Fracture mechanics

Question 1

What are the aims of fracture fixation?

Answer 1

To produce the optimum rigidity for fracture healing
To maintain the fixation until the fracture heals
To avoid excessive stress shielding of the bone caused by the fixation method

Question 2

What levels of strain promote the different types of fracture healing?

Answer 2

<2% promotes primary bone healing
>2%, <10% promotes healing by callus (secondary)
>10% no healing occurs

Question 3

What factors influence the mechanical properties of the device?

Answer 3

Stress/strain and fatigue characteristics of the device
The dimensions
The distribution of materials within the device

Question 4

When considering IM nails, the torsional stiffness is important as well as bending stiffness. What term is used to indicate a material’s resistance to torsion?

Answer 4

The polar moment of inertia (J)

Question 5

What is the working length of a device?

Answer 5

The total unsupported length

As working length increases, the total movement (strain) at the fracture site also increase

Question 6

What is the working length for torsion?

Answer 6

The distance between locking points for locked undreamed nail

Torsional stiffness across a fracture is inversely proportional to the working length

Question 7

How do stress and strain change with increased working length of plates?

Answer 7

Strain increases with working length

Stress is shared along the working length of the plate