Final answer:
An SI joint fracture with posterior disruption is considered partially unstable.
Step-by-step explanation:
An SI joint fracture with posterior disruption is considered partially unstable. The sacroiliac joint (SI joint), located between the sacrum and ilium, allows for limited movement. When the joint is fractured with posterior disruption, it means there is a disruption of the ligaments supporting the posterior side of the joint.
This disruption can lead to instability and abnormal movement of the joint. However, it is not completely unstable as the joint still has some stability and limited movement.
In summary, an SI joint fracture with posterior disruption is considered partially unstable.