Three-dimensional analysis of shoulder hand-behind-back motion in patients with frozen shoulder

Hand-behind-back (HBB) motion is commonly used to assess shoulder internal rotation; however, it involves multiple joint movements, including scapulothoracic, glenohumeral, and elbow joint motions, thus complicating the interpretation of the HBB position. This study aimed to investigate 3-dimensional joint alignment during HBB motion of the bilateral shoulders in patients with frozen shoulder and to identify which joint motions primarily contribute to motion limitation. Seventeen patients with unilateral frozen shoulder underwent bilateral shoulder computed tomography scans in the neutral and HBB positions using an upright multidetector computed tomography. Bone surface models of the thorax, scapula, humerus, and forearm were created to calculate the rotation of the scapulothoracic, glenohumeral, thoracohumeral, and elbow joints. The affected and unaffected sides were compared, and the correlation between the HBB reach level (vertebral level) and the thoracohumeral internal rotation angle was analyzed. No significant differences in joint rotation were observed between the sides in the neutral position. The affected side demonstrated significantly reduced glenohumeral internal rotation (31&#xb0; vs. 66&#xb0;, P < .001), abduction (10&#xb0; vs. 22&#xb0;, P < .001), and elbow flexion (74&#xb0; vs. 115&#xb0;, P < .001) during HBB motion. The scapulothoracic joint on the affected side had less internal rotation (18&#xb0; vs. 21&#xb0;; P = .045) and more anterior tilt (24&#xb0; vs. 19&#xb0;; P = .003). The HBB reach level was negatively correlated with the thoracohumeral internal rotation angle in the unaffected and affected sides (r = -0.518 and r = -0.675, respectively). This study clarified that limited internal rotation of the glenohumeral joint is the main cause of restricted HBB motion in frozen shoulder, which supports the use of the thumb-to-spinous process as a valid measure of thoracohumeral internal rotation in patients with frozen shoulder.