No Room for Error in the Operating Room
Annex Medical Inc., Minnetonka, Minn.
It's a medical company's worst nightmare: product failure in the middle of an operation.
In this case the end of a lathroscopic device used to remove kidney stones had broken
off outside a patient's body. So Annex Medical needed to know what caused the failure,
and how to prevent it from happening again.
A metallurgical examination was an important part of the failure evaluation.
A suspect piece of the puzzle was the hypodermic needle tubing securing the device to
the assembly. Analysis showed sensitization of the tubing, which could result in
corrosion and cause it to weaken. The sensitization was introduced during the manufacturing process. Fortunately,
it has no effect on the mechanical properties of the tubing, and was ruled out as the cause of failure of the device.
Though the sensitized tubing was not the cause of failure of the device,
the processes used in its production were closely studied for possible modification.
The needle tubing comes in four-foot lengths and in order to cut it to the required length
it has to be solidified in epoxy, cut to size, and the epoxy is then burned away. The process
inevitably leaves epoxy residue behind. In the next step of the manufacturing process, the
needle tubing had to be heated to 1900 degrees, which makes the metal more malleable so it
can be crimped without breaking. This is the point in the process where the sensitization was happening.
As the needle tubing cooled, the epoxy residue made the carbon levels increase at the grain boundaries.
The solution? A slight change in the manufacturing process: water-cooling the needle tubing,
instead of allowing it to air cool. This simple change would eliminate the possibility of corrosion.
Knowing all the facts about this particular part, its appropriate uses and correct processing,
gives Annex Medical Inc. an informed
approach in any further designs that may incorporate the part, and the assurance that devices in use are safe.
M. E. Williams and Associates, Inc. has been our consulting metallurgist for the past
14 years. They have completed many projects with regard to design, production and failure
analysis of our surgical instruments. Their professional wisdom and ability has been an
asset to our organization. I continue to strongly recommend his services to other companies.
Stuart Lind, President, Annex Medical