dude, when shit gets lodges in your body, it travels around. My mom tells me how some of her patiantes (she is a registered nurse who works with veterns in long term care) who have shratnell in their bodys from grenades and bombs and things of that sort find that it moves. Over the course of 70 years, someone with shratnell in their back can find it in their wrist. there was one case where a peice had migrated into an arterie, and they were contemplating surgery to have it removed, but decied that removing it would be more damaging then letting it sit there.
The reason why my mom told me these things is because i have tones of pebbels and rocks in my knees from falling mountianbiking. You clean it out the best you can, but sometimes you just cant get all the rocks out on a trail, and they scab over and you forget about them. She says that the numerous rocks in my knees wont cause me any trouble in my life - and i have yet to run into problems, and most of this rock has been there for 6 or 7 years. I think your good.