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An SI joint fracture with posterior disruption is classified as totally unstable due to the nature of the injury involving both the sacroiliac joint and the surrounding structures, resulting in significant displacement. In such cases, there is a complete disruption of the joint's normal anatomy and stability, often leading to the potential for further complications such as neurological injury or vascular compromise.

In this situation, the instability implies that the fracture may require surgical intervention to realign the joint and restore proper function, as well as to stabilize the pelvis. This is in contrast to partially unstable or stable classifications, which would suggest some level of retained alignment or stability in the joint. A totally unstable classification emphasizes that the normal anatomical relationships are not maintained, and immediate medical intervention is often necessary to address the instability.