95 - Bone Pathology

Question 1

Cells involved in bone mechanotransduction

Answer 1

Osteocytes

Question 2

Appearance of osteoblasts

Answer 2

Resting - flat, look like fibroblasts

Active - large, densely basophilic

Question 3

Location of RANKL

Answer 3

On surface of osteoblasts

Question 4

Location of osteoprotegrin (OPG)

Answer 4

Released by osteoblasts

Question 5

Role of PTH

Answer 5

Stimulates osteoblasts to increase RANKL, decrease OPG.

Doesn’t directly stimulate osteoclasts.

Question 6

What is osteoid?

Answer 6

Organic matrix laid down by osteoblasts in cartilage.

Contains type 1 collagen, other proteins, some GAGs, growth factors, cytokines

Question 7

Basic multicellular unit of bone

Answer 7

Module of remodelling.
Osteoclasts in front, osteoblasts behing.
Capillary in the middle.
Makes a tunnel of bone, rebuilds a new osteon (lamellar bone, not woven bone)

Question 8

Do osteoblasts release calcium and phosphate into osteoid?

Answer 8

Not directly.

Release enzymes which increase retention of calcium and phosphate.

Question 9

What can commonly cause a Colle’s fracture

Answer 9

Fall on outstretched hand

Question 10

Comminuted fractures

Answer 10

When there are more than two pieces of bone

Question 11

Stages of fracture healing
1 a, b
2 a,b
3

Answer 11

1. Inflammatory phase:
   a) bleeding and haematoma formation
   b) vascular granulation tissue
2. Reparative phase:
   a) Soft callus (mostly cartilage)
   b) Hard callus (woven bone)
3. Remodelling phase

Question 12

Inflammatory phase of bone healing

Answer 12

• Haematoma formation
– fibrin mesh creates a framework
– damaged matrix releases trapped growth factors and cytokines
– stem cells activated to start repair
– stem cells differentiate very early depending on local stability: osteoblast > chondroblast > fibroblast
• Granulation tissue formation
• Hours to days

Question 13

What determines stem cell differentiation in a fracture?

Answer 13

Stability.
If the break is very stable, most will differentiate to osteoblasts.
If very unstable, most will become fibroblasts

Question 14

Soft callus stage of fracture healing

1-3

Answer 14

• Cartilage formation
– Holds the fractured ends together but no structural rigidity (so don’t want to weight-bear with it)
• Periosteum repairs itself over the outside
• Days to weeks

Question 15

Hard callus stage of fracture healing

1-3

Answer 15

• Osteoid formation and ossification
– woven bone
– endochondral ossification (same as in growth plates)
• A thickened area of woven bone (‘fusiform’, as bulges out from surrounding bone)
– rigid, but not as strong as it was when intact
• Weeks to months

Question 16

Bone remodelling stage of fracture healing

1-3

Answer 16

• Bone remodelling
– osteoclasts followed by osteoblasts
– woven bone to lamellar bone
– along lines of stress
• Result: “completely reconstituted”
• Months to years

Question 17

Ways to promote union of fracture

1-3

Answer 17

– Minimise the gap (“reduction” of the fracture)
– Minimise the strain or movement (“fixation”)
– Minimise any other factors that slow healing
• e.g. poor blood supply, infection, smoking

Question 18

Non-union

Answer 18

A compilation of fracture healing.

Can get pseudoarthrosis (cells differentiate into synovial sheath around non-union bone)

Question 19

Malunion

Answer 19

Healing of a bone in an unacceptable position

Question 20

Infection of fracture

Answer 20

Can lead to osteomyelitis.
Very severe
Disruption of vessels in periosteum/haversian canals lead to infected fragments of necrotic bone. 
Can cause sepsis.
Often S aureus

Question 21

Osteonecrosis

Answer 21

Fractures can interrupt the blood supply and leave part of the bone ischaemic.
EG: neck of femur, scaphoid

Question 22

Examples of complications of bone healing

Answer 22

Non-union
Malunion
Infection of fracture
Osteonecrosis

Question 23

Process by which soft callus changes to hard callus

Answer 23

Endochondral ossification

Question 24

Which cells from normal bone form a soft callus

Answer 24

Osteoprogenitor cells

Question 25

At which stage in fracture healing do osteoprogenitor cells decide which cell to differentiate into?

Answer 25

Inflammaotry phase - haematoma