Biomedical Engineering Reference
In-Depth Information
6.3.2 MechanicalDurability
Apart from suitable mechanical performance, mechanical integrity and wear re-
sistance of a biomaterial are vital to its continuing success. For example, polymer
behavior is strongly influenced by temperature, moisture absorption, and other
environmental factors. The use of these materials for biomedical applications in
a surrounding with varying environmental conditions, needs design procedures,
which take into account the loading history and effect on thermomechanical char-
acteristics, in order to obtain a reliable prediction of the long-term behavior. The
material could deteriorate physically or chemically, resulting in reduced mechanical
properties. Understanding the mechanical durability under the intended loading
conditions is important, particularly for long-term implantable devices such as total
hip joint replacements, which need to function effectively over periods of 10 years
or more.
Durability of the prosthetic device is greatly influenced by the process of cy-
clic loading in which they are present. Implant materials must have a high degree
of fatigue resistance to perform over the long term. Otherwise products will not
perform well or for very long. By reducing wear, the tribologist (who deals with
friction, heat, wear, bearings, and lubrication) prevents the failure of prosthetic
components within the body. Wear depends upon the nature and geometry of the
interacting surfaces, the environment in which they interact and the mechanical
load (static, dynamic, or impact type).
There are two main types of wear: mechanical and chemical. Mechanical wear
involves processes that may be associated with friction, abrasion, impact and
fatigue. Chemical wear arises from an attack of the surface by reactive compounds
and the subsequent removal of the products of reaction by mechanical action. Me-
chanical wear results when surfaces produce local mechanical damage, unwanted
loss of material, and the resultant generation of wear particles. Fatigue wear oc-
curs as a result of repetitive stressing of a bearing material. Wear at the interface of
two components is grouped into abrasive and adhesive wear. Abrasive wear occurs
when a surface roughness cuts or plows into the opposing surface, particularly
when the two surface materials have different hardnesses and the harder material
cuts into the softer material. Adhesive wear occurs when bonding of microcontacts
exceeds the inherent strength of either material. The weaker material may then be
torn off and adhere to the stronger material. Other factors in wear include surface
roughness, material hardness, contact areas, and loads applied.
For example, in the popular head-cup design of the total hip prosthesis and
knee prosthesis [Figure 6.6(a)], UHMWPE is the weaker component. The produc-
tion of large number of particles with UHMWPE is a major factor limiting the
life of prosthetic joints. If UHMWPE is replaced by another alloy, the number of
wear particles produced decreases significantly. Cells in the immune system sense
the presence of wear debris, which leads to aseptic loosening of the hip prosthesis
(discussed in Section 5.4.4). To aid in the development of load-bearing devices,
multidirectional simulators that mimic the intended loading condition have also
been developed. To understand the wear mechanisms, wear rates, wear debris mor-
phology, and wear-surface morphology, these simulators are operated under the
appropriate conditions. The wear rate of UHMWPE against alumina ceramic is
