Final answer:
A butterfly fracture is caused by a bending force, which creates a characteristic triangular fragment in the bone due to a combination of bending and axial compression.
Step-by-step explanation:
The type of force that causes a butterfly fracture is a bending force. Butterfly fractures are a type of bone fracture characterized by a triangular fragment that is separated on two sides from the main bone, resembling the wings of a butterfly hence the name. It occurs when there is a bending moment, combined with axial compression, applied to the bone, causing it to split in such a pattern. In contrast, compressive forces would lead to a different type of break, while torsional forces and shearing forces are associated with spiral fractures and other patterns of bone injury, respectively.