2. MSK (Infection)

Question 1

Osteomyelitis features (5)

Answer 1

Radiographs will be normal for 7-10 days.
Can have any appearance, any location, any age.
Children get haematogenous spread to long bones (metaphysis).
Adults more likely direct spread (diabetics).
Generally, septic joints more common in adults, osteomyelitis more common in kids.

Question 2

Osteomyelitis in spine

Answer 2

IVDU

Question 3

Osteomyelitis in spine with kyphosis

Answer 3

Gibbus deformity (T)

Question 4

Unilateral Sacroilitis

Answer 4

IVDU

Question 5

Psoas muscle abscess

Answer 5

TB

Question 6

Hallmarks of osteomyelitis

Answer 6

Destruction of bone and periosteal new bone formation

Question 7

Brodie’s abscess (2)

Answer 7

Bone abscess due to chronic infection.
Well circumscribed, may have osseous sequestrum (piece of necrotic bone surrounded by granulation tissue).

Question 8

Bony sequestration DDx (4)

Answer 8

Osteomyelitis
EG
Lymphoma
Fibrosarcoma

Question 9

Involucrum definition

Answer 9

Thick sheath of periosteal bone around sequestrum

Question 10

Cloaca definition

Answer 10

Space/tract where the dead bone exists.

Question 11

Acute bacterial osteomyelitis - types (3)

Answer 11

3 different categories
- Haematogenous seeding (most common in child)
- Contiguous spread
- Direct innoculation of bone from surgery or trauma

Question 12

Acute bacterial osteomyelitis - trivia (4)

Answer 12

Prefers long bones, specifically metaphysis (best blood supply)
Age <1 month: Multicentric involvement, often with joint involvement (bone scan often negative)
Age <18 months: Spread to epiphysis by blood
Age 2-16 years: Trans-physeal vessels closed (primary focus is metaphysis)

Question 13

Acute bacterial osteomyelitis - spread (3)

Answer 13

In older babies (<18 months), vessels from metaphysis to epiphysis atrophy, growth plate stops the spread (although spread can still occur), creating septic tank effect.
Similar effect seen in certain cancers e.g. leukaemia.
Once growth plate fuses, this obstruction is no longer present

Question 14

Acute bacterial osteomyelitis - imaging (2)

Answer 14

MRI: Low T1 signal in bone marrow adjacent to ulcer or cellulitis is diagnostic.
Ghost sign: Poor definition of margins of bone on T1 with reappearance on T2 or post contrast.

Question 15

Discitis/osteomyelitis - spread/cause (5)

Answer 15

Infection of the disc and infection of the vertebral body often go together.
This is due to route of seeding, often involved seeding of the vertebral endplate (vascular) and subsequent eruption and crossing into disc space and eventually adjacent vertebral body.
Adults: source usually from recent surgery, procedure or systemic infection.
Kids: Usually haematogenous spread.
Staph A is most common bug, IVDU get gram negatives.

Question 16

Discitis/osteomyelitis - imaging (3)

Answer 16

Early is hard to see on XR, needs MRI.
Paraspinal and epidural inflammation, T2 bright disc signal and disc enhancement.
Gallium superior to WBC scan in the spine.

Question 17

Pott’s disease (7)

Answer 17

TB of spine. More common in developing countries.
Tends to spare disc space.
Tends to have multilevel thoracic “skip” involvement
Buzzwords
- Large paraspinal abscess
- Calcified psoas abscess
- Gibbus deformity (destructive focal kyphosis)
Mimic: Brucellosis (unpaseurised milk) can also have disc space preservation.

Question 18

Septic arthritis - pathology/cause (6)

Answer 18

Mostly large joints with abundant blood supply to metaphysis (shoulder, hip, knee).
IVDU get it in SI and sternoclavicular joint.
Risk factors
- Old age
- AIDS
- RA
- Prosthetic joints

Question 19

Septic arthritis - imaging (3)

Answer 19

Plain film: May see joint effusion
MRI: Synovial enhancement.
Pneumoarthrogram sign: Air within a joint: Can exclude joint effusion and therefore septic joint.

Question 20

Necrotizing fasciitiis (6)

Answer 20

Kills quickly, but rare.
Often seen in HIV, Transplant, Diabetic and Alcoholics.
Usually polymicrobial.
Gas only seen in minority of cases, but is highly suggestive.
Diffuse fascial enhancement on cross sectional imaging.
Fournier Gangrene is Nec Fasc in the scrotum.

Question 21

TB - trivia (5)

Answer 21

<5% of TB patients have MSK involvement.
Vertebral body involved with sparing of disc space until late disease.
Gibbus deformity - focal kyphosis seen in Pott’s disease
Rice bodies - sloughed, infarcted synovium seen in end stage RA and TB joint infections.
Tuberculous Dactylitis (Spina Ventosa) - typically affects kids > adults, involvement of short tubular bones of hands and feet.
- Often smoldering infection without periosteal reaction.
- Classic look is diaphyseal expansile lesion with soft tissue swelling.