Musculoskeletal infections

Question 1

Septic arthritis

Answer 1

• Diagnosis is clinical.
• Acute mono-articular arthritis, it should be suspected.
• Confirmed with arthrocentesis and counting White blood cell count and percentage of polymorphonuclear neutrophils.
• PMN >90% and WBV > 50.000.

Question 2

Osteomyelitis

Answer 2

• Inflammation of the bone caused by an infectious organism.
• Route of spread: Hematogenous, contiguous or direct inoculation.
• Chronicity: Acute or chronic.
• <2 weeks: Acute.
• Pathologic hallmark: Bone necrosis.
• Distribution: Depends on age. Adults: contigous or direct inoculation (polymicrobial). Children: hematogenous (S. aureus).

Question 3

Osteomyelitis progression

Answer 3

• Initial nidus of infection, usually metaphyseal.
• Brodie abscess formation.
• Sinus tract, sequestrum, involucrum and subperiosteal infection.

Question 4

Sequestrum

Answer 4

Piece of necrotic bone that is separated (sequesterated) from viable bone by granulation tissue. Needs to be resected.

Question 5

Involucrum

Answer 5

Living bone surrounding necrotic bone.

Question 6

Hematogenous osteomyelitis

Answer 6

• Often seen in infants and children.
• Mataphyses are most commonly affected.
• <12 months infections can involve the metaphysis, epiphysis and the joint.
• In older children the infection is contained to the metaphysis.
• Typically caused bt a single organism.
• Initial infection is intra-medullary.
• It later expands through the cortex and uplifts the periosteum.
• In adults most commonly affects the spine.
• Chronic osteomyelitis may produce: sequestrum, involucrum, perforation by cloacae, sinus tract to the skin predisposes to squamous cell carcinoma
• Imaging: Focal, ill-defined lucent metaphyseal lesion. Periosteal reaction appears agressive and lamellated, mimicking Ewing sarcoma.

Question 7

Contiguous focus osteomyelitis

Answer 7

• Most typical form for adults.
• Soft tissue infection, periosteal and cortical penetration and then medullary invasion.
• Most common cause: Diabetic foot ulcer.
• Common sites: Calcaneus, first and fifth metatarsals and first phalanx.
• Polymicrobial.

Question 8

Brodie abscess

Answer 8

Characteristic lesion of subacute osteomyelitis, consisting of a walled-off intra-osseous abscess surrounded by granulation tisue and sclerotic bone.

Question 9

Chronic ostelmyelitis

Answer 9

• > 6 weeks.
• Devascularized bone leads to sequestrum, granulation tissue and involucrum.
• Can cause a mixed lytic and sclerotic appearance, with thickened cortex.
• Sclerosing osteomyelitis of Garre: Sclerosis and thickening of bone. DD: Lymphoma, sclerotic MX and osteoid osteoma.

Question 10

Pott disease

Answer 10

Osteomyelitis of the spine caused by Mycobacterium tuberculosis.

Question 11

Osteomyelitis on MRI

Answer 11

• Hallmark: Replacement of normal fatty marrow signal with edema and exudates.
• T1 low-signal and T2 high-signal.
• Three patterns: Confluent intramedullary (Geographic area involving medullary canal) (OM), hazy reticular (Interspersed areas of normal marrow) (Not OM) and subcortical (The linear signal change subadjacent to the cortex) (Can’t reliably diagnose OM).

Question 12

Necrotizing fasciitis

Answer 12

• Extremely agressive infection caused by Clostridium or other gram-positive rods.
• Surgical emergency, requires immediate debridement.
• CT and RX: Gas bubbles in soft tissues.

Question 13

Pyomyositis

Answer 13

• Bacterial infection of the muscle.
• Also called Tropical myositis.
• Diabetes is a risk factor.
• Staphylococcus aureus causes most of cases.
• CT: Geographic, irregular-shaped, low attenuation collection in the muscle.