defect in the cortex that allows pus to drain from the medullary cavity into the subperiosteal space. (Figure 11)

Findings: Sequestra, cloaca, cortical destruction and the thickness of the involucrum. 

Tip: The absence of cortical hickening helps to differentiate acute from chronic OM. 

Trick: Gadolinium-enhanced sequences help to outline zones of necrosis and are useful to detect abscess.

Fig 11: Figure 11. Chronic osteomyelitis in MR (A) Chronic osteomyelitis with...

4. Nuclear medicine techniques (PET-CT)

PET/CT Distinguishes between active infection (uptake) and healing/chronic infection or post-surgical changes (no uptake). Due to that, it has more diagnostic accuracy than bone scintigraphy or leukocyte scintigraphy. (Figure 12)

Tip: Specially useful in diabetic foot

Fig 12: Figure 12. Osteomyelitis in PET/CT. (A) 60-year-old paraplegic man with left...

B. SPECIAL CASES

1. Diabetic foot

Appearances of diabetes-related OM are discussed, with emphasis on evaluating the extent of soft-tissue infection and differentiating neuroarthropathy with superimposed OM from that without a superinfection. (Figure 13)

Findings:

• Neuropathic Osteoarthropathy: subarticular bone marrow edema, subluxation, cysts, necrotic debris.
• OM: Bone marrow edema that affects the entire bone. Secondary signs: cortical destruction, cutaneus ulcer, abscess, cellulitis, sinus tract. 

Tip: 

• Neuropathic Osteoarthropathy: It affects multiple joints, specially in the midfoot (Lisfranc, Chopart joints)
• OM: It affects metatarsal heads, toes, calcaneus, malleoli

Trick: Normal marrow signal intensity on all pulse sequences excludes OM.

Fig 13: Figure 13. Pedal infection. A 55-year-old diabetic male had right great toe...

2. Chronic recurrent multifocal osteomyelitis (CRMO)

CRMO is an idiopathic inflammatory bone disorder of unclear origin. It is often a diagnosis of exclusion after infection and neoplasia have been ruled out. It affects paediatric population (5-15 yo) with female predominance. (Figure 14)

Findings: Mixed ill-defined lucent and sclerotic lesions, non-aggressive periosteal reaction. 

Tip: Typically multifocal and symmetric and most frequently located in tibial metaphysis.

Trick: Blood tests and biopsies tend to be negative. No response to antibiotics.

Fig 14: Figure 14. Chronic recurrent multifocal osteomielitis. A 12-year-old girl with...

3. Periprosthetic infections

Most prevalent in hip arthroplasties (around 1%). Risk factors include underlying infection, previous surgery, immunosuppression, inflammatory arthropathies, obesity, IV drug use, and alcoholism. (Figure 15)

Findings: periosteal reaction, wide band of radiolucency (bone resorption) at the metal-bone interface, patchy osteolysis, implant loosening, intraosseous abscess.

Tip: Plain radiographs are the main imaging method.

Trick: Using monochromatic Dual-CT with or without MAR (Metal artifact reduction) software can reduce metal artifacts.

Fig 15: Figure 15. Periprosthetic infection. Ninety-year-old woman with left hip pain...

B. DIFFERENTIAL DIAGNOSIS (DD)

A trick to make DD in suspicion of OM is to assess two patterns:

1. Bone destruction in children (Figure 16)

1. Langerhans cell histiocytosis: Tends to be centered on the diaphysis while haematogenous OM tends to originate in the metaphysis.
2. Leukemia: Diffuse osteopenia is the most frequent finding.
3. Ewing sarcoma and osteosarcoma: OM tends to cause more rapid destructive change compared to malignant bone tumours. Codman´s triangle could be seen also in OM.

Fig 16: Figure 16. DD in OM. (A) A 14-year-old girl left knee pain. A) Plain radiograph...

2. Bone marrow edema (Figure 17)

1. Stress fracture: Look for fracture lines.
2. Rheumatoid Arthritis: In RA erosions, soft tissue swelling and osteoporosis are more commonly seen.
3. Bone marrow edema syndrome: It is transient and self-limiting.
4. Stress reaction: There is no evident fracture line or inflammatory changes in the surrounding soft tissue
5. Complex regional pain syndrome: The bone edema is patchy and mainly subcortical.
6. Osteoid osteoma:  It may appear similar to a sequestrum. Osteoid osteomas are usually round whereas sequestra are irregularly shaped. On postcontrast sequences, osteoid osteomas will enhance avidly while sequestra do not enhance.
7. Bone tumours: OM tends to have a faster course compared to malignant bone tumours. Abscesses demonstrate peripheral rim enhancement whereas tumours usually enhance heterogeneously.

Fig 17: Figure 17. DD in OM. (A) Trabecular bone fracture (pink) with surrounding edema...

C. SYSTEMATIC APPROACH

Systematic and thorough evaluation of findings should be performed to reduce patient morbidity and mortality. (Table 2)

Table 2: Table 2. Systematic approach to Osteomyelitis.