A “Toolbox” for Forensic Engineers

Failure of Medical Implements 315
The injured woman received compensation for the trauma and injury
after the manufacturer accepted the findings of our investigation.
9.10 Failure of Nerve Probe during Surgical Procedure
A female patient underwent circumferential reconstruction surgery on her
lumbar spine at a private hospital in the U.K. During the procedure effective
monitoring of motor pathways was undertaken, with specifically designed
electronic nerve stimulation equipment. Such equipment is used during this
type of surgical procedure to reduce the risk of nerve damage by effective
monitoring of motor nerves. However, toward the end of the operation, a
nerve stimulation probe failed while being removed from the patient’s leg;
an operating room doctor had attempted removal by pulling on the lead of
the probe. The probe in question was of a straight needle variety, and failure
was by way of lead detachment from the probe at its soldered connection
leaving the probe in situ within the patient’s leg. A decision was made to
complete the spinal surgery and remove the needle probe at a later date. This
was accomplished 7 days later, requiring additional surgery recuperation and
scarring. The patient sought compensation from the hospital for her injuries
and, in turn, the hospital authorities sought to pass responsibility for failure
on to the equipment manufacturer.
The probe and lead in question were not available for examination
because they had been disposed of prior to any investigation. However,
similar units had a needle length of some 35 mm with leads attached by
soldering with a lead-free alloy system (Figure 9.23). Quality control procedures
for the solder joint were somewhat antiquated, relying on gently pulling
the cable lead to ensure the joint had been properly made. This was a rather
ambiguous quality control method, leaving itself open to question when
considering the integrity of a newly formed solder joint.
Strain relief is a term to describe methods used to ensure that loading is
not transmitted to vulnerable parts of an assembly. Strain relief methods used
Metallic Probe
2-mm diameter, 35 mm long
Solder Joint — accomplished by pushing
the electrode needle 2 mm into strands of
signal cable and soldering
Heat Shrink
Tubing
Signal Cable
Figure 9.23 Needle electrode schematic, showing heat shrink tubing over a
soldered joint between needle probe and signal cable.