Biomedical Engineering Reference
In-Depth Information
Fig. 2.5
Left
: one striation represents a progress of the crack by one step (bar D
1 mm);
Right
:the
fast fractured surface (bar D
20 m). The surface at this stage is normally rough but was smoothed
by wear after the fracture occurred
Each striation in the left side of the latter figure stands for one step forward in
progressing fracture of the stem of Fig.
2.1
. At the right side is the situation where
the progressing fracture weakened the prosthesis to an extent where it could no
longer bear the imposed load and was breaking catastrophically. This surface is
normally very rough and without striations but in this case, the patient did continue
to walk after the stem was broken, which smoothed the surface.
With an assumed load of 4,000 N (>400% of the patients BW) and a section of
the 165 mm
2
, the applied stress was of the order of magnitude of 26 MPa. Calculat-
ing the number of cycles the patients' prosthesis performed by summing the activity
cycles/year as given by Black, jogging excluded, and multiplying by 27, the number
of years of service the total of cycles must be something like 10
8
cycles [51]. This
is below the expected fatigue limit for a stainless steel type 304 (see Sect. 2.2.1). As
showninFig.
2.5
, fatigue is definitely at least part of the reason for failure, although
the applied stress remained below the fatigue threshold.
3
Corrosion
: Still a number of other things can go wrong. On the medial part of the
prosthesis (Fig.
2.4
), some surface deterioration either by
wear
(rubbing against the
PMMA cement) or by
corrosion
or by synergy of both phenomena is visible. Medial
means facing the median or sagittal plane of the body (Fig. 1.8) or more detailed in
any topic of anatomy, e.g., Gray's Anatomy [52, p.13]. Wear and corrosion as in
Fig.
2.4
is no longer seen in modern prostheses.
Corrosion is a potential onset of fracture, particularly for stainless steels. Close
inspection of the fracture site (metallography, microanalysis by X-ray spectrometry
in the scanning electron microscope (SEM)) could not prove that corrosion was the
cause here. However, the alloy of this prosthesis is sensitive to attack by chlorides
3
An important research center on fatigue is located in the
Graduate School of Engineering of
Osaka University (Japan)
, headed by Prof. Masary Zako. Coordinates are easily found on Internet.
