Lecture 2 - Bone

Question 1

Bone function

Answer 1

• support
• protection
• muscle attachment
• calcium store
• produce cells
• enable hearing

Question 2

Osteon structure

Answer 2

• Osteon = functional unit
• lamellae = concentric rings of bone tissue (layered)
• cement line = outer rings of bone tissue
• haversian canal = blood vessels run down centre (50-100um)
• lacunae = holes for osteocytes
• canaliculi - osteocyte communication
• Volkmann’s canal = perpendicular blood vessels

Question 3

Bone content

Answer 3

Organic = tensile strength, flexibility and durability

• Collagen I (90%)
• fibrous proteins
• GAGs

Mineral content = compressive strength
- hydroxyapatite

Question 4

Osteocytes

Answer 4

• within bone matrix
• communicate with canaliculi
• long-lived (several years)
• mechano-sensing
• maintenance: calcium homeostasis and microcrack detection

Question 5

Osteoblasts

Answer 5

• lay down bone (osteoid - type I collagen matrix) + apatite
• make collagen fibres and PGs
• can become osteocyte

Question 6

Osteoclasts

Answer 6

• remove bone: secrete H+ to dissolve mineral
• secrete collagenase to clear protein
• multinucleated

Question 7

Basic multicellular units (BMUs)

Answer 7

• bone lining cells (SC on endosteum)
• osteoclasts
• osteoblasts

Question 8

Bone turnover process

Answer 8

• damage
• bone lining cells recruit osteoclasts
• osteoclasts resorb bone
• osteoclast apoptosis
• osteoblasts fill pit with osteoid
• mineralisation of matrix
• maturation of matrix

Question 9

Absorbed energy on graph

Answer 9

area under stress-stain curve

Question 10

Factors affecting bone mechanical properties (4)

Answer 10

1. Loading rate
   - E increases with loading rate
   - ductile-brittle transition
   - bones adapted to absorb max energy
2. Orientation
   - collagen fibres aligned longitudinally
   - stronger in compression
   - stronger longitudinally
3. Creep
   - strain changes with constant load
   - viscoplastic - depends on strain and history of strain
4. Age
   - > stiffness
   - > strong (UTS > by 2%/decade)
   -

Question 11

Wolff’s Law

Answer 11

Bone will adapt to the loads placed on it

Question 12

Measuring bone mineral content (BMC)

Answer 12

DEXA - dual energy xray absorptiometry

Question 13

Trabecular bone adaptation

Answer 13

direction of trabeculae align with principal stress

Question 14

Osteoporosis

Answer 14

• affects 50% > 75yrs
• £1.7 bn/yr to NHS
• decrease in BMD (<2.5 std dev below peak bone mass in healthy 20 yr)
• > E (brittleness)
• disruption of trabecular structure
• weaker bone –> increase fracture risk

Question 15

Osteoporosis risks

Answer 15

• age: decrease estrogen and testosterone
• genetic
• diet: malnutrition - vit D deficiency
• inactivity

Question 16

Osteoporosis treatments

Answer 16

• calcium supplements
• bisphosphonates
• estrogen replacement therapy
• exercise

Question 17

Types of fracture

Answer 17

• compression
• segmental
• commuted
  Simple:
• transverse
• spiral
• oblique

Question 18

What methods are used to stabilise a fracture?

Answer 18

• Plaster cast (simple, closed)
• Nails, screws, plates, wires (complex, open)
• External fixation: control position of independent segments

Question 19

Fracture healing

Answer 19

1. Hematoma - blood clot
2. Granulation tissue - loose connective tissue (2-3 days)
   cells in blood clot die, fibroblasts live
   small vessel formation
3. cartilage forms (3 weeks)
   chondroblasts migrate to fracture gap
4. bone formation (6-12 weeks)
5. Remodelling (12-16 weeks)

Question 20

Bone hierarchical structure

Answer 20

Each structural element itself contains a structure:

• cortical bone is made up of osteons
• osteons are made up of fiber arrays
• fiber arrays are made up of mineralised collagen fibrils (crosslinking adds strength to overall structure)
• collagen fibrils are made up of individual collagen molecules