Biomedical Engineering Reference
In-Depth Information
returned to hospital with fatigue failures in their implants. One major issue
in joint implants is the requirement to use them for younger, more active
people, who are victims of accidents or early onset arthritis. The magnitude
of this problem is realized when one notes, as shown in Table 12.1, that the
forces (and therefore stresses) involved in running are much higher than
those sustained during slow walking.
12.4 Case study 1: fatigue design in the
artificial hip joint
This author makes no apology for returning frequently to the AHJ in this
chapter. Because, whilst it was mentioned in the Introduction that the problem
of failure in this implant has to a considerable extent been solved, this is
thanks to the large body of research which has been conducted and which
therefore makes this an excellent example to study. Furthermore, the lessons
learned on the hip joint can certainly be applied (though with some additional
complications) to other implants and to orthopaedic devices in general.
The development of the AHJ was greatly aided by studies, mostly conducted
in the 1980s, which considered the interaction of component shape and
material properties in determining the stresses in the implant and ultimately
the long-term integrity of the chosen design (Prendergast
et al
., 1989; Fagan
and Lee, 1986; Rohlmann
et al
., 1987). Figure 12.8(a) shows some results
synthesised from these various studies, which illustrate the behaviour of a
Table 12.1
Activity levels as defined by Whalen
et al
. (1988)
Activity
Slow
Normal
Fast
Slow
Distance
walking
walking
walking
jogging
running
(hr/day)
(hr/day)
(hr/day)
(hr/day)
(hr/day)
Sedentary
4
Sedentary
4
0.67
plus exercise
Normal
4
4
Active
4
8
Active plus
4
8
0.67
exercise
Athletic
4
4
0.75
Action
Frequency
Force (normalised by the
(cycles min
-1
)
single-leg stance force)
Slow walking
35
1.01
Normal walking
51
1.17
Fast walking
68
1.33
Slow jogging
84
1.7
Distance running
91
2.62
