The rotator cuff assists in shoulder movement and provides dynamic stability to the glenohumeral joint. Specifically, the anterior–posterior (AP) force balance, provided by the subscapularis anteriorly and the infraspinatus and teres minor posteriorly, is critical for joint stability and concentric rotation of the humeral head on the glenoid. However, limited understanding exists of the consequences associated with disruption of the AP force balance (due to tears of both the supraspinatus and infraspinatus tendons) on joint function and joint damage. We investigated the effect of disrupting the APforce balance on joint function and joint damage in an overuse rat model. Twenty-eight rats underwent 4 weeks of overuse to produce a tendinopathic condition and were then randomized into two surgical groups: Detachment of the supraspinatus only or detachment of the supraspinatus and infraspinatus tendons. Rats were then gradually returned to their overuse protocol. Quantitative ambulatory measures including medial/lateral, propulsion, braking, and vertical forces were significantly different between groups. Additionally, cartilage and adjacent tendon properties were significantly altered. These results identify joint imbalance as a mechanical mechanism for joint damage and demonstrate the importance of preserving rotator cuff balance when treating active cuff tear patients.

Background Pathology in the long head of the biceps tendon often occurs in patients with rotator cuff tears. Arthroscopic tenotomy is the most common treatment. However, the role of the long head of the biceps at the shoulder and the consequences of surgical detachment on the remaining shoulder structures remain unknown. Questions/purposes We hypothesized that detachment of the long head of the biceps, in the presence of supraspinatus and infraspinatus tears, would decrease shoulder function and decrease mechanical and histologic properties of both the subscapularis tendon and the glenoid articular cartilage. Methods We detached the supraspinatus and infraspinatus or the supraspinatus, infraspinatus, and long head of the biceps after 4 weeks of overuse in a rat model. Animals were gradually returned to overuse activity after detachment. At 8 weeks, the subscapularis and glenoid cartilage biomechanical and histologic properties were evaluated and compared. Results The group with the supraspinatus, infraspinatus, and long head of the biceps detached had greater medial force and decreased change in propulsion, braking, and vertical force. This group also had an increased upper and lower subscapularis modulus but without any differences in glenoid cartilage modulus. Finally, this group had a significantly lower cell density in both the upper and lower subscapularis tendons, although cartilage histology was not different. Conclusions Detachment of the long head of the biceps tendon in the presence of a posterior-superior cuff tear resulted in improved shoulder function and less joint damage in this animal model. Clinical Relevance This study provides evidence in an animal model that supports the use of tenotomy for the management of long head of the biceps pathology in the presence of a two-tendon cuff tear. However, long-term clinical trials are required.

The objective of this study was to identify acute responses and chronic adaptations of supraspinatus tendon to noninjurious exercise. We hypothesized that chronic exercise (EX) increases tendon mechanical properties, and a single exercise bout increases matrix metalloproteinase (MMP) activity acutely. Rats were divided into acute or chronic EX or cage activity groups. Animals in acute EX groups were euthanized, 3, 12, 24, 48, or 72 h upon completion of a single bout of exercise (10 m/min, 1 h) on a flat treadmill. Animals in chronic EX groups walked on a flat treadmill for 3 days or 1, 2, or 8 wk. Tendon histology, MMP activity, and mechanics were measured. A single bout of exercise trended toward reducing tendon mechanical properties, but 2 or 8 wk of chronic EX increased tendon mechanics. Cell density was not affected. Cells became rounder with chronic EX. All tendons were highly organized. MMP activity decreased after a single bout of exercise and returned to baseline by 72 h. MMP activity decreased after 8 wk of chronic EX. Decreased MMP activity may indicate an anabolic instead of catabolic response in contrast to injury. Results suggest that mild, acute decreases in MMP activity and tendon mechanics following a single exercise bout lead to enhanced tendon mechanical adaptations with repeated exercise bouts. This study defines acute and chronic changes of MMP activity, mechanical properties, and histology of the rat supraspinatus tendon in response to beneficial exercise and proposes a mechanism by which acute responses translate to chronic adaptations. The line between beneficial exercise and overuse has not been elucidated. This study defines the acute and chronic temporal response to exercise of supraspinatus tendon in an in vivo model. We found that decreased matrix metalloproteinase activity and tendon mechanics after a single bout of exercise are followed by beneficial chronic adaptations of the tendon with repeated bouts. How the acute responses to exercise lead to chronic adaptations may distinguish beneficial exercise from overuse.

Exercise protocol induces muscle, tendon, and bone adaptations in the rat shoulder tendon collagen content, cross-sectional area, and mechanical properties were measured. Supraspinatus muscle cross-sectional area, weight, and the expression of mitochondrial oxidative phosphorylation (OXPHOS) proteins were measured. Humeri were analyzed with μCT and mechanically tested. Results: exercise decreased fat pad mass. Supraspinatus muscle hypertrophied and had increased OXPHOS proteins. Humerus trabecular bone had increased anisotropic orientation, and cortical bone showed increased bone and tissue mineral density. Importantly, the supraspinatus tendon did not have diminished mechanical properties, indicating that this protocol was not injurious to the tendon. Conclusion: this study establishes the first rat exercise protocol that induces adaptations in the shoulder. Future research can use this as a comparison model to study how the supraspinatus tendon adapts to loading and undergoes degeneration with overuse.

Background Previous studies have shown that ibuprofen is detrimental to tissue healing following acute injury; however, the effects of ibuprofen when combined with non-injurious exercise are debated. Hypothesis We hypothesized that administration of ibuprofen to rats undergoing a non-injurious treadmill exercise protocol would abolish the beneficial adaptations found with exercise but have no effect on sedentary muscle and tendon properties. Study Design Controlled laboratory study Methods Rats were divided into exercise or cage activity (sedentary) groups and acute (a single bout of exercise followed by 24 hours of rest) and chronic (2 or 8 weeks of repeated exercise) time points. Half of the rats received ibuprofen to investigate the effects of this drug over time when combined with different activity levels (exercise and sedentary). Supraspinatus tendons were used for mechanical testing and histology (organization, cell shape, cellularity), and supraspinatus muscles were used for morphological (fiber CSA, centrally nucleated fibers) and fiber type analysis. Results Chronic intake of ibuprofen did not impair supraspinatus tendon organization or mechanical adaptations (stiffness, modulus, max load, max stress, dynamic modulus, or viscoelastic properties) to exercise. Tendon mechanical properties were not diminished and in some instances increased with ibuprofen. In contrast, total supraspinatus muscle fiber cross-sectional area decreased with ibuprofen at chronic time points, and some fiber type-specific changes were detected. Conclusions Chronic administration of ibuprofen does not impair supraspinatus tendon mechanical properties in a rat model of exercise but does decrease supraspinatus muscle fiber cross-sectional area. Clinically, these findings suggest that ibuprofen does not detrimentally affect regulation of supraspinatus tendon adaptions to exercise but does decrease muscle growth. Individuals should be advised on the risk of decreased muscle hypertrophy when consuming ibuprofen. This fundamental study adds to the growing literature on the effects of ibuprofen on musculoskeletal tissues and provides a solid foundation on which future work can build. Clinical Relevance Ibuprofen is a commonly used drug by sedentary individuals and athletes. This study suggests that ibuprofen has tissue-dependent effects that should be considered when prescribing the drug.