Literally could have been anything. Sinter time not long enough, sinter temp too low, mix imbalance, press pressure not high enough, too much oil in the mix, the list goes on. Most gears are powder metal these days and will look like that on the inside. You can’t find the root cause from a picture, just a suspected root cause.
Edit: For everyone saying to use a pair of pliers or a hammer: we will absolutely NOT do this unless it is life or death. The risk of further injuring a patient is far too high. Let the doctors do it with tools that can put precise pressure on the ring, I’m not going to whack it with a hammer and hope for the best. This isn’t Chicago Fire.
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