Increased Critical Shoulder Angle Impairs Tendon-Bone Healing in a Rat Model of Chronic Rotator Cuff Tears

Background: The role of an elevated critical shoulder angle (CSA) in rotator cuff healing following rotator cuff repair (RCR) remains a subject of clinical controversy. The present study aimed to investigate the effect of increased CSA on tendon-bone interface healing following RCR. Methods: A bilateral chronic rotator cuff tear model was established in 48 Sprague-Dawley rats. Acromion lateralization (Acr) surgery was performed unilaterally to increase CSA. After 4 weeks, bilateral RCR was performed. Micro-computed tomography was utilized to measure CSA. Tendon-bone interface healing was assessed at 3, 6, and 9 weeks post-RCR with use of magnetic resonance imaging (MRI), biomechanical testing, gait analysis, and histological evaluation. Results: The mean CSA in the Acr group was significantly greater than that in the RCR-only group (37.2° ± 2.6° versus 29.7° ± 3.1°; p < 0.001). At 6 and 9 weeks postoperatively, the Acr group demonstrated significantly poorer outcomes on MRI (i.e., higher signal-to-noise quotient), biomechanical strength (i.e., lower ultimate failure load and stiffness), and gait parameters compared with the RCR-only group (p < 0.05). Histological analysis revealed inferior tendon-bone interface integration in the Acr group (p < 0.01), including reduced fibrocartilage formation, disorganized collagen fibers, and a lower collagen I/III ratio. Immunohistochemistry showed significantly higher Piezo1 expression in the Acr group (p < 0.001), suggesting a mechanobiological response to increased mechanical stress. Conclusions: An increased CSA impaired tendon-bone interface healing following RCR in a rat model. Although these findings were preclinical, they provide experimental evidence that an increased CSA may influence rotator cuff healing, supporting the potential role of CSA modification (e.g., with acromioplasty) in reducing the risk of retear. Clinical Relevance: The present study provides experimental evidence to support the consideration of CSA reduction in selected high-risk patients undergoing RCR to promote rotator cuff healing and potentially reduce retear rates.