I like dentists. I just don’t like going to see them. The thought of needles being stuck in my mouth makes me cringe. Speaking of cringing, here’s a past project that Engel Metallurgical worked on.
A one inch, 30 ga hypodermic needle cannula (tube) fractured near the needle hub during a dental injection. The needle disappeared into the patient’s jaw. Subsequent surgical removal of the needle resulted in nerve damage to the patient’s face. Engel Metallurgical was tasked with determining the failure mode and root cause of the needle failure. Examination of the fractured cannula material showed that the hardness and microstructure were typical for a stainless steel needle. The fractured cannula was bent 30° at the fracture. The fracture surface in the cannula was clearly ductile fracture. The failure mode for the cannula is ductile fracture.
Exemplar needles were tested by bending the cannula at the hub 90° then bending 180° back, and back another 180°. The exemplar needles didn’t fracture until the second 180° bend. Metallographic examination of the failed cannula at the fracture revealed partial plugging in the cannula with epoxy that is used to bond the cannula to the hub. Further examination of the fractured incident needle and exemplar needles revealed that the exemplar needle crossections became eliptical in the bend area while the failed needle retained its round cross section at the fracture where the epoxy plug is located. The epoxy plug resulted in the failed needle being stretched further on the outer side of the bend, exceeding the ductility of the material, and failing by ductile fracture. The root cause of the failure was failure of the manufacturer to keep epoxy out of the needle cannula.
Fractured Hypodermic Needle, Hub Side
Fractured Hypodermic Needle, Needle Side
Scanning electron microscope image of hub side of fractured needle with epoxy blocking the opening.