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_Ssc/Isp+TM) and computed tomography-density ratio (CT-DR_Ssc/Isp+TM) 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_Ssc/Isp+TM decreased in all sections in the early postoperative period but increased thereafter, whereas the mean CT-DR_Ssc/Isp+TM 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_Ssc/Isp+TM group were significantly different and had clinical implications, whereas changes in the CT-DR_Ssc/Isp+TM group were not significantly different. This method may provide a potential indicator for evaluating muscle balance and recovery after Bankart repair surgery for ASI.