Increased Deltoid Muscle Volume After Scope-Assisted Lower Trapezius Tendon Transfer.

Background: Scope-assisted lower trapezius tendon transfer (SALTT) has emerged as a viable treatment option for posterosuperior irreparable rotator cuff tears (PSIRCTs) due to its biomechanical advantages and favorable clinical outcomes. However, there is limited research on deltoid muscle volume (DMV) and muscle strength recovery after SALTT. Purpose: To evaluate the changes in DMV and its effect on clinical outcomes-including muscle strength after SALTT. Study Design: Case-control study; Level of evidence, 3. Methods: This retrospective study analyzed 54 patients with PSIRCT who underwent SALTT from January 2021 to January 2022, with a minimum follow-up of 2 years. The deltoid muscle area of each 2-dimensional axial magnetic resonance image was identified and segmented using ITK-SNAP automatic segmentation software. The 3-dimensional (3D) deltoid muscle model was generated by combining each segmented 2D axial image, and the volume was automatically calculated. The clinical assessment included shoulder pain, patient-reported outcome measures (PROMs), active range of motion (aROM), and muscle strength of aROM. Results: The postoperative clinical outcomes were significantly improved after SALTT. The postoperative body mass index-adjusted DMV (aDMV) showed a statistically significant increase compared with the preoperative aDMV. Based on the mean postoperative Constant score, 32 patients had scores above the mean (optimal function group), while 22 patients had scores below the mean (suboptimal function group). The pre-aDMV and post-aDMV in the optimal function group were significantly larger than those in the suboptimal function group. However, there was no significant difference in ΔaDMV (the difference between pre-aDMV and post-aDMV) between the 2 groups. A graft rupture occurred in 6 patients. The graft-intact group showed statistically significantly better outcomes than the graft-rupture group. Although there was no difference in pre-aDMV between the 2 groups, post-aDMV and ΔaDMV were significantly larger in the graft intact group. The ΔaDMV is significantly correlated with postoperative PROMs, aROM, and muscle strength. Conclusion: This study confirmed that the postoperative aDMV was significantly increased after SALTT, and the increase of aDMV was significantly associated with good clinical outcomes-including PROMs, aROM, and aROM strength. These findings contribute to understanding the mechanism of aROM strength recovery by demonstrating an increase in DMV after SALTT.