Final answer:
A hip spica cast is used to immobilize a femoral shaft fracture completely. This cast extends from the chest or abdomen down the injured leg to stabilize the fracture.
Step-by-step explanation:
The kind of cast used to completely immobilize a femoral shaft fracture is the hip spica cast. This type of cast extends from the chest or abdomen down the injured leg, sometimes including the foot, and up the opposite side to the groin, ensuring that the fracture is kept stable and secure. Hip spica casts are typically used for femoral fractures in children, but may also be utilized in adults in certain circumstances.