Review of bone fracture mechanisms

Question 1

What factors determine the location and mode of a fracture?

Answer 1

Geometry and structure of the bone 
Loading mode (compression, bending, torsion etc) 
Loading rate

Question 2

What is the second moment of area?

Answer 2

Measure of resistant a structure is to bending

Question 3

What does a larger second moment of area mean?

Answer 3

Stronger, stiffer bone

Question 4

Why do long bones have a high second moment of area?

Answer 4

Much of the bone tissue is distributed at a distance from the neutral axis

Question 5

What factors affect bone strength and stiffness in torsion?

Answer 5

Cross-sec area

Distribution of bone tissue around a neutral axis

Question 6

What is the polar moment of area?

Answer 6

Measure of how resitant a structure is to torsion

Question 7

Why does torsional fracture of the tibia happen more commonly distally?

Answer 7

As although has a smaller cross-sec area more of the bone tissue is distributed away from the neutral axis (higher J)

Question 8

If the distal tibia is fracture where would the accompanying fibular fracture occur? Why?

Answer 8

Within the proximal third - fibula does not have same geometry as tibia and the fracture will occur at the weakest point of the fibula

Question 9

Why do supracondylar and tibial plateau fractures occur?

Answer 9

Cancellous bone is significantly weaker under axial compressive loading and will fail before cortical bone

Question 10

What type of fracture usually occurs in pure bending in adults? Why?

Answer 10

Transverse - convex side is loaded in tension and concave side in compression
Convex side will fail first as adult bone is weaker in tension than compression

Question 11

What type of fracture usually occurs in pure bending in children? Why?

Answer 11

Transverse - convex side is loaded in tension and concave side in compression
Concave side will fail first in children as childrens bone is weaker in comrpession than tension

Question 12

What fracture pattern occurs in pure compression?

Answer 12

Oblique

Question 13

Describe the fracture pattern if bone is subjected to bending superimposed on axial compression

Answer 13

Bending produces transverse crack on tension side
+
Compression results in oblique fracture
=
“Butterfly segment” on the compressed side of bone

Question 14

What type of fracture results from pure torsion?

Answer 14

Spiral - fracture line results from failure of the bone in tension, perpendicular to the crack

Question 15

What angle is the fracture line at in a spiral fracture caused by pure torsion?

Answer 15

About 45 degrees to the axis about which the torque is applied

Question 16

What fractures result from pure axial compression? Why?

Answer 16

Fractures close to or within the metaphyses - because cancellous bone is weaker than cortical bone

Question 17

How does loading rate affect bone strength?

Answer 17

Bone is stronger at a higher loading rate than at a lower loading rate

Question 18

What type of fracture results when a bone is loaded to failure by impact?

Answer 18

High energy fracture

Question 19

Describe a high energy fracture

Answer 19

Comminuted with severe soft tissue damage

Question 20

Summarise the fracture process

Answer 20

Energy delivered to the limb

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

Bone breaks and energy is released back to the soft tissues

Broken bone and soft tissues bleed - haematoma forms

Acute inflammatory response occurs around damaged area leading to pain and starting processes which lead to healing