Thumb to spinous process is a false metric for glenohumeral internal rotation

BACKGROUND: The measurement of internal rotation by noting the maximal vertebral level reached by the patient’s thumb behind their back is an established physical examination technique, as outlined in the American Shoulder and Elbow Surgeons Shoulder Assessment Form.(7) The purpose of the present study is to correlate real-time glenohumeral internal rotation with thumb to spinous process movement to determine the accuracy of the technique. METHODS: Healthy volunteers with no previous history of shoulder injury or symptoms were recruited from the local medical school population. Ultrasound probe was placed over the anterolateral shoulder, and relevant anatomy was identified. Internal rotation was evaluated by measuring displacement of the peak of the medial aspect of the bicipital groove relative to the anterior glenoid rim with the arm held in defined positions of progressively increasing internal rotation. The difference in displacement between arm positions was calculated and recorded. RESULTS: A total of 20 participants (11 women/9 men, aged 22-42 years) were recruited for measurement. A mixed-model repeated-measures analysis of variance was used. The most significant differences in displacement, and therefore internal rotation, were observed between the neutral and anterior superior iliac spine (0.21 ± 0.39 mm, P= .0269) and between the anterior superior iliac spine and peak iliac crest (0.26 ± 0.44 mm, P= .0163). After the peak iliac crest, there was no further statistically significant change in rotation. CONCLUSION: The present study suggests that most glenohumeral internal rotation occurs before reaching the arm behind the back. Although not directly studied, this supports the notion that the maximal vertebral level reached involves an interplay of various joint motions. While the thumb to spinous process maneuver remains a functional evaluation, our results suggest a different examination technique be used to more accurately test glenohumeral internal rotation.