When the mitral valve becomes too tight, hardened or calcified, then the condition is called mitral valve stenosis. This particular condition prevents blood from moving forward or when they become too loose, condition called mitral regurgitation in which blood tends to flow backwards to atrium and then back to lungs.
Another condition where mitral valve replacement is done is mitral valve prolapsed. This disease can occur due to infection, calcification, or inherited collagen disease.
A minimally damaged mitral valve can be repaired instead of being replaced, but those which are damaged to the extent of not being in the condition to get repaired are replaced surgically via surgical mitral valve replacement.