Why doesn't my left thumb naturally bend?

I was born with six fingers on my left hand. I had an extra thumb beside my unbendable thumb. Both thumbs didn't bend. I surgically got the extra thumb removed by a doctor. Now I have my another thumb which doesnt bend at all. There are no creases over thumb where there is a bending point. It does not hurt at all, its just a very smooth and straight thumb. My right hand is normal. My right thumb bends perfectly. I dont have any idea why my left thumb doesnt bend at all, even if i try very hard. It is really weird to have one thumb that bends nomally and the other thumb that just stays straight. Any ideas as to what this may mean? Thanks.