Fracture Mechanisms and Healing

Question 1

What factors determine the mode and location of a fracture?

Answer 1

geometry and structure of the bone 
loading mode (compression, bending, torsion)
loading rate

Question 2

What quantity takes into account the cross-sectional area of the bone and its distribution of bone tissue in bending?

Answer 2

second moment of area

Question 3

What quantity takes into account the cross-sectional area of the bone and its distribution of bone tissue in torsion?

Answer 3

polar moment of inertia

Question 4

How are long bones structured so as to increase their resistance to fracture?

Answer 4

much of the bone tissue is distributed at a distance from the neutral axis and thus they have a much larger second moment of area than a tube of the same shape that is solid

Question 5

How does strength change as the polar moment of inertia increases?

Answer 5

increases

Question 6

Where in the tibia does torsional fracture commonly occur?

Answer 6

distally

Question 7

Where is the weakest point of the fibula?

Answer 7

proximal 1/3

Question 8

Which type of bone is weakest?

Answer 8

cancellous

more likely to fail under axial compression causing supracondylar and tibial plateau fractures

Question 9

Describe the loading on opposite sides of bone in bending

Answer 9

one side is loaded in tension and the other in compression

Question 10

Why does the side of the bone in tension fail first in bending?

Answer 10

bone is weaker in tension than compression

Question 11

What fracture patter results from bending?

Answer 11

transverse

Question 12

In pure compression, what fracture pattern occurs?

Answer 12

oblique

Question 13

When bending is superimposed on axial compression, a combination of two fracture patterns occurs, which two?

what pattern is this?

Answer 13

transverse
oblique

butterfly segment

Question 14

In pure torsion, what fracture pattern occurs?

Answer 14

spiral

Question 15

True or False: Most fractures occur due to a single method of loading

Answer 15

False - most are a combination

Question 16

How does bone strength vary with loading rate?

Answer 16

Higher loading rate = higher strength

Question 17

What happens in a high energy fracture?

Answer 17

the high energy is suddenly released and the bone fails - resulting in a comminuted fracture with severe soft tissue damage

i.e. high velocity gun shot fracture

Question 18

Name an example of a low-velocity fracture

Answer 18

spiral fracture due to a skiing injury

Question 19

Describe the fracture process (5 steps)

Answer 19

energy is delivered to the limb

the energy is transferred via the soft tissue to the bone which absorbs the energy

the bone breaks and energy is released back to the soft tissues

the broken bone and damaged tissues bleed causing a haematoma

an acute inflammatory response occurs which causes pain and commences the healing process

Question 20

Which type of bone healing is natural bone healing?

Answer 20

secondary

Question 21

What is the relationship between fracture energy and healing time?

Answer 21

increased energy = longer healing time

Question 22

What is a pseudoarthrosis?

Answer 22

excessive movement causes a false joint to form between rapidly proliferating cartilage cells at either end

Question 23

Describe the secondary fracture healing process

Answer 23

Weeks 0-2 - The haematoma is invaded by macrophages in surrounding tissue which are responsible for “mopping up” dead and damaged tissue. The haematoma and dead cells are absorbed into the macrophages.

Weeks 2-6 - New capillaries grow into the fracture haematoma bringing with them cells of healing and repair including fibroblasts, which form fibrin (scar tissue) and also other cells including bone forming osteoblasts. At the same time the surviving periosteum begins to regenerate and grow between the bone fragments.

Weeks 6-12 - New bone tissue is laid down in the endosteal space from the residual living bone and eventually the two ends are reunited as a ball of “provisional callus” which appears as a dense area on an X-ray.

Up to about 12 months - If circumstances are correct the provisional callus continues to form woven bone which gradually remodels to form a cortex.

Up to 2 years - The callus matures so that the trabecular pattern is reformed and the bone remodels to accommodate the stresses that the bone experiences in that anatomical region.

Question 24

What is primary bone healing?

Answer 24

if no movement takes place between fracture fragments it heals without callus formation

Question 25

How does primary bone healing take place?

Answer 25

New Haversian systems grow directly across the fracture gap

Question 26

What is an advantage and a disadvantage of primary bone healing

Answer 26

A - radiologically the fracture resolves quicker

D - the bone takes longer to regain its original strength

Question 27

What causes a bone to heal by primary healing

Answer 27

rigid fixation which removes all loading - thus the bone becomes weaker from stress shielding

Question 28

What properties does the callus give to the bone as it heals?

Answer 28

callus increases cross-sectional area and therefore second moment of area

which increases the structural support to compensate for the lower strength and rigidity of the callus material

Question 29

What is the equation for rigidity?

Answer 29

R= EI

Young’s Modulus x Second moment of area

Question 30

Name 3 possible factor which may explain why movement at the fracture site influences bone healing

Answer 30

electrical effects by moving hydroxyapatite crystals (HAp is piezoelectric i.e. develops a charge when it is loaded)

hormonal factors - substance P produced at fracture sites

electro-magnetic effects produced through electron flow away from the fracture site