O&T: They Are Too Important To Miss

Question 1

Periosteum

Answer 1

• Produces new bone every minute and second, just that we do not know it is happening
• New calcium is deposited to existing skeleton (on periosteum), while old calcium is removed
• New bone is formed by:
  —> Child: Endochondral ossification
  —> Adult: Periosteum

Question 2

DDx of periosteal new bone formation

Answer 2

1. ***Callus formation after a Traumatic fracture
   - take at least 3-4 weeks to form
   Other types of fracture:
   - Pathological fracture: bone already weakened by underlying pathology
   - Stress fracture: repeated minor stress (need to ask history of occupation e.g. in solder, heavy stamping, marching metatarsal fracture)
2. ***Tumour
   - Primary malignant bone neoplasm (e.g. osteosarcoma)
   - Usually occurs first in the medullary canal of the metaphysis of long bone (∵ most actively growing part of bone in children)
3. ***Pus (infection)
   - Bone: osteomyelitis (not just ostitis but also myelitis as medullary cavity involvement is usually the most important)
   - Joint: septic arthritis
   - Routes:
   —> Haematogenous
   —> Direct inoculation
   —> Local spread, e.g. soft tissue abscess

Why does bacterial infection in medullary cavity cause periosteal new bone formation?
- Bacteria proliferate —> pus accumulates —> bone is rigid so the pressure inside medullary canal increases —> abscess is forced out the bone to subperiosteum area —> strip up the periosteum
—> But note: when the subperiosteal abscess has caused periosteal new bone formation, the infection is no longer acute, but instead subacute or chronic; hence, antibiotics alone will not suffice, surgery will be needed to remove all pathogens

4. ***Blood (subperiosteal haematoma)
   - Traumatic: blunt injury to blood vessels in bone accumulation of blood (haematoma formation) periosteal new bone formation
   - Bleeding tendency in haemophilia
   - Non-accidental injury in a child

Question 3

Radiological features of Bone tumour

Answer 3

1. Codman’s triangle
   - formed by **periosteum, not by the tumour
   - elevation of periosteum: because tumour gradually erodes cortical bone and grows out, but still under periosteum, so periosteum is **lifted up
   - tumour itself is invisible because it takes a long time to be mineralized and ossified, but it has already stripped up the periosteum which gives a ***triangular shaped shadow below it
   - non-specific sign, but should raise the suspicion (can’t afford to miss)
2. Sunray / Sunburst appearance
   - radiating spicules of new bone = new bone formed by ***tumour itself (specific to osteosarcoma but late, so don’t rely on this for diagnosis)
3. Heterogenous (lytic / sclerotic), ill-defined border, EXPANSILE
4. Thinned cortex (from erosion)
5. Any pathological fracture, soft tissue swelling

DDx:
- **Primary / **Secondary bone tumour
- **Infection
- **Myeloma (if pure lytic)
- ***Brown’s tumor (Osteitis fibrosa cystica) (hyperPTH)

Complications:
- Pathological fracture
- Malignant hypercalcemia

(From JC O/T:
1. Infection
- ***Osteomyelitis
- esp. TB

2. Metabolic disorders
   - **Gout
   - Paget’s disease
   - **Renal osteodystrophy
3. Endocrine disorders
   - **Hyperparathyroidism
   - **Cushing’s disease)