Chapter 39 Bone Biomechanics and Fracture Biology

Question 1

Materials vs. composites vs. structures

Answer 1

A material may be composed of one or more elements
A composite is a material made from 2 or more primary materials
A structure may be composed with one or more materials or composites

Question 2

Stress/strain vs load/deformation analysis

Answer 2

Load-deformation analysis is conducted on the body as a whole
Describes overall changes in geometry of the sample in response to an applied load

Stress-Strain analysis is conducted point-by-point within the body
Describes the materials response to loading

Question 3

Difference between cancellous and cortical bone in load/deformation curve

Answer 3

Cortical bone = more dense, stiffer and stronger under compressive load
Structure and orientation of osteons make cortical bone resistance to deformation under axial compression
2% strain without failing
Becomes stronger and stiffer with higher strain rate

Cancellous bone = Can handle more strain
75% strain without failing

Question 4

What are the 5 growth plate zones

Answer 4

Resting/reserve zone
Proliferative zone
Hypertrophy zone
Zone of calcification
Zone of ossification

Question 5

Describe the resting/reserve zone of the growth plate

Answer 5

Only vascularized zone
Hyaline cartilage matrix with small chondrocytes – identical to hyaline cartilage

Question 6

Describe the proliferative zone

Answer 6

Chondrocytes undergo mitosis
Align in longitudinal columns
Primarily type II collagen
Growth factors are in this zone

Question 7

Describe hypertrophy zone

Answer 7

Chondrocytes hypertrophy and undergo apoptosis
Chondrocytes produce collagen X and decrease expression of type II collagen

Question 8

Describe the zone of calcification and its result

Answer 8

Matrix mineralization
Chondrocytes release ALP and other enzymes that scavenge Ca and P
Result: Calcium=phosphate aggregates and matrix calcification

Question 9

Describe the zone of ossification

Answer 9

Osteoblasts produce woven bone
Osteoclasts remodel woven bone to lamellar bone
Until activity within the zone of ossification > chondrocyte repopulation of the resting zone then no more growth plate

Question 10

What is the force on the medial column of the femur during weight bearing

Answer 10

compressive

Question 11

What forces are needed to generate an oblique fracture

Answer 11

oblique shear, transverse tensile, and compressive stress

Question 12

Types of forces on transverse fracture

Answer 12

Concentric tensile loads
Oblique shear stress
Transverse compressive stress
Axial tensile stress

Question 13

Types of forces on oblique fracture

Answer 13

Oblique shear stress - main
Concentric axial compressive loads
Transverse tensile stress

Question 14

Types of forces on spiral fracture

Answer 14

Torsional load
Shear stress axially and transversely
Tensile stress
Compressive stress
Failure parallel to axis of the twist and propagates along tensile lines

Question 15

Types of forces on butterfly segment and where would it fail

Answer 15

Bending moment
Induced by axial compressive loading = buckling
Compressive stress
Tensile stress
Failure on the tensile side

Question 16

How does granulation tissue stabilize fracture

Answer 16

End result of inflammatory phase is a scaffold that remodels into granulation tissue to form external callus
Granulation tissue can withstand nearly 100% deformation - is primary stabilizer

Question 17

How to achieve primary/direct bone healing

Answer 17

Anatomical reconstruction
Strain has been eliminated
Absolute rigidity and stability

Question 18

How does primary/direct bone healing occur

Answer 18

Intermembranous ossification – osteoblasts and osteoclasts directly deposit bone at the fracture

Question 19

Describe contact healing

Answer 19

Gap is < 0.01mm
Interfragmentary strain <2%
Lamellar bone deposited parallel to long axis

Question 20

Describe gap healing

Answer 20

Gap < 1mm*
Interfragmentary strain <2%
Initially filled with fibrin matrix
Then remodeled with type I and type III collagen
Lamellar bone oriented transverse to the long axis

Question 21

How does interfragmentary strain decrease in secondary bone healing

Answer 21

Osteoclasts resorb bone at fracture leading to a wider fracture gap*
Widened gap decreases strain*
Granulation tissue can now form and survive within the gap
External callus forms on the abaxial surface of bone
Bigger callus = increase in area moment of inertia = greater stability

Question 22

How fast do osteoblasts get activated in bone injury

Answer 22

within 24 hours

Question 23

Define Wolff’s law

Answer 23

Ability of bone to remodel adaptively in response to mechanical load

Question 24

Describe elastic osteosynthesis

Answer 24

Hypothesis that distributing stress along the entire plate to limit stress at the screw-bone interface is more appropriate for juvenile patients
Best for animals < 5-6 months old
Compliance of bone-plate construct is increased to promote plate deformation within its elastic range to spare the weak bone-screw interface from overwhelming shear stresses

Question 25

How to achieve elastic osteosynthesis

Answer 25

Minimal manipulation of fracture
Bridging plate – metaphysis to metaphysis
2-3 screws at each end of the bone plate
Screws should be diverging and not tapped

Question 26

Define distraction osteogenesis

Answer 26

method of bone formation via intramembranous ossification

Question 27

What is the fibrous interzone in distraction osteogenesis

Answer 27

Radiolucent zone in the osteotomy gap where in response to tension, the gap is filled with collagen type I that is deposited in a longitudinal manner by fibroblasts and mesenchymal stem cells in response to linear tensioning of the provisional fibrin matrix

Question 28

What is platelet alpha granules role in bone healing

Answer 28

Hemorrhage and the resulting clot have activated platelets. Platelets are degranulated and platelet alpha granules release growth factors necessary for all wound healing especially bone healing. Alpha granules release IGF-1, PDGF, and TGF-beta which are osteopromotive (promotes de novo formation of bone).

Question 29

Describe disadvantages of collagen X in the hypertrophy cartilage zone

Answer 29

Collagen X – stiffer and prevents nutrient diffusion – leading to tissue hypoxia
Structurally the weakest and predisposed to Salter-Harris fractures