In Gaenslen's test, which SI joint is being stressed with the top leg extended?

In Gaenslen's test, the focus is on the sacroiliac (SI) joints and how they respond to specific positioning. When the patient is positioned with one leg extended off the examination table, the top leg, which is the leg that is extended, causes a stress on the top SI joint. This positioning places tension on the ligaments and structures associated with that SI joint, thereby allowing the clinician to assess for potential instability or pain that may indicate pathological conditions in that joint.

By extending the top leg, there is a directional force through the pelvis that emphasizes the stress on the side where the leg is hanging. This is critical for evaluating the SI joint function and pain response. Consequently, it is understood that Gaenslen's test primarily evaluates the stability and integrity of the SI joint on the side of the extended leg, making the choice about the top SI joint accurate in this context.