Thanks for the suggestions. I think Colin's hypothesis that it must have
been bumped was correct, even though there was no physical damage on the
outside. I used a screwdriver to wedge the plastic helicoid apart from the
aluminum one. The threads had been knocked out of place, which I guess is
what can happen given the flexibility of plastic. (I don't think a mating
of plastic and metal is such a good idea, but I guess it's cheap, and most
of the time it works.) I was able to move it up a bit, and then the threads
popped into place.
Unfortunately, part of the plastic cracked in the process, so the operation
was a success but the patient died.
