Compensatory Activation of Periscapular Muscles Aids Active Abduction in Patients With Massive Rotator Cuff Tears

Background Patients with massive rotator cuff tears can present with shoulder pain with preserved ROM, yet the compensatory mechanisms remain poorly understood. Identifying these mechanisms, particularly the role of periscapular muscles, could guide nonsurgical therapeutic strategies. Questions/purposes (1) In patients with massive rotator cuff tears who achieved acceptable active ROM with physical therapy, which periscapular muscles provide compensatory motor activity? (2) What is the correlation between muscle metabolic activity and fatty infiltration and atrophy? Methods Between January 2019 and April 2019, we evaluated 39 consecutive patients who presented to our outpatient clinic and were diagnosed with massive rotator cuff tears through sonographic screening. Of these, 41% (16) were excluded because of concomitant diseases. Of the remaining 59% (23) of patients who met the inclusion criteria and began the rehabilitation program, 13% (5) discontinued because of partial adherence or conversion to surgery. Ultimately, 46% (18) of patients completed the rehabilitation program and were included in the final analysis, comprising 8 with anterosuperior tears (median [IQR] age 56 years [54 to 61]) and 10 with posterosuperior tears (median [IQR] age 59 years [58 to 64]). Additionally, four nuclear medicine technicians without a history of shoulder injury or pain were recruited as the control group (median [IQR] age 54 years [52 to 56]). To address our first research question—identifying the muscles responsible for compensatory motor activity after a massive rotator cuff tear—participants underwent positron emission tomography/CT with fluorodeoxyglucose (FDG-PET/CT) imaging after performing a scaption exercise to assess muscle metabolic activity. Standardized uptake values (SUVs), reflecting glucose-based metabolic activity, were calculated for the periscapular, rotator cuff, and deltoid muscles using FDG-PET/CT and compared among groups using Kruskal-Wallis tests. To address the second research question—examining the correlation between muscle metabolic activity and fatty infiltration or atrophy—MRI was used to assess the Goutallier classification and occupation ratio, and Spearman correlation analysis was performed to evaluate their relationship with SUVs. Continuous variables were expressed as median and IQR. Results In patients with posterosuperior rotator cuff tears who regained acceptable active ROM, several periscapular muscles exhibited increased activity after shoulder abduction exercises compared with controls. This pattern was not observed in those with anterosuperior tears. SUVs were significantly higher in the posterosuperior group than in controls for the levator scapulae (0.75 [95% confidence interval (95% CI) 0.73 to 0.81] versus 0.65 [95% CI 0.60 to 0.71], mean rank difference 9.33; p = 0.04), rhomboids (0.80 [95% CI 0.70 to 0.85] versus 0.65 [95% CI 0.62 to 0.68], mean rank difference 9.58; p = 0.03), pectoralis major (0.54 [95% CI 0.49 to 0.55] versus 0.47 [95% CI 0.40 to 0.51], mean rank difference 6.58; p = 0.04), and teres major (0.62 [95% CI 0.55 to 0.75] versus 0.51 [95% CI 0.47 to 0.55], mean rank difference 9.28; p = 0.03). In the anterosuperior group, SUV was significantly higher than in controls only for the teres major (0.63 [95% CI 0.55 to 0.69] versus 0.51 [95% CI 0.47 to 0.55], mean rank difference 8.69; p = 0.04). Among patients with massive rotator cuff tears who regained acceptable active ROM, greater fatty infiltration and lower occupation ratios—reflecting more severe muscle atrophy—were associated with reduced muscle activation. Moderate correlations were observed between fatty infiltration and SUV (ρ = -0.531 [95% CI -0.805 to -0.071]; p = 0.02) and between occupation ratio and SUV (ρ = 0.493 [95% CI 0.018 to 0.786]; p = 0.04) in the supraspinatus, but not in other rotator cuff muscles. Conclusion Compensatory activation of the periscapular muscle can be observed when abduction ROM is restored in patients with massive rotator cuff tears after physical therapy. Increased metabolic activity in the scapular elevators and eccentric humeral head depressors suggests that they may contribute to this functional adaptation. Clinical Relevance Based on these findings, future studies may explore whether physical therapy protocols that specifically target periscapular muscle activation can enhance functional outcomes in patients with massive rotator cuff tears managed nonsurgically.