Biomedical Engineering Reference
In-Depth Information
PROBLEM 7.6
The relative disadvantage of annealed cast stainless steel as an implant
material is that it (select the best answer[s])
A. Has a low elastic modulus
B. Deforms plastically at low stress
C. Has poor ductile properties
D. Has too high an elastic modulus
E. Has an ultimate strength below standards
ANSWER:
Fully annealed cast stainless steel has the lowest yield stress of any
implant alloy. This produced varus deformation in the stems of early
THR devices, leading to increased bending moments and stem fatigue
failure; thus,
B
is the best answer. All implant alloys have moduli well
above that of bone; this must be accounted for in design (not
A
or
D
).
Fully annealed stainless steel may have greater than 50% ductility (not
C
). An individual part may have strength “below standards,” owing to
poor manufacture; however, there are no categorical standards for mini-
mum strength of implant materials (not
E
).
PROBLEM 7.7
A 14-year-old girl is treated for a distal femoral osteosarcoma by en
bloc excision of the distal third of the femur followed by prosthetic
reconstruction of the knee 1 year later. The best metal for the prosthesis
would be
A. Cast stainless steel
B. Wrought stainless steel
C. Cast cobalt-base alloy
D. Pure titanium
E. HIP cobalt-base alloy
ANSWER:
One cannot say categorically which metal alloy would be best for this
application, in absence of design details. Alloys
C
and
E
would prob-
ably be satisfactory,
B
would be marginal, and
A
and
D
would not be
recommended.
PROBLEM 7. 8
Phase diagrams (true or false)
1. Predict ductility
2. Allow calculations of time and temperature required for alloy heat
treatment
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