Evaluation of the lag sign tests for external rotator function of the shoulder.

Rotator cuff lesions pose a serious clinical challenge. The objective of this study was to verify the biomechanical basis for the lag sign clinical tests for rotator cuff dysfunction. The lag sign tests were simulated in vitro by a sensor-guided robotic simulator configured to reproduce in vivo testing conditions. The ability of the test to isolate supraspinatus and/or infraspinatus dysfunction was investigated from 20 degrees to 90 degrees of scapular plane arm elevation. The test was 100% sensitive for lack of all infraspinatus-teres minor force at all elevations tested (6/6 specimens at 20 degrees, 30 degrees, and 60 degrees elevation; 5/5 at 90 degrees elevation). The test was less sensitive to complete loss of supraspinatus force (1/6 specimens testing positive at 20 degrees, 0/6 at 30 degrees, 3/6 at 60 degrees, and 3/5 at 90 degrees elevation). The results of this biomechanical study suggest the lag sign tests to be highly sensitive only for infraspinatus-teres minor muscle dysfunction.