The failure of synthetic polymeric medical devices - Durability and Reliability of Medical Polymers - page 190

covered with dust from the solicitor's office, there appeared to be no trace
of the score marks claimed by the claimant's expert. Another new catheter
had been deliberately damaged by withdrawal through the Tuohy needle,
and its surface (Fig. 9.9) was quite different to the failed sample. it does
show score marks from tiny defects in the sharp edge of the needle blade,
and cut debris at the edge. so how had the catheter actually failed? Another
part of the proximal end was also examined, and its failure surface examined
using conventional SEM (made conducting by a thin gold film). It exhibited
a brittle glassy fracture over most of its end surface and a longitudinal crack,
confirming what the nurses had said (Fig. 9.10). Most interesting, however,
was the presence of a ductile tear (at left in the picture) of very similar type
to that shown in the failed tip.
My interpretation of the features shown by the distal end (Fig. 9.11)
indicated that the part had fractured in a mainly brittle, but also partly ductile
way. There were two large flat zones next to the infusion hole with a thinned
and torn part at the furthest extremity of the surface. There was no trace of
the cut marks at all. Far from indicting the hospital staff, the evidence at
both the distal and proximal ends showed a brittle catheter. such a device
should remain tough and ductile in response to loads, so how could it have
become brittle?

￿ ￿ ￿ ￿ ￿ ￿

9.9 New catheter pulled back through Tuohy needle.