Chapter 48: Bone Grafts and Substitutes

Question 1

What lineages of differentiation are possible for mesenchymal stem cells?

Answer 1

• Osteoblastic
• Chondroblastic
• Adipocytic
• Tenocytic
• Myoblastic

Question 2

Where are the most accessible sources of mesenchymal stem cells?

Answer 2

• Cambium layer of the periosteum
• Bone marrow
• Fat

Question 3

What are the three mechanisms via which growth factors act on target cells?

Answer 3

Autocrine: influences on cell of identical or similar phenotype in local environment
Paracrine: Influences seen in adjacent cells of different phenotype
Endocrine: Influences seen in different and remote populations of cells

Question 4

List the (5) main growth factors in relation to bone production:

Answer 4

Transforming growth factor-Beta (TGF-B)
Bone morphogenic proteins (mostly BMP-2, -4, -7)
Fibroblast growth factor
Insulin-like growth factor
Platelet-derived growth factor

Question 5

In what tissues is TGF-B most commonly found?

Answer 5

• Bone
• Platelets
• Cartilage

Question 6

From what superfamily are bone morphogenic proteins (BMP) part of?

Answer 6

Transforming growth factor family

Question 7

What is the main role of fibroblast growth factor?
What are the most abundant forms in the adult?

Answer 7

• Critical role in angiogenesis and proliferation of mesenchymal stem cells (embryologically important)
• FGF-1 and FGF-2

Question 8

What is the role is insulin-like growth factors?

Answer 8

• Facilitate bone and limb lengthening and cellular proliferation
  CRITICAL to skeletal development

Question 9

Where is platelet-derived growth factor released from?

Answer 9

The alpha granules of platelets

Question 10

What is gene therapy?
How it is delivered?

Answer 10

• The introduction of genetic material via noncellular or cellular material
• Introduced by physical mechanisms (electric pulsed or ultrasonic waves) or by viral vectors

Question 11

What are the basic tenets of bone regeneration and grafting?

Answer 11

Osteogenesis: directly supplies and supports bone-forming cells
Osteoinduction: capacity to induce bone formation when placed into a site where no bone formation will otherwise occur
Osteoconduction: Provides a scaffold for mesenchymal stem cells and their progeny to migrate into and proliferate
Osteopromotive: A material or physical impetus that results in enhancement of regenerating bone

Question 12

What are the 2 main forms of osteogenic material?

Answer 12

Autogenous cancellous bone graft
Bone marrow (not the same magnitude of osteogenesis)

Question 13

What percentage of cells in autogenous cancellous bone graft survive at room temperature in normal saline for up to 3hr after harvesting?
When does maximum osteogenesis from autogenous cancellous bone graft occur?

Answer 13

• 60%
• Maximum osteogenesis at 8wk

Question 14

What is the most common form of osteoinductive agent?
How does it function?
What growth factors are present in the above agent?

Answer 14

• Demineralised bone matrix

Function:
- Recruit mesenchymal stem cells (chemoattraction and migration)
- Induce multipotent cells to multiply and become cells that make up the regenerating bony callus (proliferation and differentiation

Growth factors:
- TGF-B
- BMP-2, -4, -7

Question 15

What are the characteristics of an osteoconductive agent?

Answer 15

Offer a scaffold for adherence of mesenchymal stem cells, osteoblasts, osteocytes, chondroblasts and chondrocytes with interconnecting porosity for cellular proliferation and, most important, vasular ingrowth

Question 16

List some (3) examples of osteopromotive materials:

Answer 16

PRP
Hydrogels
Biphasic calcium phosphate

Cannot induce bone formation alone - is just enhancing

Question 17

How does autogenous cancellous bone graft provide all the tenets of new bone formation?

Answer 17

Osteogenesis - Provided trabeculae lined with osteoblasts
Osteoinduction - cytokines and growth factors within the extracellular matrix
Osteoconduction - maintains a structural scaffold
Osteopromotive - hemorrhage and the resulting clot contain activated platelets and therefore growth factors from the alpha granules (IGF-1, PDGF, TGF-B)

Question 18

How long do you need to wait before reharvesting bone graft from the proximal humerus and the proximal tibia?

Answer 18

Humerus: 8 weeks
Tibia: 12 weeks

Question 19

What type and size of curette is recommended for cancellous bone graft harvesting?

Answer 19

• Brun or Spratt Volkmann curette
• Size #000 - #2 (equates to cancellous particles between 3 and 6mm)

Question 20

How much can a properly applied autogenous cancellous graft speed up healing?

Answer 20

By as much as 4 weeks

Question 21

Describe the healing process of an autogenous cancellous bone graft:

Answer 21

Minutes-hours: inflammatory response attracts lymphocytes, plasma cells, and mononuclear cells
5 days: Capillary loops enter
20 days: Necrotic tissue is resorbed and graft is fully vascularized
- Woven bone initially placed which is remodeled over several months into lamellar bone
- Remodeling of lamellar bone leads to corticalization
- Remodeling of deeper trabecular bone leads to medulization

Question 22

What is the method of choice for final sterilisation of allogenic bone graft materials?

Answer 22

low-dose gamma irradiation (less than 2.5mrad)

Question 23

What is creeping substitution?

Answer 23

Method by which cortical allografts heal
- Haversian systems of the graft are not available for host integration
- Host vascular penetration slowly resorbs cortical allograft by immunologic and/or osteoclastic activity
- Substituted with host bone
- Takes months to years

Question 24

What is demineralized bone matrix?

Answer 24

Bone which has been ground to specific particle sizes and has been decalcified with the use of acids (typically hydrochloric acid)
- Decreases Ca content from 22-25% to less than 3%
- Osteoinductive material
- Can be mixed with cancellous bone chips for osteoconduction and with patients own blood for osteopromotion

Question 25

What are three strategies associated with stem cells for bone regeneration?

Answer 25

• Cultured and expanded autologous stem cells
• Cultured and expanded allogenic stem cells
• Selective mesenchymal stem cell retention

Question 26

Which BMPs promote differentiation of mesenchymal stem cells to an osteoprogenitor lineage?
Which promote differentiation of osteoprogenitor cells into osteoblastic cells?

Answer 26

• Differentiation into osteoprogenitor cells : BMP-2, -6, -9
• Differnetiation into osteoblastic cells: BMP-2, -4, -7, -9

Question 27

List some examples of bone graft substitutes:

Answer 27

Ceramics
Calcium phosphate ceramics (modelled off hydroxyapatite)
Coralline substitutes (marine coral, treated to convert calcium carbonate to hydroxyapatite)
Tricalcium phosphate (old school, Rapidly absorbed)
Biphasic calcium phosphate (combination of tricalcium phosphate and hydroxyapatite)
Nanocrystalline calcium phosphate
Calcium sulfate (plaster of Paris, rapid absorption)
Hydrogels (polymers containing aqueous solvent capable of controlled release of incorporated proteins such as growth factors