Today Justin Morneau was reportedly diagnosed with a "stress fracture on his back", which will keep him out of the year.
There are not many details yet, but a fracture in the "back" is a fairly scary thing, and there is not such a diagnosis, btw, but I will try to give some medical and scientific context about it before we learn of more details:
A. What is a stress fracture:
A stress fracture, is a fracture (i.e. break, as in "sticks and stones will break my bones") caused by repetitive strenuous activity. These are hairline fractures (which means that they are "not all the way through" or there is no bone separation and there are no bone fragments around)
B. Anatomy of the spine
A fracture of "the back" does not mean anything... the assumption here is that Justin has a fracture in one of his vertebrae. As far as the seriousness goes, fractures on the Pedicles (the little protrusions that help the back bend but making it not bend so much so it doesn't put too much stress in the spine) are very easy to heal. Fractures on the Facet Joints that do what the Pedicles do plus hit the Pedicles above to keep the "back straight" are a little harder to heal, but still ok. On the other hand, fractures of the Verebral Bodies are much harder to heal and might cause things like slipped discs (which is really painful and that's what Joe Crede had; btw, Joe Mauer's condition was totally different, but this is another story).
Wishing the best for Justin and I will try to keep updated with information about exactly what's going on
Update 12:34 AM EDT:
Apparently the fracture is on L5 (or Lumbar 5) which is the lowest Lumber vertebra (the one where the Pedicle line points to on the right hand side of the above picture). Still no word of which part of the vertebra is affected.