Verified answer

your story made me think of an instance with a work colleague a few years back

on a building site a piece of steel hit him in the leg

it was cut and a little painful but otherwise ok

as the next few days passed he realised he couldnt straighten his leg all the way

so, he went to the doctor who did an xray

surprise surprise, a small piece of steel was lodged in between the bone stopping his knee from coming all the way together

anyway .. if its both knees Im sure youve nothing to worry about but if its just one knee ..

could you have suffered an injury at some point resulting in small bone fragment jamming or calcification around joint

It's a muscle thing, it happens to my family too. Don't stress over it, it's genes