Why are hairline fractures painful?

It's a fracture. Even though it is a "hairline" fracture, it is a fracture just the same....And fractured bones are painful.
Nerves.... Hairline fractures are painful because of the bone as well as the lining of the bone. Bone itself has nerves in it although they are not as plentiful as those in the lining of the bone (periosteum). Small motion that is present with hairline fractures can cause significant pain because the nerves are stimulated in both of these structures. Find an orthopedic surgeon at aaos.Org.