I think I've convinced myself that injuries never fully heal.  I sprained my ankle when I was a junior in high school (8 years ago).  I did all the proper rehab and was fully healed.  I went on to live a happy and productive (that's questionable) life.  I'm physically active and have regularly done things to test the ability of my healed ankle.  For most activities, I'm fine.  I'm even fine walking along rocky trails in the woods and occasionally twisting my ankle.  But certain things I do have a very specific effect on the exact part of my ankle that was sprained.  It's been 8 years and I've proven myself to be fully recovered, but some things still hurt.  This isn't an isolated incident or injury, and it doesn't only happen with me.  I see it in professional sports all the time.  A player gets injured, recovers, and ends up either re-injuring the same thing or having problems later on down the road.  It also happens with people who can "feel" a storm coming in because their knees or hips ache.  It seems like there's a certain amount of permanent damage done with serious and semi-serious injuries. #health