Rotator cuff injuries cause pain, disability, and loss of shoulder function in over 17 million individuals in the United States that result in over 500,000 surgeries performed annually, though with alarming failure rates of 20-94% (Colvin et al., 2012;​ Galatz et al., 2004;​ Harryman et al., 1991;​ Jain et al., 2014;​ Mather et al., 2013;​ Oh et al., 2007;​ Vitale et al., 2007;​ Yamaguchi et al., 2006). These surgeries involve repair or reconstruction of the damaged rotator cuff tendon(s) along with enlargement of the subacromial space by debriding the overlying bone (acromion) and removing the subacromial bursa (Beard et al., 2018;​ Burkhart et al., 2016;​ Dines et al., 2006;​ Lo & Burkhart, 2003;​ Rossi & Ranalletta, 2020).

The subacromial bursa is a synovial-like tissue that is situated between the acromion and the tendons of the rotator cuff. This tissue has been long understood to serve a primarily mechanical role by providing cushioning and protecting from friction-wear from the acromion on the underlying tendons. More recently, the identification of a robust vascular network within the bursa, a resident population of mesenchymal stem cells, and inflammatory responsiveness to rotator cuff pathology have supported the existence of a biological role of this tissue in addition to its mechanical one (Blevins et al., 1997;​ Gotoh et al., 1998, 2001;​ Põldoja et al., 2017;​ Rathbun & Macnab, 1970;​ Steinert et al., 2015;​ Yepes et al., 2007).

hese observations make surgical excision of the bursa problematic, given our current lack of understanding of the implications of removing the bursa on the biological response to tendon injury. Therefore, the goals of this dissertation were three-fold:​ (1) to determine the role of the subacromial bursa in the rotator cuff tendon response to injury and healing, (2) to interrogate patterns of cellular crosstalk between the subacromial bursa and the rotator cuff following injury, and (3) to demonstrate therapeutic potential of targeting the subacromial bursa for modulating inflammation and improving tendon healing.

Motivated by clinically observed phenotypic changes in the subacromial bursa with rotator cuff pathology, the profiles of human bursa and rotator cuff tendon tissues were assessed using histology, proteomics, and transcriptomics. This data set, analyzed in the context of patient demographics and diagnoses, revealed distinct bursa proteomes according to tissue phenotype (i.e., fibrous, vascular, or fatty), patient age, and presence of a tear in the underlying rotator cuff. These results suggested the presence of crosstalk between the rotator cuff and the bursa that had not been previously appreciated.

Employing multiple methods of validation, including histology, microcomputed tomography, gene expression, and flow cytometry, the rat bursa was established as an appropriate animal model of the human bursa. Therefore, we used the rat model to investigate the role of the bursa in tendon injury response and healing;​ tendon injuries were created surgically with or without a subsequent repair to study healing and responses to injury, respectively. The role of the bursa in the response to injury was assessed using gene expression, transcriptomics, and histology. The bursa promoted inflammatory gene expression in the injured supraspinatus but resolved inflammatory gene expression in the intact infraspinatus. The role of the bursa in tendon healing was assessed using gene expression, histology, microcomputed tomography, and tensile mechanical testing of the cuff tendons. Consistent with responses during the inflammatory phase of healing, the bursa promoted expression of genes related to aberrant, scar-mediated healing in the supraspinatus, whereas it promoted tenogenic and tendon extracellular matrix gene expression in the intact infraspinatus. Mechanical testing demonstrated that the bursa protected the infraspinatus from the inflammatory environment caused by the supraspinatus injury but had a limited functional effect on the healing supraspinatus. Microcomputed tomography also indicated bursa-dependence in cortical and trabecular bone remodeling following tendon injury.

Cross-talk between the bursa and the tendon was then studied in a novel tissue explant co-culture platform using gene expression and nitric oxide release as outcome measures. These experiments revealed that the activated bursa engaged in immunomodulation of tendon fibroblast responses to inflammatory stimulus. The in vitro platform also established the glucocorticoid dexamethasone as a viable therapeutic candidate for bursa-targeted treatment based on its capacity to regulate the bursa’s response to an inflammatory stimulus and enhance the bursa’s immunomodulatory potential. Therefore, in the final component of this thesis, dexamethasone was delivered via PLGA microspheres in vivo to the bursa to modulate the post-injury inflammatory response in the supraspinatus and the infraspinatus tendons. Results supported the therapeutic potential of this treatment approach to improve rotator cuff healing outcomes.

This body of work demonstrated a robust involvement of the bursa in rotator cuff responses to injury, with distinct roles in the underlying injured and intact tendons. This work also established, for the first time, the immunomodulatory capacity of the activated bursa and provided strong evidence against the clinical practice of bursectomy. Finally, use of sustained-release dexamethasone to dampen the inflammatory responses to rotator cuff injury offers a new direction for harnessing the inherent properties of the subacromial bursa therapeutically for improved rotator cuff tendon healing.