"The medics stay up nights blunting the needle tips with a file," was the legend in my army days of many years ago. Hypodermic needles were reused, with only a rinsing between injections. The needles got dull and I hate to think of the bacteria and viruses transmitted from one soldier to the next.
Fine, very sharp, one-use needles came into use just after my army hitch. These are a great improvement in both sanitation and comfort. Still, no one likes to get stuck, and one of the least favorite regions is the mouth.
One of my earliest cases centered on a hypodermic needle which broke off in a patient's gum during dental work. To make things worse the needle was so fine (12 mils) that it hardly showed up on x-rays. The dental surgeon had a tough time finding the needle fragment to remove it.
The lady with the needle fragment in her gum was understandably perturbed and sued all the usual suspects. I was called into the case as an expert metallurgist. My client was the insurer of both the needle manufacturer and dentist.
My job was to find the cause of the fracture. Was there something wrong with the materials or manufacture of the needle or was there an error in usage? A dentist friend tells me that broken needles are extremely rare, so I did not have prior cases for guidance.
I was supplied with the large end of the subject needle and a box of exemplar needles. Chemical analysis showed the exemplars to be of the common 18-percent chromium 8-percent nickel, so-called austenitic stainless steel. Metallography showed that the exemplar needle had been welded and had been reduced from some larger diameter. I estimate that the tubing had been electron beam welded from strip and drawn from a diameter of perhaps a quarter inch to the final 12-mil diameter. The microstructure was heavily cold worked by the drawing, which strengthened the needle and also rendered it partly ferromagnetic.
A scanning electron microscope fractographic study showed the fracture surface to be of the ductile "fork-split English muffin" morphology. There was no evidence of any defect that would have contributed to premature fracture.
The remaining part of the subject needle showed a bend to about 90 degrees, so I proceeded to find what sort of bending would cause fracture. I could bend an exemplar needle double without fracture. Further bending to a very small loop resulted in fracture, but such bending could not occur in the patient's mouth. However, bending double, followed by reverse bending caused fracture.
My scenario for fracture was the following. The patient turned her head with the needle inserted. The dentist held onto his end of the hypodermic and the needle bent double, but did not fail. The patient then jerked her head back and the dentist still held onto the hypodermic. The reverse bending was what caused the fracture. The fracture would not have occurred had the patient not twisted her head back and forth or had the dentist released the hypodermic prior to reverse bending.
I spent two days in my office waiting a call to testify which never came. The case settled out of court, but I have no idea of the terms.
I add a sad postscript. Needles such as those I studied must be mind-numbingly cheap to produce, as the tubing may readily be drawn from steel costing less per pound than peanut butter. Yet, I read of needles still being re-used in the third world, where HIV and other horrific infectious diseases run rampant. I wish there were some way to put cheap, disposable needles in the hands of health care people who currently cannot afford them.