Biomedical Engineering Reference
In-Depth Information
6.3 Material Characteristics
6.3.1 Mechanical Performance
The starting point in selecting an appropriate biomaterial for an application is the
identification of the loading conditions (discussed in Chapter 5) under which the
material has to perform. For example, a hip prosthesis must be strong and rigid
as it is exposed to high load bearing conditions. For hip prosthesis, the most com-
monly selected biomaterials are ceramics as they possess high load bearing capac-
ity. However, brittle materials exhibit high ultimate stress and high modulus and a
low breaking strain and therefore are not as tough (strong ductile materials have
a moderate ultimate stress but large ultimate strain). Thus, using ceramics is re-
stricted to designs involving limited tensile loading, and no impact loading condi-
tions. High elastic modulus of alumina and zireconia also limits their effectiveness
as bone interface materials. In total hip joint arthroplasty where both articulat-
ing surfaces of the joint are replaced, the most widely used implant configuration
[Figure 6.6(a)] includes a metal component articulating against a polymeric com-
ponent fabricated from UHMWPE. Although other bearing couples such as metal-
on-metal or ceramic-on-ceramic total joint replacements have been investigated,
metal-on-UHMWPE total joint replacements provide better care for degenerative
joint disorders. Commonly used metal alloys are made of cobalt, chromium, and
molybdenum. Metal-on-UHMWPE total joint arthroplasty is a popular treatment
modality providing remarkable restoration of mobility for patients with disabilities.
Other physical properties also need to be considered. The dialysis membrane
needs specified permeability, the articular cup of the hip joint must have high lu-
bricity, and the intraocular lens has clarity and refraction requirements. Optimizing
the mechanical properties of biomaterials is an important step in determining their
clinical performance. Forming composites with good engineering design provides
an opportunity for long-term clinical survival prosthesis.
Figure 6.6
Total hip replacement: (a) femoral head/cup prosthesis and (b) wear behavior of differ-
ent head and cup combinations [2].
