What indicates a positive Jerk test?

• A. Posterior instability; sudden jerk or clunk as the humeral head subluxes posteriorly
• B. Inferior instability; increased sulcus width
• C. Anterior instability; apprehension with external rotation
• D. Posterior instability; pain with internal rotation

Answer: (A)

The main idea is posterior shoulder instability detected by a clear jerk or clunk. In the Jerk test, the arm is brought into flexion with internal rotation, and an axial load is applied through the humerus as the arm is then adducted across the body. If the humeral head has posterior laxity, it will suddenly subluxate posteriorly, producing a distinct jerk or clunk that you can feel or hear. That abrupt movement of the humeral head in the posterior direction is what confirms a positive Jerk test.

This aligns with the option describing posterior instability accompanied by a sudden jerk or clunk as the humeral head subluxes posteriorly. Other choices point to different signs: inferior instability is typically tested with the sulcus sign; anterior instability shows up as apprehension with external rotation; and posterior instability with pain during internal rotation lacks the specific clunk/jerk trigger that defines a positive Jerk test.