MSK Growth/Injury and repair – bone

Question 1

What are the differences between cortical and cancellous bone?

Answer 1

Cortical bone – diaphysis, resists – bending + torsion, laid down circumferentially, less biologically active.
Cancellous bone – metaphysis, resist/absorbs – compression, site of longitudinal growth, very biologically active

Question 2

Describe what is meant by a fracture

Answer 2

A fracture is break in structural continuity of bone. May be a crack, split, crumpling, buckle.

Question 3

Why may bones fail?

Answer 3

High energy transfer in normal bones – takes a lot of energy.
Repetitive stress in normal bones – stress fracture.
Low energy transfer in abnormal bones – osteoporosis, osteomalacia, metastatic tumour, other bone disorders.

Question 4

Describe what is involved in stage 1 of fracture repair

Answer 4

Inflammation stage, Begins immediately after fracture.
Haematoma and fibrin clot
Platelets, PMN’s, neutrophils, monocytes, macrophages.
By products of cell death – lysosomal enzymes.
Involves:
- Fibroblasts
- Mesenchymal and osteoprogenitor cells
- Angiogenesis

Question 5

What are the mesenchymal and osteoprogenitor cells (stage 1 fracture repair)?

Answer 5

Mesenchymal: Transformed endothelial cells from medullary canal and/or periosteum.
Osteoprogenitor: Osteogenic induction of cells from muscle and soft tissues.

Question 6

Describe what is involved in stage 2 fracture repair

Answer 6

Begins when pain and swelling subside. Lasts until bony fragments are united by cartilage or fibrous tissue. Some stability of fracture although Angulation can still occur. Continued increase in vascularity.

Question 7

Describe what is involved in stage 3 fracture repair

Answer 7

Conversion of cartilage to woven bone. Typical long bone fracture – endochondral bone formation, membranous bone formation. Increasing rigidity – “secondary” bone healing, obvious callus (bony healing tissue).

Question 8

Describe stage 4 fracture repair

Answer 8

Conversion of woven bone to lamellar bone. Medullary canal is reconstituted. Bone responds to loading characteristic Wolff’s Law (bone in a healthy person or animal will adapt to the loads under which it is placed.)

Question 9

What is the role of strain in fracture healing?

Answer 9

Degree of instability is best expressed as magnitude of strain (% change of initial dimension). If strain is too low mechanical induction of tissue differentiation fails. Too high and healing process does not progress to bone formation.

Question 10

What are the different categories of abnormal healing of bone?

Answer 10

Delayed union – failure to heal in expected time. Non-union – failure to heal.

Question 11

What may be the reason for delayed union?

Answer 11

High energy injury. 
Distraction (increasing osteogenic jumping) 
Instability 
Infection 
Steroids
Immunosuppressant 
Smoking 
Warfarin 
NSAIDs
Ciprofloxacin

Question 12

What can cause non-union?

Answer 12

Failure calcification fibrocartilage.
Instability – excessive osteoclasts. 
Abundant callus formation. 
Pain + tenderness. 
Persistent fracture line.
Sclerosis.

Question 13

What would be the approach to delayed healing?

Answer 13

Consider alternative treatment:

• Different fixation
• Dynamisation
• Bone grafting