The Jerk Test is a provocative test used to diagnose posterior shoulder instability. Traumatic posterior instability is underdiagnosed and is less common than anterior instability. Posterior instability of the shoulder is common in athletes such as football players (linemen in the blocking position). Posterior instability of the shoulder is usually traumatic in the majority of patients. Injury typically occurs in football linemen who sustain injury during blocking. Flexion, adduction, and internally rotation of the arm is a high-risk position. A posterior labral tear or detachment is common in this injury. This is called a Kim lesion or a Reverse Bankart lesion. The patient will have pain with posteriorly directed force or pressure. The patient may also have the feeling like the shoulder is “slipping” out. There will be no apprehension in abduction and external rotation of the shoulder.
There are several provocative tests used for posterior instability, the posterior load and shift test, jerk test, and the posterior apprehension test. A combination of the Jerk Test and the Kim test will give approximately 97% sensitivity. During the Jerk Test, the patient’s arm is abducted to 90 degrees and internally rotated and the elbow is bent. The examiner axially loads the humerus while the arm is moved horizontally across the body. The arm is then adducted and the shoulder will be flexed. Axial loading will be continuously applied at this point. A positive test is indicated by sharp pain in the shoulder with or without a clicking sound. In some cases, the patient may have the sense of instability, but no pain.
Surgery, either open or arthroscopic, will be performed for posterior labral repair; especially for recurrent posterior shoulder instability that is not getting better with rehab. The Jerk Test is used as a predictor for the prognosis of nonoperative treatment to the posterior shoulder instability. A painful Jerk Test will have a higher failure rate with nonoperative treatment for the posterior labral lesion.
