Three-Dimensional Sectional Measurements of Shoulder Muscle Volume and Computed Tomography Density to Monitor Serial Postoperative Volume Changes in the Transverse Force Couple of Shoulder Muscles in Anterior Shoulder Instability.

Purpose: In this study, we evaluated serial changes in shoulder muscle volume and computed tomography (CT) density of the transverse force couple (i.e., subscapularis (Ssc) vs. infraspinatus and teres minor (Isp+TM) muscles) after Bankart repair surgery in patients with anterior shoulder instability (ASI).

Methods: Four consecutive CT scans (obtained preoperatively and postoperatively at 1 month, 6 months, and 12 months) of 24 shoulders from 24 patients who underwent arthroscopic rotator cuff repair were examined. To generate muscle models, the Ssc and Isp+TM muscles were segmented. The reconstructed models were separated by the Y-view plane, and the planes were situated 2.5 cm and 5 cm medial to the Y-view plane. Balance was evaluated using volume ratio (VR) and computed tomography-density ratio (CT-DR) in each section of both groups, employing a three-dimensional sectional approach. Changes in these values were correlated with the preoperative background factors.

Results: Shoulder muscle volume and CT density were decreased in the early postoperative period compared with the preoperative period but recovered through the late postoperative period. The mean VR decreased in all sections in the early postoperative period but increased thereafter, whereas the mean CT-DR in all sections remained mostly unchanged throughout the perioperative period. Furthermore, the perioperative differences in muscle volume were correlated with sex, work and sports activity level, Hill-Sachs lesion, and range of motion. Perioperative differences in CT density were not correlated with most items, except for sulcus signs.

Conclusion: Serial changes in the VR group were significantly different and had clinical implications, whereas changes in the CT-DR group were not significantly different. This method may provide a potential indicator for evaluating muscle balance and recovery after Bankart repair surgery for ASI.