Mechanical properties - Orthopaedic Biomaterials in Research and Practice - page 75

the presence of unstable long bone nonunion, owing to the lack of bony
support and load sharing.

PROBLEM 3.10
The femoral head resurfacing component shown in Figure 3.7 was
removed after a sudden onset of pain and radiographic evidence of frac-
ture 3 years after insertion. Complete the following statements:
The failure is a fracture.

The failure is a stress, cycle fatigue

failure.
Modifying the design to the thickness/diameter of the

stem or to use a material with a higher might

have delayed or prevented this failure.

ANSWERS:
Brittle, low, high, increase, endurance limit. Increasing the thickness/
diameter of the device reduces stress; selecting a higher endurance
limit tends to compensate for the design. However, material or design
changes need to be considered in light of impact on other design fac-
tors or implant performance. Generally speaking, design factors inter-
act with one another in terms of trade-offs in different performance
measures.

PROBLEM 3.11
The hardness of structural ceramic implants is (select the best answer)

A. Less than that of metals

B. Less than that of polymers

C. Greater than that of metals

D. Less than that of metals but greater than that of polymers

E. None of the above

ANSWER:
The best answer is C since ceramics lack any appreciable plasticity their
hardness correlates with σ u rather than σ y , as is the case of metals. Since
useful structural ceramics are stiffer and stronger (in the defect-free
state) than most metals, they are also much harder. Polymers are usually
softer than all but the softest metals.

annotated bibliography

1. Black J, Hastings G: Handbook of Biomaterial Properties . Chapman
& Hall (Springer), London, 1998.

Useful compilations of reliable mechanical properties of tissues and
prosthetic materials.