A **complicated or open fracture** is one in which, in addition to the bone breaking, there is an **injury to the blood vessels**. This can lead to a **hemorrhage**, which is the significant loss of blood due to the rupture of these vessels. Hemorrhage can be dangerous, as losing a lot of blood can cause a person to go into **shock** or even die. Therefore, a complicated or open fracture not only involves damage to the bone but also an additional risk due to the possible loss of blood.

In contrast, a **closed fracture** does not involve an injury to the blood vessels that causes external hemorrhage, and a **cross fracture** is not mentioned in the provided texts, so it is not associated with blood vessel injuries.