A “Toolbox” for Forensic Engineers

Failure of Medical Implements 307
Figure 9.16 Markings on site glass.
2. Some marks on the molding could only have been caused by a blunt
instrument (such as a screwdriver tip) levering against the precision
tool surfaces (Figure 9.16). However, these markings could in no way
be responsible for the variation of process conditions observed.
The expert for the plaintiff refused to consider evidence contained on
the sample moldings, and reiterated his original views. The case collapsed
after the plaintiff was cross-examined on the witness stand on the third day
of the trial.
9.8 Failure of Surgical Tweezers
Two cases of the failure of medical tweezers are considered in this section.
Both problems were encountered before introduction of the products onto
the market, showing the intense effort that goes into research and development
before taking the substantial risks of premature failure.
9.8.1 In-Plane Cracking of Surgical Tweezers
After specializing in production of steel-based surgical instruments, a manufacturer
decided to introduce a new range of titanium microsurgical tweezers.
The production lines were installed and commissioned, and the new
instrument range introduced to the market place. Fabrication of the tweezers
required an initial stamping of blanks from sheet stock, followed by a pressing
sequence to form a dog-leg (double bend) toward one end (Figure 9.17A).
This dog-leg section was then riveted to a mirror image blank, to form the
final tweezer profile. In addition, a dog-leg geometry provided the “spring”
necessary for normal operation.
Production and sales went well for the first year and, in order to increase
output, it was decided to introduce larger titanium sheets for the initial
blanking process. Although this pressing operation had been undertaken on