How hard to do have to hit in the nutsack for it to cause fertility problems?

Severe trauma: Where there is rupture of the testicle. This would have to be result of severe trauma and cause scrotal swelling and pain sufficient to require emergency room care. These injuries are quite rare, & caused by more than a hit. Ruptured testicles need to be repaired because sperm escaping into the tissue could cause sperm antibodies to be produced which in turn could cause infertility.