Mechanical properties - Orthopaedic Biomaterials in Research and Practice - page 74

PROBLEM 3.7
The endurance limit (select the best answer)
A. Refers to a creep test
B. Exists for all metals
C. Is reduced by surface defects
D. Is increased by a corrosive environment
E. Is none of the above
ANSWER:
The best answer is C . An endurance limit is found during fatigue rather
than creep testing and does not exist for all metals. When an endurance
limit exists, corrosive conditions tend to remove it, especially for iron-
base alloys.
PROBLEM 3.8
Stress relaxation is desirable in internal fixation since it tends to elimi-
nate stresses in the fixation devices (true or false).
ANSWER:
False. Fixation devices remain secure owing to frictional forces gen-
erated by stresses produced in them during insertion. Relief of these
stresses secondary to either stress relaxation or creep processes may
result in loosening of the devices and loss of fixation integrity.
PROBLEM 3.9
A 37-year-old male patient, who works as a mail carrier, sustains a spi-
ral fracture of the right femur that is treated with dynamic compression
plates applied to both lateral and anterior sides of the femoral shaft. He
is seen on referral 30 months after surgery. He reports pain at the frac-
ture site at the end of his route. Radiographic examination results in a
diagnosis of nonunion. Is there any aspect of the fixation one should be
concerned with?
ANSWER:
This individual is obviously placing significant cyclic loading on his
internal fixation hardware. It may already have one or more partial
cracks associated with screw holes. These are usually not seen on radio-
graphs, although stress films may be of some value. The fixation is at
risk; whatever therapy is decided for the nonunion, adjunctive measures
should be taken to remove or support the hardware or reduce the cyclic
loading upon it. Fixation hardware may be expected to fail eventually in