Review of bone and fracture biology

Question 1

Composition of bone

Answer 1

Structural composite
- 70% organic, 25% inorganic, 5% cells
- 90% organic –> type I collagen
- inorganic component is hydroxyapatite

Collagen is stronger in tension
HA is stronger in compression

Question 2

Osteoblasts

Answer 2

Mesenchymal cell origins
Express osteoblastic markers (ALP, OC)
Deposit mineralised matrix
Morphology varies from flattened to cuboidal
Mature cells become encased in bone (osteocytes)

Question 3

Osteoclasts

Answer 3

Formed from circulating mononuclear precursors
Multinucleate cells capable of resorbing bone
Acid phosphatase (TRAP), cathepsin K

Question 4

Two mechanisms of bone formation

Answer 4

Endochondral ossification - get longer

Intramembranous ossification - get wider

First tissue to be formed is woven bone

Both forms can lead to formation of compact or cancellous bone

Question 5

Endochondral ossification

Answer 5

For the formation of short and long bones of the appendicular skeleton

Hyaline cartilage model is sequentially resorbed and then replaced with bone

Question 6

Longitudinal bone growth

Answer 6

Cartilage grows at epiphyseal growth plate

Cartilage replaced by bone

Question 7

Intramembranous bone formation

Answer 7

The direct conversion of mesenchymal tissue into bone.

Condensation - differentiation - mineralisation

Question 8

Circumferential bone growth

Answer 8

Appositional bone growth occurs by direct deposition of bone at the periosteal surface

At the same time, bone is resorbed from the inner (endosteal) surface to cause the marrow cavity to increase in size

Question 9

Gross structure of bone

Answer 9

Compact or cancellous ( sponge like in a hard ‘shell’)

Question 10

Microscopic structure of bone

Answer 10

Unmineralised (osteoid)

Over time becomes mineralised.

Mineralised bone is either woven (immature), or lamellar bone (mature)

Question 11

Diaphysis

Answer 11

Shaft of long bone

Question 12

Epiphysis

Answer 12

End of the long bone, separated by physis

Question 13

Metaphysis

Answer 13

Region between diaphysis and epithysis

Question 14

Cancellous bone

Answer 14

Plates of extensively interconnected bone
Increased porosity and increased surface area compared to cortical bone
Increased metabolic activity, so a sentinel for systemic disease

Question 15

Vascular supply to bones

Answer 15

Nutrient artery - enters at foramen, branches proximal and distal to endosteal surfaces

Metaphyseal vessels - arise from periarticular tissues, primary supply to metaphyseal cancellous bone

Periosteal capillaries - arise from soft tissues and attach to bone

De-vascularising during surgery slows healing

Question 16

Sequence of bone turnover

Answer 16

Resorption
Osteoblast recruitment
Osteoid recruitment
Osteoid formation
Quiescence
Activation
Osteoclast recruitment

Question 17

Bone remodelling difference between cortical bone and cancellous bone

Answer 17

Cortical bone - occurs in haversian canal

Cancellous bone - occurs on the trabecular surface

Question 18

Bone turnover vs age

Answer 18

Skeletally immature animal: formation»resorption
Both longitudinal and appositional growth

Skeletally mature: formation = resorption

Endocrine changes can cause bone loss - earlier in females

Question 19

What drives bone remodelling

Answer 19

mechanical loading
trauma
microfracture
systemic hormones

Question 20

Wolff’s law

Answer 20

Bone will align along the line(s) of stress

Question 21

Stages of fracture healing

Answer 21

Inflammatory, repair, remodelling

Question 22

Mechanical stability

Answer 22

Optimal - plated osteotomy
Good - external fixator
Acceptable - IM pin, cast

Question 23

Primary bone healing

Answer 23

Abscence of callus
Contact healing (full apposition with plated repairs) or gap healing (apposed within 1mm)

Question 24

Secondary healing

Answer 24

Fragments are not close and so heal with a callus
Not necessarily worse - can lead to a stronger repair

Question 25

Downsides of rigid fixation

Answer 25

Plates and screws decrease cortical vascularity
Plates and screws may need to be removed
Plates affect load transer through bone (‘stress shielding’)
Remodelling can take much longer
Can act as an opening for infection