Bone injury, h ealing and grafting

Question 1

Perren’s strain theory of fracture healing

Answer 1

Fracture healing is dependent on its strain environment
Fracture gap strain is defined as the relative change in the fracture gap divided by original fracture gap
Fracture strain decrease with
I. Larger fracture gap eg: Metaphyseal fracture (larger bone diameter)
Ii. Multifragmentary or segmental fracture (Overall strain is shared among the individual fragments)
In absolute stability: Fracture site strain is low, inhibit callus formation and allow direct Haversian remodelling
In relative stability (Intramedullary fixation/ splint): callus is required to stiffen the fracture site before hard woven bony callus forming and replacing

Question 2

Physiology of Primary healing

Answer 2

Anatomical reduction
Interfragmentary compression
Absolute stability

Question 3

Physiology of Secondary healing

Answer 3

Relative stability
Periosteal bony callus (Intramembranous ossification)
Fibrocartilaginous bridging callus (endochondral ossification)

Question 4

Stages of callus formation

Answer 4

Stage 1: Haematoma and inflammation
Haematoma formation
Migration of inflammatory cells
Bone resorption mediated by osteoclast

Stage 2: Soft callus (1-4 weeks)
Intramembranous callus: Primary callus. Type 1 collagen laid down from periosteal osteoblast
Endochondral callus: Bridging external callus

Stage 3: Hard callus (1-4 months)
Soft calcified chondroid callus becomes hard mineralised osteoid callus
Painless and solid
Fracture united

Stage 4 (Remodeling)
Hard callus remodelled from woven bone to hard, dense lamellar bone by osteoclastic resorption and osteoblastic bone formation
Bone assumes configuration and shape based on stresses acting on it (Wolff’s law)
Osteoclastic activity predominant on electropositive tension side
Osteoblastic activity on electronegative compression side

Question 5

Hypertrophic nonunion
Atrophic non union

Answer 5

Hypertrophic nonunion
Good blood supply, but excessive strain at fracture site

Atrophic non union
Poor blood supply due to soft tissue damage, periosteal stripping, fracture comminution

Question 6

Properties of bone graft

Answer 6

Osteogenicity
Contains living precursor cells
Capable of differentiation into bone

Osteoconduction
Provides 3-D scaffold that support in growth of capillaries, perivascular tissue and osteogenic cell precursors

Osteoinduction
Provides biological stimulus that stimulates mitosis and differentiation of undifferentiated mesenchymal cells into osteoprogenitor cells

Question 7

Genetics of bone grafts

Answer 7

Autograft
Same individual

Allograft
Another individual of same species

Xenograft
Different species