What mechanism primarily causes an unstable pelvic fracture?

An unstable pelvic fracture primarily results from anteroposterior compression because this type of force can significantly disrupt the normal alignment and support structures of the pelvis. When anteroposterior compression occurs, it creates a mechanism of injury where the front (anterior) and back (posterior) aspects of the pelvis are subjected to extreme compressive forces that can lead to fractures and separation of the pelvic ring. This disruption affects the stability of the pelvis, causing it to be unstable as it can no longer maintain its integral structure under load.

In contrast, other mechanisms, such as direct blows or direct trauma from weight, can cause fractures, but they typically result in stable injuries when they do not involve significant displacement or disruption of the pelvic ring. Excessive twisting can lead to injuries like ligamentous tears or stable fractures but is less likely to result in the devastating instability associated with anteroposterior compression. Thus, the nature of anteroposterior compression as a mechanism highlights the severity and instability of the resulting pelvic injuries.