Back Joint

Back Joint
What could cause a strange permanent callus on the back of my thumb on the joint?

It’s been there for a couple years now. It’s not painful or anything, just a toughened white bump that does not seem to go away. There is also a much less distinct one on the other thumb also. Someone told me it had something to do with a future in arthritis? Have any of the medical professionals out there heard of this?
didn’t suck my thumb

It can be from sucking your thumb if you do or used to do this in the past.