Biomedical Engineering Reference
In-Depth Information
the Young's modulus between the bone and the metallic implant. assuming
that the system bone/metal is rigid and has an isoelastic behaviour, for a
given strain, the amount of stress carried out by the bone will be lower, as
illustrated in Fig. 6.9. This phenomenon, so-called 'stress shielding', affects
the bone remodelling and healing process, leading to bone resorption and
eventual aseptic loosening. This is particularly a problem with young and more
active patients. as deduced from Table 6.4, the stress shielding effect would
be more pronounced with austenitic stainless steels and cobalt alloys.
overall, the mechanical properties of metallic alloys are well above those
of natural bone. Here it is worth remarking that bone is a living tissue that
has the capability to self repair microdefects generated during the daily
activity. Metallic implants, however, have to be over designed since, although
loads are rather low, they are cyclic and occasionally very high. Moreover,
these stresses fluctuate depending on the daily activity. For instance, during
movement, the main load on a femoral head is about twice the body weight.
The load varies with the position in the walking cycle and reaches a peak of
about four times the body weight at the hip and three times the weight at the
knee. Peak loads during jumping can be as high as 10 times body weight.
The frequency of loading and load cycles are also important. Typically a
person may take one to two million steps per year.
33
Thus, it is not strange
that most of the mechanical failures of orthopaedic implants are due to a
fatigue mechanism. often the fatigue failure is associated with design-induced
stress concentration.
34
Therefore, fatigue strength becomes relevant for most
load-bearing components.
In this sense it is important to note that the presence of defects at the
surface produced during machining or at the moment of the surgery may
impair fatigue strength, irrespectively of the alloy microstructure. In Fig.
Metal
Bone
Strain
6.9
Schematic representation of the dependence of the stress and
elastic strain of a metallic biomaterial and the natural bone.
