Biomedical Engineering Reference
In-Depth Information
change in the elasticity modulus, heat conduction and thermal expansion.
this effect is known as plasticizing and will cause the
T
g
of bone cements
to drop by 20°c. With this temperature clearly above body temperature, safe
use of the cements is assured.
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8.3.3 Mechanical properties of PMMA-based
bone cements
the function of bone cements in cemented hip arthroplasty is to locate the
implant components in the bony skeleton and to transmit loads through
the joint into the bone and muscle surrounding the joint over very long
periods of time. Bone cement is subjected to high stresses since the forces
transmitted through the hip joint are high and, in addition, it has to function
in the relatively aggressive environment of the body.
31, 32
As has been mentioned before, PMMA bone cement is a thermoplastic
polymer and its properties change as temperature changes. similarly, the water
in the surroundings will act as a plasticizer and change the characteristic
properties of the cement.
33
therefore, bone cement must be tested in conditions
that replicate the body environment as far as possible.
34
There are five mechanical properties of bone cement that should be
measured by testing samples in the laboratory.
35
these are tensile strength,
shear strength, compressive strength, bending strength and modulus of
elasticity.
25, 36, 37
the average results for these different properties are:
∑ tensile strength: 35.3 MPa
∑ shear strength: 42.2 MPa
∑ compressive strength: 93.0 MPa
∑ bending strength: 64.2 MPa
∑ bending modulus: 2.55 GPa.
In addition to the five elastic mechanical properties described above, there
are three long-term viscoelastic properties that are of importance. these
properties are creep, stress relaxation and fatigue.
Creep is defined as the change in strain with time in a sample held at
constant stress.
34
All PMMA cements creep and the creep rate is affected by
the environment, ageing of the cement (creep rate reduces with age of the
cement), temperature and stress level (an increase of the temperature and/
or stress level will increase the creep rate).
Stress relaxation is defined as the change in stress with time under
constant strain (deformation).
38
stress relaxation in bone cement takes place
by a similar molecular relaxation process within the polymer to that which
causes creep. similarly, the same effects as for creep are observed for stress
relaxation, that is environment, ageing of the cement, temperature and stress
level. creep and stress relaxation increase the compressive stress in the
