I have a weird thumb. Anyone else have this?!

Okay so I was born with this disability. Fortunately it's a really small disability and it really makes me no different then any other person. Well sort of. I can do anything a regular person can do, but with a different thumb. I was born with a thumb that only has one joint. Notice how people can bend their thumb in two places? Yea I only have one, and it's the one that connects to the palm. The one by my fingernail is the one that doesn't bend. Not just one thumb, both of them. I don't know what it's like to bend my thumb the way most people do. I'm 13 and my friends love it, they say "can I see your thumb?" and wiggle it all around and are like "COOOOOOOL!!!!!!!!!" so I've never gotten bullied from it, which is good. But I wanna know why I'm like this. Plus I'm getting calcium buildup around the area you're suppose to bend it [the joint near the thumbnail] because I've never moved it. My mom says I'm going to get artheritis when I'm older because of it, unless I do something about it. I told my doctor like once, but he said it wasn't a huge deal. But then again, he barley looked at it, without Xrays or anything. Because I can only bend my thumb one way my thumb is REALLY flexible!! Like... INSANELY. I can't explain how flexible on text but if you saw it, you would be like this:

:O!!!!

So does anyone know what this is? I'm a pre-me baby. About one month premature. I was a twin, but it died in my mom's stomach before it hit like 7 or 8 weeks. Does that have to do with anything? Anyone else have this problem? I don't mind it i actually like my thumb but I've met NOBODY with the same thing I have. NOBODY. Any information is welcome! Thanks for readin this whole thing I appreciate it hahahaha :D