Final answer:
The thickened region after a diaphysis fracture has healed is known as the external callus. It is a mix of cartilage and bone providing stability during healing.
Step-by-step explanation:
After a fracture of the diaphysis has healed, the thickened region that results is called the external callus. The external callus is a collar of hyaline cartilage and bone that forms around the outside of a fracture to stabilize and heal the bone. As the healing process progresses, the callus becomes more calcified and eventually is replaced mostly by bone. However, a slight swelling may remain as evidence of the fracture.